So this post was supposed to be about the discovery of the most distant galaxy ever found, at a redshift of about 8.2 (13.1 billion light years from us, or, to put it another way, only about 630 million years after the Big Bang), but I didn’t get round to it yesterday and I’ve now been distracted by a paper out today on telescopes, laser beams and aeroplanes instead! (For more details on the distant galaxy see this post over at the always excellent In the Dark. There’s a lot of effort being put into this field at the moment, as already discussed here by Rita, so I expect there’ll be a new ‘most distant’ one that I can catch up with soon.)
One of the main problems astronomers face when trying to observe things in the sky is the turbulence in the Earth’s atmosphere, from clouds, wind etc (this is known as ‘seeing’ and is what causes the stars to twinkle). To get away from this telescopes are normally built high up on mountains, above as many of the clouds as possible, or, like the Hubble Space Telescope, put into orbit where there’s no atmosphere to worry about.
Putting a telescope on a mountain doesn’t completely avoid all the atmospheric distortions, but a technique called adaptive optics can help to correct it. This needs a nice bright star in the field of view, but when this isn’t available an artificial star can be created by shooting a laser beam into the sky, as shown for the Keck Telescope in the picture above. The telescope then observes this ‘star’, and uses its fluctuations in shape (due to the current patch of atmosphere overhead at the time) to correct the astronomical images as they are observed. I should add here that this isn’t the only time lasers are used alongside telescopes – for example they are also fired at the reflectors left on the moon by the Apollo missions, as I’ve blogged about before.
The only flaw in this telescopes-lasers-great corrected, pictures of the sky/laser ranging plan is aeroplanes. Laser beams can dazzle pilots so understandably they can’t be used at the telescopes when planes are in the vicinity. What this has meant in practice up till now is someone sitting outside all night, ready to close the laser-shutter when one comes too close. Not very practical and a pretty boring job! Well, now some researchers in California have come up with a solution. Aviation regulations require that all aircraft are fitted with transponders so that they can be tracked by air-traffic control. By measuring the ratio of transponder signal power from two antennae aligned with the laser, one broad and one narrow beam, the position of the aircraft can be found. An automated shutter can then be activated to turn the laser off, and the cold plane-watchman can go back inside.
W. A. Coles, T. W. Murphy Jr., J. F. Melser, J. K. Tu, G. A. White, K. H. Kassabian, K. Bales, & B. B. Baumgartner (2009). A Radio System for Avoiding Illuminating Aircraft with a Laser Beam submitted to PASP arXiv: 0910.5685v1
Just a quick post to point you all towards this week’s Carnival of Space over at The Gish Bar Times, the blog to go to for all things Io-related.
Quick post to let you know of two new blogs (well, new to me) that have caught my eye. Firstly, let me welcome Duncan and Well-Bred Insolence to my RSS feeder – keep an eye over there for news on planet formation, discovery and possible inhabitants amongst other things. He beat us to telling you about HARPS latest addition to the list of known extra-solar planets. Glad he did too, firstly because we would have been intolerably late with those news, and secondly because he knows a fair bit more about it than we do.
And then there’s the The Big Blog Theory, a blog by David Saltzberg – the science advisor to The Big Bang Theory (the American sitcom, not the beginning of the Universe) – who explains the science behind the episodes. My favourite TV sitcom just got better!
In our first post exploring galaxy evolution, we saw how observing galaxies at different distances from us is crucial for our understanding of how galaxies form and evolve. It also naturally follows that the larger the range of distances we can study, the better we can constrain our theories. So it’s only natural that astronomers have always been hunting for the most distant galaxies – it’s a sort of high-flying game in the astronomy community, and breaking the record for the most distant galaxy observed is no mean feat.
More distant objects appear on average fainter, and they are harder to detect. So traditionally one looks at technological improvements in order to make advancements in this area. For example, a larger telescope has a wider light-collecting area. Therefore it’s more sensitive, and is able to detect fainter objects in a given time. One can also observe a region of sky for a longer period of time, which again increases the number of photons that we collect. Astronomers call this deep imaging.
