Astronomy

Comparing Star Formation rate in different galaxies

Comparing Star Formation rate in different galaxies


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I was looking for some scale to compare my result of star formation rate in order to find out whether it is moderate or high or low, but I couldn't find any scale. Is there any paper related to such scale where Star formation rate form minimum to maximum of each galaxy is given and once can compare their result with scale.

I found that one of dwarf galaxies has 0.2 star formation rate, but since I don't have scale to compare, thus I have no clue whether it is high or low or moderate? What is the maximum star formation rate?


Not being an expert in star formation, I found a well-written paper summary from which I conclude that typical star formation rates range between $6 ldots 24 M_odot / yr$.

The blog quotes the following graph by M. Boquien, V. Buat, and V. Perret, see https://arxiv.org/abs/1409.5792

In this paper we investigate in isolation the impact of a variable star formation history on the measurement of the SFR. We combine 23 state-of-the-art hydrodynamical simulations of 1

In summary: To me, $0.2 M_odot / yr$ looks rather small, but I cannot judge whether it is unrealistic for a dwarf galaxy.


A Turbulent Law of Star Formation

Title: A Universal Turbulence-Regulated Star Formation Law: From Milky Way Clouds to High-Redshift Disk and Starburst Galaxies
Authors: Diane M. Salim, Christoph Federrath, Lisa J. Kewley
First Author’s institution: Australian National University
Status: Accepted to Astrophysical Journal Letters

Stars form when dense cores of gas collapse inside molecular clouds. To measure the star formation rate in nearby clouds, like the Taurus Molecular Cloud, we can simply count up the number of newly formed stars. To measure star formation in distant galaxies, we do not have this luxury. We must rely on integrated properties – such as H-alpha emission – to trace the overall star formation rate in the galaxy.

Because dense gas forms stars, there is a well-known observed relation between the star-formation rate and gas density. This observed relationship is the famous Kennicutt-Schmidt law. Described in this Astrobite, the Kennicutt-Schmidt law is often used in galaxy evolution models in order to convert gas density to star formation rate to compare with observations. But the Kennicutt-Schmidt law only relates the average gas density in a galaxy to the star-formation rate. In reality, only the densest gas clumps in larger clouds form stars, so a more physical treatment would relate the distribution of gas densities to the observed star formation rate. This is the goal of today’s paper: a more physically realistic star formation law that works in our galaxy and beyond.

Turbulence in molecular clouds

One of the major problems in star-formation theory has been to understand why star formation is so inefficient. Only about 1% of gas in molecular clouds is converted to stars. The updated star formation law presented by the authors relies on the recent consensus that turbulence – random particle motions – is crucial for preventing too much gas from collapsing into stars. Check out this Astrobite for a video that shows how important turbulence is for regulating star formation and take a look at this article for an artistic treatment of turbulence.

How do we define just how turbulent a gas is? One way is to use the Mach number of the gas. The Mach number is a measure of the turbulent motions in the gas compared to the speed of sound, and can be determined by observing the width of emission lines from molecules such as carbon monoxide. The higher the Mach number, the more turbulent the gas, and the greater range in gas densities in the cloud. Therefore, a higher Mach number means more gas will have sufficient density to collapse into stars, resulting in a higher star formation rate. This relation between Mach number and density variance forms the basis for a new star formation law.

Accounting for turbulence in the star formation law

By comparing the star formation rate to the amount of gas consumption expected given the distribution of densities in the gas – as governed by turbulence – the authors fit observations of clouds in the Milky Way to determine the new star formation law. The figure below shows how the Kennicutt-Schmidt law (far left), which simply compares gas density to star formation rate, compares to the newly derived star formation law (far right). Because the variation in Mach number is taken into account, the scatter in the new relation is reduced by a factor of 3-4 compared to the Kennicutt-Schmidt law.

Star-formation laws. The Kennicutt-Schmidt law is on the far left, while the authors’ new law is on the far right. The points represent observations of star forming regions in the Milky Way and Small Magellanic Cloud that span several orders of magnitude in gas density and star formation rate. Including turbulence by way of the Mach number produces a more physically realistic relation with less scatter.

What about other galaxies?

In the Milky Way, the Mach number is determined by measuring turbulent velocity width and gas temperature using high-resolution molecular emission line maps. This is very difficult for other galaxies. By inverting the new star-formation law, the Mach number can be predicted for these distant systems if star formation rate and gas density have been measured. Specifically, the authors predict that starburst galaxies at high redshift have more turbulent gas than local spiral galaxies.

As Arthur Dent could tell you, turbulence is a strange beast, but a deep study of this phenomenon is important in understanding how gas turns into stars, in our galaxy and beyond.


Exploring the Host Galaxies of Changing-Look AGN

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites the original can be viewed at astrobites.org.

Title: The Landscape of Galaxies Harboring Changing-Look Active Galactic Nuclei in the Local Universe
Authors: Sierra A. Dodd et al.
First Author’s Institution: University of California, Santa Cruz
Status: Published in ApJL

As we grow older and learn more about ourselves, we sometimes think about changing our appearance: a new hairstyle, clothes, or maybe even a tattoo! Active galactic nuclei (AGN) are, in this respect, no different. Over the course of a galaxy’s lifetime it will accrete gas and dust that will make its way to the centre, where its supermassive black hole (SMBH) resides. As the SMBH feeds on this material, it will emit huge amounts of radiation and become an AGN. However, certain conditions can cause disruptions in the AGN’s gas supply. Whilst the cause of these disruptions is unclear, we can see the effects in the AGN’s changing spectrum. Either they “turn on” as broad optical emission lines emerge or “turn off” as those lines disappear. Today’s authors are interested in identifying what kind of galaxies host these changing-look AGN (CL-AGN) to try to isolate the conditions that might trigger these changes.

Constructing the AGN Lookbook

Identifying CL-AGN involves a comparison between two sets of spectra of the same galaxy taken at two different times to find the changes in the broad emission lines. Different studies will approach this task in their own way but are ultimately following this principle. This is true for all identification techniques, but CL-AGN have only been identified as a phenomenon relatively recently, so very few have been found. To produce today’s sample of 17 CL-AGN, the authors have had to combine detections from three different studies that follow this broad approach but with their own unique characteristics. So, what one study calls a CL-AGN might be slightly different from another. In addition, all but two of these detections are turn-on AGN. The authors argue that the lack of turn-off AGN is due to the relative abundance of quiescent galaxies in the nearby universe. Within these galaxies, it is much easier to see the emergence of broad emission lines. As a result, their sample may not be particularly representative of the underlying CL-AGN population, but the authors are very forthcoming about these issues and have shown care in constructing their sample.

