miércoles, 11 de enero de 2012


Galaxy overlaps at the AAS

Galaxy Zoo Blog

Galaxy overlaps at the AAS
X-ray observations of IC 2497 in the can!
Seeing mergers in a different light
Galaxy Zoo on the “Curious” Podcast
Galaxy Crash Debris: Post-merger Spherodials paper now out!
Post-starburst galaxies paper accepted!
Bars in Spain!
GZ1 used for the fractions of early-types in clusters
First look at Hubble’s first look at the first Voorwerpje
New Bar Paper Submitted to MNRAS

Posted: 10 Jan 2012 10:03 PM PST

Wednesday’s session at the Austin meeting of the American Astronomical Society will include new results from the Galaxy Zoo sample of overlapping galaxies. Extending the work in Anna Manning’s Master’s thesis, this marks an extension that helps us look ahead to comparison with the higher-redshift Hubble Zoo overlaps. Specifically, we compared visible-light data with ultraviolet data (from the GALEX satellite or a UV/optical monitor instrument on the European Space Agency’s XMM-Newton) to compare the amounts of optical and ultraviolet absorption in galaxies. This tells us, for example, how much we should correct Hubble measurements for high-redshift galaxies, where visible-light filters sample light which was emitted in the ultraviplet, to compare them with the rich SDSS data which see the visible range emitted by nearby galaxies. This is a key tool in trying to use backlit galaxies to search for changes in the dust content of galaxies over cosmic time, by comparing Hubble and Sloan results. Along the way, we see evidence that a common result – the flat so-called Calzetti extinction law in star-forming galaxies – results from the way dust clumps into regions of larger and smaller extinction that we usually see blurred together, since we see this in regions so far out in some galaxies that internal illumination by the galaxy’s own stars doesn’t matter. Here’s the poster presentation:

UV overlap poster paper

(That had to be shrunk to fit the blog size limits but should still be just legible – click for a bigger PNG). NGC 2207 is outside the SDSS footprint but had such good data that gave nice error bars that it wound up featuring a whole image series. Now to go back and apply that new set of analysis routines to more GZ pairs…

In other news, a Canadian astronomer working with NED found a new use for the overlap catalog including the “reject” list – to distinguish galaxies in pairs which are seen moving together or apart, since we often have both redshifts and from the dust we know which one is in front.

And to reiterate what it says at the end of the abstract – we thank all the Zooites who have contributed to the overlap sample and made this work possible!

X-ray observations of IC 2497 in the can!

Posted: 10 Jan 2012 02:51 PM PST

As we tweeted about and as Chris noted in his blog post about the Zooniverse success at the American Astronomical Society meeting in Austin TX, half of the Chandra X-ray observations of IC 2497 (the galaxy next to Hanny’s Voorwerp) have been executed and so with bated breath we awaited the results.

From previous X-ray observations with Suzaku and XMM, we know that the quasar that lit up Hanny’s Voorwerp is dead, and that there’s just a weak source in the center of IC 2497 where the black hole lives and some evidence for hot gas. So we had turned to Chandra to figure out what was going on in the center of IC 2497. To puzzle apart the faint black hole at the center and the gas around, and Chandra has the sharpest X-ray eyes in the sky.

We got the notification from the Chandra X-ray Center that the observations had concluded and that we could have a preview of the raw frame. Bill and Chris happened to be near, so after Chris finished his talk on the latest Planethunters.org results (two new planets!), we got together in (possibly) the exact same spot where Chris and Bill viewed the first spectrum of the Voowerp at another AAS meeting in Austin four years ago.

Chris (left), Kevin (right)

From left to right: Bill, Kevin, Chris, all looking at the data.

So, without much further ado, here’s what we got:

Well that’s a bit underwhelming!

Or not!

First, we know that we have a bright source, so we can study the X-ray data in detail. Also, this is just a JPEG screenshot, so we can’t even zoom in and change the scaling to see if there’s anything else there. We don’t even know which way is North, so we don’t know where the Voorwerp is. So for now, all we can do is wait for the actual raw data to be available. This should take a few days. Stay tuned….!

Seeing mergers in a different light

Posted: 02 Jan 2012 01:02 PM PST


My name is Alfredo and I’m a Ph.D. student at Imperial College London. I’ve been asked to write a blog about how we take an idea and turn it into a paper, showing exactly what the man behind the scene does.

