# How do we have photos of galaxies so far away?

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A possible answer for this is that, light emitted from the galaxies travelled a billion miles all the way to earth, where the hubble space telescope picked up this light through its sensors, and was able to construct an image of the galaxy

but if this is true, and galaxies are billions of miles away, shouldn't the light particles emitted from the galaxies be scattered all over the place? after all they have been travelling from millions of years, and have probably collided with asteroids and other foreign objects. What were the chances that about 95% of the photons actually reached earth, giving us a very detailed image.

Consider the andromeda galaxy which has a distance of 1.492 × 10^19 mi from earth. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy, evident from the photo below?

Shouldn't like half of the galaxy be missing since photons could have hit other objects, and "never have reached earth"?

There are two reasons that often - but not always - light from galaxies millions and even billions of lightyears away make it through the Universe and down to us:

Particle number and particle size
1. First, the intergalactic medium (IGM) is extremely dilute. The number density of particles out there is of the order $nsim10^{-7},mathrm{cm}^{-3}$, or roughly 26 orders of magnitude lower that the air at sea level! That means that if you consider a tube from Andromeda to the Milky Way with cross-sectional area of $1,mathrm{cm}^{2}$, it will contain roughly one microgram of matter (thanks to Rob Jeffries for catching a factor $10^6$ error).

2. Second, even if a photon comes close to an atom, it will only be absorbed if its energy matches closely some transition in the atom. Since most of the atoms are ionized (and thus should be called plasma instead, but in astronomy the distinction if often not made), there are no electrons to absorb the photon. The photons are more likely to interact with the free electrons via Thomson scattering, but the Thomson cross section is immensely small $(sim10^{-24},mathrm{cm}^{2})$, so even if you consider the CMB photons - which have traveled through the Universe almost since the Big Bang - only around 5% of them have interacted with electrons on their way.

In other words: The amount of transmitted light depends on two factors: 1) The amount of matter along the line of sight, and 2) that matter's ability to absorb the light. In the IGM, both are tremendously small. When the light enters the interstellar medium (ISM) inside our galaxy, it may encounter denser clouds with atoms that are able to absorb the light. But usually (although not always) "dense" is still very dilute compared to Earth's atmosphere.

Mathematical expression

In general, if a beam of light traverses a region of particles, each with a cross section $sigma$ (measured e.g. in cm$^2$), passing $N$ particles per area of the beam (measured e.g. in cm$^{-2}$), then the opacity of the medium is given by the optical depth $au$, defined by $$au equiv N , sigma.$$ The transmitted fraction $f$ of photons is then $$f = e^{- au}.$$ In general $sigma$ depends on the wavelength, and thus part of the spectrum may pass unhindered, while another part may be completely absorbed.

The figure below (from here) shows the spectrum of a quasar lying at a distance of 22 billion lightyears, i.e. $10,000$ times farther away than Andromeda. You see that there are several thin absorption lines (caused by intervening hydrogen clouds whose densities are a factor of 10-100 higher than the IGM), but still most of the light makes it down to us.

Because the light we see from this quasar was emitted so long ago, the Universe was considerably smaller at that time, and thus the density was larger. Nonetheless, only a small fraction is absorbed. The farther away the light is emitted, the longer ago it was, which means smaller Universe, and higher density, and thus the more light is absorbed. If you consider this quasar (from here) which lies 27 billion lightyears away, you see that much more light is absorbed in part of the spectrum. Still, however, much light make it through to us.

The reason that it is only the short wavelengths that are absorbed is quite interesting - but that's another story.

As Rob Jeffries says, the universe is mostly empty space. A photon can easily travel thousands of light years without interacting with anything. Most of the interaction would occur when photons entered the earth's atmosphere. The Hubble avoids this. These photos were most likely from combining several viewing sessions giving basically an extended time period for observing the galaxy.

Sorry if this logic seems a bit circular, but we can get unobscured pictures of galaxies because they are unobscured.

As has been mentioned - space is really, really big and really, really empty. This is hard for us to contemplate, because there's so much stuff right next to us - but this is actually a really unusual condition. The next star to the Sun is over 4 light-years away, but we get almost all (99.9999999999… %) the light from it that heads in our direction - the same with light from further away - we get a huge number of photons sent to us from objects very far away.

Hubble also uses the simple camera techniques of lensing and long exposures to take images of distant objects - so more light is received to construct the image.

But, the other part of this, it is almost impossible to take a picture of a galaxy (or star) that is behind another galaxy or dust cloud. For example, we can't easily see past the centre of our own galaxy, because there's a lot of dust and gas and stars in the way. The picture in your question, on the other, seems to be Andromeda, which is above the plane of the galaxy. Our galaxy is quite thin compared to its diameter, and we're a decent way out of the galactic centre, meaning there's a lot less stuff in the way.

And there are some galaxies we've taken images of which are obscured by dust:

If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy

Light is emitted from the galaxy in all directions. Only a tiny, tiny fraction of it is directed to Earth, and of that, an even tinier fraction is collected by any given telescope. But we can still see it, because galaxies are very, very bright. Andromeda contains about a trillion stars.

How do we have photos of galaxies so far away?

Because there's nothing much between them and us that interferes with the light that reaches our cameras.

A possible answer for this is that, light emitted from the galaxies travelled a billion miles all the way to earth, where the Hubble space telescope picked up this light through its sensors, and was able to construct an image of the galaxy

It's a billion miles to Saturn. Well actually the distance varies with the orbits, but see this Space.com article: "At their most distant, when they lie on opposite sides of the sun from one another, they are just over a billion miles (1.7 billion km) apart". The Andromeda galaxy is circa fifteen billion billion miles away. Or circa fifteen quintillion miles.

but if this is true, and galaxies are billions of miles away, shouldn't the light particles emitted from the galaxies be scattered all over the place?

Don't forget that photons have an E=hf wave nature. And that even though they are scattered in the air, you can still see the Moon. Yes, there's a bit of light going astray in space. But not so much that the night-time sky is some blank foggy fug. You can see Saturn too. And the stars. And the galaxies, but they are rather dim.

after all they have been travelling from millions of years, and have probably collided with asteroids and other foreign objects. What were the chances that about 95% of the photons actually reached earth, giving us a very detailed image.

The chances are high. We have pictures of planets and things because the chances are high.

Consider the Andromeda galaxy which has a distance of 1.492 × 10^19 mi from earth. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy, evident from the photo below?

If I was covered in lights, I would emit light in all directions, and you would see me because some of that light goes into your eye. The Andomeda galaxy is similar.

Shouldn't like half of the galaxy be missing since photons could have hit other objects, and "never have reached earth"?

No. And if half the photons didn't reach Earth, you'd just see a dimmer galaxy, that's all.

Let me give some simple explanations.

No, no, no. 95% of the photons don't reach Earth. Even if 5% of photons emitted (within a few seconds) just by one star, say, by our Sun had reached Earth, our planet would have been completely scorched! Now then, Andromeda has hundreds of billions of stars (or suns). Nothing of that reaches us, except for an infinitesimally small number. It's mind-boggling how small the percentage of photons that reach us is! You can try to calculate that very roughly. It is very easy to calculate what percentage of photons emitted by the Sun reaches Earth. And the Sun is only 8 minutes away from Earth, while Andromeda is more than 2.5 million years away! So, actually, it's not that difficult to imagine how many photons reach us.

Now, why don't asteroids, planets or stars block everything? Andromeda is way too large to be blocked like that! It's easier to block the view of the Pacific Ocean from space by placing a few specks of dust in between! The diameter of Andromeda is more than 200 million light years. Can we block it from view? Actually it can be blocked by something as large as a nebula close to our solar system. Such a nebula must be many light years in diameter; it must be dense enough; and not too far away. Thankfully nothing like that blocks this beautiful galaxy from our view. However it happens with some other galaxies and deep space objects. As to very distant nebulas, they won't block Andromeda from our view because they will look way too small against the background of Andromeda which is much further away.

Why is light not scattered? Why should it be scattered that much to make Andromeda blurry? When the Moon is on the horizon, its light travels through many hundreds of miles of dense atmosphere almost parallel to the surface of Earth; yet, we can still train our telescopes on it and see the various features of the Moon. It would not be a very clean view but we would still see a lot. Now, in space light travels through an almost complete vacuum, especially empty is the void between galaxies. So, there's no reason for light to be scattered too much. Photons and many other particles are stable enough and can travel much larger distances: billions of light years. Another way to look at it is to ask a question how much photons should deviate from their straight path so that Andromeda becomes blurry to us. Well, they have to go sideways a lot, and the diameter of Andromeda is too huge for that. That doesn't seem logical, as photons travel in straight lines. Large objects, like stars and black holes will affect their path but the diameter of Andromeda is so huge that it is not an option, unless we artificially place trillions of black holes along the line between Andromeda and our solar system in an attempt to warp the image of Andromeda or to make these black holes gobble up all the light from the galaxy! So, when astronomers say most of the light reaches us, they mean that intergalactic space is almost complete vacuum, and the photons that go exactly in our direction are “free” to go. Yet, only an infinitesimally small number of them go exactly in our direction and it is still enough for nice photos. Why? That's why:

The absolute magnitude (relative luminosity against that of an object $40$ times brighter than the Sun at a distance of $33$ light years away) of Andromeda is around $-21.5$. Our Sun is only around $5$. The higher the number the dimmer the object. An object with an absolute magnitute of $1$ would be $2.5^{5-1}=40$ times brighter than the Sun. The difference between Andromeda and our Sun is $-21.5-5=-26.5$. This means Andromeda is very roughly $2.5^{26.5}approx 40,000,000,000$ times brighter than the Sun.

As to how large it is in the night sky, well, lengthwise it is roughly six times the diameter of the moon but you can only see the bright central part. To see the whole extent you need a large aperture telescope and long exposure photography to gather more light and produce a better, more detailed image.

Hope, this primitive explanation will be of some help. Andromeda is visible today if weather permits :)

## Early Galaxies Looked Similar

A group of the newly discovered galaxies by the Lyman-break technique. Image credit: Astronomy & Astrophysics. Click to enlarge
An international team of astronomers have performed one of the most detailed surveys of the most distant galaxies. These galaxies are so far away, we see them as they looked when the Universe was less than half its current age. One of the big surprises of this survey however, is how much these young galaxies match the structures we see in the current Universe. This means that galaxies probably evolved through collisions and mergers much earlier than previously believed.

A team of astronomers from France, the USA, Japan, and Korea, led by Denis Burgarella has recently discovered new galaxies in the Early Universe. They have been detected for the first time both in the near-UV and in the far-infrared wavelengths. Their findings will be reported in a coming issue of Astronomy & Astrophysics. This discovery is a new step in understanding how galaxies evolve.

The astronomer Denis Burgarella (Observatoire Astronomique Marseille Provence, Laboratoire d’Astrophysique de Marseille, France) and his colleagues from France, the USA, Japan, and Korea, have recently announced their discovery of new galaxies in the Early Universe both for the first time in the near-UV and in the far-infrared wavelengths. This discovery leads to the first thorough investigation of early galaxies. The discovery will be reported in a coming issue of Astronomy & Astrophysics.

The knowledge of early galaxies has made major progress in the past ten years. From the end of 1995, astronomers have been using a new technique, known as the “Lyman-break technique”. This technique allows very distant galaxies to be detected. They are seen as they were when the Universe was much younger, thus providing clues to how galaxies formed and evolved. The Lyman-break technique has moved the frontier of distant galaxy surveys further up to redshift z=6-7 (that is about 5% of the present age of the Universe). In astronomy, the redshift denotes the shift of a light wave from a galaxy moving away from the Earth. The light wave is shifted toward longer wavelengths, that is, toward the red end of the spectrum. The higher the redshift of a galaxy is, the farther it is from us.

The Lyman-break technique is based on the characteristic “disappearance” of distant galaxies observed in the far-UV wavelengths. As light from a distant galaxy is almost fully absorbed by hydrogen at 0.912 nm (due to the absorption lines of hydrogen, discovered by the physicist Theodore Lyman), the galaxy “disappears” in the far-ultraviolet filter. Figure 2 illustrates the ?disappearance? of the galaxy in the far-UV filter. The Lyman discontinuity should theoretically occur at 0.912 nm. Photons at shorter wavelengths are absorbed by hydrogen around stars or within the observed galaxies. For high-redshift galaxies, the Lyman discontinuity is redshifted so that it occurs at a longer wavelength and can be observed from the Earth. From ground-based observations, astronomers can currently detect galaxies with a redshift range of z

6. However, once detected, it is still very difficult to obtain additional information on these galaxies because they are very faint.

For the first time, Denis Burgarella and his team have been able to detect less distant galaxies via the Lyman-break technique. The team collected data from various origins: UV data from the NASA GALEX satellite, infrared data from the SPITZER satellite, and data in the visible range at ESO telescopes. From these data, they selected about 300 galaxies showing a far-UV disappearance. These galaxies have a redshift ranging from 0.9 to 1.3, that is, they are observed at a moment when the Universe had less than half of its current age. This is the first time a large sample of Lyman Break Galaxies is discovered at z

1. As these galaxies are less distant than the samples observed up to now, they are also brighter and easier to study at all wavelengths thereby allowing a deep analysis from UV to infrared to be performed.

Previous observations of distant galaxies have led to the discovery of two classes of galaxies, one of which includes galaxies that emit light in the near-UV and visible wavelength ranges. The other type of galaxy emits light in the infrared (IR) and submillimeter ranges. The UV galaxies were not observed in the infrared range, while IR galaxies were not observed in the UV. It was thus difficult to explain how such galaxies could evolve into present-day galaxies that emit light at all wavelengths. With their work, Denis Burgarella and his colleagues have taken a step toward solving this problem. When observing their new sample of z

1 galaxies, they found that about 40% of these galaxies emit light in the infrared range as well. This is the first time a significant number of distant galaxies were observed both in the UV and IR wavelength ranges, incorporating the properties of both major types.

From their observations of this sample, the team also inferred various information about these galaxies. Combining UV and infrared measurements makes it possible to determine the formation rate for stars in these distant galaxies for the first time. Stars form there very actively, at a rate of a few hundred to one thousand stars per year (only a few stars currently form in our Galaxy each year). The team also studied their morphology, and show that most of them are spiral galaxies. Up to now, distant galaxies were believed to be mainly interacting galaxies, with irregular and complex shapes. Denis Burgarella and his colleagues have now shown that the galaxies in their sample, seen when the Universe had about 40% of its current age, have regular shapes, similar to present-day galaxies like ours. They bring a new element to our understanding of the evolution of the galaxies.

## Spitzer Finds Hidden Galaxies

How do you hide something as big and bright as a galaxy? You smother it in cosmic dust. NASA’s Spitzer Space Telescope saw through such dust to uncover a hidden population of monstrously bright galaxies approximately 11 billion light-years away.

These strange galaxies are among the most luminous in the universe, shining with the equivalent light of 10 trillion suns. But, they are so far away and so drenched in dust, it took Spitzer’s highly sensitive infrared eyes to find them.

“We are seeing galaxies that are essentially invisible,” said Dr. Dan Weedman of Cornell University, Ithaca, N.Y., co-author of the study detailing the discovery, published in today’s issue of the Astrophysical Journal Letters. “Past infrared missions hinted at the presence of similarly dusty galaxies over 20 years ago, but those galaxies were closer. We had to wait for Spitzer to peer far enough into the distant universe to find these.”

Where is all this dust coming from? The answer is not quite clear. Dust is churned out by stars, but it is not known how the dust wound up sprinkled all around the galaxies. Another mystery is the exceptional brightness of the galaxies. Astronomers speculate that a new breed of unusually dusty quasars, the most luminous objects in the universe, may be lurking inside. Quasars are like giant light bulbs at the centers of galaxies, powered by huge black holes.

Astronomers would also like to determine whether dusty, bright galaxies like these eventually evolved into fainter, less murky ones like our own Milky Way. “It’s possible stars like our Sun grew up in dustier, brighter neighborhoods, but we really don’t know. By studying these galaxies, we’ll get a better idea of our own galaxy’s history,” said Cornell’s Dr. James Houck, lead author of the study.

The Cornell-led team first scanned a portion of the night sky for signs of invisible galaxies using an instrument on Spitzer called the multiband imaging photometer. The team then compared the thousands of galaxies seen in this infrared data to the deepest available ground-based optical images of the same region, obtained by the National Optical Astronomy Observatory Deep Wide-Field Survey. This led to identification of 31 galaxies that can be seen only by Spitzer. “This large area took us many months to survey from the ground,” said Dr. Buell Jannuzi, co-principal investigator for the Deep Wide-Field Survey, “so the dusty galaxies Spitzer found truly are needles in a cosmic haystack.”

Further observations using Spitzer’s infrared spectrograph revealed the presence of silicate dust in 17 of these 31 galaxies. Silicate dust grains are planetary building blocks like sand, only smaller. This is the furthest back in time that silicate dust has been detected around a galaxy. “Finding silicate dust at this very early epoch is important for understanding when planetary systems like our own arose in the evolution of galaxies,” said Dr. Thomas Soifer, study co-author, director of the Spitzer Science Center, Pasadena, Calif., and professor of physics at the California Institute of Technology, also in Pasadena.

This silicate dust also helped astronomers determine how far away the galaxies are from Earth. “We can break apart the light from a distant galaxy using a spectrograph, but only if we see a recognizable signature from a mineral like silicate, can we figure out the distance to that galaxy,” Soifer said.

In this case, the galaxies were dated back to a time when the universe was only three billion years old, less than one-quarter of its present age of 13.5 billion years. Galaxies similar to these in dustiness, but much closer to Earth, were first hinted at in 1983 via observations made by the joint NASA-European Infrared Astronomical Satellite. Later, the European Space Agency’s Infrared Space Observatory faintly recorded comparable, nearby objects. It took Spitzer’s improved sensitivity, 100 times greater than past missions, to finally seek out the dusty galaxies at great distances.

The National Optical Astronomy Observatory Deep Wide-Field Survey used the National Science Foundation’s 4-meter (13-foot) telescope at Kitt Peak National Observatory, located southwest of Tucson, Ariz.

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington, D.C. Science operations are conducted at the Spitzer Science Center. JPL is a division of Caltech. The infrared spectrograph was built by Ball Aerospace Corporation, Boulder, Colo., and Cornell its development was led by Houck. The multiband imaging photometer was built by Ball Aerospace Corporation, the University of Arizona, Tucson, Ariz., and Boeing North American, Canoga Park, Calif. its development was led by Dr. George Rieke of the University of Arizona.

The Infrared Astronomical Satellite was a joint effort between NASA, the Science and Engineering Research Council, United Kingdom and the Netherlands Agency for Aerospace Programmes, the Netherlands.

## Astronomy, HWL04

A) all galaxies were born at the same time, and all will die at the same time.

B) all galaxies contain billions of stars, and all galaxies have spiral shapes.

C) all galaxies outside the Local Group are moving away from us, and the farther away they are, the faster they're going.

D) all galaxies outside the Local Group are orbiting the Local Group.

A) that portion of the universe that we have so far photographed through telescopes

B) the portion of the universe that is not hidden from view by, for example, being below the horizon

C) the portion of the universe that can be seen by the naked eye

A) Earth's speed of revolution about the Sun, typical speeds of stars in the local solar neighborhood relative to us, Earth's speed of rotation on its axis, the speed of our solar system orbiting the center of the Milky Way Galaxy, the speeds of very distant galaxies relative to us

B) Earth's speed of rotation on its axis, Earth's speed of revolution about the Sun, typical speeds of stars in the local solar neighborhood relative to us, the speed of our solar system orbiting the center of the Milky Way Galaxy, the speeds of very distant galaxies relative to us

C) the speed of our solar system orbiting the center of the Milky Way Galaxy, Earth's speed of revolution about the Sun, Earth's speed of rotation on its axis, the speeds of very distant galaxies relative to us, typical speeds of stars in the local solar neighborhood relative to us

D) Earth's speed of revolution about the Sun, Earth's speed of rotation on its axis, the speed of our solar system orbiting the center of the Milky Way Galaxy, typical speeds of stars in the local solar neighborhood relative to us, the speeds of very distant galaxies relative to us

## How Can We Count Galaxies If They're Evolving Or Dying Out?

Does the estimate of the number of galaxies in the universe only contain those that exist now, or those that we can see now that their light has traveled to us? I mean, some of what we see now must have evolved, and died out. When you look into the sky are you seeing the same thing repeating, like a star for example, only at different times and distances?

This false-colour image shows a patch of the sky known as the ‘Lockman Hole’, as observed by the . [+] SPIRE instrument on board Herschel. Located in northern constellation of Ursa Major, The Great Bear, the ‘Lockman Hole’ is a field on the sky almost devoid of foreground contamination and thus ideally suited for observations of galaxies in the distant Universe. Almost every dot in the image is an entire galaxy, each containing billions of stars and appearing as they did 10–12 billion years ago, when the Universe was only a couple of billion years old. The blue, green and red colours represent the three far-infrared wavelengths used for Herschel’s observations: 250, 350 and 500 micron, respectively. The galaxies shown in white have equal intensity in all three wavebands and are the ones forming the most stars. Detecting these galaxies individually is particularly challenging, as they are both extremely faint and numerous, so many of them overlap in Herschel’s images. This creates a fog of infrared radiation known as the Cosmic Infrared Background (CIB), which reflects the clustering pattern of the galaxies responsible for this fog. Studying the CIB and its fluctuations is thus an extremely powerful tool to explore the way galaxies tend to be grouped on both small and large scales. Image credit: ESA & SPIRE consortium & HerMES consortium

Trying to count the total number of galaxies in the universe is a difficult task, made harder by the part where no one wants to spend an infinite amount of time counting galaxies. Instead, what we usually do is count the number of galaxies in a very small area of the sky. Usually what happens is that we point a telescope at a very empty, dark patch of sky, and wait for a while. We’ve done this a few times with Hubble, creating what we now call the Deep Fields. We now have the Hubble Deep Field, the Hubble Ultra Deep Field, and the Hubble eXtreme Deep Field. (Once more, astronomers prove themselves eminently practical namers.) Once we have a really deep image, we can then assume every other patch of the sky is roughly going to look the same (as far as we can tell, a valid assumption). We can then multiply the number of galaxies in that one piece of sky by the fraction of sky we looked at, and get a very rough estimate of the total number of galaxies. Hey presto: several hundred billion galaxies!

Like photographers assembling a portfolio of best shots, astronomers have assembled a new, improved . [+] portrait of mankind's deepest-ever view of the universe. Called the eXtreme Deep Field, or XDF, the photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon. The Hubble Ultra Deep Field is an image of a small area of space in the constellation Fornax, created using Hubble Space Telescope data from 2003 and 2004. By collecting faint light over many hours of observation, it revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. The new full-color XDF image reaches much fainter galaxies, and includes very deep exposures in red light from Hubble's new infrared camera, enabling new studies of the earliest galaxies in the universe. The XDF contains about 5,500 galaxies even within its smaller field of view. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. Image Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

But you’re absolutely on the money with your suggestion that we can only see the galaxies whose light has reached us. This fundamentally limits our observing, so that the galaxies which are most distant from us are also very far removed from our current time.

And here we begin a giant puzzle, because the most distant galaxies, which we see the farthest in the past, are very different from the galaxies we see in our local, nearby universe, which we see much closer to our current time. This means, again as you suggest, that the ancient galaxies we can spot in images such as the Hubble Deep Fields, must have evolved and changed between the time when the light we observe left them, and now. Galaxies which were once independent will have merged in the billions of years which have passed while that light was making its long way to us. Some galaxies will have used up or lost the gas they need to create new stars - one of the few ways a galaxy can “die out”, though its existing stars will disagree with you on how dead they are. Some galaxies will have uneventful evolutions, though they will still evolve. At a base level, galaxies will be creating more stars over time and adding to their own mass, though the number of new stars they make each year will drop over time.

It is known today that merging galaxies play a large role in the evolution of galaxies and the . [+] formation of elliptical galaxies in particular. However there are only a few merging systems close enough to be observed in depth. The pair of interacting galaxies picture seen here — known as NGC 3921 — is one of these systems. NGC 3921 — found in the constellation of Ursa Major (The Great Bear) — is an interacting pair of disc galaxies in the late stages of its merger. Observations show that both of the galaxies involved were about the same mass and collided about 700 million years ago. You can see clearly in this image the disturbed morphology, tails and loops characteristic of a post-merger. The clash of galaxies caused a rush of star formation and previous Hubble observations showed over 1000 bright, young star clusters bursting to life at the heart of the galaxy pair. Image credit: ESA/Hubble & NASA

Untangling the complex line which can connect a nearby galaxy to the sort of galaxy it might have been, billions of years ago, is a whole subfield of astronomy, under the moniker of galaxy evolution.

It’s important to keep in mind that it’s not quite as simple as seeing the same things repeated over and over again. The galaxies we see much earlier in their lives than our own are truly, physically, very far away, which is why we see them so far removed in time. They will be evolving over time in their own physical space, but they should evolve into something that looks like the galaxies near us. Distant galaxies seem to be the same everywhere we look, so we shouldn’t be looking at a special group of distant galaxies that would evolve in a unique way. They’re not the same galaxies as the ones that built our own galaxy, but they should be pretty similar. It’s up to us to learn what the pathway between ancient and current day must have been.

## Hubble Finds Most Distant Galaxy Yet

By: Monica Young March 4, 2016 2

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Astronomers using the Hubble Space Telescope have measured a precise distance to a galaxy dwelling in the cosmic dawn.

The Hubble Space Telescope has enabled a precise distance measurement to the galaxy dubbed GN-z11, which was furiously forming stars 400 million years after the Big Bang. The galaxy appears red in this infrared image, but if its light didn't have to traverse space and time to reach us, its new stars would be burning blue.
NASA / ESA / P. Oesch / G. Brammer / P. van Dokkum / G. Illingworth

Does the title give you a sense of déjà vu? Surely, you might think, we’ve probed the farthest stretches of the universe before. And we are indeed approaching the limit to what the Hubble Space Telescope can see of the cosmic dawn — yet the fortune-favored satellite still has some surprises in store for us.

Astronomers had already used Hubble to find hundreds of galaxies that existed less than 1 billion years after the Big Bang, as well as a handful that were around even earlier than that. Now, observations have zeroed in with greater precision than ever before on one particular object that breaks all previous distance records: a humdinger of a galaxy dubbed GN-z11 that dwells in a universe just 400 million years old (at a redshift of 11.1, in astronomer-speak).

Though the universe only started forming stars when it was about 100 million years old, this galaxy already holds a billion Suns’ worth of mass in its stars. It’s churning out even more at a rate between 14 and 34 solar masses a year — dozens of times higher than Milky Way’s stars formation rate.

"It's amazing that a galaxy so massive existed only 200 million to 300 million years after the very first stars started to form,” said Garth Illingworth (University of California, Santa Cruz) in a press release. “It takes really fast growth, producing stars at a huge rate, to have formed a galaxy that is a billion solar masses so soon."

This diagram shows a timeline of the universe, stretching from the present day (left) all the way back to the Big Bang (right). The previous record-holder at a spectroscopic redshift of 8.68.
NASA / ESA / B. Robertson / A. Feild

Thanks to its stars, GN-z11 is incredibly luminous, radiating three times the ultraviolet light typical of galaxies in that early era. That trait was key to measuring its distance with such high precision.

### Once More, With Precision

When galaxies lie so far away, not only do they appear faint, but their light also shifts redward, the wavelengths stretching out as they traverse space and time. Light that was initially emitted at ultraviolet wavelengths has lengthened to infrared radiation by the time it arrives at Hubble’s detectors.

Astronomers measure distance to these faraway galaxies by looking for a clear imprint in their spectra. Hydrogen that fills the early universe absorbs virtually all light at wavelengths shorter than 121 nanometers. So while a galaxy’s light at longer wavelengths may pass through the universe relatively unmolested, light at wavelengths shorter than that magic number will disappear, absorbed into interstellar and intergalactic gas.

Galaxies emit light at many wavelengths, but some light will be absorbed by clouds of hydrogen gas. That break, where light is absorbed into gas clouds and disappears from the spectrum, shifts to longer wavelengths as the galaxy's light travels the long way to Earth. Taking a spectrum can identify the location of this break precisely, but often astronomers are limited to images rather than spectra. In the latter case, they look for a galaxy to disappear from images taken at shorter wavelengths.
NASA / ESA / C. Christian and Z. Levay

Locating this break in a spectrum gives astronomers an easy measure of how far a galaxy’s light has shifted redward — its spectroscopic redshift. But easy is only as easy does: Hubble’s deep looks capture regions of space teeming with faint and faraway galaxies (such as CANDELS), and it’s far from easy to take the spectrum of every object in the field of view.

Instead astronomers collect the poor man’s version of a spectrum by taking pictures of the same field at many different wavebands. They locate the drop-off in light when they see a galaxy disappear from images taken at shorter wavebands. This process gives an object’s so-called photometric redshift.

But photometric redshifts can be deceiving. A previous record-holder for the “most distant galaxy” title had the lovely name UDFj-39546284. Its photometric redshift of 11.9 placed it in a universe just 380 million years old. But follow-up spectra called that result into question: the galaxy might actually lie much closer to Earth than originally thought. The next most distant galaxy has a photometric redshift of 10.7.

Spectroscopic redshifts remain the golden standard for measuring distance — it’s just been difficult to find galaxies bright enough to pass through a spectrograph with a legible result. GN-z11 happens to be exceptionally bright, and that’s what enabled Pascal Oesch (Yale) and his team to measure its redshift so precisely. The result will be published in the Astrophysical Journal.

This result may represent the edge of Hubble’s reach, but it’s only the beginning when it comes to future telescopes such as James Webb and WFIRST, whose longer-wavelength detectors will probe hundreds of galaxies even further back in cosmic time.

Watch the video below to find the whereabouts of this galaxy in your night sky:

## Truth Behind the Photos: What the Hubble Space Telescope Really Sees

The nearly 20-year-old Hubble Space Telescope has taken many iconic images of the cosmos and is even the star of a new 3D IMAX movie that gives viewers a chance to fly through those snapshots. But does Hubble show us what the universe really looks like?

Yes and no, according to NASA.

When Hubble beams down images, astronomers have to make many adjustments, such as adding color and patching multiple photos together, to that raw data before the space observatory's images are released to the public.

Hubble doesn't use color film (or any film at all) to create its images. Instead, it operates much like a digital camera, using what's called a CCD (charge-coupled device) to record incoming photons of light. [Spectacular Photos From The Revamped Hubble Space Telescope]

Hubble's CCD cameras don't measure the color of the incoming light directly. But the telescope does have various filters that can be applied to let in only a specific wavelength range, or color, of light. Hubble can detect light throughout the visible spectrum, plus ultraviolet and infrared light which is invisible to human eyes.

The observatory will often take photos of the same object through multiple filters. Scientists can then combine the images, assigning blue light to the data that came in through the blue filter, for example, red light to the data read through the red filter and green light to the green filter, to create a comprehensive color image. [Most Amazing Hubble Discoveries ]

"We often use color as a tool, whether it is to enhance an object's detail or to visualize what ordinarily could never be seen by the human eye," NASA officials explain on the agency's Hubble Web site.

For some Hubble photos, such as the galaxy ESO 510-G13 for example, the end result is a close approximation of the colors people would see with their own eyes were they to visit the distant sight in a spacecraft.

Though even these photos are an enhanced version, since most celestial objects, such as nebulas, emit colors that are too faint for human eyes to make out. It takes a telescope, letting light build up in its CCD over time, to see the rich hues in Hubble photos.

And for other Hubble images, scientists assign colors to the filters that don't correspond to what that light would look like to human eyes. They do this when using light from infrared and ultraviolet filters, since those wavelength ranges have no natural colors, or when combining light from slightly different shades of the same color.

"Creating color images out of the original black-and-white exposures is equal parts art and science," NASA said.

For example, Hubble photographed the Cat's Eye Nebula through three narrow wavelengths of red light that correspond to radiation from hydrogen atoms, oxygen atoms, and nitrogen ions (nitrogen atoms with one electron removed). In that case, they assigned red, blue and green colors to the filters and combined them to highlight the subtle differences. In real life, those wavelengths of light would be hard to distinguish for humans.

The Hubble Space Telescope launched in April 1990 and has been visited by NASA astronauts multiple times for vital repairs, maintenance and upgrades.

The most recent visit was in May 2009, when astronauts performed five tricky spacewalks to add a new camera, spectrograph, and make unprecedented repairs and upgrades that left Hubble more powerful than ever before.

NASA scientists hope those upgrades will add at least five more years of life to the aging Hubble Space Telescope.

## Why explore a galaxy, far, far away?

Looking at billion-year old galaxies will help us understand life, the universe and everything, writes Kylie Andrews.

The Andromeda Galaxy, which is only 2.5 million light-years away from Earth, is a spiral galaxy. The galaxies in Galaxy Explorer are much further away. (Source: NASA/JPL-Caltech)

It's part of being human to want to make sense of the world around us.

These days, expanding our knowledge involves looking further and further out into space, grappling with some of the mind-bogglingly vast questions about the existence of the universe and trying to understand the creation of everything in it.

That's the purpose of Galaxy Explorer, ABC Science's citizen science project asking regular people to help scientists classify galaxies to help out astronomers.

The research project behind Galaxy Explorer is the Galaxy and Mass Assembly (GAMA) project — a global research project led by Professor Simon Driver from the International Centre for Radio Astronomy Research (ICRAR) in Western Australia.

"GAMA is blue sky," says Driver, "we want to understand the evolution of energy, the evolution of mass and the evolution of structure."

Most of us are more concerned about whether our energy is renewable or not, and how much it costs! But the astronomers involved in Galaxy Explorer want to understand the origin of energy in the universe.

"We want to understand all the processes in the universe that generate energy, and we want to understand how this has evolved," says Driver.

And that's just one of the big questions they're interested in.

### Why look at images of far-off galaxies?

There are over 200,000 images of galaxies between 800 million to 4 billion light-years away to be classified by citizen scientists in Galaxy Explorer.

Comparing distant galaxies will help scientists understand inconsistencies with what's observed in the universe and what's predicted by Einstein's equations, and as a result may change our fundamental understanding of dark matter and dark energy.

They will also help astronomers understand how galaxy evolution has changed through time, which will provide insight into how the processes in the universe have evolved.

"We're drilling a hole right through the universe, collecting samples of galaxies," says Driver.

"Because it takes so long for light to travel when we're looking at something further away we're looking at something as it was in the past."

It's very analogous to drilling a core sample in Antarctic, he says, where every layer tells you something about what conditions on Earth were like at different times.

Similarly, astronomers can look at examples of nearby galaxies and far-away galaxies and work out how they might have changed.

### Galaxy evolution has changed through the history of the universe

Astronomers believe the process that drives galaxy growth has changed from the time of the early universe.

"So far we think that right after the Big Bang, gravity started to pull galaxies together, and then they went through a period when there was lots of merging, lots of collisions, and violent episodes leading to distorted looking galaxies," says Driver.

Galaxies undergoing collisions tend to be highly distorted and have an asymmetrical shape.

While mergers may have been the dominant process early on, gas accretion is much more common now, he says. This occurs when a galaxy swallows gas and it results in a symmetrical flattened rotating disk of stars, often with spiral arms.

"That's when we start to see the beautiful spiral arms and those sort or ordered symmetrical structures," says Driver.

Comparing galaxies of differing age should allow astronomers to confirm this theory of galaxy evolution.

"It will also provide detailed information about when mergers were taking place, when the gas accretion began, and when galaxies first started to develop spiral arms and other features of more ordered systems," says Driver.

### What does the shape and size of the galaxy tell us?

These are some of the different types of galaxies you'll see in Galaxy Explorer

Each galaxy carries a record of how it formed and its evolutionary history is encoded in its shape, colour, and features.

If a galaxy has a central bulge then it's probably the result of a merger that has had time to re-organise itself into a spherical shape. If it's a thin disc then it's grown by slowly swallowing gas.

"A lot of galaxies we see have a central bulge and a thin disk. Which suggests that the galaxy first formed by merging and then later on it formed a disc through gas accretion," says Driver.

If a galaxy looks messy or irregular then it's undergoing a significant evolutionary event — either merging with another galaxy or accreting gas very fast. These galaxies aren't in equilibrium.

Alternatively, if a galaxy is left alone and hasn't undergone any major mergers for a long time then a bar may start forming. This begins if there's a region where there are slightly more stars. With time, these stars tend to pull others towards them.

"An over-density of stars rotating round a galaxy's centre will pull on the ones in front, slowing them down, and accelerate the ones behind," says Driver.

"And over time, you go from a flat Frisbee-like structure to a galaxy with a bar. But it only happens if a galaxy is left alone. If another galaxy goes by, it gives enough of a kick that disrupts that bar process."

When it comes to spiral arms, the process that forms them is not fully understood.

"They're believed to be a shock-wave or density-wave that permeates out from the centre of the galaxy," says Driver.

Many galaxies have all three features — bulges, bars and spiral arms — telling a complex story of evolution.

### Why we need citizen scientists to help

With the help of citizen scientists, the astronomers will be able to very quickly build up statistics as to how many galaxies have bars, how many have spiral arms, how many have bulges, how many are in a state of merging, and how many look very smooth.

"These statistics can be used to build a model of how the entire galaxy population in the universe has evolved," says Driver.

"It would take an enormous amount of time for us to go through every galaxy one by one — we're just a team of five or six here."

The astronomers understand that the process of classifying galaxies will be challenging for some people. It's a process they themselves often have trouble with.

"That's why we have to make sure that every galaxy is looked at by five people," says Driver.

"We should then be able to look at the average of those five. If they all agree it's a dead certainty. If all five disagree then it's probably one of those ones that's really hard to classify and we need to have a look at it ourselves. And if four agree and one doesn't then we can probably weed that one person out, and go with the majority."

Besides the evolution of galaxies and the origin of energy, Driver and his team are also interested in other big questions like how mass built up in the universe and the processes that created gravity.

"So maybe if we carry on studying the distribution of galaxies, carrying on studying the motions of galaxies we'll start to get some insight.

"We're just trying to understand this strange force called gravity in all its glory, and then one day we may find a way to harness it, just like we harnessed electromagnetism and use it to our benefit."

How you can helpCan you volunteer some time to be a citizen scientist? Visit Galaxy Explorer and start classifying galaxies for astronomers as part of a real research project. Get involved in August and you could win a wi-fi telescope. Schools can join in too.

Use these social-bookmarking links to share Classify a galaxy, far, far away.

## Astronomers accidentally discover a nearby galaxy in a Hubble image!

Stars, gas, clusters, galaxies… even though they're separated by vast distances in space, we don’t directly notice that distance when we look up at the sky because that third dimension is compressed. Something very far away can appear to be right next to something much closer to us, like looking out a window and seeing a nearby tree apparently right next to a distant mountain.

There's so much stuff out there, though, that coincidental superposition happens a lot. We just don't see it often in astronomical photos because for the most part these objects are so far away they're really, really faint, so they don't show up in the pictures unless the telescope is big, the camera sensitive, and the exposure time long.

Hey, Hubble's kinda big, has sensitive cameras, and can take long exposures. So yeah, it sees these sorts of things all the time. Many shots from Hubble show nearby stars, distant galaxies and everything in-between in a single image.

Well, not everything. Even nearby objects can be dim if they're intrinsically faint… and that brings me to an observation that I simply love.

The globular cluster NGC 6752 (bottom), when observed near its core with Hubble (right), reveals it’s hiding a very dim dwarf galaxy (upper left). Credit: NASA, ESA, L. Bedin (Astronomical Observatory of Padua, Italy), and Digitized Sky Survey 2

At the bottom of that image is a ground-based image of the globular cluster NGC 6752, a roughly spherical collection of over 100,000 stars all orbiting each other. As a unit they orbit our Milky Way and are about 13,000 light years from Earth.

Astronomers pointed Hubble at NGC 6752 for a long, long time, taking incredibly deep exposures of it to see the very faintest stars they could, so they can better understand the population of faint stars residing there. The science behind this is interesting and important, but it yielded an extra dividend: while they were examining the Hubble images taken near the core, they found a small clutch of faint stars all together in one spot.

The tiny and faint dwarf galaxy Bedin I is almost hidden in the background among the much brighter stars in the globular cluster NGC 6752. Note several other far more distant galaxies can be seen, too. Credit: NASA, ESA, and L. Bedin (Astronomical Observatory of Padua, Italy)

As you can see in the image, most of the stars in NGC 6752 are brighter and evenly spread around, so these being dimmer and clumped together meant they were looking at something different: An extremely faint galaxy, far in the background of the globular cluster!

But how far? Given that individual stars can be seen in the galaxy — which the astronomers named Bedin I, after the lead investigator on the team, Luigi Bedin — it can’t be too far away, but the exact distance is important to figure out. Analyzing the stars in this galaxy is really hard, though, because for one thing it's incredibly dim. Their combined light brings this galaxy to a magnitude of about 20 — the faintest star you can see with your naked eye is 400,000 times brighter. And that's for the whole galaxy the individual stars are hundreds of times fainter.

The globular cluster NGC 6752, in a Hubble image taken in 2012. Credit: ESA/NASA/Wikisky

Worse, it's sitting right near the core of NGC 6752 (well, from our point of view), so there are lots of far brighter stars sitting near and even on top of it, contaminating the sample. And finally, as bad luck would have it, the galaxy is near the edge of the image’s field of view. The technique used to combine all the observations into one super-deep image tends to make observations near the edge shallower, so we don't see the faintest stuff in the galaxy as well as if it were near the center of the field of view.

Still, enough individual stars are visible for the astronomers to examine them carefully. They were able to find quite a few red giant stars, which are great benchmarks: The very brightest of these old, dying stars tend to always give off the same amount of light, so by measuring how bright they appear we can measure their distance.

The astronomers determined that Bedin I is about 28 million light years away. That's close, relatively speaking, though well outside the Local Group of galaxies which holds our Milky Way, Andromeda, and a couple of dozen other galaxies. Given how bright it is, that makes Bedin I a dwarf galaxy for sure. It's about 2,700 x 1,100 light years in size, which is tiny — the Milky Way is 100,000 light years across!

But this gets better. Looking at the colors of the stars, the astronomers found that the stellar population is old, and I mean old: something like 13 billion years old, almost as old as the Universe itself! This galaxy formed right after the cosmos did, made a bunch of stars right away, then… stopped. It doesn't appear to have any gas or dust left in it, so it doesn't have the materials needed to make more stars.

The spiral galaxy NGC 6744, a nearby spiral that’s quite similar to the Milky Way. Credit: ESO

That makes Bedin I a dwarf spheroidal galaxy — a relic galaxy, a fossil of truly ancient times. It also appears to be isolated, not near any other galaxies, so it's likely to have been untouched since it formed eons upon eons ago. It does appear to be in the same region of sky and at about the same distance as the big spiral NGC 6744 (which coincidentally, I wrote about just a few months ago), but even then they're separated by at least two million light years, a huge distance (Andromeda is about that far from the Milky Way), so even then it looks very likely that Bedin I is quite isolated.

What a great discovery! This is probably the least luminous galaxy every seen at such a distance in fact second place is held by a galaxy much, much closer to us. Intrinsically faint galaxies are incredibly difficult to detect past about 13 million light years, so Bedin I is a treasure at more than twice that distance. We don't have many examples of these dwarf spheroidal galaxies because they're so hard to detect, so this one can help astronomers understand how they form, how they've lived, and what this means for bigger galaxies like ours (which grew to their present huge size in part due to gobbling down dozens of smaller galaxies like this one).

Like I said, I love this. When I worked on Hubble one of my favorite self-imposed tasks was looking at all the images we got to see if anything interesting showed up in the background. I found a lot of cool stuff (including a planetary nebula in a nearby galaxy that we got enough info on to publish a short paper), but nothing as important as this.

## The Galactic Eye of Sauron

Oooo, what a pretty galaxy! That’s NGC 4151, a spiral galaxy in the constellation of Canes Venatici. This image was taken by my friend Adam Block using the 81 cm Schulman Telescope at the Mount Lemmon SkyCenter in Arizona. It’s an amazing 20 hour exposure! And oh my yes, you want to see the full-res version. It’s a stunner.

As you can see though, as beautiful as it is, NGC 4151 is a bit odd. It has far-flung and faint spiral arms, but also a brighter ring of stars and gas closer to the center. The ring is blue, indicating the presence of lots of hot, young, massive stars these blast out bluer light than the Sun does, and at much higher rates, making their existence clear.

NGC 4151 is one of the closest galaxies in the Universe with an actively feeding black hole in its center. As far as we know, all big galaxies have a supermassive black hole at their hearts, ranging from millions to billions of times the Sun’s mass. But most are quiescent, dark. Some, though, have material falling into them. This stuff piles up around the hole, forming a flat disk called an accretion disk. The material in the disk can get very hot, and also very bright. So, while the black hole itself is dark, that material can be so bright it can make the galaxy visible clear across the Universe.

So how far away is NGC 4151? Most catalogs list it as roughly 40 million light-years away. But a paper from 2014 shows that it’s much farther than that: more like 60 million light-years! The new study used a nifty technique, called echo mapping.

When matter falls into the black hole, it is not always a smooth flow. A star can fall in, or a big clump of material. When that happens, a flare of high-energy light is emitted. This can light up the disk of material around the black hole, but there’s a delay in how long it takes the disk to brighten because of the huge distances involved it takes time for the light to reach it. By carefully measuring the time it takes for the disk to respond to a flare, and comparing that with the measured size of the disk, the distance to the galaxy can be found.

The new measurement of 62 million light-years looks pretty good. The old measurements are based on things like the galaxy’s redshift, and that can be misleading for nearby galaxies (if it’s in a cluster, for example, it can be moving rapidly and mess up the redshift measurements). Apparently, something like that is the case here, and NGC 4151 is half again farther away than we thought.

I find it rather humbling that we are still figuring out the distances to even nearby galaxies. Sometimes we have a lot of confidence in the distance, and other times we’re off by a factor of 50 percent or more. It’s a good reminder that we need to be careful about such things.

But at the same time, it’s astonishing that we can measure the distances to galaxies at all! Even nearby galaxies are so far away that the light we see left them when humans barely existed, and for many we see light so ancient the Earth itself hadn’t formed when they sent their photons our way. And yet, despite distances so terrible they crush our sense of scale, we can still get a decent measurement just how large that distance is.

I love that we’re so curious as a species that we want to find out answers to questions like these: How far away are galaxies? Why are they shaped the way they are? How are they formed, how do they age, how do they die? Why do they all have monster black holes in their cores?