Their comparison sample is a set of 500,000 local galaxies with measured stellar masses, star formation rates and numerous other spectral properties. Most of these quantities are measured across the whole galaxy and are also broken down into bulge and disk components. These data, mostly drawn from the SDSS, will allow today’s authors to place the CL-AGN within the wider galaxy population and isolate the conditions that trigger this change.

Figure 1: Distribution of host galaxies’ star formation rates and stellar masses. Blue circles show the distribution of CL-AGN compared to the underlying comparison galaxies (grey contours). Dashed lines indicate different star formation classifications: green-valley galaxies lie between the dashed blue and orange lines. [Dodd et al. 2021]

Results

From their analysis, we can determine three key consistencies about the preferences of CL-AGN. Figure 1 shows that the CL-AGN are all consistent, within errors, of being hosted in green-valley galaxies. These are a rarer form of galaxy that lie in between the blue, actively star-forming galaxies and their red and dead counterparts. Green-valley galaxies are believed have recently undergone a burst of extreme star formation, possibly implying the presence of large amount of cold gas at the centre which could fuel the mostly turn-on activity seen in this sample.

Figure 2: Asymmetry of the host galaxy against SMBH mass. Blue circles show the tight distribution of CL-AGN compared to the underlying comparison galaxy distribution (grey contours) and merging galaxies (yellow diamonds). [Dodd et al. 2021]

Figure 3: Sersic index (indicating the concentration of galaxy light) against SMBH mass. Both the squares and stars show how CL-AGN are distributed compared to the underlying comparison galaxy distribution (grey contours). [Dodd et al. 2021]

Original astrobite edited by Alison Crisp.

About the author, Keir Birchall:

Keir is a PhD student studying methods to identify AGN in various populations of galaxies to see what affects their incidence. When not doing science, he can be found behind the lens of a film camera or listening to the strangest music possible.


Tracing star formation rates in distant galaxies

An image of distant galaxies forming stars. Image credit: NASA, ESA and Bahram Mobasher. When we think of a galaxy the first thing that comes to our minds is an assembly of stars. Indeed, the stars of a galaxy are one of its most important characteristics.

To understand the physics of the evolution and formation of galaxies it is crucial to know at what rate galaxies form stars, referred to as the star-formation rate. This rate shows how active a galaxy is: young galaxies with large amounts of gas form many stars, while red and old galaxies that have depleted their gas reservoirs do not actively form stars.

Cosmological events such as mergers between galaxies can also boost the star-formation rate. However, unless we are observing the Milky Way and very close local galaxies, we cannot detect individual stars and star-forming regions in distant galaxies. Therefore, we need to rely on global observable characteristics to estimate the star-formation rate of galaxies located far away.

The best way to fully understand the properties of galaxies is by studying them at a broad range of wavelengths as each type of light is emitted from a different actor in a galaxy. For example, the ultraviolet light comes from the youngest and most massive stars, while the optical and near-infrared continuum light is emitted mostly from more evolved stars. Infrared light, on the other hand, traces dust in a galaxy, and emission lines that are detected in spectral lines trace the gas clouds.

Irene Shivaei. Image credit: University of California, Riverside. In a recent paper published in The Astrophysical Journal Letters, a group of researchers, led by Irene Shivaei, a University of California, Riverside graduate student, observed 17 bright distant galaxies with the MOSFIRE high-resolution near-infrared spectrometer at the W. M. Keck Observatory telescopes. Then, they combined the spectra with infrared images of the NASA’s Spitzer Space Telescope, ESA’s Herschel Space Observatory, and optical images of the NASA/ESA Hubble Space Telescope, to create a complete multi-wavelength picture of their galaxies: from rest-frame ultraviolet to rest-frame far-infrared.

They looked at various observables that are commonly used to estimate the star-formation rates in galaxies and compare them with each other. These star-formation rate estimators include the ultraviolet light that is emitted from young stars, the infrared light that shows how much of the ultraviolet light was absorbed by dust, and the nebular emission lines that are caused by young stars making the clouds of gas around them glow and radiate. These diagnostics have been observed and tested for local galaxies extensively in the past decade, but for distant galaxies it is challenging to acquire complete multi-wavelength datasets.

This study makes the first direct comparison between the optical emission line and the ultraviolet and infrared tracers of star formation and indicates that, despite the underlying uncertainties, astronomers can trust the nebular emission lines as robust indicators of the star-formation rate and the amount of light that is obscured by dust in distant galaxies.

These results help to build the foundations of galaxy evolution studies, in other words, help predict a physical quantity (in this case, the star-formation rate) of a distant galaxy from the light that our telescopes capture.

This analysis is part of the MOSFIRE Deep Evolution Field (MOSDEF) survey, which is conducted by astronomers at UC Riverside, UCLA, UC Berkeley, UC San Diego. The MOSDEF team uses the MOSFIRE spectrometer on the the W. M. Keck Observatory telescopes to obtain spectra for many galaxies that are located at 1.5 to 4.5 billion years after the Big Bang, the interval in which the universe formed the highest amount of stars in its history. The goal of the survey is to study the stellar, gaseous, and blackhole content of galaxies at this important era in the history of the universe.


Over a few short decades, our understanding of the evolution of galaxies and how stars and planets form has grown exponentially. But, in many cases, this increase in understanding serves to highlight the questions that still remain. One of the most pressing of those questions arises from the fact that older galaxies seem to be birthing less stellar objects. How does star formation cease in some galaxies? In a new study published in New Astronomy, Giorgio Manzoni from the Institute for Computational Cosmology, Durham University, UK, and his co-authors examine the timings of this halt in star formation to select a mechanism that could possibly be used to answer this and other cosmological questions.