I’m working with galaxy mergers so the field from which we are going to pluck our idea has to be that one. Merger properties have been described extremely well by the Galaxy Zoo team, which used the Sloan Digital Sky Survey optical data so we thought it might be interesting looking at the GZ merger catalogue in different wavelengths, specifically in the infrared.

You can study pretty much every object in the infrared because what we call heat is simply the emission of infrared light. If you can measure it’s temperature then it radiates in the infrared. In astronomy infrared radiation allow us to see objects that are not too bright in the visible spectrum (cold stars, gas clouds), to probe regions that are obscure in the optical and to explore the early Universe. Our project will use the infrared fluxes to extrapolate interesting characteristics, mostly to do with the star formation process of the galaxies.

In the past, a huge number of papers have shown that galaxies which were very bright in the infrared ( called LIRGs – Luminous infrared galaxies, U(ltra)LIRGs and H(yper)LIRGs) were mostly mergers or post-mergers. We are going in the opposite direction: since we have a strong visually selected merger catalogue, thanks to your hard work, we can now see what’s the real connection between mergers and warm galaxies.

Galaxy Zoo on the “Curious” Podcast

Posted: 14 Dec 2011 12:07 PM PST

Just wanted to put up a quick post to point out that the latest podcast from the people who run Ask an Astronomer @ Cornell discusses citizen science, and I’m interviewed on it about Galaxy Zoo stuff.

Link to the podcasts in iTunes.

Merry Christmas! Karen.

Galaxy Crash Debris: Post-merger Spherodials paper now out!

Posted: 29 Nov 2011 10:35 AM PST
Today’s post is by Alfredo Carpineti, a Ph.D student at Imperial College:
I’m happy to inform you that a paper on the properties of spheroidal post-mergers (SPMs) has been accepted for publication by the Monthly Notices of the Royal Astronomical Society. The arXiv link to the paper is http://arxiv.org/abs/1111.5008.
We are interested in post-mergers because we want to study in the hierarchical model of galaxy evolution and understanding the evolution of galaxies along a merger sequence is necessary to achieve this. We define post-mergers as single-core galaxies with tidal feature or disruption that can only be explained as merger related.

The specific subset we chose are the likely predecessors of elliptical galaxies, and we compared them to the general merger and an elliptical control sample to see how the properties of galaxies evolve along the merger. The SPMs are part of a sample classified by Galaxy Zoo as post-mergers. We looked at this sample again and we picked the ones which look mostly bulge dominated, a key feature of galaxies that are likely to be precursors of elliptical galaxies. You can see in the figure below how, even though these galaxies are similar in morphology to elliptical galaxies, they appear to be in the process of relaxing into relaxed ellipticals.

In our paper we found that the vast majority of the SPMs inhabit low-density environments and that they sit between mergers and ellipticals in colour space, which indicates that the peak of star formation activity takes place during the merger phase. However the AGN fraction peaks in the post-merger phase (compared to the mergers) suggesting that the AGN phase probably becomes dominant only in the very final stages the merging process.
In general the SPMs are very active, with 84% of the galaxies in our sample showing some emission-line activity compared to the 63% of the mergers and the 27% of the relaxed ellipticals. The post-merger phase might be less showy than the merger phase, but it’s clear that the dust is yet to set in these galaxies.
Finally we compared the colours of the SPMs to models in which a young stellar population (with an age of 500 million years) is superimposed on an old population that forms 10 billion years in the past (since the bulk of the stars in elliptical galaxies are known to be old). We found that, under these assumptions, the vast majority of the SPMs are likely to have formed more than 5% of their stellar mass in the recent merger-driven burst. Since ellipticals themselves are rather gas-poor objects, our results indicate that ∼55% of the SPMs are products of major mergers in which at least one of the progenitors is a late-type galaxy.

Post-starburst galaxies paper accepted!

Posted: 10 Nov 2011 06:35 AM PST

Great news everybody!

The post-starburst galaxies paper has now been accepted by MNRAS. You can find the full paper for download on astro-ph.

Bars in Spain!

Posted: 09 Nov 2011 04:48 AM PST

Dear all,

Recently the Spanish media has described the Google funded GZ bar drawing project. The article, which can be seen here and was based on this MNRAS paper , was written by members of the Spanish Public Agency for the Dissemination of Scientific Knowledge, (see here), which is a leading news agency in Spain.

Thanks again for making the bar drawing project so successful.

Ben (on behalf of Bob, Karen, and the GZ bar drawing team)

GZ1 used for the fractions of early-types in clusters

Posted: 09 Nov 2011 03:06 AM PST

We’re happy to report that we have once again used your (now public) GZ1 classifications to find an interesting result.

We use the classifications in a study we just submitted to MNRAS (or see the arXiv entry for a copy) looking at the observed fractions of early-type galaxies (and spiral galaxies), in groups and clusters of galaxies.

Recent work (De Lucia et al. (2011), which posted to the arxiv in September), used sophisticated semi
analytic models to determine the properties of galaxies found in massive
clusters in the Millennium Simulation. They identified elliptical galaxies
(or more accurately early-type galaxies) in the simulation, and found that the fraction these
galaxies, remained constant with cluster halo mass, over the range 10^14 to
10^14.8 solar masses. They compared their results with previous
observational studies which each contained less than 100 clusters.

With GZ1 we realised we could put together a much larger sample. We
used galaxies with GZ1 classifications, cross matched with cluster and
group catalogues, to compare the above results with almost 10 thousand
clusters. We found that the fraction of early-type galaxies is indeed
constant with cluster mass (see the included figure), and over a much larger range of 10^13 to 10^15
solar masses (with covers small groups of galaxies to rich clusters), than previously studied. We also found the well known result (to astronomers) that outside of groups and clusters, the fraction of early-type galaxies is
lower than inside of groups and clusters.

Plot showing the fraction of early-type galaxies (red lines) as a function of halo mass. We used two different halo mass catalogues, and the agreement between them is excellent. We also examine the fraction of spiral galaxies with halo mass (blue lines)

This work suggests that galaxies change from spiral to early-type when individual
galaxies join together to form small groups of galaxies, but that going from groups to rich clusters does not significantly change the morphologies of galaxies.

Without the GZ1 results at our finger-tips, this work, which was devised,
implemented, and written up in less than 2 months, would have taken much
longer to complete.

Thanks again for making the Zoo such a wealth of information,

Ben Hoyle (on behalf of Karen Masters, Bob Nichol, Steven Bamford, and Raul Jimenez)

First look at Hubble’s first look at the first Voorwerpje

Posted: 08 Nov 2011 09:44 AM PST

The bits are still warm, having just been downlinked from Hubble overnight. There is still a good bit of processing to be done, cleaning up cosmic rays and so forth. But that said, here is our first look at SDSS 2201+11, first of the Galaxy Zoo AGN cloud galaxies (AKA voorwerpjes) to come up on the telescope’s schedule. As a reminder, as waveney just posted in yesterday’s Object of the Day, here it is in the SDSS images:

And now what we’ve all been waiting for! First up, the galaxy in a narrow filter that includes the strong [O III] emission from the clouds at this redshift:
emission”]Hubble image of SDSS 2201+11 with [O III] emission

And one in a filter including H-alpha emission, which is several times fainter in such highly ionized gas:
SDSS 2201+11 HST image with H-alpha

SDSS 2201+11 HST image with H-alpha

And finally, in the tradition of vacation photographs everywhere, a shot of just the galaxy (in this case a medium-width filter near the standard i band to show the stars and dust but not the gas):
SDSS 2201+11 Hubble i image

SDSS 2201+11 Hubble i image

First inspection shows that the galaxy has been disturbed – the dust lanes twist. One of them trails right off into one of the gas clouds, adding to our evidence that ionized tidal debris often shows up in this way. That also suggests which cloud is on the near side, so we have a clue about the time delays experienced by the radiation we see from each one as it has been affected by possible changes in the nucleus. There are interesting holes and curlicues in the gas, as well.

Further processing will show us more. And there are six galaxies to go! (These should dribble in throughout 2012 – we just got an appetizer).

New Bar Paper Submitted to MNRAS

Posted: 07 Nov 2011 12:00 AM PST

I’m happy to report that this week we submitted a new paper (to the Monthly Notices of the Royal Astronomical Society) using your bar classifications from Galaxy Zoo 2. The paper appears on astroph this morning (paper link here).

The title of the paper is shown below.

Ramin Skibba (who wrote one of the early Galaxy Zoo papers on the morphology-colour-environment relations) led this study into how the chance of finding bars and bulges in disc galaxies depends on the environment the galaxy is found in. We’re interested to find this out – particularly how the chance of finding a bar depends on environment – to help with the interpretation of our finding that redder disc galaxies are much more likely to host bars (which has now also been seen in other samples, and illustrated beautifully in one of the first figures from the paper, below – look at the blue lines which show the probability of finding a bar in a disc galaxy against it’s colour, with red to the right). Previous studies of the effect of environment have not had as many galaxies as we have (thankyou again), and have come up with contradictory answers as a result (for references see Ramin’s paper), so being able to do this study with a huge sample of galaxies with visually classified bars has been really fantastic.

Ramin used a technique called marked correlation functions to look for the effect of environment on the chance of a disc galaxy having a bar (or bulge). A correlation function (in astronomy at least) gives the probability that you’ll find two galaxies separated by a particular distance. We plot the “correlation” as a function of this separation (usually called r). The higher the correlation function (w(r)) is at some separation the more likely it is that you’ll find galaxies clustered on that scale. Practically this is measured by counting pairs of galaxies separated by each separation, r, and comparing it to the number of such pairs you’d find in a completely randomly arranged sample of galaxies.

The only difference with a marked correlation function is that in addition to simple counting of pairs, each galaxy is weighted by some “mark” (or number from zero to one). For example, in our paper we mark the galaxies by the fraction of you who classified the galaxy and clicked that you saw a bar – which we called p_bar. So two galaxies with a high fraction of “bar clicks” separated by a distance r would count more in the marked correlation function than two with a low fraction of “bar clicks”.

Our main result is shown above. The top panel shows the correlation functions of the whole sample, both with and without the weighting by p_bar. The bottom panel shows the ratio between those two correlation functions – so basically it shows the scales at which you’re more likely to a pairs of barred disc galaxies than a pair of any two random disc galaxies. This shows us that on the smaller scales barred disc galaxies are more strongly clustered than disc galaxies in general. The clustering peaks at about 0.4 Mpc/h (where Mpc/h are fantastic astronomers distance units which show our mistrust of the value of the Hubble constant; h = H0/100 km/s/Mpc, so is probably about h=0.7; so 0.4 Mpc/h is probably about 0.6 Mpc, or about 2 million light years) which is interestingly about the scale where most galaxies will be satellites of larger halos (ie. galaxies in groups). It is also interesting that on the very smallest scales the ratio drops back down to almost one – showing that for very close pairs disc galaxies are not more likely to have a bar.

Ramin (and of course also Steven using a different method) had previously shown that redder disc galaxies are more strongly clustered than bluer ones, so we had to wonder how much of the extra clustering of barred disc galaxies was just due to them being preferentially in red discs…. Ramin tested this using a really neat little trick, which he called “shuffling the marks”. Basically he took all galaxies of a similar colour and randomly shuffled the p_bar number within the group. If the bar-environment correlation was entirely due to the colour-environment correlation doing this for all the galaxies should result in no change in the marked correlation function. And in fact this is almost what we saw (below: the red triangles almost match the white circles). On most scales the bar-environment correlation can be explained by red discs being more strongly clustered, except right around the 0.4 Mpc/h scale (which likely represents galaxies in small groups) where in simple terms – we see some excess clustering of red discs with bars over the clustering of red discs in general.

This finding suggests that something about the group environment may be triggering bar formation. In addition the downturn at small scales (if real) suggests that once galaxies get really close and start interacting they are not more likely to have a bar.

There is a whole lot more information in Ramin’s paper, which as usual is an excellent and densely packed piece of work, so I hope you’ll forgive me for stopping here after explaining only the main result on the barred disc galaxies. It’s really been a pleasure working with Ramin on this study, and I just wanted to give you a flavour of the interesting results we have found.

The relative importance of bars (and other forms of “secular evolution” – generally used to mean slow internal processes acting on a galaxy) is turning into a hot topic in astronomy; with an entire session at the upcoming International Astronomical Union General Assembly (being held in Beijing in August 2012) devoted to “Galaxy Evolution through Secular Processes” so I expect you’ll be hearing a lot more about this.

Fuente...Galaxy Zoo Blog feeds@zooniverse.org via google.com

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