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Supernovae of type Ia can be used as standard candles to determine extra-galactic distances. But these event only occur (roughly) once every 200 years in any given galaxy and rapidly fade away. So to actually see a supernova in another galaxy, you have to be observing a lot of galaxies for a long time. This makes it seem like this isn't a very practical method to determine extra-galactic distances. To how many galaxies has the distance been determined using type Ia supernovae?
15,000 Galaxies in One Image
Here’s an image to fire your imagination: Fifteen thousand galaxies in one picture — sources of light detectable today that were generated as much as 11 billion years ago.
Of those 15,000 galaxies, some 12,000 are inferred to be in the process of forming stars. That’s hardly surprising because the period around 11 billions years ago has been determined to be the prime star-forming period in the history of the universe. That means for the oldest galaxies in the image, we’re seeing light that left its galaxy but three billion years after the Big Bang.
This photo mosaic, put together from images taken by the Hubble Space Telescope and other space and ground-based telescopes, does not capture the earliest galaxies detected. That designation belongs to a galaxy found in 2016 that was 420 million years old at the time it sent out the photons just collected. (Photo below.)
Nor is it quite as visually dramatic as the iconic Ultra Deep Field image produced by NASA in 2014. (Photo below as well.)
But this image is one of the most comprehensive yet of the history of the evolution of the universe, presenting galaxy light coming to us over a timeline up to those 11 billion years. The image was released last week by NASA and supports an earlier paper in The Astrophysical Journal by Pascal Oesch of Geneva University and a large team of others.
And it shows, yet again, the incomprehensible vastness of the forest in which we are a tiny leaf.
Some people apparently find our physical insignificance in the universe to be unsettling. I find it mind-opening and thrilling — that we now have the capability to not only speculate about our place in this enormity, but to begin to understand it as well.
For those unsettled by the first image, here is the 2014 Ultra Deep Field image, which is 1/14 times the area of the newest image. More of the shapes in this photo look to our eyes like they could be galaxies, but those in the first image are essentially the same.
In both images, astronomers used the ultraviolet capabilities of the Hubble, which is now in its 28th year of operation.
Because Earth’s atmosphere filters out much ultraviolet light, the space-based Hubble has a huge advantage because it can avoid that diminishing of ultraviolet light and provide the most sensitive ultraviolet observations possible.
That capability, combined with infrared and visible-light data from Hubble and other space and ground-based telescopes, allows astronomers to assemble these ultra deep space images and to gain a better understanding of how nearby galaxies grew from small clumps of hot, young stars long ago.
The light from distant star-forming regions in remote galaxies started out as ultraviolet. However, the expansion of the universe has shifted the light into infrared wavelengths.
These images, then, straddle the gap between the very distant galaxies, which can only be viewed in infrared light, and closer galaxies which can be seen across a broad spectrum of wavelengths.
The farthest away galaxy discovered so far is called GN-z11 and is seen now as it was 13.4 billion years in the past. That’s just 400 million years after the Big Bang.
GN-z11 is surprisingly bright infant galaxy located in the direction of the constellation of Ursa Major.
In addition representing cutting-edge science — and enabling much more — these looks into the most distant cosmic past offer a taste of what the James Webb Space Telescope, now scheduled to launch in 2021, is designed to explore. It will have greatly enhanced capabilities to explore in the infrared, which will advance ultra-deep space observing.
But putting aside the cosmic mysteries that ultra deep space and time astronomy can potentially solve, the images available today from Hubble and other telescopes are already more than enough to fire the imagination about what is out there and what might have been out there some millions or billions of years ago.
A consensus of exoplanet scientists holds that each star in the Milky Way galaxy is likely to have at least one planet circling it, and our galaxy alone has billions and billions of stars. That makes for a lot of planets that just might orbit at the right distance from its host star to support life and potentially have atmospheric, surface and subsurface conditions that would be supportive as well.
A look these deep space images raises the question of how many of them also house stars with orbiting planets, and the answer is probably many of them. All the exoplanets identified so far are in the Milky Way, except for one set of four so far.
Their discovery was reported earlier this year by Xinyu Dai, an astronomer at the University of Oklahoma, and his co-author, Eduardo Guerras. They came across what they report are planets while using NASA’s Chandra X-ray Observatory to study the environment around a supermassive black hole in the center of a galaxy located 3.8 billion light-years away from Earth.
In The Astrophysical Journal Letters, the authors report the galaxy is home to a quasar, an extremely bright source of light thought to be created when a very large black hole accelerates material around it. But the researchers said the results of their study indicated the presence of planets in a galaxy that lies between Earth and the quasar.
Furthermore, the scientists said results suggest that in most galaxies there are hundreds of free-floating planets for every star, in addition to those which might orbit a star.
The takeaway for me, as someone who has long reported on astrobiology and exoplanets, is that it is highly improbable that there are no other planets out there where life occurs, or once occurred.
As these images make clear, the number of planets that exist or have existed in the universe is essentially infinite. That no others harbor life seems near impossible.