&ldquoThe evolution of galaxies, although studied for a long time, still holds unanswered questions, and sometimes very basic ones. For example, how do galaxies stop forming new stars? Why do similar galaxies behave in different ways? What is the mechanism that makes them die? All of these are questions that the general public often asks, but that still there are no clear answers to,&rdquo says Manzoni. &ldquoMy research gives a hint to the mystery of the death of galaxies: it shows that whatever the mechanism is, it has to be fast, almost instantaneous less than 100 million years seeming instantaneous in comparison to the time-scales at which the Universe evolves.&rdquo

The researchers came to the conclusion that star formation ceases quickly by studying two properties of galaxies: their intrinsic luminosity and their colour, combined together in a &lsquocolour-magnitude diagram&rsquo. They were also able to conclude that there is some mechanism that drives the transition between an active star-forming phase in galaxies to a quieter stage with less or no star formation &ndash a mechanism referred to by astronomers as &lsquoquenching.&rsquo &ldquoIn the field of galaxy evolution, it is clearly established that galaxies form from gas falling into the gravity of a dark matter halo and then collapsing into new stars &ndash the active stage of galaxies. When the gas is spent, the galaxies die &ndash the passive stage of galaxies,&rdquo says Manzoni. &ldquoMy research shows that this natural process of gas exhaustion can&rsquot be the only thing responsible for the death of galaxies.&rdquo

Manzoni explains that he and his colleagues actually found that every galaxy, at a random point in their life, experiences a process that empties the reservoir of gas and kills &ndash or quenches &ndash star formation, thus bringing the galaxy into its passive stage very quickly. &ldquoTo introduce some constraints into the timing of the quenching of star formation, I used two properties of galaxies combined together in the colour-magnitude diagram. When studied at different distances from us &ndash different redshift &ndash this diagram tells us a lot about the population of galaxies that were present at different epochs of the Universe,&rdquo Manzoni says.

The researcher gives the example that, in the very past at high redshift and greater distances, galaxies were forming stars at a rapid rate that is not present anymore in our epoch, thus showing bluer colours and higher luminosities. Today, that population of galaxies has moved into a redder and weaker luminosity part of the diagram.

&ldquoSeveral mechanisms have been proposed as candidates for the quenching of star formation, such as active galactic nuclei feedback, galaxy harassment, and galaxy mergers, but, since different quenching mechanisms have different time-scales, my study can help to identify the main mechanism responsible for quenching over a large number of galaxies,&rdquo Manzoni says. &ldquoThis study is based on a simple concept we observe: the colour-magnitude diagram evolves with redshift, meaning the average properties of galaxies are changing with time.&rdquo

In order to reach their conclusion Manzoni and colleagues used a sample of about 90,000 galaxies taken from the VIMOS Public Extragalactic Redshift Survey (VIPERS) survey, now available publicly, which were observed with one of the eight-meter Very Large Telescope instruments in Chile.

Manzoni compares the quest to answer such cosmological questions to hunting for clues at a crime scene. &ldquoStudying the evolution of galaxies is for me similar to the job of a detective,&rdquo he says. &ldquoWe can&rsquot change the scene of the crime, as we can&rsquot modify the Universe or move a galaxy as we want, but we have to observe every little detail to try and understand what happened in the past 13 billion years of the Universe&rsquos life.&rdquo


Star formation rate vs. color in galaxy groups

Toady’s guest blog is from Andrew Wetzel, a postoc at Yale University. We asked Andrew to write this blog since he and his collaborators had used the public Galaxy Zoo 1 data in their own work (that is, they weren’t part of the team). Without any further ado, here’s Andrew’s experience with the Zoo data:

Recently, Jeremy Tinker, Charlie Conroy, and I posted a paper to the arXiv (click the link to access the paper) in which we sought to understand why galaxies located in groups and clusters have significantly lower star formation rates, and hence significantly redder colors, than galaxies in the field. Among the interesting things we found is that the likelihood of a galaxy to have its star formation quenched increases with group mass and increases towards the center of the group. Furthermore, galaxies are more likely to be quenched even if they are in groups as low in mass as 3 x 10^ <11>Msol (for comparison, the `group’ comprised of the Milky Way and its satellites has a mass of about 10^ <12>Msol). All together, these results place strong constraints on what quenches star formation in group galaxies. However, many of the above results disagree with what some other authors have found recently, and here is where Galaxy Zoo has been useful for us.

Because galaxies that are actively forming stars have a significant population of young, massive, blue stars, while galaxies that have very little star formation retain just long-lived, low-mass, red stars, astronomers often differentiate between star-forming and quenched galaxies based on their observed color. But using observed color can be dangerous, because if a galaxy contains a significant amount of gas and dust, it can appear red even if it is actively star-forming (analogous to how the sun appears redder on the horizon as the light passes through more of Earth’s atmosphere). To get a more robust measurement of a galaxy’s star formation, we used star formation rates derived from their spectra, because spectroscopic features are fairly immune to dust attenuation. But, we wanted to check how these spectroscopically-derived star formation rates compare with the color-based selection that many previous authors have used. What we found was striking: in lower mass galaxies, over 1/3 of those that appear red and dead actually have high star formation rates!

What is going on? Here is where Galaxy Zoo provided us with insight. We examined the Galaxy Zoo morphologies of these red-but-star-forming galaxies, and the result was telling: 70% of these galaxies are spirals (which have particularly high gas/dust content) and furthermore, 50% are edge-on-spirals (for which the dust attenuation is particularly strong). The image shows a good example of a galaxy which has a high star formation rate but appears red. You can even see the dust lane.

So, Galaxy Zoo helped to confirm our suspicion that many spiral galaxies that appear red are in fact actively forming stars, but their colors are reddened via dust (Karen Masters has done a lot of work in this direction as well). This gave us further confidence in our spectroscopic star formation rates and insight into why previous authors, using observed color, came to such different conclusions. Thanks to the Galaxy Zoo team and all the volunteers.


Emission lines trace star formation rate in distant galaxies

When we think of a galaxy the first thing that comes to our minds is an assembly of stars. Indeed, the stars of a galaxy are one of its most important characteristics. To understand the physics of the evolution and formation of galaxies it is crucial to know at what rate galaxies form stars, referred to as the star-formation rate. This rate shows us how active a galaxy is: young galaxies with large amounts of gas form many stars, while red and old galaxies that have depleted their gas reservoirs do not actively form stars. Cosmological events such as mergers between galaxies can also boost the star-formation rate. However, unless we are observing the Milky Way and very close local galaxies, we cannot detect individual stars and star-forming regions in distant galaxies. Therefore, we need to rely on global observable characteristics to estimate the star-formation rate of galaxies located far away.