Recently, the public release of very deep imaging from the Hubble space telescope‘s new Wide Field Camera 3 generated a rush of papers which were precisely looking for very high-redshift galaxies. Look for example at Bunker et al., McLure et al., Oesch et al., among others, which were mostly submitted within a few hours of each other, and just a few days after the data was publicly released – astronomy doesn’t get much more immediately competitive (or stressful?) than this! The work of these particular papers requires not only deep imaging, but also a wide range in terms of electromagnetic spectrum – i.e., they need sensitive images of the same region of the sky in different colours, and the redder the better.
These papers detected galaxies at redshifts between 7 and 8.5. Or, in more common units, these galaxies are at least 12,900,000,000 light-years away. That means the light that was detected by the Hubble space telescope, and on which these papers and scientific analysis are based, left those galaxies 12,9000,000,000 years ago. The Universe was only around 778,000,000 years old by then. To go back to our previous analogy, it’s like looking at a snapshot of me when I was less than 2 years old. Admittedly I had bathed by then, but that’s still very very young!
These papers are interesting and important in their own right, but what prompted me to come and tell you all of this was actually the work of Bradac et al., which has the same goal as the above, it uses the same basic techniques as the above, but it cheats. And it’s the way it cheats that makes it really rather neat.
Bradac and co-authors use not only human-made telescopes, but also harvest the power of gravitational lensing to turn a galaxy cluster (the Bullet Cluster) into an enormous cosmic telescope. We have covered here before how mass affects space which in turn affects the way light travels. Matter can act to focus light from distant objects – and galaxy clusters have a lot of matter. This makes them rich and exciting playgrounds for astronomers who have now long used gravitational lensing to probe the distant Universe.
Bradac and friends have additionally showed just how effective it can be at measuring the density and properties of distant galaxies, and how much there is to gain from a given image (with a given sensitivity limit) when there is a strong and appropriately focused cosmic lens in the field of view. Distant galaxies are also magnified – their angular size on the sky increases, compared to an unlensed image – and that allows us to look at them in more detail, and study their properties. As a bonus, given that cosmic telescopes often produce more than one lensed image of any one given galaxy, they can use these multiple images to help with the distance measurement and avoid some contamination.
They didn’t set any distance records as the wavelength range of their imaging wasn’t quite right for that. But with the right imaging, the right clusters and the right analysis, the authors argue that this is the way forward for this sort of study. Galaxy evolution is hard and full of technical challenges, so using galaxy clusters as gigantic telescopes can certainly go a long long way.
M. Bradač, T. Treu, D. Applegate, A. H. Gonzalez, D. Clowe, W. Forman, C. Jones, P. Marshall, P. Schneider, & D. Zaritsky (2009). Focusing Cosmic Telescopes: Exploring Redshift z~5-6 Galaxies with the Bullet Cluster 1E0657-56 Accepted for publication in ApJL arXiv: 0910.2708v1
R. J. McLure, J. S. Dunlop, M. Cirasuolo, A. M. Koekemoer, E. Sabbi, D. P. Stark, T. A. Targett, & R. S. Ellis (2009). Galaxies at z = 6 – 9 from the WFC3/IR imaging of the HUDF Submitted to MNRAS arXiv: 0909.2437v1
P. A. Oesch, R. J. Bouwens, G. D. Illingworth, C. M. Carollo, M. Franx, I. Labbe, D. Magee, M. Stiavelli, M. Trenti, & P. G. van Dokkum (2009). z~7 Galaxies in the HUDF: First Epoch WFC3/IR Results submitted to ApJL arXiv: 0909.1806v1
Andrew Bunker, Stephen Wilkins, Richard Ellis, Daniel Stark, Silvio Lorenzoni, Kuenley Chiu, Mark Lacy, Matt Jarvis, & Samantha Hickey (2009). The Contribution of High Redshift Galaxies to Cosmic Reionization: New
Results from Deep WFC3 Imaging of the Hubble Ultra Deep Field Submitted to MNRAS arXiv: 0909.2255v2
This week’s Carnival of Space is now up at the always interesting Orbiting Frog. Hot topics this week include the misunderstandings surrounding NASA’s LCROSS mission, an impressive view of the Martian landscape and a new way to see one billion dollars. Oh, and a mysterious header image which looks astronomical but turns out to be from much closer to home.
Last week I was in Milan doing something I traditionally don’t really do – getting my hands dirty on data, and doing my bit to make it usable for science.
By data, in this case, I mean 100,000 spectra which were collected by the VIMOS spectrograph which is one of the instruments on the Very Large Telescope in Paranal for a new project caller VIPERS. A spectrum is simply the light of an object decomposed in a relatively large number of components. So instead of decomposing an image in, say, 3 colours, a spectrum will often have hundreds to thousands of different pixels – or colours, really – so we can see exactly how “red”, or “blue” an object is. Think of it as a rainbow – which is simply the light of the Sun decomposed in a number of colours. And yes, astronomers spend a large amount of effort effectively making rainbows out of the light of galaxies.
Spectra of objects (stars, galaxies, quasars, planets, etc) are one of the most useful observables of our Universe. Depending on whether you are a cosmologists or someone who studies galaxy evolution, you want them mainly for different reasons. Today I’m going to put my cosmologist hat on.
As we’ve covered here before, galaxies have a very rich range of properties which we can use to trace their evolution. With our hat today, none of it matters – the astonishing this is that we can learn an awful lot about the evolution of the Universe as a whole simply by studying the position of these galaxies. This is the realm of observational cosmology - the study of the birth, evolution, dynamics, composition and ultimate fate of our Universe by studying the spacial distribution of galaxies. This may be a somewhat narrow description of a huge field of modern-day Astronomy, but to first order it’s perfectly correct.
So why is the spacial distribution of galaxies so revealing? Well, let us start by considering the components of our Universe. In our current standard model we have four main components: radiation, baryonic matter, dark matter and dark energy. The first two we are very familiar with – baryonic here just refers to the matter that makes up all we see in the Universe: ourselves, the solar system, distant galaxies, far away planets, etc. The last two are more of a mystery, and excitingly also the two most important components of our Universe today. I must leave a more thorough explanation for another post, but for now let us state that there is 5 times more dark matter out there than baryonic matter, but we can’t see it. It interacts gravitationally with baryonic matter so we can detect its presence and because it is so much more abundant than baryonic matter, it’s dark matter that has the most influence in the master game of tug of war that is the evolution of our Universe – not baryonic matter. The other player is dark energy. Now, I can’t tell you what dark energy is (nobody can), but I can tell you that it behaves in the opposite way to gravity. Whereas one attracts, the other repels. Whereas one brings things closer together, the other takes them apart.
The amount of dark matter and dark energy govern the dynamics of our Universe, but we can’t see either of them directly. So we turn to what we can see: baryonic matter, via the radiation produced mainly (but not exclusively) by stars. The neat thing is that baryonic and dark matter attract each other gravitationally, so baryonic matter traces dark matter. And this is why simply measuring where galaxies sit is so insightful – it gives you a tool to track dark matter and see how its spacial distribution evolves. This in turn tells you about the interplay between dark matter and dark energy – depending on which one dominates, by how much and for how long, the evolution of the spacial distribution of dark matter (and therefore galaxies) will be different. And that we can measure! The spacial distribution (or clustering) of galaxies is one of the most promising tools to tell us about dark energy and how structure has grown in our Universe.
But before all of this can be done, we need to know the distance to each galaxy. This means measuring their redshift, by using the spectrum of each galaxy to determine how fast it is receding from us – which in turn tells us how far away it is. If the data coming out of the telescope and spectrograph are good enough, the whole process can be fully automatised. However, our position is slightly different in that due to some instrumental artifacts, the red side of the spectra is much below standard and the computerised pipeline often fails in assigning the correct redshift.
This is where I (and around 25 other people) come in. Humans are much much better at spotting mistakes than computers, so each of the 100,000 spectra is being looked at by at least two different people to make sure that the redshift is correct and that when the time comes to do science, we are doing it with reliable data. As I mentioned, this is not something I’d ever done before, so my time in Milan was used to get me trained and up to scratch with the software. I measured 300 redshifts this week, which is rather slow especially when I think that I have to do 1500 more or so in the next couple of months!
It’s a huge task but one that really needs to be done. And on the upside, I will never get bored on a train or airport ever again!
So today the Nobel Memorial Prize in Economics (not one of the original true Nobel Prizes) will be announced. I thought this would be a good time to write about the work of last year’s winner. And yes, you are still reading an astronomy blog.
In 1978 Paul Krugman was a bored young assistant professor at Yale. To amuse himself he let his thoughts wander onto how economics could evolve in the far future. This resulted in him writing a paper entitled The Theory of Interstellar Trade, which is quite frankly fantastic.
This is a light-hearted work and with the claim on the front page that the work has been funded by the Committee to Re-elect Willian Proxmire it starts as it means to go on. Proxmire was a US Senator who crusaded against what he considered wasteful public spending. His cross-hairs sometimes fell on scientific projects which were often recipients of his Golden Fleece Award, a sort of insulting IgNobel for spending government money on bizarre research programmes. Proxmire is also credited with killing off NASA’s space colonisation research programme. Hence Krugman’s ironic funding claim for his odd research work on the economics of space trade.
What makes interstellar trade unusual is the great distances involved and the problem of the Universe’s speed limit. Stars are separated by distances of light years and it is likely those with habitable planets are rare and could be even further apart. As it isn’t possible to travel faster than the speed of light through empty space, trade missions could take tens or hundreds of years to travel between worlds. Additionally as some for of propulsion that carries a craft close to the speed of light will be required for such journeys, time dilation will prove a problem.
Time dilation is a effect of travelling close to the speed of light where time appears to go slower for an observer moving at high speed compared to one that is stationary in a particular frame of reference. You might have heard of a consequence of this, the twin paradox, where one twin remains on Earth, while the other goes off on a rocket at high speed. When the rocketeering twin returns he finds he has aged less than his brother. The same principle applies to a space trader on a long voyage and raises the problem, who’s time frame do you you use for calculating money gained from compound interest? Krugman uses an argument based on trade between the fictional planet of Trantor (Galactic capital in Azimov’s Foundation Series) and Earth to show that the time lapsed in the inertial reference frame of the trading planets should be used for calculating interest costs (this is his “First Fundamental Theorem of Interstellar Trade”).
Krugman further uses the example of Trantorian’s trading with and living on Earth to assert his second law, that interest rates on the different planets will eventually equalise.
I may have come across as glossing over some of the details, this is mostly because it would lead to a pretty long blog post. But also it’s because I want to encourage you to read the original paper because it is one of the smartest and most amusing pieces of writing I’ve read in a long time. Krugman is clearly both a science and sci-fi geek and uses his knowledge wonderfully to weave together a thoroughly fun read.
Paul Krugman was awarded his Nobel Memorial Prize for his terrestrial work. This is lucky because to be awarded the prize for his stellar interstellar work he’d have had to wait longer than Peter Higgs to be proved right.
So we are hosting Carnival of Space this week. The idea of this is to collate together the writing on astronomy blogs over the last week. If you want more info have a look at the Carnival Homepage.
Let’s start with the Nobel Prize for Physics which this year was partly won by the scientists responsible for inventing the CCD. The Chandra Blog and Commercial Space celebrate this and explain how CCDs have been indispensable to astronomy and our understanding of the Universe. As well as, of course, giving us lots of pretty desktop pictures.
Speaking of pretty pictures, over at Bad Astronomy Phil Plait gives us a visual treat in the form of an image of our nearest galactic neighbour, Andromeda, as you have never seen it before. Taken as 330 separate images, it shows Andromeda in Ultra Violet light. The UV traces both the regions of star formation in the spirals, where young, incredibly bright stars pump out UV light, and the older, densely packed population of stars in the central region of the galaxy.
And one of the biggest stories of this week is also about looking at a familiar object in new wavebands, I am talking of course about the discovery of a new ring around Saturn. Spotted by the Spitzer space telescope, which sees the sky in the infrared, this ring is like no ordinary ring. Spanning a volume which could fit a billion Earths, this gargantuan collection of dust and ice would appear in the sky to be the size of two full moons ether side of Saturn, if only our eyes could see infra-red like Spitzer. See Cosmic Ray for details and pretty pictures and Universe Today for an interview with Anne Verbiscer, a member of the team that found it.
While the rings of Saturn contain small clumps of ice, the solar system is full of their bigger brothers. You might vaguely remember hearing about one of these, a comet named Lulin, which was briefly visible in very dark skies last February. Well Lulin won’t be visiting our part of the solar system for a million years or more, but space_disco writes about a group of astronomers over at the Lowell Observatory who were watching Lulin closer than most. Using their telescope they were able to work out the rotational period of Lulin and even hope to use their data to create a 3D model of its nucleus.
Talking of comets, this October the Earth will be ploughing through the dusty trail left by Halley’s Comet. When it does so the night sky will (hopefully) play host to a spectacular meteor show. Head over to Visual Astronomy for the details. Cheap Astronomy latest podcast has also been considering some of our solar system’s icy bodies. Tune in to learn all about trans-Neptunian objects.
While you are outside enjoying these celestial fireworks why not dust off your old pair of binoculars and have a look at what else there is to see in the sky. What’s that you say… you find it hard to hold your beloved binoculars steady and so the stars are all shaky? Well never fear, just head over to this fantastic post about fixing your binocs to a mount. And speaking of fireworks Wierd Warp has some thoughts about the effects of sunspots.
Getting the telescopes like Spitzer in to space, so they can show us beautiful images of our cosmos, is no easy feat. While this has traditionally been the job of massive rockets like the Arianne rockets, or the Saturn 5, Next Big Future wonders if in the future payloads couldn’t be “shot” in to orbit using massive Orbital Gun Launch Systems.
While this might be fine for inanimate payloads, I think it might be a suggestion to worry the “chumps”. This affectionately nicknamed crowd is the new class of future NASA astronauts. To learn more about who the “chumps” are, and who gave them their nickname, head over to Collect Space .
It seems nostalgia is in the air as Beyond Apollo, 21st Centrury Waves, Cumbrian Sky, and The Gish Bar Times are all taking time to remember times gone by. Beyond Apollo looks to 1972 when, because of a faulty Apollo Command and Service module, three astronauts almost had to be rescued from the Skylab Orbital Workshop. Gish Bar, ten years on, takes a look back at some of the stunning pictures of the Galileo fly by of Jupiter’s moon Io. In other solar system exploration news Cumbrian Sky looks at the award of the Sagan Medal to the man behind the Mars Rover. Meanwhile 21st Century Waves takes a look at a new book by Fred Kaplan called “1959 — The Year Everything Changed”.
Another good book review comes from Simostronomy who gives us the low down on a new book by Douglas Isbell and Stephen E. Strom, about the observatories of the American southwest , imaginatively entitled “Observatories of the Southwest”. While Paul at Centauri Dreams uses his latest reading material as a springboard for musings about how bizarre and alien our ancestors’ civilizations would look to us.
Finally, Habitation Intention is looking for speakers from the aerospace or engineering industry for a conference at Columbia University later this month. They might find what they are looking for over at Kentucky Space, where cubesat pioneer Bob Twiggs described his new role at Morehead State University.
Is there water on the Moon, how did it get there and why should we care? The third question is definitely the easiest to answer since if we ever want to build the moon base science fiction has been promising for years, a local water supply will be an essential resource – transporting it from Earth would be both time-consuming and expensive. The second answer is also relatively simple; it’s thought to have accumulated over time from cometary impacts. More information on the first question however will begin to be gathered on Friday when the upper stage of the LCROSS satellite is due to be crashed into the Moon.
The lunar surface is not a very hospitable place for water. Daytime temperatures in the exposed areas can reach up to ~120 degrees Celsius, boiling it off into space. However, ice could survive in the permanently shadowed craters in the polar regions where the Sun’s rays never reach.
Right, now we know where to look for it, finding it should be simple surely? Well yes it would if we wanted to send up some astronauts on a dangerous, costly, mission down into one of these polar craters. A much cheaper option is to send probes and satellites instead.
In 1998 the first tantalizing hints of the presence of lunar ice came from NASA’s Lunar Prospector satellite which detected hydrogen signatures (a possible indicator of water) in polar craters. However, trying to find water from lunar orbit is obviously always going to be harder than finding it from the surface. I’ve already ruled out sending people and a robotic rover (like Spirit and Opportunity on Mars) would also be too inefficient. The simplest and easiest method is to crash a large object into a crater and see what comes out…
This Friday at 4:30 am PDT (12:30 pm in the UK I think) the 2000 kg Centaur upper stage of NASA’s Lunar CRater Observation and Sensing Satellite (LCROSS) will be crashed at 2.5 km/s into the Cabeus crater, the best candidate for finding the signature of water. Four minutes later the remaining part of the spacecraft will then fly through the massive plume of vaporized material thrown up by the impact and analyze what it’s made of. It will then also crash and create a debris plume of its own. Incidentally LCROSS was launched in June along with the Lunar Reconnaissance Orbiter which took the pictures of the Apollo landing sites that Stuart blogged about earlier in the year.
The plume should also be visible from Earth with relatively small telescopes. As a result NASA is encouraging people in America to hold ‘impact parties’ to observe and photograph the crash and then to send them the resulting pictures to help in the data analysis – another example of ‘citizen science’ which I’ve talked about before.
This isn’t the first man-made thing thrown at the moon but I think it’s the biggest so presumably it has the best chance of throwing up enough material to find the water that could be there. It’s just a shame that it’s happening in daytime in the UK so people aren’t going to be able to get their telescopes out and see it here! I’ll be doing the next best thing and watching it live on NASA TV.
(Quick update: you can also watch astronomers observing the impact live at http://mmto.wordpress.com/2009/10/08/watching-lcross-impact/ )
I think it’s about time that we start covering some ground in galaxy evolution, here in weareallinthegutter. We won’t do it in one post. We won’t do it in 100 either, simply because galaxy evolution is not yet solved. But of course, that only makes it more exciting.
Let us start with some of the basics then, and lay down our aims. The goal of galaxy evolution, in its broader terms, is to explain how galaxies are born and how they evolve throughout cosmic history. A successful theory will give a framework which, given an ensemble of galaxies at one point, can predict how these same galaxies (or another ensemble just like it) will end up in the future.
We have an advantage here, as Astronomers, in that we can look at the Universe during different stages of its past evolution. The trick is in the finite speed of light – for example we see a star which is 10 light years away as it was 10 years ago. In other words, light takes 10 years to travel from this star to us, and an observer sitting on a planet around this star would see me not sitting at my computer right now, but 10 years younger and sitting someplace else (probably a lot warmer).
So the further we look, the further back in time we’re travelling. If you’re studying galaxy evolution, then, this is incredibly advantageous: by looking at galaxies which are at different distances from us, we are looking at how galaxies looked at different stages of the cosmic evolution. Our job is to draw a coherent story line through these stages.
What, then, should our observations be? Let us start simply, with two sets of galaxies – one set near us, and one set far away from us. Each galaxy has a set of characteristics which we may want to study – for example its shape, known in the business as morphology; its colour; its brightness; its mass; its chemical composition; its dynamics (the way it moves); or even its neighbourhood, or environment. The truth is, there are many ways in which one could describe a galaxy, much in the same way as I could choose a variety of characteristics to describe a person. I could go for height, arm length, hair colour, eye colour, number of eye lashes, gender, etc. Some, you will agree, are more useful than others, depending on what I’m studying about a person or group of people. It’s the same with galaxies.
It turns out that one of the most defining characteristics of a galaxy is its colour. And not just any colour – galaxies tend to either be blue, or red. The colour is related to the age of the dominant stellar component – old stars are red, young stars are blue – so the colours themselves are easily explained. But what is surprising is that galaxies tend to sit very much in either the red or in the blue side of the fence. There are very, very few galaxies which sit on the fence and are, for example, green. This on itself is very revealing – it means that whatever process makes galaxies go from red to blue (or the other way around) must happen quickly. If this transition is fast, it means we are less likely to observe a galaxy in this period which explains why we see so few galaxies perching on the fence.
Good. Now, remember that we have two sets of galaxies – one near, and one far from us. If we have some theory of how galaxies go from blue to red and vice-versa, we should be able to predict the fraction of red and blue galaxies in the present (those near us) by measuring it in the past (in those far from us). Our observations of the near Universe should therefore help prove or disprove our theory for galaxy evolution.
This is the mantra of many a paper in galaxy evolution. Observables get more or less complicated – for example, instead of just looking at how the number of red and blue galaxies evolves, we can look at how bright they are, how fast they make stars, how they’re distributed in space, their environment, etc. But essentially, this is what galaxy evolution is all about – and it’s hard!
Two papers recently have caught my eye on this particular matter, so let me very briefly tell you about them. Last month, Tinker et al. looked at these sets of clouds of red and blue galaxies at different distances from us, and tried to make sense of the time-scale of the process which drives the blue-to-red transition. The process itself is still unconstrained, but what they did find is that whatever dominates this evolution today is different from what dominated it in the early Universe. And a bit later in the month, Zucca et al. studied how this transition depends not only on the epoch, but also on the environment of the galaxies. Interestingly, they found that in very dense regions (i.e., more packed regions of the Universe, where there are more galaxies per unit volume) most of the blue-to-red transition happened over 7 Gyr ago. However, in more sparse regions of the Universe, this transition is still happening today.
So the picture is complex – galaxies appear to evolve via different processes according to the age of the Universe, and according to their environment. This is not a surprise, but it is exactly this sort of observational constraints which help test, prove and most often disprove several ideas for galaxy evolution – they are as important as they are technically and instrumentally hard.
I’ll leave you now with this very brief and basic first introduction to galaxy evolution, but I promise to come back with more observational constraints, and with some explanation of what the theorists have to offer.
Jeremy L. Tinker, & Andrew R. Wetzel (2009). What Does Clustering Tell Us About the Buildup of the Red Sequence? ApJ arXiv: 0909.1325v1
E. Zucca, S. Bardelli, M. Bolzonella, G. Zamorani, O. Ilbert, L. Pozzetti, M. Mignoli, K. Kovac, S. Lilly, L. Tresse, L. Tasca, P. Cassata, C. Halliday, D. Vergani, K. Caputi, C. M. Carollo, T. Contini, J. P. Kneib, O. LeFevre, V. Mainieri, A. Renzini, M. Scodeggio, A. Bongiorno, G. Coppa, O. Cucciati, S. delaTorre, L. deRavel, P. Franzetti, B. Garilli, A. Iovino, P. Kampczyk, C. Knobel, F. Lamareille, J. F. LeBorgne, V. LeBrun, C. Maier, R. Pello`, Y. Peng, E. Perez-Montero, E. Ricciardelli, J. D. Silverman, M. Tanaka, U. Abbas, D. Bottini, A. Cappi, A. Cimatti, L. Guzzo, A. M. Koekemoer, A. Leauthaud, D. Maccagni, C. Marinoni, H. J. McCracken, P. Memeo, B. Meneux, M. Moresco, P. Oesch, C. Porciani, R. Scaramella, S. Arnouts, H. Aussel, P. Capak, J. Kartaltepe, M. Salvato, D. Sanders, N. Scoville, Y. Taniguchi, & D. Thompson (2009). The zCOSMOS survey: the role of the environment in the evolution of the luminosity function of different galaxy types A&A arXiv: 0909.4674v1