Talking about Colliding Galaxies on the Sky at Night

I was lucky enough to be invited to participate in filming for this month's Sky at Night. The theme of the show is Impacts, and I was invited to talk with Chris Lintott about colliding galaxies.

We filmed in Oxford in a back room at the Natural History Museum. It was a really fun experience for me, and fascinating to hang around and watch them film other segments for the show around the museum.

HST image of merging galaxies Arp274

The show will air on BBC4 this Sunday 8th June, at 10pm, and be repeated three further times on BBC. It will also be available to download via the BBC iPlayer. For more details on how to watch visit the BBC Website for Sky at Night: Impacts

Talking Black Holes and Galaxies on the Today Show

The interview I recorded with Tom Fielden from the BBC R4 Today show aired at 11:46 UK time, Saturday, 10 November 2012 . Tom also did a nice write up of it for the website: article "Gas Guzzler"

You can listen to the show in which this appears for another 4 days at this link: Today Show 10th November 2012.

I recorded the segment and posted in on Audioboo as well.

listen to ‘Talking about the Black Hole at the Centre of Our Galaxy on Radio 4 Today Show’ on Audioboo

My favourite colour magnitude diagram

Cross post from the Galaxy Zoo blog.


I was embarrassed to discover today that I never got around to writing a full blog post explaining our work studying the properties of the red spirals, as I promised way back in October 2009. Chris wrote a lovely post about it "Red Spirals at Night, Astronomers Delight", and in my defense new science results from Zoo2, and a few other small (tiny people) things distracted me.

 I won't go back to explaining the whole thing again now, but one thing missing on the blog is the colour magnitude diagram which demonstrates how we shifted through thousands of galaxies (with your help) to find just 294 truly red, disc dominated and face-on spirals.

 A colour magnitude diagram is one of the favourite plots of extragalactic astronomers these days. That's because galaxies fall into two distinct regions on it which are linked to their evolution. You can see that in the grey scale contours below which is illustrating the location of all of the galaxies we started with from Galaxy Zoo. The plot shows astronomical colour up the y-axis (in this case (g-r) colour), with what astronomers call red being up and blue dow. Along the x-axis is absolute magnitude - or astronomers version of how luminous (how many stars effectively) the galaxy is. Bigger and brighter is to the right. So you see the greyscale indicating a "red sequence" at the top, and a "blue cloud" at the bottom. In both cases brighter galaxies are redder.

 The standard picture before Galaxy Zoo (ie. with small numbers of galaxies with morphological types) was that red sequence galaxies are ellipticals (or at least early-types) and you find spirals in the blue cloud. The coloured dots on this picture show the face-on spirals in the red sequence (above the line which we decided was a lower limit to be considered definitely on the red sequence). The different colours indicate how but the bulge is in the spiral galaxy - in the end we only included in the study the green and blue points which had small bulges, since we know the bulges of spiral galaxies are red. These 294 galaxies represented just 6% of spiral galaxies of their kind.

 So this is one of my favourite versions of the colour magnitude diagram.

What is a Galaxy (QI style)?

I'm a big fan of QI, so it was fun when Brian Cox was on it recently and the whole show was peppered with astronomy.

I was amused to find them mention the interesting paper by Duncan Forbes and Pavel Kroupa in which they consider what the definition of a galaxy is. Luckily someone posted the segment on You Tube:



Earlier this year when they posted the discussion on the arxiv, Duncan and Pavel opened it up for a vote - in good "citizen science" style. Of course this caught my "Galaxy Zoo" eye so I blogged about it last Feb (2011).

It's on my mind tonight as I prepare to talk about "What is a Galaxy?" tomorrow night as part of the Intech Science Centre Adult Only Evening.

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.

John Huchra Measuring Redshifts at the Whipple Observatory

One of the things I've been working on recently is the final publication of the 2MASS Redshift Survey (2MRS). This is the largest complete map of the local universe (ie. it looks at as much of the sky as is possible - about 95% when you consider our Galaxy blocks some of it), and represents the culmination of decades of work on redshift surveys by John Huchra (1948-2010). I worked with John on the 2MRS as a postdoc for 3 years (2005-2008) and myself and Lucas Macri (a former student of John's) have been working on finalizing the 2MRS and publishing it in John's name (based largely on text he wrote in various unpublished descriptions of the survey). We plan to release the data very soon.

As part of this whole effort I'm going to be speaking in a special session at the upcoming American Astronomical Society meeting (#AAS218) in Boston, MA about "John and the 2MRS". For my presentation I've been looking for pictures of John observing to get redshifts, and with some help (particularly from Dan Brocious at the Whipple Observatory, and Boyd Estus at Heliotrope Studies, Ltd) I managed to get my hands on the below clip of John Huchra observing redshifts at FLWO in the 1980s, presumably as part of his famous CfA redshift survey (it's a segment from "So Many Galaxies, So Little Time", narrated by Margaret Geller).

By definition this was exactly what I was looking for!




By the way you can read an article I wrote about John (shortly after his death in October 2010) on the Galaxy Zoo blog. 

Some recent Galaxy Zoo work

I'm a bit behind, but I wanted to point out two papers I was involved in from the Galaxy Zoo project which have recently come out on the arXiV.

Both were blogged about on the Galaxy Zoo blog (in both cases by the first author, and in both cases I was the second author).

Ben Hoyle: GZoo2 Bar Paper Accepted in MNRAS

Lucy Fortson: Galaxy Zoo and Zooinverse Review Article posted today on arXiv

This last ones use the above Hubble Tuning Fork I made using Galaxy Zoo style SDSS images of galaxies.

SDSS Telescope Getting Ready to Observe (on YouTube)

In most of my research, I use data taken by the Sloan Digital Sky Survey. In the last few months I have been particularly focussed on looking at the morphologies of galaxies which are being observed as part of the ongoing Baryon Oscillation Sky Survey (or BOSS). This survey is in the process of taking spectra (to measure redshifts) of 1.5 million distant galaxies. I've just been looking at a small fraction of them which have high resolution images taken by the Hubble Space Telecope. I promise a post about that work once I submit the paper. :)

Anyway, the BOSS collaboration had a science meeting in New Mexico last week. Unfortunately I couldn't go. I phoned in to give a talk about my work, which was great, but I was still disappointed to not be physically present because the meeting included a trip to visit the actual telescope used to take all the SDSS data (both the images and spectra).

 I love telescope, particularly as the sun sets and they prepare to observe. Usually they are so peaceful and full of hope at that time of night. Who knows what they'll discover as they work hard during the darkness.

So you can imagine, I was delighted to learn that a video had been taken of the visit, and put up on YouTube. You can see lots of scientists getting in the way of the telescope operators as they prepare the telescope for a night's observing (so not as peaceful as normal!). It's all set to nice classical music, and ending with a beautiful sunset.



Watch out for the "BOSS plates" going in. These are big metal plates (a couple of metres across) which have holes drilled in them. Fibres are connected to each hole, and the plate carefully lined, so the light from a single galaxy goes down each hole. That's how SDSS can take so many spectra - hundreds are taken at once using this method.

Also watch out for the building moving off the telescope. Instead of a classic dome with an opening, or even a building with a removable roof, the SDSS telescope is covered during the day by a building which is rolled completely off the telescope at night.

To learn more about SDSS3 and BOSS you can follow the SDSS3 Blog.

Testing Gravity with Galaxies

A gas rich galaxy. Credit: THINGS

A few weeks ago a bunch of news articles came out suggesting that there was some new evidence that ruled out dark matter. It was all based on a paper in the journal Physical Review Letters (PRL) by Stacy McGaugh: "A Novel Test of Modified Newtonian Dynamics with Gas Rich Galaxies", which had appeared as a preprint on the arXiV, and was the subject of a press release: "Gas rich galaxies confirm prediction of modified gravity theory".

The press went a bit crazy about the paper, as was well discussed by Sean Carroll (@seanmcarroll) over at Cosmic Variance: "Dark Matter: Just Fine Thanks". He held this up as an example of the problems of communication between scientists and the science media. I think he has a point. He also presents a nice list of problems with the "alternate theory" held up in the paper: MOND (Modified Newtonian Dynamics).

Ethan Siegel has also written about the problems with the press coverage on his blog: "Good ideas, Bad ideas, MOND, and Dark Matter" illustrated very nicely with lots of pictures.

At about the same time as all this press (it was even on BBC online: Dark Matter Theory Challenged by Gassy Galaxies Result) a request to write a PRL Viewpoint on the article came across my desk.

PRL publishes Viewpoints (articles written by and for scientists) on papers they think are of note. The audience is supposed to be non-specialist physicists, so this was a fun article to write - equations allowed, but not obscure astronomical terms! I roped in my good friend Kristine Spekkens (an Assistant Professor at the Royal Military College in Canada) and we got to work.

The result appeared in PRL yesterday: "Testing Gravity in Gas Rich Galaxies", by Karen Masters and Kristine Spekkens. I was going to write a version for non-scientists for this blog, but having found the two articles by Sean Carroll ("Dark Matter: Just Fine Thanks") and Ethan Siegel ("Good ideas, Bad ideas, MOND, and Dark Matter") while looking for links to the press articles (just to be clear I only read those this morning, after writing the viewpoint) I find they've already done the job for me.

Just like them, our main conclusion is that MOND cannot compete with the standard cosmological model. It is a way of explaining the rotation curves of galaxies without dark matter, and it does that impressively well but it's not a theory of gravity, and it fails at a lot of tests that our standard gravity + dark matter + dark energy model (clunky as it is) passes with flying colours.

I think the most interesting result from McGaugh's paper is the finding that the baryon fraction scales with the rotation velocity of the galaxies (ie. is smaller for lower mass galaxies). This is presented as being an obscure fine tuning for the standard model, but actually it has the right qualitative sense (ie. there are known processes which produce something a bit like that), and can now provide a constraint to the models we have. So let me explain what it means a bit more......

Baryons are the scientific term for normal matter - atoms, electrons, protons etc. that people, stars, planets are made of. We actually have a really good measurement of what the baryon fraction averaged over the whole universe must be (compared to the radiation content). In the standard cosmological model, that fraction is set by our understanding of nuclear physics and the fractions of different light elements which are made right after the Big Bang (known as Big Bang Nucleosynthesis: wikipedia article). The baryon fraction can also be measured in CMB experiments like WMAP.

However it is well known that the baryon fraction in galaxies is lower than the cosmic average. And it makes sense that this would be so. Once the dark matter gets into a galaxy it can only be thrown out by gravity. But baryons do things. They form stars which have stellar winds, and blow up in immensly energetic supernovae. Baryons both flow into and out of dark matter halos, and the balance between those two processes depends on the mass of the halo. Massive halos have stronger gravity and so may be able to hold onto their baryons better - so they should have a higher baryon fraction, just as McGaugh's result shows.

We end our viewpoint by talking about how the next generation of radio telescopes (like SKA) will be able to make a much bigger (and more complete) sample to repeat the test McGaugh proposes. And if the result still holds, we'll have to explain it with our models of how galaxies form. But I seriously doubt it means MOND is the answer.

In fact as part of the research for this paper I also came across the following Physics World article (thanks Chaz!) which presents the best rebuttal for MOND I've seen yet: "New lower limit set for Newton's Law". At it's heart, MOND is a modification of Newton's 2nd law at low acceleration scales (not really a modification of gravity). This article presents the results of a test of Newton's 2nd Law at low acceleration scales well below where MOND proposes a change.  The only problem is that in this experiment the attractive force is the electromagnetic force rather than gravity. But that means that MOND only modifies Newton's 2nd Law for gravity, not for other forces - so that makes it an even stranger theory than it already is!

365 Days of Astronomy Podcast - Do Bars Kill Spirals?

The podcast today over at 365 Days of Astronomy is "Galaxy Zoo 2 - Do Bars Kill Spirals?" by Chris Lintott and me. We had great fun talking about the first result from Galaxy Zoo 2 - that bars are more common in redder (deader?) spirals. Hope you enjoy listening to it (should be available later today).

You can read more about that project in my Galaxy Zoo blog posts about it.

I will also be interviewd about this by Darren Gamblen of Express FM 93.7 (local Portsmouth radio) this morning at 11.30am. You can listen to the segement at this link.

Beautiful Galaxies on the BBC Big Screen

As part of National Science and Engineering Week, 11-20th March 2011 in the UK I was involved in the production of a series of 5 short videos called “From the Earth to the Edge of the Universe” which were made as a collaboration between Creative Technologies and the Institute of Cosmology and Gravitation at the University of Portsmouth. They are going on the BBC Big Screens, apparently right across the UK and continuing up until the 2012 Olympics.

Karen - From Earth To The Edge Of The Universe from CCi Live on Vimeo.

My segment is all about galaxy morphologies. I talk (briefly) about Galaxy Zoo and show the HST image of Hanny’s Voorwerp. I also describe some of the main morphological features of galaxies, and what I like about them.

You can watch all 5 videos here

I also blogged about this for Galaxy Zoo. 

The Hubble Tuning Fork at Galaxy Zoo

Over at the Galaxy Zoo Blog this week I posted an article about the Hubble Tuning Fork after I made the below example using images like the one used by Galaxy Zoo.