The best way to fully understand the properties of galaxies is by studying them at a broad range of wavelengths as each type of light is emitted from a different actor in a galaxy. For example, the ultraviolet light comes from the youngest and most massive stars, while the optical and near-infrared continuum light is emitted mostly from more evolved stars. Infrared light, on the other hand, traces dust in a galaxy, and emission lines that are detected in spectral lines trace the gas clouds.

In this study, led by UC Riverside graduate student Irene Shivaei, the authors observed 17 bright distant galaxies with the MOSFIRE high-resolution near-infrared spectrometer at the W. M. Keck Observatory telescopes. Then, they combined the spectra with infrared images of the Spitzer Space Telescope, the Herschel Space Observatory, and optical images of the Hubble Space Telescope, to create a complete multi-wavelength picture of their galaxies: from rest-frame ultraviolet to rest-frame far-infrared. They looked at various observables that are commonly used to estimate the star-formation rates in galaxies and compare them with each other. These star-formation rate estimators include the ultraviolet light that is emitted from young stars, the infrared light that shows how much of the ultraviolet light was absorbed by dust, and the nebular emission lines that are caused by young stars making the clouds of gas around them glow and radiate. These diagnostics have been observed and tested for local galaxies extensively in the past decade, but for distant galaxies it is challenging to acquire complete multi-wavelength datasets.

This study makes the first direct comparison between the optical emission line and the ultraviolet and infrared tracers of star formation and indicates that, despite the underlying uncertainties, astronomers can trust the nebular emission lines as robust indicators of the star-formation rate and the amount of light that is obscured by dust in distant galaxies. These results help to build the foundations of galaxy evolution studies, in other words, help predict a physical quantity (in this case, the star-formation rate) of a distant galaxy from the light that our telescopes capture.

This analysis is part of the MOSFIRE Deep Evolution Field (MOSDEF) survey, which is conducted by astronomers in four University of California campuses, including UC Riverside, UC Los Angeles, UC Berkeley, and UC San Diego. The MOSDEF team uses the MOSFIRE spectrometer on the the W. M. Keck Observatory telescopes to obtain spectra for many galaxies that are located at 1.5 to 4.5 billion years after the Big Bang, the interval in which the universe formed the highest amount of stars in its history. The goal of the survey is to study the stellar, gaseous, and blackhole content of galaxies at this important era in the history of the universe.

The paper was published in the Astrophysical Journal Letters. It was authored by researchers at UC Riverside, UCLA, UC Berkeley, UC San Diego, Harvard University and the National Optical Astronomy Observatory in Tuscon, Ariz. Besides Shivaei , the other UC Riverside researchers were: Naveen Reddy, an assistant professor Brian Siana, an assistant professor William Freeman, a graduate student working with Siana and Bahram Mobasher, a professor.

Images:
[Top right] Examples of galaxy evolution as seen through infrared filters. Credit: ESA–C. Carreau/C. Casey (University of Hawai’i) COSMOS field: ESA/Herschel/SPIRE/HerMES Key Programme Hubble images: NASA, ESA.
[Bottom] Keck Telescopes by Irene Shivaei.


Comparing Star Formation rate in different galaxies - Astronomy

There are several senses in which galaxies evolve with time, not all necessarily running at the same rates for various kinds of galaxy. We may distinguish for convenience evolution of stellar populations, chemical abundances, and dynamics of a galaxy, remembering that in real galaxies these are all tied together at some level. I will cover the basic principles needed to understand modelling these processes, and observational approaches to each one.

The likelihood of observing galaxy evolution can be seen from comparing several relevant timescales. The Hubble time is in the range 1-2 × 10 10 years. This matches the main-sequence lifetime of solar-mass stars, making it many generations of more luminous massive stars. And dynamically, the rotation period on our part of the Milky Way is 0.25 Gyr, so the relaxation time for a massive galaxy to form structure should be several Gyr.

Population evolution combines the history of star formation in a galaxy and stellar evolution of its constituents, giving changes in the HR diagram of a galaxy (generally changing from place to place in the galaxy) with time. To predict the population at some time, we thus need the SFR at all previous times and an understanding of stellar evolution for all relevant masses see the discussion under spectrum and population synthesis. Elliptical galaxies, or the bulges of spirals, are popular tests of synthesis models, since they are assumed to have had most of their stars formed in a short period long ago, and undergone changes due only to stellar evolution since then (the condition of passive evolution). As a sample calculation, here's a series of models of a rapid burst of star formation, from the code by Rocca-Volmerange and Guiderdoni, sampled at approximately 1-Gyr intervals:

One piece of software in particularly wide use is the GISSEL package by Bruzual and Charlot (see ApJ 405, 538, 1993, with later improvements). One can also predict from such results the expected history of any particular color index or line strength. Color indices usually have K-corrections folded in, since we can seldon tailor filters to cover wide redshift ranges line indices have less of a problem since they are measured spectroscopically and are nearly monochromatic for all the galaxies in a sample. Still dealing with ellipticals, Hamilton (1985 ApJ 297, 371) searched for evolution in an index of the spectral break at 4000 Å (the so-called H-K break) for redshifts up to about z=0.9, finding at most marginal evidence of any evolution. He presents a figure that nicely connects look-back time and the stellar evolution timescale (and shows why such work indirectly constrains H0, connecting a redshift-scaled time to the absolute timescales of stellar lifetimes), which I have gleefully cribbed below (courtesy of the AAS). Note that the redshift-time mapping is sensitive to cosmological parameters including both the Hubble constant H0 and deceleration parameter q0, plus for good measure any nonzero value of the cosmological constant &Lambda.

Hamilton's results suggested a very small H0 (formally less than 42 km/s Mpc). Later work on both radio- and optically-selected galaxies shows that a clear envelope in amplitude of the 4000-Å break is seen with redshift. Here I combine Hamilton's data, a set of low-power radio galaxies for z< 1 from Owen and Keel (1995 AJ 109, 14), and the large number of (less precise) measures from the Hawaii survey from Songaila et al. (1994 ApJS 94, 461):

The very limited role of evolution for redshifts below unity was to some extent contravened by strong claims of color evolution of radio galaxies at redshifts z=1 and larger, and interpretation of counts in both color and magnitude for faint galaxies. If the most powerful radio galaxies, seen to large redshift, are to be identified with normal elliptical galaxies seen at more recent epochs, and if their colors are generally dominated by starlight and not scattered nonthermal radiation, their properties are barely consistent with Hamilton's results.

Many of these objects show spectacular levels of star formation, both in emission lines and in the detailed shape of their continua (Chambers and McCarthy 1990 ApJLett 354, L9). A first comparison of the emitted-UV spectra of low-redshift radio galaxies (from IUE) with those at high redshift has been done by Keel and Windhorst (1991 ApJ 383, 135). The UV upturn below about 1400 Å is ubiquitous in low-redshift objects, so much so as to suggest a connection between radio sources and Gyr-old starbursts, and lacking in high-redshift objects. This is consistent with the idea that it requires a few Gyr to produce evolved hot stars, so high-redshift galaxies are seen while too young to have any such stars. The composite spectrum shown by Chambers and McCarthy (190 ApJL 354, L9) indeed shows absorption features from a very young stellar population.

The Butcher-Oemler effect may be a case of population evolution - is this the tail of the same process? Starbursts are seen in "field" galaxies at redshifts z=0.1 and greater maybe the Butcher-Oemler effect is only easier to see in clusters and not tightly coupled to dense environments.

Population evolution also drives luminosity evolution (along with mergers and the evolution of obscuring ISM). Thus, in principle, color-magnitude arrays can test for evolution if we have suitable zero-point models for comparison. An early example is shown by Spinrad 1977 (Evolution of Galaxies and Stellar Populations, p. 326. This approach can reach very deep, in that all the redshift information is in the model rather than in the data, but for the same reason can't produce very detailed results. At this point, its greatest success was in setting forth the faint blue galaxy problem. This detection of color evolution based on data by Tyson and Seitzer (1988 ApJ 335, 552) at first rested on difficult calibrations of surface brightness at extremely faint levels (see also Guhathakurta et al 1990 APJLett 357, L9). The issue is that there are more blue galaxies at magnitudes fainter than about B=22 than nonevolving models from the local neighborhood suggest, and indeed it has proven nontrivial to make a model that shows so much evolution once it became clear that most of the objects around this magnitude are at redshifts to z=0.5 or so rather than being extremely distant. Much of the interest in very deep high-galactic latitude surveys has centered on the detection of galaxy evolution. A new population of radio galaxies, with blue colors and redshifts z=0.3-1.0, appears at very faint radio fluxes (Windhorst et al 1985 ApJ 289, 494). Their log N - log S behavior suggests a cosmologically evolving set of galaxies.

We are finally finding fairly normal galaxies in large numbers at redshifts 3 and above, either radio-quiet or weak (see Windhorst et al 1991 ApJ 380, 362), in some cases by identifying QSO absorption-line systems with faint galaxies at the same redshift (Turnshek et al 1991 ApJ 382, 26). Observational progress on population evolution requires substantial and well-understood samples of faint galaxies with measured redshifts and colro properties (of course, absorption lines would be even better). IR spectroscopy may be important in tracing the same spectral features across large redshift spans.

A huge leap forward in seeing galaxy evolution came with the Hubble Deep Fields, North and South (see Williams et al. 1996 AJ 112, 1335 and Gardner et al. 2000 AJ 119, 486), then surpassed in depth by the Hubble Ultra-Deep Field. These data sets were based on carefully planned long series of multicolor HST images, reaching very deep with unique morphological information. As a community-wide project, redshift surveys have built up a substantial number of objects in and around these fields. These data form the core for recent studies of galaxy evolution. An important aspect has been extending the spectroscopic redshifts by estimating photometric redshifts, the redshift at which each galaxy's colors best match some empirical or calculated template. The more bands and longer wavelength baseline measured, the better in a sense this is extremely low-resolution spectrophotometry, running the k-correction backwards by assuming the spectral energy distribution to be one of the known forms and solving for the redshift at which all the colors can be matched for some value of internal reddening. The idea dates back at least to Loh and Spillar (1986 ApJ 303, 154), though its serious exploitation had to await data of increased precision, depth, and sky coverage. For many redshift ranges, photometric redshifts are accurate to about 0.05 in z, with problems occurring between z=1 and 2 unless near-IR data are available. One interesting example of how to do this is provided by the hyperz public code by Bolzonella, Miralles, and Pello, which incorporates the current practice of generating a probability distribution for the galaxy lying at each redshift.

One goal of deep field surveys is the star-formation history of galaxies as a population, which is clearly linked to morphologies, present-day metallicities, and the unresolved background in the UV and deep infrared. From the HDF data, Madau et al. (1996 MNRAS 283, 1388) presented an estimate of the comoving SFR as a function of redshift, which has since been both elucidated and vilified as the Madau plot, shown here in their Fig. 9 from the ADS:

Uncertainties in this history come from the imprecision of optical photometric redshifts at z=1-2, and more basic, the poorly-known role of extinction. Extinction is especially important since most of these data sample emitted ultraviolet light. Deep mid- and far-infrared surveys, and radio detections sensitive enough to see non-AGN galaxies, are important in telling whether the apparent broad peak is real or an artifact of dust obscuration at larger redshifts. The verdict is still out on the early SFR history of galaxies.

High-redshift galaxies can now be identified wholesale by a particularly simple kind of photometric redshift, using that fact that any non-AGN galaxy goes black at the Lyman limit (912 Å). Thus faint objects which are blue in some passband like V-R but undetectable to low limits in U or B are likely to be high-redshift systems whose Lyman limit is redshifted into the optical band. This approach was described by Steidel and Hamilton (1992 AJ 104, 941), and exploited by many starting with Steidel et al. (1996 ApJL 462, L17 and AJ 112, 352) to find a rich field of objects for followup spectroscopy. Most galaxies known at z > 3 started detection in this way. This selection - deep in the emitted UV - will impose unavoidable biases in what kinds of galaxies show up, favoring objects with high star-formation rate, high UV luminosity, and low net extinction. Galaxies have been identified in this way out to z=5.5, and maybe to 6.7. The panels below show the brightest of the Lyman-break galaxies above z=3 in the original Hubble Deep Field, with wavelengths near 3000, 4500, 6060, and 8140 A. The clumpy object in the center is comparably bright in the longer wavelengths, showing a flat spectrum, and vanished in the UV shortward of its Lyman break.

Lyman-break galaxies include systems which are quite luminous and probably massive. There is some evidence that they are less metal-rich than today's luminous galaxies. Additional kinds of high-redshift galaxies can be selected in ways less biased to unobscured star-forming systems. EROs (extremely red objects) are known over a wide redshift range, usually selected by a color index involving K. These include elliptical galaxies at substantial redshifts, where the passband correction means that optical filters sample their very weak UV radiation, and dust-reddened objects. EROs cluster very strongly, suggesting a link to present-day ellipticals. On the other side of the thermal hump, we can now detect very luminous far-IR galaxies in the submillimeter. The steep rise of even a modified Planck spectrum makes it easier to find high-redshift galaxies than their nearer counterparts. These submillimeter galaxies may comprise a significant fraction of the total star formation at z=3 efforts to understand their energy source are important, possibly connecting them to processes such as merging and starbursts seen in local far-IR-bright galaxies. Until recently, the poor precision of submillimeter continuum positions has hampered identification of these objects. In some cases both AGN and starbursts may be involved in their high luminosity. These are extreme examples of how strongly biased an optical/UV perspective can be against just those environments that might be the most important stellar birthplaces.

Chemical evolution. The stars are not evolving in a vacuum (well, not quite). They are marked by initial composition, and change the composition of the ISM and later-formed stars through winds, planetary nebulae, and supernovae the relative abundances of heavy elements and dust increases with time. The best place to study this is our own galaxy, for which (in our little neighborhood) we can count stars in bins of age and metallicity.

The simplest expectation is based on the one-zone closed-box model. This assumes a closed system with an initial complement of gas, in which star formation proceeds. It assumed instantaneous recycling of the elements under consideration divide the stars into those that live longer than the time of interest and those with shorter lifetimes. In this case, the rate of recycling into the ISM depends only on SFR and IMF (at that time, if the IMF changes). This is justified as a first guess since so much nucleosynthesis goes on in the most massive, short-lived stars. Define some basic parameters:

R returned fraction of gas
y the yield, fraction of stellar mass turned into heavy elements

This allows one to solve for the abundance as a function of gas content of the galaxy, not as a function of time. For this simple model, as described by Audouze and Tinsley 1976 (ARA&A 14, 43), the abundance Z of some element is related to the gas fraction (by mass) &mu according to Z = y ln &mu -1 and the fraction of all stars so far formed with abundance less than some value Z is S/S1 = [1 - &mu (Z/Z1)]/ (1 - &mu1 ) where &mu1 is the present gas fraction. These equations apply to primary elements, those produced directly from hydrogen the case for secondary elements, those requiring some seed abundance of a heavier nuclide, is more complicated. Note that different elements are recycled on different timescales, depending on what stellar masses produce them most efficiently (Wheeler, Sneden, and Truran 1989 ARA&A 27, 279).

This simple model flagrantly fails to describe the sitiation in our galaxy, a condition classically known as the G-dwarf problem there are too few low-metallicity dwarfs in our galactic disk. The usual way out is to postulate introduction of new, pristine gas into the disk at a rate comparable to that of star formation in the convenient case of exact mass balance, Z = y (1 - e 1- 1/&mu ) (Larson 1972 Nature Phys. Sci. 236, 7). To do better, one must do a detailed numerical model incorporating the full range of stellar properties, as described by Tinsley 1975 (Mem. Soc. Ital. Astron. 46, 3). These results will affect predictions of stellar population spectra, since increased abundances mean more obscuration by more dust. There have been a few premature attempts to incorporate this into predicted spectra, but it is becoming clear that the results depend entirely on the relative geometry of stars and ISM.

The best chance for progress here may be in a detailed detailed stellar census in our neighborhood (what Sandage was doing with the Mt. Wilson telescope when it was shut down). We may have to work backwards. Note that we suffer from the fact the we are surrounded by stars that came from a large part of the galaxy, and thus see results of chemical evolution mixed with

Dynamical Evolution. The linkage of kinematic and chemical properties for at least two populations (Eggen, Lynden-Bell, and Sandage 1962 ApJ 136, 735 this is what is known as A CLASSIC PAPER) has long been interpreted as showing evolution in our galaxy's stellar dynamics. More detailed distinctions probably don't have the same meaning, since scattering by molecular clouds can mimic internal evolution of the disk (and in fact cause such evolution in the opposite direction, Freeman in Nearly Normal Galaxies). The notion of quick bulge production with associated violent relaxation, followed by remaining gas collpasing to a disk with dissipation, then leisurely star formation and chemical evolution in the disk, stands up well. However, there are still major questions that should be answerable once we get clear looks at enough high-redshift galaxies (assuming they're not all distorted out of recognition by gravitational lensing). Just how did bulges form? Can they be made by merging disks? And how many mergers have there been? Simple calculations of merging timescales and current rate suggest that the number of mergers (of fairly equal galaxies) per present-day luminous galaxy has been greater than one. High-redshift radio galaxies often show multiple lumps, which got people thinking about this sort of piecemeal galaxy formation (see Djorgovski in NNG, for example), but the interpretation has since clouded. The excellent alignment of optical and radio emission suggests that we are not seeing starlight from a normal galaxy, and Hammer and collaborators have argued that we may be misled by a gravitational mirage (though that doesn't account for the cases with different optical and radio structure).

The peculiar knotty and lumpy structures of many high-redshift galaxies has led to models for wholesale morphological evolution. This issue is still unresolved because many galaxies become less symmetric when observed in the emitted UV, and because cosmological surface-brightness dimming strongly favors the detection of galaxies with high-surface brightness regions of star formation. Work in the emitted optical range (observed near-IR) should make real progress here now that adaptive optics can deliver even better resolution than NICMOS, and will surely be a major product of NGST.

Mergers certainly drive some present-day morphological evolution. Toomre in 1977 estimated a merger rate which is pretty close to later estimates like Keel and Wu (1995 AJ 110, 129) and Borne and Cheung (2000 DDA abstract 32.1101). The upshot is that bright galaxies suffer major mergers at a rate 0.3-0.4 per Hubble time, with most of this concentrated in systems that are part of bound pairs.

Structures such as rings also indicate that galaxies evolve dynamically at different rates models suggest that stars may be driven into a stable bulge+ring configuration, and that even disk structure may not be stable over very long times. On the longest timescales (10 20 years and longer), gravitational radiation will turn galaxies into massive black holes plus escaped individual stars, if protons don't decay first (Dyson 1979 Rev. Mod. Phys. 51, 447 Barrow and Tipler 1978 Nature 276, 453 Dicus et al 1982 ApJ 252, 1). The thermodynamic properties of the universe over such timespans make for interesting speculation (see Krauss & Starkman 2000 ApJ 531, 22 and Fred Adams has a whole book on this).

The monolithic collapse picture suggested by the results of ELS (and many subsequent ones) is appealing and explains the early appearance of some of the most massive galaxies. In contrast, the most straighforward interpretation of simulations of structure growth in a universe dominated by cold dark matter (CDM) is that galaxies form hierarchically - the earliest units are subgalactic by present standards, building up via repeated mergers until the present acquisition of dwarfs by large galaxies is the latest stage. Some facets of galaxy evolution evidently work like this - but others don't. Luminous galaxies are found out to around z=5, and the color-absolute magnitude diagrams of clusters to z=1.5 are so narrow that they suggest the completion of major star formation very early on. A whole range of techniques describes the star-formating behavior of galaxies as one of downsizing (apparently first so called by Cowie et al. in 1996). One way of describing it is that the characteristic mass of star-forming galaxies has declined monotonically with time - it can also be described, of course, in terms of history as a function of mass. It is probably important that something closely analogous has been derived for the growth of black holes - the characteristic mass of actively accreting black holes, as seen in AGN, has been declining roughly in a parallel (and thus, perhaps, connected, fashion). One lesson of all this is that interpreting the cosmological simulations in too much detail may be hostage to the details of gas viscosity and collapse ("gastrophysics"). The Durham group, in particular, has advocated a hybrid semi-analytical approach, in which analytical results are used to guide interpretation of statistics from simulations to go beyond the resolution of the numerical work.

The connections between galaxy bulges and central black holes, upper mass limit for individual galaxies, and star-formation histories of dwarf systems, all suggest an important role for feedback in galaxy history. This may take the form of massive stars blowing apart surrounding or neighboring clouds through winds, radiation pressure, or supernova explosions, or act as accretion onto central black holes drives radiative and mechanical pressure into the surroundings. AGN feedback may even shut down the cooling flows that would otherwise continue to grow central galaxies in clusters. (Chandra examples: cluster MS0735 | bubbles and acoustic waves in Perseus | M87 | bubbles around Cygnus A). This feedback may be related to the shutdown of star formation inm massive galaxies. (And in the context of S0 galaxies, Dressler has pointed out tat keeping our eyes on when it shut down may be as fruitful as on when it happened).


Comparing Star Formation rate in different galaxies - Astronomy

Star formation is galaxy evolution caught in the act. Measurements of the star-formation rate (SFR) usually apply strictly only to OB stars and may be extrapolated to all masses, since less massive ones can't be distinguished from the older background population. Some observational indicators of star formation include:

Recombination lines (especially H&alpha): Line emission is characteristic of H II regions, zones of ionized gas around young star clusters that still contain OB stars (spectral types B0.5 and hotter). These stars are special in that they are hot enough to produce significant fluxes of ionizing radiation, shortward of the Lyman limit. H II regions may be found wholesale by narrow-band imaging at H&alpha if internal extinction is not too large, and also by radio surveys for recombination-line emission (though this works best in our own galaxy). To turn measures of emission-line intensity into star-formation rate, first consider the physics of photoionization and recombination. I use the notation from Osterbrock in Astrophysics of Gaseous Nebulae and Active Galactic Nuclei, which will be no great surprise to anyone looking at my graduate institution.

in terms of the electron velocity distribution f(v) and cross-section for capture to quantum state n, 2 L given by &sigma<>. We deal only with the state 2 L since cascades to here occur very rapidly. The velocity distribution may be taken as a thermal (Maxwellian) form in the electron temperature T (in fact defining this temperature):

because, even though the electrons are released with a different f(v), Coulomb interactions with ions (predominantly protons) thermalize their velocities on a timescale short compared to the recombination time. In the so-called nebular approximation, the atom is undisturbed after recombination, so that the subsequent radiative cascade is governed only by the radiative decay coefficients (Einstein A values) that is, the cascade is rapid compared to the photoionization timescale. Then the total capture coefficient (to all quantum states) is &alphaA = &Sigman &alphan and the recombination time is

for Ne in cm -3 . At high Lyman &alpha optical depth (in most H II regions, for example), the cascade stops at n=2 due to resonant scattering of Lyman &alpha line photons, so we also define a "case B" recombination coefficient, starting the sum above at n=2.

In equilibrium, there will be a balance between photoionization and recombination rates, which allows us to measure the ionizing UV luminosity of a star solely from the intensity of line radiation from surrounding ionized gas. In the ionization-bounded case, all of a star's ionizing flux is absorbed within the nebula (as opposed to matter-bounded, where all the material is ionized but some ionizing radiation escapes). Then the total number of Lyman-continuum photons produced by the star is

for electron density Ne (see Osterbrock's Astrophysics of Gaseous Nebulae and Active Galactic Nuclei for the gory details).

If we are dealing with a single star, the theory of stellar atmospheres predicts NLC as a function of spectral type (or effective temperature) and luminosity class (or evolutionary stage). Blackbodies are not necessarily good approximations to stellar spectra in this range. A conversion to rate of star formation schematically uses the luminosity (actually for this application the Lyman-continuum photon emission rate), lifetime, and mass, with a properly weighted average to account for the mix of stellar masses which contribute. As a benchmark, the number of O7 stars needed to produce a given H&beta luminosity is N(O7) = 3.4 x 10 -37 L(H&beta) erg/sec. H&alpha is about 2.9 times as strong as H&beta under low-density conditions. Following Kennicutt 1983 (ApJ 272, 54) we may extrapolate using a representative initial-mass function (IMF) to the total star-formation rate above 10 solar masses: SFR = L(H&alpha) / 7.0 x 10 41 erg/sec, in solar masses per year, and to a total SFR SFR = L(H&alpha / 1.12 x 10 41 erg/sec. Different assumptions about the initial mass function will change these constants (especially a different lower-mass cutoff or slope on the lower main sequence). Recombination lines are sometimes spoken of as photon counters for these purposes.

Measurements of optical recombination lines (again, H&alpha is strongest and easiest to deal with) may use either narrow-band aperture photometry or imaging. Imaging is a bigger reduction headache, but can also tell you where the action is going on. Reddening is always a potential problem across some regions, if seldom on galaxy-wide scales. One also needs to allow for the contribution of the adjacent [N II] forbidden lines in deriving pure H&alpha intensities. There are now hundreds of integrated H&alpha fluxes for galaxies, spanning Hubble types, luminosity, and environment. Here again, it is frequently useful to deal with an equivalent width for the line, giving a normalized SFR compared to its past average. Kennicutt and Kent (1983, AJ 88, 1094) show this relation between Hubble type and EW(H&alpha) (as usual, courtesy of the AAS):

Not surprisingly, the SFR per unit starlight climbs to later Hubble types (toward Sc). To some extent, this reflects the dominant bulges of earlier types, and without information on the gas content it is not so clear what happens within the disk itself.

Models of an evolving population with various SFR histories can also predict broad-band colors as well as EW(H&alpha) it turns out the H&alpha is vastly more sensitive to bursts of small relative mass. These figures from Kennicutt in Stellar Populations, and Kennicutt, Tamblyn, and Congdon 1994 (ApJ 435, 22) compare models with various SFR(T) to spiral galaxies the dispersion and thus discrimination among models is much less for continuum colors. One way to think of this is that the H&alpha equivalent width is a color index with baseline running all the way from 6563 Å to the Lyman continuum in the far UV.

The curves are models with different (declining) star-formation histories. The uppermost systems in their Fig. 2 (above, courtesy AAS) are those in which transient bursts of star formation must be taking place. There will be more to say about starburst systems later.

Emission-line images allow mapping of the distribution in both space and luminosity of large numbers of individual H II regions (more properly complexes with unresolved or barely resolved structure). The example at right, NGC 5427, shows numerous H II regions and clumps of them in a continuum-subtracted H&alpha+[N II] image. The luminous H II regions often align very well with optical spiral features, but not usually so well that the arms are visible in the H II regions alone. Rings of star formation are not uncommon. There is a range of concentration to spiral arms Hodge (1985 PASP 97, 688) has quantified this with a mean vs. variance statistic. The luminosity function of H II regions is frequently close to a power-law form (Kennicutt et al. 1989, ApJ 337, 761) but there are variations among galaxies such that it is not clear whether the luminosity distribution or clumping properties change. This kind of analysis is particularly vulnerable to effects of changing linear resolution, in making a number of small H II regions look like a single luminous one. The proper distinction is not obvious. Note that in Soviet literature, the terminology concentrates on stars and not the gas - giant H II regions are generally identical with superassociations. Both tend to clump on somewhat larger scales - complexes - which might be a more robust way to quantify star-forming regions.

Not all the star-powered H&alpha emission originates in discrete H II regions. Most spirals have a substantial diffuse interstellar medium which contributes as much as half the integrated line flux (see, for example, Ferguson et al. 1996 AJ 111, 2265). Several studies of its ionization level and energetics suggest that much of its ionization comes from Lyman continuum photons which manage to leak out of individual H II regions. This component to some extent makes up for whether individual regions are matter - or ionization-bounded in estimating a global SFR.

Ultraviolet intensity The mass-lifetime relation for stars tells us that stars that are bright in the UV (beyond the level of the hot evolved stars in ellipticals) must be fairly young, less than 10 9 years for early A stars. Thus, there has been interest in seeing massive and young stars directly in the UV, where one traces stars not massive or hot enough to produce H II regions. The measurement is analogous to the scaling used for H&alpha interpretation, except one deals now with luminosity in energy units rather than photon units, and we see the photospheric light directly. However, reddening (extinction) is a strong effect and correspondingly serious uncertainty. At these wavelengths, dust gives a "picket-fence" effect, with very little column density range between no significant extinction and no significant transmission. Comparison between H&alpha and UV imagery has proven very interesting, with the detailed correspondence being less than exact.

Cowie et al. (1988 ApJL 332, L29, and more detail in two conference papers referenced therein) have noted that the stars responsible for UV light are the same ones that go supernova and produce most of the metals that can be recycled for subsequent star formation, as well as being measurable through line emission. There is this an integral constraint relating (emitted-frame) UV light from galaxies and the rpesent-epoch metallicity of galaxies. Current results (for example from redshift surveys in the Hubble Deep Field) indicate that most of the star formation has happened at z 6 -10 7 solar masses, giant molecular clouds or GMCs) revolutionized our view of star formation. These clouds are the immediate precursors of star formation, the "placental clouds" of giant H II regions. Most optically visible H II regions in fact exist as blisters blown on the edges of such clouds. If one assumes that cloud clumps are everwhere similar, and that any threshold mass for forming star clusters is also constant, the CO intensity will indirectly tell us the SFR. Existing correlations in fact are consistent with this notion for normal galaxies.

This notion will clearly break down under extreme conditions, and in fact starburst systems have much of their molecular gas in compact high-density configurations. Also, at low metallicity, the amount of CO per H2 mass will change, largely because C and O abundances are lower.

Integrated Starlight - Time-integrated Constraints: The total luminosity of a galaxy at some given wavelength gives us an important clue to the star-forming history. It is the sum of the luminosities of all stars that have not reached their final compact states. Blue light is emitted most efficiently by stars of a few solar masses if their formation has continued so that such stars (not older giants or main-sequence stars) dominates the optical light, their formation rate is given by Gallagher, Hunter, and Tutukov (1984, ApJ 284, 544) as SFR (solar masses/yr)= 2.9 x 10 -9 LB/LSun.

The Star-Formation Law in Galactic Disks: Since stars come from gas, and we can now measure more or less directly the appropriate gas components, it is useful to ask how SFR depends on local gas properties. A favorite, simple contender for many years has been the Schmidt law: SFR = constant x &sigmagas N where N is the significant parameter. Fits to this law have given contradictory results. The problem has been accounted for by the realization that there is a threshold surface density of gas below which essentially no star formation takes place. If the Schmidt law applies, it is far above this threshold. There is a plausible dynamical explanation (Kennicutt 1989 ApJ 344,685 Zasov and Simikov 1988 Astrofizika 29, 190) involving stability of disk gas in the presence of rotational shear gas doesn't collapse unless it is dense enough to be unstable under the local shear conditions (Quirk 1972 ApJL 176, L9 Toomre 1964 ApJ 139, 1217). The dependence of SFR on surface density and the relation of the extent of the star-forming zone to the stability criterion are shown in Figs. 9 and 10 of Kennicutt 1989: