Friday Galaxy - 2 - NGC 720

This is a real hot off the press Friday Galaxy, the data for this was only taken last night, in Chile, by those very helpful people at the Gemini-South telescope. I'm using the 12 images they took (4 each of g',r' and i', that's blue, green and red to you and me) to pick out Globular Clusters for spectroscopic follow up.

The image below shows the image produced, there are a couple of odd things about this image. The first is that the galaxy itself is very blue, for an elliptical galaxy this is odd, these are usually known as being red and dead, because they don't form stars. At first I thought I had mixed up the blue and red channels, however in the top left corner you can see a very pretty background edge on spiral and this appears to be just the right colour for a spiral, hmm.

If we assume that the colours are correct and that we are seeing a blue elliptical galaxy, then their are two possibilities: Either some fraction of the galaxy is made up of hot young blue stars, which can't have been formed more than a few Gyrs ago (not that common for ellipticals), or else the galaxy must be very old and lacking in metals. This is one of the major drawbacks of using the colours of astronomical objects like galaxies to tell you about them, there is a degeneracy between the age of stellar populations and the amount of elements heavier than Helium that they contain (their metallicity). Basically things can appear blue for one of two reasons, they are either young, or they have a low metallicity (and are old).



To solve this conundrum will require spectroscopy, which hopefully we will be getting in the next few months. One other interesting thing that appears in this image can be seen in the top left, when you look at the spiral galaxy (shown enlarged below) you can just about make out what appears to be a stream of material trailing to the bottom right. This trail, if real, could be a trail of stars produced by a minor merger event, if I get the chance and have the space when I make up the mask for the spectroscopy I may well try and get a sly spectrum of this just to see whats going on.


That just shows you one of the cool things about astronomy, serendipity means you can find all sorts of interesting things in the unlikliest of places. There are loads of interesting things in the background of deep images like this one, if anyone finds anything interesting in the large version of the image, let me know, you never know I may try and get a spectrum of anything you find that looks interesting enough.

Friday Galaxy - 1 - NGC 524

I've decided to institute a new tradition of posting images of a favourite galaxy on a friday. For the first example I have decided to choose NGC 524, this galaxy is an S0 galaxy that I am working on at the moment.

The images I'm going to show are from archival HST images taken using WFPC2 (PI Brodie project 6554), I have produced a colour image of the galaxy, though I have cheated somewhat, as I only had access to two filters the F555W and the F814W, which I am going to treat as blue and red respectively, I'm then going to use an average of the two as the green channel. The downside is you're pretty much guaranteed to get something that looks red or blue, still this is interesting in itself, as blue galaxies tend to be young and red ones old. So here is the image.



You can see the centre of the galaxy is located towards the top left of the image, the diffuse glow around this is the halo of NGC 524, many foreground stars and background galaxies are also obvious. In this image the galaxy looks fairly boring, a very smooth looking ellipical galaxy, however I had seen some hints of something odd going on in the inner regions in some data I had from the Gemini telescopes and decided to investigate it. What I did was to average the images from the blue and red exposures, as this tends to pick out structures and dust in galaxies, this is because dust tends to absorb different amounts of the two wavebands. What I found was this:


First of all you can see that many objects disappear, this is just because they have similar amounts of flux in the blue and red, the centre of the galaxy however doesn't. You can see that some very pretty spiral structure emerges, so you can see that on closer inspection NGC 524 is being observed face-on, the Milky Way would probably look very similar if you stopped forming stars and then looked at it from above the disc after a few Billion years.

Science In Pictorial Form

I've just come across a cool image that encapsulates the level of interconnection within science. It even appears Astrophysics has some connections to other sciences, who would of thought. The full article can be found at Seed. The description is fairly interesting.

This map was constructed by sorting roughly 800,000 published papers into 776 different scientific paradigms (shown as pale circular nodes) based on how often the papers were cited together by authors of other papers. Links (curved black lines) were made between the paradigms that shared papers, then treated as rubber bands, holding similar paradigms nearer one another when a physical simulation forced every paradigm to repel every other; thus the layout derives directly from the data. Larger paradigms have more papers; node proximity and darker links indicate how many papers are shared between two paradigms. Flowing labels list common words unique to each paradigm, large labels general areas of scientific inquiry.

Click on the picture for a very big version (< 5Mb).

The Scale Of Things

Wow, check out this composite picture of the moon partially occulting Saturn. Even as an Astronomer my mental image just didn't have Saturn appearing that big relative to the moon, click for the big version.

Here is the description from LPOD:

In England on March 2nd, 2007 the Moon slowly moved past Saturn. From Pete’s observing site in Selsey the Moon just barely nicked the planet itself but pleasingly covered about 40% of the rings. Here are Pete’s words on how he captured this event: The difference in brightness of the Moon compared to Saturn was huge at the time and in order to get both objects imaged simultaneously, one must suffer. In this case I exposed correctly for Saturn which meant that the Moon’s limb was burnt out. A number of shots were taken at 30s intervals (10s movie captures at 60fps, fixed on Saturn) which gave me the positional information I needed to build the composite you see here. The RGB image of Saturn was captured just before the occultation and the lunar limb just after (this is a three frame mosaic). The interval positions shown are separated by 90s in time. South is up in the image and the Moon would be moving towards the upper right.

Hubble Heritage

Click for VERY Big.

Thanks to the BadAstronomer I have found out about a cool new HST image from the Hubble Heritage site, what's even better is that two of the people that worked on the project to get the data work along the corridor, hence I am actually better informed than the BadAstronomer for the first time. The picture released by Hubble Heritage shows a cluster of Galaxies called Abell S0740. There are two versions of the image one without annotations and one showing zoomed regions of interest. See them both here.

This data is of particular interest because of what was found in the inner regions of the large elliptical galaxy at the centre of the cluster, when examined carefully it was discovered (by Russell Smith, here's the paper to prove it) that there were 3 gravitational arcs within the galaxy, you can see them in the inset in the top right of the image above. These arcs are the result of the light from a background galaxy being bent by the gravity of the large foreground elliptical galaxy, this has the effect of making several images of the same galaxy which are brighter and larger than they would usually be, making it possible to see galaxies further away. The gravitational arcs in this galaxy are the closest known example of strong lensing by an individual galaxy, as opposed to lensing by a cluster of galaxies.

The reason these lenses are so interesting is that when the position and brightness of the lenses are combined with the distance to the actual background galaxy (which can be worked out from its redshift) it becomes possible to measure precisely the amount of mass in the lensing galaxy within the radius of the arcs, with little or no assumptions. By looking at the way brightness of the lensing galaxy it is then possible (assuming that brightness is related to the amount of stars) to work out fraction of the mass in the lensing galaxy that is from normal matter and the fraction from dark matter. But wait that's not all.

By using the mass determined with the observed brightness of the lensing galaxy and the measured motion of stars in the galaxy (measured from spectroscopy) it becomes possible to work out exactly how the stars in the galaxy move as well, allowing a determination of parameters that can be used to constrain models of how galaxies form. Essentially these lenses could allow determinations of all the physical parameters of the lensing galaxy at accuracies previously unheard of.

Great you say, lets do it, but this is where things get difficult, it turns out that the arcs are so narrow that with current technology is actually impossible to get enough signal from them to determine a redshift, bugger. Basically they are so narrow that you add much more lensing galaxy light then you do lensed galaxy light. If STIS on the HST is ever repaired it may be possible, but if not it could be 10 years before its feasible. Note: if you happen to be someone sitting on a TAC (telescope allocation committee) and I, R. Smith or J. Lucey have a proposal to do what I've just claimed is impossible, I was wrong, we can do it, please give us the time.

Another thing I like about this work is shown in the other inset, a whole bunch of globular clusters. I study globulars, and when observing them from the ground it becomes very difficult to observe them at distances probably around 30-50Mpc away, these ones are located at 142Mpc, makes me even more annoyed that ACS has broken down, there will be no more new data like this for at least two years.

New Links.

I'm planning on periodically adding to the links section on the sidebar to the right, I'm not sure how often this will be, it all depends on how much work I have on. Anyway, here is the first set of new links, all to do with pictures of Science:

The Hubble Heritage site has a very good selection of some of the HST's greatest hits, many will be familiar but there are one or two new ones, and others that have changed slightly since they were first released. See them in all their glory here.

In a similar vein, but looking the other way, here is a link to the US Geological Survey's image gallery, check out the galleries called Earth As Art.

The Earth Science Picture of the Day, is set up along much the same lines as the Astro-Picture of the Day, in particular check out the archives for many spectacular pictures of the Earth and natural phenomena.

The National Geographic also has a picture of the day, the archive found here is again worth a look (its where the picture at the top comes from).