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Astronomers model, determine how disk galaxies evolve so smoothly
Computer simulations are showing astrophysicists how massive clumps of gas within galaxies scatter some stars from their orbits, eventually creating the smooth, exponential fade in the brightness of many galaxy disks.
Researchers from Iowa State University, the University of Wisconsin-Madison and IBM Research have advanced studies they started nearly 10 years ago. They originally focused on how massive clumps in young galaxies affect star orbits and create galaxy disks featuring bright centers fading to dark edges.
(As Curtis Struck, an Iowa State professor of physics and astronomy, wrote in a 2013 research summary: "In galaxy disks, the scars of a rough childhood, and adolescent blemishes, all smooth away with time.")
Now, the group has co-authored a new paper that says their ideas about the formation of exponential disks apply to more than young galaxies. It's also a process that is robust and universal in all kinds of galaxies. The exponential disks, after all, are common in spiral galaxies, dwarf elliptical galaxies and some irregular galaxies.
How can astrophysicists explain that?
By using realistic models to track star scattering within galaxies, "We feel we have a much deeper understanding of the physical processes that resolve this almost-50-year-old key problem," Struck said.
Gravitational impulses from massive clumps alter the orbits of stars, the researchers found. As a result, the overall star distribution of the disk changes, and the exponential brightness profile is a reflection of that new stellar distribution.
The astrophysicists' findings are reported in a paper just published online by the Monthly Notices of the Royal Astronomical Society. Co-authors are Struck Jian Wu, an Iowa State doctoral student in physics and astronomy Elena D'Onghia, an associate professor of astronomy at Wisconsin and Bruce Elmegreen, a research scientist at IBM's Thomas J. Watson Research Center in Yorktown Heights, New York.
Stars are scattered, disks are smoothed
The latest computer modeling -- led by Wu -- is a capstone topping years of model improvements, Struck said. Previous models treated the gravitational forces of galaxy components more approximately, and researchers studied fewer cases.
The latest models show how star clusters and clumps of interstellar gases within galaxies can change the orbits of nearby stars. Some star-scattering events significantly change star orbits, even catching some stars in loops around massive clumps before they can escape to the general flow of a galaxy disk. Many other scattering events are less powerful, with fewer stars scattered and orbits remaining more circular.
"The nature of the scattering is far more complex than we previously understood," Struck said. "Despite all this complexity on small scales, it still averages out to the smooth light distribution on large scales."
The models also say something about the time it takes for these exponential galaxy disks to form, according to the researchers' paper. The types of clumps and initial densities of the disks affect the speed of the evolution, but not the final smoothness in brightness.
Speed in this case is a relative term because the timescales for these processes are billions of years.
Over all those years, and even with model galaxies where stars are initially distributed in a variety of ways, Wu said the models show the ubiquity of the star-scattering-to-exponential-falloff process.
"Stellar scattering is very general and universal," he said. "It works to explain the formation of exponential disks in so many cases."
University of California, San Diego Center for Astrophysics & Space Sciences
Variable stars have proven to be one of the most reliable types of "standard candles". Cepheid Variables are giant stars which have instabilities in their envelopes that cause them to pulsate in size, temperature and luminosity over timescales of a few days. Below is the light curve (brightness as a function of time) for Cephei, the variable star in the constellation Cepheus from which the stars derive their name. This Mpeg animation shows the pulsation of a cepheid.
Light Curve of Delta Cephei
(You can determine your own light curve from the AAVSO)
The pulsations of Cepheids are very regular Cephei has a period of pulsation (time between maxima or minima) of 5.366341 days. Furthermore the period is directly related to the luminosity of the star as shown from the Period-Luminosity Relation.
- Their luminosities may be determined from the P-L relationship which has very little scatter.
- They are giant stars so they are pretty luminous and can be seen out to relatively large distances. With HST Cepheids are being studied in galaxies in the Virgo Cluster. about 50 million light-years away.
- The variability of Cepheids makes them easy to pick out. Photographs (now digital images) are obtained of galaxies over several nights. When the images are compared, most stars don't change, but the Cepheids blink brighter and fainter and are easily distinguished from the billions of other "normal" stars.
Cepheid Variable in M100
(may also be seen in this MPEG animation)
- extremely bright, outshining the entire galaxy of stars in which they reside.
- easily detected, but very rare , occurring only about once every 500 years in our galaxy.
- very uniform in brightness at maximum light.
The doppler shift is a shift in wavelength or frequency of a wave due to relative motion of the source and receiver. If the source of waves is moving toward the receiver (or vice versa) each successive wavecrest is emitted a small distance closer to the receiver, the wavecrests will be closer together and the wave will be"squashed as shown below (shorter wavelengths & higher frequency). Similarly, if the source of waves is moving away from the receiver each successive wavecrest is emitted a small distance further away, the wavecrests will be further apart and the wave will be"stretched" (longer wavelengths, lower frequency). This is the phenomenon that causes the apparent shift in pitch (from higher approaching to lower receding) as a train passes by sounding its whistle.
Diagram from Ned Wright's ABC's of Distances © Edward L. Wright (UCLA) used by permission.
