My latest paper accepted in MNRAS.

A cross posting from the Galaxy Zoo blog. I'm delighted to announce that the latest paper based on Galaxy Zoo classifications was accepted to appear in the Monthly Notices of the Royal Astronomical Society earlier this week, and appears on the arxiv this morning (link). Usually there is a long delay between submission and acceptance of papers (something Kevin discussed on the Galaxy Zoo blog in "What Happens Next - Peer Review"), but in this case the initial referee report came back after 2 days, and the paper was accepted only 2 weeks after the first submission so I never got time to post to the arxiv or write a blog post about it before it was accepted! This was certainly the smoothest and fastest referee process I've been through. ;) Here's the title page.

 

 So what was new about this paper was that we combined information on the morphologies (whether or not the spiral galaxies had bars) with information on the amount of atomic hydrogen gas the galaxies contained and and our main result was that galaxies with more atomic gas in them, are less likely to have a bar. But I want to back up a bit first and tell you about where we get this information on the atomic gas content, and why it  might be interesting. As you might guess from the title of the paper it's from something called the ALFALFA survey (and the new names in the author list for a Galaxy Zoo paper - Martha Haynes and Riccardo Giovanelli - are from Cornell University who are running this survey). Atomic hydrogen emits radio waves at a frequency of 1.4 GHz (or 21cm). This is detectable by a classic radio telescope (in what we call the "L"-band which makes up the second L of ALFALFA). In the case of ALFALFA, we use the Arecibo radio telescope (two of the "A"s in the acronym stand for Arecibo, the third is for array), which is the worlds biggest single dish radio telescope deep in the jungle of Puerto Rico.

Aerial shot of Arecibo. Credit: NAIC.

 ALFALFA is a massive survey which will map the location of atomic hydrogen over basically the whole sky visible to the Arecibo radio telescope. What's neat about a survey for something which emits as a specific frequency is that you actually get a 3D map of where the hydrogen is - both redshift and sky position! Anyway, we made use of about 40% of the survey which is already complete, and which covers about 25% of the area of the sky in which the Galaxy Zoo galaxies are found (the Sloan Digital Sky Survey Legacy Area). Adding some cuts on how face-on the galaxies are so that the bars can be identified, and to make sure the sample contains the same size galaxies right through it's volume we ended up with 2090 galaxies with both atomic hydrogen detections and bar classifications from you guys. This is an order of magnitude larger than any similar sample! So thanks. :) Atomic hydrogen is the basic building block of galaxies (after dark matter). It represents the fuel for future star formation in a galaxy - a galaxy with a lot of atomic hydrogen could in principle make a lot of new stars. Many spiral galaxies have a lot of atomic hydrogen (with perhaps as much as 10 times as much mass in hydrogen as in stars!), while a typical elliptical galaxy has very little atomic gas, and so cannot form lots of new stars. So our observation that bars are more likely to be found in spiral galaxies with less atomic gas supports our earlier ideas about bars possibly "killing" spirals (ie. helping to stop them form stars). 

Trends of bar fraction with atomic gas content, galaxy colour and how many stars are in a galaxy.

 Of course it's never quite that straightforward with galaxies. To start with correlation is not the same as causation, and to that we add that lots of things are correlated. We show some of that in the figure above. Bars are more likely in redder spirals which have more stars ("log Mstar" represents stellar mass in units relative to the mass of our Sun) and which also have less atomic gas. So the skeptical astronomer could say this has nothing to do with the gas content at all, just that the types/sizes of galaxies with less bars have more gas. To test that idea we measured the typical gas content of a spiral galaxy with a given number of stars, and from that we calculated how "deficient" or rich in atomic hydrogen any given galaxy was. Then we plotted the bar fraction against that. The convention in astronomy is to call how much less atomic hydrogen a galaxy has than normal it's "HI deficiency" which gets bigger the less atomic hydrogen there is (from the people who brought you the magnitude scale!).

Bar fraction against how much more or less atomic gas a galaxy has than is typical for the number of stars it has. Bigger HI deficiency = less atomic gas than is normal for a galaxy's size.

Example images.

 Anyway you can see we still see a clear trend, which demonstrates that it's likely to be the atomic gas driving the correlation. Where a galaxy is richer in atomic hydrogen than normal it's less likely to host a bar, and vice versa. Very atomic hydrogen rich galaxies which are massive and have bars are really quite rare! Here are some examples of low and high mass galaxies which are gas rich or poor and with or without bars. :) [/caption] I made images of the whole sample we use available here.

 At the end of the paper we put forward three possible explanations for the correlation, all of which fit in with the observations we presented. It's possible that the bars are causing the atomic gas in galaxies to be used up faster - "killing" the galaxy. The bar does this by driving the gas to the centre of the galaxy where it gets denser, turns into molecular hydrogen and from that stars (but only in the centre). It's also possible (based on dynamical studies of galaxies) that gas slows down the formation of a bar in a spiral galaxy, and/or destroys the bar. Finally it's possible that as a galaxy interacts with its neighbours, a bar gets triggered and its gas gets stripped (ie. the correlation between the two is caused by an external process). We'll need to do more work to figure out which of these (or which combination of them) is the most important.

 To my mind the most interesting result was a hint that if a gas rich galaxy does (rarely) host a bar, it's optically redder than similar galaxies without bars. It's just possible that bars hold back infall of gas from the outer regions of a spiral galaxy and slow down star formation over all in that galaxy. That idea needs testing, but if it's true it's saying that an internal structure like a bar plays an important role in the global star formation history of a galaxy.

 Anyway thanks again for the classifications! ;)

What is Hoag's Object?

Perhaps one of the most striking looking galaxies I know of is "Hoag's Object" (seen below by the Hubble Space Telescope; NED information, Wikipedia article, lookUP information). The wikipedia article talks of it as an object which fascinates both amateur and professional astronomers.

 I am really curious to know how it might look through an amateur telescope, and I'd love to see it for myself some day (RA=15 17 14, Dec=+21 35 08 in the Serpens constellation), Astrometry.net can help give an idea - see Hoag's Object images on Flickr found by the service.

HST Image of Hoag's Object. Credit: NASA.

Through HST as you can see the object appears to be made up of a red spheroidal core, surrounded by a blue ring of star formation (with a gap between the two). The ring shows some spiral structure. In my opinion, one of the most fun things about the object is the more distance ring galaxy which can be seen through the gap (just to the right of 12 o'clock). This to me demonstrates the sheer size of the universe. To find such a rare object behind such a rare object seems quite extraordinary. 

So why am writing about Hoag's Object today, well appearing on the arXiV this morning is a paper addressing the formation scenarios for Hoag's Object (Finkelman et al. 2011, MNRAS in press), which struck my interest so I thought I'd write about it. It puts forward a new scenario for the formation of this unusual object as well as talking about the two previously suggested models. In addition they present some new data for our consideration.

 The three models discussed are:

1. Ring formed as a collisional ring.

In this model another galaxy would have passed through the centre of Hoag's object, and what is observed is the merger remnant. The Cartwheel Galaxy is perhaps the most famous of this class of objects. It's shown below in a HST image, and illustrates the most obvious problem with interpreting Hoag's Object in this way.

Cartwheel Galaxy. Credit: HST, NASA

You can see quite clearly in the above image the culprits in the galactic collision. No such neighbours exist for Hoag's Object. In addition the ring appears to be at rest with respect to the central spheroid - which would be unlikely if the ring were collisional.

2. Ring formed through a bar instability which has since dissolved. 

 In this model at an earlier time there would have been a strong bar, and material would have flowed out along it to form the ring. The main objection to this theory appears to be the lack of evidence for any residual bar in the central spheroid. I should perhaps point out that this was a theory previously put forward by one of the co-authors of today's paper (Noah Brosch), so presumably his co-authorship on this new paper is an indication that he no longer believes this to be the best model.

3. Gas Accretion. 

This is the new theory put forward (although I should say it seems rather similar to me to one discussed by Schwiezer et al. 1987). In this model the object has a very low density HI disk which accreted at early times onto the spheroid and is only dense enough to form stars in the ring.

In fact the kinematic data does seem to suggest that Hoag's object (other than having its only visible disk light in a ring of course) is a normal disk galaxy, with the spheroidal component playing the role of a central classical bulge. At the risk of getting too technical in a blog, check out the Halpha velocity map and HI line profile below.

Ignoring the gap in the Halpha velocity map these two observations look to me basically identical to what you would expect from a normal nearly face-on disk galaxy. The classic "double horned" HI profile is usual interpreted as a coming from a rotating disk of HI with a central gap. HI (atomic hydrogen in its ground state) emits (due to hyperfine spitting of the ground state) at a single frequency of 1420 Mz (21cm) - the broadening of the line is caused by Doppler shifting of the emission (indicated along the x-axis is the velocity of the Doppler shifter HI line) and the peaks at the maximum velocity are interpreted as a pile up of HI in the flat part of a galaxy rotation curve (see global HI profiles, or galaxy rotation curves for more information on this).

HI in Hoag's Object from Schwiezer et al. 1987

The Halpha velocity map is showing a similar thing. Halpha is a spectral line emitted by excited hydrogen. Again this emits at a single frequency (656.28 nm), which is shifted due to the Doppler effect. The map shows the velocity of this line colour coded such that red indicates a greater velocity than the mean for the galaxy and blue a smaller. This is again interpreted as a rotating disk of hydrogen with the upper left moving away from use and the lower right towards us. The velocities are very consistent with what's seen in the HI profile, suggesting the HI is indeed coming from the ring.

Halpha velocity field in Hoag's Object from Finkelman et al. 2011

 What is needed to complete this picture is a HI map of Hoag's Object. I think this could be fairly easilly done with the EVLA, and I will be interested to see it when it is.