Because shorter wavelength is toward the blue and longer wavelengths are toward the red for visible light, approaching velocities are said to produce "blueshifts" and receding velocities produce redshifts. for speeds small compared to the velocity of light the shift is given by:
From the work of Slipher in the early part of the 1900's astronomers knew that most of the spiral nebulae (galaxies) are receding from us. Edwin Hubble demonstrated that there was a linear relationship between distance and velocity for galaxies.
Hubble's (1929) Velocity-Distance Relation for Nearby Galaxies
Diagram from Ned Wright's ABC's of Distances
© Edward L. Wright (UCLA) used by permission.
The data from Hubble's original velocity-distance relationship. Hubble's distance scale was in error by a factor of several, but his result was a most important one, demonstrating that the Universe is expanding and providing a means of estimating the distances to galaxies at the "edge of the Universe. Hubble's "law" is
with the modern value of the slope, H, called the Hubble Parameter (sometimes called Hubble Constant, but as we shall see it is not constant):
The precise value of the Hubble Parameter is a matter of very hot debate with different groups proposing values between 15 km/s/million l. y. to 25 km/s/million l. y. It is interesting to note that the units for the Hubble Parameter are 1/time and the current estimate corresponds to a time of about 15 billion years. What might this time correspont to?
- Ned Wright's ABC's of Distances offers detailed discussion of a variety of distance determination methods.
- NASA's Tutorial on the Hubble Parameter.
Galaxies The Interstellar Medium Education & Outreach CASS Home Comments? Gene Smith
Prof. H. E. (Gene) Smith
CASS 0424 UCSD
9500 Gilman Drive
La Jolla, CA 92093-0424
Last updated: 22 April 1999
How Many Galaxies Are There in the Universe?
The best science is skeptical of itself, always examining its own theories to find out where they could be wrong, and seriously considering new ideas to see if they better explain the observations and data.
What this means is that whenever I state some conclusion that science has reached, you can’t come back a few years later and throw that answer in my face. Science changes, it’s not my fault.
I get it, VY Canis Majoris isn’t the biggest star any more, it’s whatever the biggest star is right now. UY Scuti? That what it is today, but I’m sure it’ll be a totally different star when you watch this in a few years.
What I’m saying is, the science changes, numbers update, and we don’t need to get concerned when it happens. Change is a good thing. And so, it’s with no big surprise that I need to update the estimate for the number of galaxies in the observable Universe. Until a couple of weeks ago, the established count for galaxies was about 200 billion galaxies.
Jacinta studies distant galaxies like those shown in this image from the Hubble Space Telescope, using the new ‘stacking’ technique to gather information only available through radio telescope observations. Credit: NASA, STScI, and ESA.
But a new paper published in the Astrophysics Journal revised the estimate for the number of galaxies, by a factor of 10, from 200 billion to 2 trillion. 200 billion, I could wrap my head around, I say billion all the time. But 2 trillion? That’s just an incomprehensible number.
Does that throw all the previous estimates for the number of stars up as well? Actually, it doesn’t.
The observable Universe measures 13.8 billion light-years in all directions. What this means is that at the very edge of what we can see, is the light left that region 13.8 billion years ago. Furthermore, the expansion of the Universe has carried to those regions 46 billion light-years away.
Does that make sense? The light you’re seeing is 13.8 billion light-years old, but now it’s 46 billion light-years away. What this means is that the expansion of space has stretched out the light from all the photons trying to reach us.
What might have been visible or ultraviolet radiation in the past, has shifted into infrared, and even microwaves at the very edge of the observable Universe.
Since astronomers know the volume of the observable Universe, and they can calculate the density of the Universe, they know the mass of the entire Universe. 3.4 x 10^54 kilograms including regular matter and dark matter. They also know the ratio of regular matter to dark matter, so they can calculate the total amount of regular mass in the Universe.
In the past, astronomers divided that total mass by the number of galaxies they could see in the original Hubble data and determined there were about 200 billion galaxies.
Now, astronomers used a new technique to estimate the galaxies and it’s pretty cool. Astronomers used the Hubble Space Telescope to peer into a seemingly empty part of the sky and identified all the galaxies in it. This is the Hubble Ultra Deep Field, and it’s one of the most amazing pictures Hubble has ever captured.
The Hubble Ultra Deep Field seen in ultraviolet, visible, and infrared light. Image Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)
Astronomers painstakingly converted this image of galaxies into a 3-dimensional map of galaxy size and locations. Then, they used their knowledge of galaxy structure closer to home to provide a more accurate estimate of what the galaxies must look like, out there, at the very edge of our observational ability.
For example, the Milky Way is surrounded by about 50 satellite dwarf galaxies, each of which has a fraction of the mass of the Milky Way.
By recognizing which were the larger main galaxies, they could calculate the distribution of smaller, dimmer dwarf galaxies that weren’t visible in the Hubble images.
In other words, if the distant Universe is similar to the nearby Universe, and this is one of the principles of modern astronomy, then the distant galaxies have the same structure as nearby galaxies.
It doesn’t mean that the Universe is bigger than we thought, or that there are more stars, it just means that the Universe contains more galaxies, which have less stars in them. There are the big main galaxies, and then a smooth distribution curve of smaller and smaller galaxies down to the tiny dwarf galaxies. The total number of stars comes out to be the same number.
The Fornax dwarf galaxy is one of our Milky Way’s neighbouring dwarf galaxies. Credit: ESO
The galaxies we can see are just the tip of the galactic iceberg. For every galaxy we can see, there are another 9, smaller fainter galaxies that we can’t see.
Of course, we’re just a few years away from being able to see these dimmer galaxies. When NASA’s James Webb Space Telescope launches in October, 2018, it’s going to be carrying a telescope mirror with 25 square meters of collecting surface, compared to Hubble’s 4.5 square meters.
Furthermore, James Webb is an infrared telescope, a specialized tool for looking at cooler objects, and galaxies which are billions of light-years away. The kinds of galaxies that Hubble can only hint at, James Webb will be able to see directly.
So, why don’t we see galaxies in all directions with our eyeballs? This is actually an old conundrum, proposed by Wilhelm Olbers in the 1700, appropriately named Olber’s Paradox. We did a whole article on it, but the basic idea is that if you look in any direction, you’ll eventually hit a star. It could be close, like the Sun, or very far away, but whatever the case, it should be stars in all directions. Which means that the entire night sky should be as bright as the surface of a star. Clearly it isn’t, but why isn’t it?
In fact, with 10 times the number of galaxies, you could restate the paradox and say that in every direction, you should be looking at a galaxy, but that’s not what you see.
A partial map of the distribution of galaxies in the SDSS, going out to a distance of 7 billion light years. The amount of galaxy clustering that we observe today is a signature of how gravity acted over cosmic time, and allows as to test whether general relativity holds over these scales. (M. Blanton, SDSS)
Except you are. Everywhere you look, in all directions, you’re seeing galaxies. It’s just that those galaxies are red-shifted from the visible spectrum into the infrared spectrum, so your eyeballs can’t perceive them. But they’re there.
When you see the sky in microwaves, it does indeed glow in all directions. That’s the Cosmic Microwave Background Radiation, shining behind all those galaxies.
It turns out the Universe has 10 times more galaxies than previously estimated – 2 trillion galaxies. Not 10 times the stars or mass, those numbers have stayed the same.
And, once James Webb launches, those numbers will be fine-tuned again to be even more precise. 1.5 trillion? 3.4 trillion? Stay tuned for the better number.
Hubble Drills Deep Into the Universe
What happens when you take a 2.4-meter telescope, launch it into space, and command it to stare at one spot in the sky for a solid 14 hours, taking data both in visible light (like our eyes see) and infrared?
Can I get a “Yowza!” from the congregation? No? Maybe that’s because when I shrink this Hubble Space Telescope picture down to fit the blog you can’t really get a sense of what you’re seeing here. So click the picture to get the 1280 x 1280 image, or better yet, do yourself and your eyeballs a favor and take a poke at the huge 3900 x 3900 pixel version, because holy wow.
What you’re seeing here is a view of thousands of galaxies. Thousands. Sure, there are some stars in our own Milky Way punctuating this picture here and there but they are few, and just stomped flat by the number of whole galaxies you’re seeing. The stars can be distinguished from galaxies because they’re point sources small dots. They also might have those lines going through them called diffraction spikes. Galaxies don’t usually get those because they’re fuzzier, spread out over many pixels. That suppresses the diffraction spikes.
So for example that bright point with pretty spikes you see toward the upper right is a star, probably a few thousand light years from Earth. That’s a long way to be sure, but even the nearest galaxies you can see in this image are hundreds of millions of light years away! Some are billions the most distant objects in this shot are at least 9 billion light years distant. That’s a million times farther away than any star in the picture.
When the light we see here left those galaxies, the Sun hadn’t yet formed. When the Earth itself was coalescing from countless specks of dust, that light still had half its journey here ahead of it.
CLASS B1608+656, a cluster of galaxies far, far away.
In fact you’re seeing galaxies at all different distances from Earth in this image, but the observation itself was taken to look at the cluster of galaxies in the center. Called CLASS B1608+656, it’s a clump of galaxies about five billion light years away. The mass of that cluster acts like a lens, bending space, magnifying objects behind it. This gravitational lens has distorted and amped up the brightness of a luminous galaxy located an additional several billion light years behind it, creating the weirdly shaped mess you see in the close-up above. Rings and arcs are common in such events.
But there’s so much more to this image just scanning across it reveals an incredible variety and diversity of galaxies. Remember, too, you’re looking at objects as they existed eons ago many are still growing, suffering collisions with other galaxies, giving them fantastic shapes. As an example, I’m fond of this little group near the top of the main image:
A cosmic train wreck a million light years long.
I’m not precisely sure what to make of this. The bigger galaxies look to all be about the same distance from us, but that could be a coincidental alignment. Some of the galaxies are blue and clumpy looking, indicating they’re aggressively forming stars (hot, young, massive stars are preferentially blue), while some are quite red. The red ones may be very dusty, which reddens the light from stars, or they may be farther away, their light redshifted as it fights against the expansion of the Universe itself, losing energy along the way. It may be a mix of both. Unfortunately, this image was made using only two filters, so colors can be difficult to interpret, and don’t yield a lot of subtle information. The only way to know more about the galaxies would be to measure their distance, and I didn’t find anything in the literature about them.
That’s worth taking a moment to ponder, actually. These are entire galaxies, collections of tens of billions of stars, planets, dust, and gas clouds, each and every one a monstrous object on scales that dwarf our everyday experience … yet there are so many of each of them in this image alone we can’t possibly know their details. We can determine their coordinates on the sky, get a rough estimate of their distance, but there is simply no way to get a measure of them as individuals. They are too many. It’s like trying to get the life history of everyone who passes you on a busy New York City street corner. The task is too overwhelming.
And just in case I have not yet crushed your puny human mind, this image represents a tiny fraction of the entire sky perhaps only one ten-millionth of it. That means there are hundreds of billions of galaxies like these scattered throughout the Universe.
So gaze again at that image, one that drills a narrow but incredibly deep view through our cosmos, one that shows us both the awe-inducing grandeur and soul-squeezing immensity of it, and remember: The Universe is far, far larger than this still.
And yet here we are, pondering it. To those galaxies, we are the ones who are lost in the anonymous throng. Yet I would argue we are as important and interesting a piece of the Universe as any other we can imagine. We are part of it at the same time as we study it, and to me, that is part of what makes us great.
How SDSS Uses Light to Measure the Distances to Galaxies
Here at the Sloan Digital Sky Surveys our mission is to explore and map the Universe, from planets to the edges of the observable Universe. The way we do this is to collect light from specially selected objects we see in the night sky – but we can’t visit them in order to measure how far away they are. So how do we actually know how far away they are in order to make a map of the Universe?
Measuring the distance to objects in the Universe has always been one of the biggest challenges for astronomers. Until we know the distance to something we cannot really understand its physical properties, and the history of astronomy is full of examples where new techniques for measuring distances opened up entirely new areas of study. For example when the “spiral nebulae” were first discovered there was a long debate over if they were small clouds of gas in our own Galaxy, or external galaxies in their own right each made up of millions or billions of stars. Only by measuring their distances was this finally settled, and our understanding of the size of the Universe suddenly jumped many orders of magnitude.
A collection of “spiral nebulae”. But how can we tell that they are distant galaxies rather than nearby gas clouds? Credit: SDSS
There’s some really useful bits of physics we can use to help measure distances to the galaxies from their light. To do this we need to understand spectroscopy. Once SDSS had finished imaging more than a quarter of the sky with its camera, it became entirely focused on “spectroscopic” surveys. Our telescope in New Mexico collects the light from stars and galaxies and uses instruments called spectroscopes to split it up into its different colours (we actually have two different spectroscopes working right now – the APOGEE spectroscope and the BOSS spectroscope). These measurements split the light into a rainbow (or a spectrum), and we look for the precise colours of series of emission and/or absorption lines to tell us all sorts of things about the light source we’re looking at.
A hot bright light source (like a star) will have a “continuous spectrum” (with the peak colour depending on its temperature – hot things glow red, even hotter things glow white or blue hot). If the light from that passes through a cool cloud of gas before we measure it, that will create “absorption lines” where very specific colours (or “wavelengths” in proper scientific terms) are absorbed by atoms in the gas cloud. The exact pattern of colours/wavelengths which are absorbed tell you which atoms are in the gas cloud. If the gas cloud gets heated up enough we might instead see emission lines – at the same specific colours, where the atoms are now re-emitting these very specific colours/wavelengths. Each atom has a very distinctive pattern of lines it emits – for example hydrogen (the most abundant element in the Universe) has a very distinctive and bright emission/absorption line in the red part of the spectrum (at a wavelength of 656.3nm).
Emission spectrum of hydrogen in visible light (wikimedia commons)
Astronomers have been using this technique to work out the materials which make up the Sun and other stars for decades. It’s not always easy (it has been compared to trying to reconstruct a piano from the noise it makes falling down the stairs), but it works. When astronomers first used the technique to look at galaxies however they were very surprised by what they found. The patterns of lines seemed to be in completely the wrong places – for example the famous hydrogen lines weren’t even visible in some cases – they had moved right into the infra-red part of the spectrum.
In order to understand why this could happen we need to learn about another part of physics – the Doppler effect. First proposed in 1842, by a Physicist named Christian Doppler this is the observation that when a source emitting a wave is moving, the waves are shortened if the source is moving towards the observer, and lengthened if it is moving away. Most people are familiar with this effect when they have listened to ambulance sirens passing them on the street the siren is higher in pitch when the ambulance is moving towards you and lower when it’s moving away (when sound waves are lengthened the pitch drops, and when they are shortened the pitch rises).
Wikimedia commons illustration of the Doppler effect.
Since light is a wave, the same effect happens when light is emitted from a moving source. When the waves of light are shortened the light becomes bluer, and when they are lengthened the light becomes redder.
An astronomer named Vesto Slipher, was the first person to try this out on galaxies, and he found that almost all galaxies he looked at showed enormous “redshifts”, implying that almost all the galaxies were moving away from the Earth at very high speeds.
Edwin Hubble is given the credit for explaining this observation by realising that we live in a Universe which is constantly expanding. In such a Universe any observer will observe almost all other galaxies moving away from them. Hubble published the first description of a relationship between how fast galaxies appear to be moving away from us (their “redshifts”) and their distances – this relationship is now called Hubble’s Law.
It is this relationship that we use to measure the distances to the galaxies from detailed observations of the light they emit, and astronomers are now used to describing the distances to galaxies as simply their “redshift”.
A map of the Universe from SDSS where the distance to galaxies is given in terms of their redshift. Credit: SDSS
This post is part of the SDSS Celebration of the International Year of Light 2015, in which we aim to post an article a month about how SDSS uses light in our mission to study the Universe.
Type Ia Supernovae Could Use Some More Color
Type Ia supernovae (SNIa) are brilliant stellar explosions that have become very important for cosmology. They are one of the primary tools that we can use to calculate how fast the universe is expanding! This is done by calculating distances to the SNIa through comparisons of intrinsic and observed properties of the explosions. In today’s paper, the authors explore the utility of also considering the properties of the galaxies that host the supernovae. In order to better understand how this can help us, let’s start with a quick introduction into how we use SNIa right now.
Type Ia Supernovae
SNIa are unique for a variety of reasons. There are two different hypothesized methods for the production of SNIa: two white dwarfs merge and explode, or a single white dwarf accretes matter from a companion star until it reaches the Chandrasekhar Limit. At this point, it undergoes a runaway nuclear reaction and explodes. SNIa also happen to be standardizable candles. This means that, using what we know about how SNIa behave and correcting for it, we can determine their true luminosities (Fig. 1) – roughly a few billion times as luminous as the sun!
Figure 1: Light curves for many different SNIa are plotted on the left. A brightness correction is made for each one based on how fast its light curve declines in time on the left, leading to a relatively standard graph of luminosity vs time for all SNIa on the right. (Figure taken from Durham University Department of Physics)
We can compare the brightness we observe with their true luminosities to figure out how far away SNIa are from us. By also looking at how much their light is redshifted, we can determine how quickly these objects are moving away from us. Coupling this distance and velocity information enables us to calculate exactly how fast the universe is expanding (spoiler: it’s accelerating!). This very calculation lead to the idea of dark energy and the Nobel Prize in Physics in 2011.
Corrections with a Splash of Color
Today’s paper suggests that a new correction needs to be added to our distance calculations using SNIa light curves. Normal corrections applied to SNIa are based on two factors: stretch, which is how fast the explosion declines in brightness and color, which is essentially B-V color at peak brightness. These corrections enable us to get pretty consistent true luminosities for SNIa the largest modern samples have scatters of only around 1% or so at peak brightness. However, the authors of today’s paper suggest that another correction for the color of the local environments of the supernovae should be included (Fig. 2). In other words, they believe that we should not just consider the supernova itself – we should also consider its surroundings.
Figure 2: Examples of galaxies and the local environments of supernovae. In each image, the
galaxy is encircled by the dashed line and the local environment of the supernova is encircled by the solid line. The supernova occurred at the red cross inside the solid circle. The authors of the paper suggest that we should also consider the region inside the solid circle when applying corrections, rather than just the supernovae itself. (Figure taken from Figure 2 in the paper)
One way to test whether this hypothesis is valid is by looking for a correlation between Hubble residuals and local color. Hubble residuals are the differences between our best fit cosmological model and the SNIa data on a distance vs. cosmological redshift plot (known as a Hubble diagram). If there is a correlation between these residuals and some property of SNIa or their surroundings, it means that our model lacks some information that could help fit the data better.
Evidence of the Need for a New Correction
Figure 3 compares Hubble residuals to the local U-V color. A statistical analysis reveals that there is evidence of a correlation between these two variables with a significance of roughly 7, meaning there is roughly a 1 in a trillion chance that this result is due to chance. This suggests that local U-V color should also be used to correct SNIa light curves, improve our model, and reduce the scatter in our sample.
Figure 3: Plot of Hubble residuals (y-axis) vs. local U-V color (x-axis) in magnitudes. Points are essentially colored by redshift – the green points are the closest, the red are at medium redshift, and the blue are at the farthest. The blue histogram on the right corresponds to the points left of the dashed line, and the red histogram corresponds to the points right of it. The purple dashed line shows the apparent correlation between Hubble residuals and local U-V color, indicating we should correct for this parameter in our model. (Figure taken Fig. 13 in the paper)
The major takeaway from the paper is that we should consider both the properties of the supernova explosion and its surroundings in our cosmological analyses. With major observational projects (LSST and WFIRST) planned for the coming decades that will help us observe more SNIa than ever before, corrections like these could help us pin down what dark energy is!
One technique for studying galaxies that astronomers have been using with modern, space-based observatories is to take deep fields. The longer you leave the shutter of your camera open while pointing at a particular part of the sky, the fainter the objects you will be able to see. In astronomical jargon, the fainter the faintest object in your image is, the "deeper" the image. In 1996, Hubble released the "Hubble Deep Field," which was a revolutionary image in its day. It is reproduced below.
If you look closely, the few objects with spikes on them (a white one near the lower left and a yellow one on the center-left) are stars in the Milky Way Galaxy, but every other object in this image is a distant galaxy. Thus, by taking a deep field, astronomers are presented with an excellent representative sample of the types of galaxies in the universe. We will return to the Hubble Deep Field for Lab 3.
For the most part, all of the galaxies that we observe locally, and that we find in deep fields, are normal galaxies. They contain billions of stars, clouds of gas, and dust. When we take a spectrum of the integrated light from the entire galaxy, it looks like the sum of the light from a large group of stars. Remember, stars have spectra that are very similar to blackbodies, but with absorption lines. So, for the most part, a galaxy's spectrum looks like a blackbody spectrum, but also with a lot of absorption lines.
See for yourself the spectra of galaxies. The Sloan Digital Sky Survey has measured the spectra for many galaxies. In the window of data on each SDSS object, click on the thumbnail image of the spectrum in the lower left to bring up a full sized image of the spectrum.
- Bring up the SDSS Object Explorer for Object ID 587722982271090881.
- Bring up the SDSS Object Explorer for Object ID 588015507682230507.
- Bring up the SDSS Object Explorer for Object ID 587731513690095801.
At the same time that astronomers were spending much of their time studying galaxies and classifying them, several astronomers were finding peculiar objects unlike many of the other galaxies. One type of peculiar galaxy, identified by astronomer Carl Seyfert, had the following properties that were unique compared to the average galaxy:
- The total luminosity of the galaxy is much larger (perhaps 1,000 times larger) than a normal galaxy.
- The spectrum of the galaxy is very different from a typical galaxy it gives off more light at all wavelengths than a normal galaxy does, and there are many bright emission lines in its spectrum.
The extra luminosity from these “Seyfert Galaxies” comes almost entirely from the nuclear region of the galaxy, not from all areas of the galaxy equally. In general, any galaxy that shows evidence for a source of radiation that is not being created by stars is called an active galaxy. Because the non-stellar light from these galaxies is concentrated in the nucleus, they are usually called Active Galactic Nuclei or just AGN. Below is a sample image of a Seyfert galaxy, and it is a particularly nice example where it is quite obvious how much brighter the bulge region is compared to some of the other spiral galaxies you have seen previously.
AGN are bright radio and x-ray sources. In particular, in the radio, the galaxies show narrow jets of plasma being ejected out of the galaxy at high speeds, which end in extremely large lobes of bright radio emission. Remember, in the optical, the extent of the Milky Way is something like 30,000 parsecs in diameter. In general, the distance from lobe to lobe in a radio galaxy can be several hundred thousand parsecs, and the two largest are 4 million and 6 million parsecs from lobe to lobe! The National Radio Astronomy Observatory has spent more than 50 years studying AGN and has a large archive of images from their observations of these objects. For example, see:
There are many different types of galaxies that are called AGN, but broadly speaking, astronomers have separated them into three main classes with the following properties:
- Seyfert Galaxies:
- These appear superficially to be normal spiral galaxies.
- One clue to how they differ from normal galaxies is that the "extra" luminosity from Seyferts appears to come from an extremely bright core.
- In some cases, the core of a Seyfert galaxy is brighter than the entire Milky Way.
- Seyfert galaxies are particularly bright in the infrared, and this is likely due to dust surrounding the core.
- The light from the core of a Seyfert can vary on a very short timescale, which suggests that the energy emitting region is less than 1 light-year across.
- Radio Galaxies:
- Radio galaxies are often found to be associated with elliptical galaxies, but not always normal elliptical galaxies. Instead, radio galaxies tend to be associated with "disturbed" ellipticals like Centaurus A.
- Much of the nonstellar radiation from these objects is in the radio part of the spectrum. Also, the morphology of the radio emission isn't always just a point source, instead, we often see a core, jet, lobe structure.
- In other wavelengths (X-ray, for example), the core of the radio galaxy stands out as a bright point source.
- The total energy output of these objects is roughly 1,000 times larger than the Milky Way, making them some of the brightest single objects in the universe.
- In the optical part of the spectrum, Quasars look like bright, blue stars. The name Quasar stands for "Quasi-stellar radio source," because they look like stars, but are in many ways unlike stars.
- They are bright in the optical, x-ray, and radio portions of the spectrum, and the brightest of these objects is 100,000 times brighter than the Milky Way.
See for yourself the spectra of active galaxies (AGN). The Sloan Digital Sky Survey had as one of its major science goals the discovery and study of a large number of quasars. Identical to the data on the normal galaxies, in the window of data on each SDSS object, click on the thumbnail image of the spectrum in the lower left to bring up a full sized image of the spectrum.
- Bring up the SDSS Object Explorer for Object ID 587725074457821263.
- Bring up the SDSS Object Explorer for Object ID 587725074995019808.
How do these spectra appear? How are they similar to each other? How do they differ from the galaxy spectra you studied above?
If you want to explore more objects beyond the few listed here, you can bring up all of the SDSS objects from a single observation with the Plate Explorer.
A question that astronomers asked, and a question you should be asking yourself now, is: What is the source of the difference between normal galaxies and active galaxies?
To answer this question, we need to return briefly to the topic of black holes. When a massive star creates a black hole, the mass of the black hole is a fraction of the mass of the star's core. We think that stellar black holes have masses of about 3 times the mass of the Sun. In the core of the Milky Way, we have found that the central black hole is several million solar masses. We think that supermassive black holes (SMBHs) like Sgr A* in the Milky Way are common in the cores of galaxies, and we are able to theorize what happens when a star or other object gets too close to the SMBH.
When matter gets close to a SMBH, it will swirl around the SMBH, and we expect this to create a disk of material around the SMBH. This is exactly analogous to the accretion disks we considered when we studied binary star systems. We believe that in many ways they look similar. As shown below, the material in this disk falls into, or accretes onto, the SMBH. As the material swirls closer and closer to the SMBH, it speeds up. Friction among the particles in the disk causes the gas to heat up, and it can reach temperatures of millions of kelvin. These disks full of hot, fast moving gas emit radiation, and the amount is large enough to power quasars and other AGN.
The jets that we see in radio galaxies and quasars are likely to come from the disk, too, but these are not well understood at all. It is thought that the disk helps to “focus” the jets (see the image above for an example), but the exact mechanism of the creation of the jet and how it stays so narrow over such large distances (a few hundred thousand parsecs) is a topic of ongoing research.
The Milky Way has an SMBH in its core, but the Milky Way is not an AGN. So, clearly, just the presence of an SMBH alone cannot be the only requirement for the creation of an AGN. The answer to this again rests with the accretion disk. The accretion disk fuels the central power source in AGN, and if there is no fuel or not enough fuel, then the galaxy will not be seen as an AGN. When we look at AGN in detail, it appears that many of them appear to show evidence for the host galaxy absorbing another galaxy. This act of galaxies merging with each other can funnel gas into the core, providing ample fuel to power the SMBH. This idea does imply that AGN will "shut off" over time as they run out of fuel. In 2016, Penn State astronomer Jessie Runnoe and her collaborators discovered a quasar that appears to have run out of fuel in between observations!
How many galaxies have had their distance determined using SNIa? - Astronomy
A galaxy is a gravitationally bound entity, typically consisting of dark matter, gas, dust and stars. Galaxies populate the Universe, mainly residing in clusters and groups. There are thought to be over 100 billion galaxies in the observable Universe. The most well-known galaxy is our own Milky Way – and indeed, the term galaxy comes from the Greek “gala” meaning “milk”.
Until the early 20th century, it was widely believed that the Milky Way was the only such structure in the Universe. Around the middle of the 18th Century, German philosopher Immanual Kant proposed “island Universes” that were similar to the Milky Way and that populated the Universe. Sir William and Caroline Herschel were the first to systematically catalogue the night sky – they catalogued around 2500 objects, including “spiral nebulae” that appeared to have a similar structure to the Milky Way. Later, using the largest telescope of its day, optical astronomer Lord Rosse agreed with Kant’s view, based on observations he made of M51 with his home-made 72 inch telescope.
In April 1920, two eminent scientists – Harlow Shapley and Heber D. Curtis held a public debate about the size of the Milky way and the nature of “nebulae”. Shapley believed that the Milky Way was vastly greater in size than previous estimates, and that spiral nebulae were a part of it. On the other hand, Curtis believed that spiral nebulae were in fact island Universes, that lay beyond the Milky Way. There was no winner as such to this “debate” which was finally settled in 1923, when, using the period-luminosity relationship of cepheid variable stars, young Edwin Hubble was able to determine the distance to the Andromeda “nebula” was around 750 kpc, and that it had a diameter larger than that of the Milky Way. This proved that Andromeda was not some small “spiral nebula” within the confines of the Milky Way, but an enormous stellar system in its own right.
Size and Mass of galaxies
Most galaxies have a total mass between
10 7 M⊙ and 10 12 M⊙. They range in size from a few kiloparsecs, to over one hundred kiloparsecs in diameter. Our own Milky Way contains over 100 billion stars, including our Sun, and the stellar disk extends to about 50 kpc in diameter. The spherical stellar halo extends up to 100 kpc, and the dark matter halo may extend even beyond this.
Classification of galaxies
Galaxies are classified according to how they appear, or their optical morphology. The first attempt at a classification scheme for “Nebulae” was by Sir William Herschel, and his son, Sir John Herschel. However, the most common classification scheme in use today is the Hubble classification scheme. Galaxies can be classified into the following broad categories, although there are many sub-catagories within each classification: