IAU2006

The previous post was the last one for the “Convection in Astrophysics” symposium. Note that some of the posters are available on the conference website as pdf files. A few more posts for the “Binary stars” symposium will follow.

I will not comment on any particular of the hundreds of posters presented during this week (of which the majority are presented at the Binary Stars Symposium). I only took a brief walk through the poster rooms of the Convection and Binary Stars Symposia and took some of the few A4 copies that were left. In general, one could say that the posters got less attention than deserved, at least in the Convection symposium, since there was not a single session dedicated to posters and the poster rooms were located far away from the coffee tables. But pdf files of the posters will be made available on the conference web page. In the Binary Stars Symposium there was one session with poster highlights each day, allowing the poster authors to make short oral presentations. Many of the posters (not presented orally) were about one particular binary system.

I noticed that for my own poster I had supplied too few A4 copies, they were all gone after a short time. You can download an A4 pdf file of my poster, presented at both the Convection and Binary Stars Symposia here.

There are some big antennae around that are not used anymore, because communication has been replaced by other means. Making telescopes out of them requires expertise which not necessarily available. For this project in Peru, they collaborated with japanese astronomers.

The tansformation of this satellite communication antenna shall start radio astronomy in Peru, create radio anstronomers by gathering knowlege and of course promote international collaborations. The antenna is good enough to go up to 2.2 GHz and the site is high up, remote and has good conditions. The location on the globe also makes it interesting for Very Long Baseline Interferometry. They have a working reciever and are well underway.

I just tried to convince a collegue, who will be here during the next week, to continue what I started. I’ve said it before: If you yourself are an astronomer at this meeting and are interested in blogging here, please drop me an email (thomas.marquart (at) astro.uu.se) and I’ll add you to the list. The technical part is easy.

A very apt title, the topic of downsizing have been discussed in almost all of the talks in the galaxy session so far.

Five roads to downsizing: – Massive red and dead galaxies – Mass density evolution – Mass-segregated SFH – Abundant post starburst pop at z~1 – Evolutio of mass-metallicty relation

R.A. worry that this might be a “bandwagon” that everyone is jumping on. Galaxy evolution sweetspot is at z~1-2, highest derivative of mass assembly is at this time. Playing devil’s advocate he finds that if the z~0 points of mass essembly is correct, 50 % of massive galaxies are from between z~1-2. But this might be a problem of large errors in the derivations.

Use ACS observations of galaxy morpholgies at z~1-2 to investigate how the mass in stars have changed from 2 to 0. When doing this you have to make sure you’re going deep enough, that you have a sufficiently large area (should be larger than HDF) and to misapply the assymetry vs concentration diagram (S/N or completeness problems perhaps, not sure I got that, it could also be a question of which filters are used?).

How to measure morphology? R.A. thinks that concentration is not the best way to do this, rather use a more general statistic => Gini statistic/coefficient. This coefficient seems to be more robust than the concentration parameter. He finds that at z~1 about 70% of the stellar should exist in the early-type galaxies and that there is strong evolution in this fraction in z~1-2. Another conclusion is that mass density evolution of early types + assymetric early types is similar to the evolution of post-starbursts at z~3.

I missed some talks in the morning, but sleep is important, too. I came in time for the last three talks of S237 on “Triggered Star Formation in a Turbulent ISM” and all three were interesting.

C. Norman was talking about his theoretical work on disk simulations and what struck me a little was that people have enough confidence in the simulations being a good approximation for the real world, to do physics with “observations” on the simulated world and derive properties and laws from there. Of course, this is tempting since one has complete control over the simulation and can get much better “data” than in the real world. There are tons of arguments and tests that reassure and convince people that this really works and, indeed, why should a program that implements a well-understood physical process fail, except for (also well-understood) limitations like resolution and other approximations. But still, if one could observe it, one would not need simulated data, so since we cannot, there is also a lack in testing the simulations to the real world.

Using the density distribution function of the ISM (log-normal except at very low densities) and a scaling relation for the critical density where SF sets in, Norman showed an alternative to the Schmitt-law (also called Kennicutt-law) that has a more shallow slope and flattens out at high gas surface densities. The observed slope then has to be understood by a change in SF-efficiency, which offset the model with respect to each other. Since the observed correlation is very tight, this would mean that also SFE correlates tighly with gas density.

The next speaker, M. Krumholtz, started from simple arguments to understand why SF is so inefficient in the sense that the SFR would be 50 higher (both in the Milky-Way and in extreme cases like Arp220) if all the molecular clouds that are present would collapse and form stars. I cannot reproduce the whole line of argument now, but he also used the density distribution (depending on mass and virial parameter) in the turbulent medium and integrated over the region above the critical value to get SFRs which fell into the observed regime. This could be tested, if the census of molecular clouds in nearby galaxies would stretch to smaller masses, by simply comparing the predicted SFR as derived from the molecular gas content with the observed one.

The last talk and summary of this symposium was given by B. Elmegreen and he again stressed the importance of turbulence for SF, instead of the older picture of monolithic collapse of a molecular cloud.

In the afternoon, there will be a “poster session”, which basically means that everyone who has a poster, stands by it to answer the questions of the people coming by. The problem is that, although not all have posters, many do and one has to find the balance to also look at the other posters and talk to the author, but that won’t work if they just do the same thing. Surprisingly, it always works out somehow anyway and I am happy that they scheduled this also for S235 for which no poster session was planned initially.

This is the final talk of Symposium 235 on Galaxy Evolution, but there will be much more other things going on tomorrow that are worth writing about.

Downsizing was mentioned frequently during the last days and in the presented survey, red, emission-line-less cluster-galaxies are used to measure the “fossil record” of galaxies, i.e. the old stars. The thousands of spectra are sorted by velocity dispersion (a measure of the total mass) and stacked together to get high quality average spectra with many spectral features that can be analysed to get ages of the stellar population.

Tey find that the smaller galaxies have smaller ages, i.e. downsizing. The age-spread is much larger at low masses than at the high-mass end. There was no morphological selection but of course it is ellipical that dominate the sample. The S0-type galaxies are slightly younger than Es with the same sigma, but this trend is weaker than the trend with sigma itself.

Comparisons with the total dynamical mass (also using Sauron-data) there is little room for dark matter (25%).

The ages also correlate with environment, i.e. distance from cluster center (16% change). Again, this is not a strong trend. Metallicity does not show a trend, but alpha-enhancment does. The tilt of the distribution in a color-agnitude diagram comes half from ages, half from metallicity.

By calculating backward, how te CMD would have looked for these galaxies at some earlier time, they can be compared to CMDs at some redshift.

When do galaxies merge? The merger fraction does evolve lowly up to z=1.2 but at around 2-3, 50% of all high-mass systems are mergers. The small ones again have only slightly higer merging rate. So at z=1 most of the high-mass objects were in place.

Using the same methodology to find mergers on numerically simulated data (with C. Mihos), they derive absolute merger rates (per volume) and a sharp drop after z=1 is found for all masses. A typical massive elliptical galaxy (today) will have undergone 3-5 major mergers since z=3.

A significant fraction (maybe the majority) of SF at z

By looking at the Hubble Ultra Deep Field (UDF), one can classify galaxies by how they look. The number of clumpy and irregular looking galaxies increases as one looks at further and further distances. Disk galaxies seem to disappear at a certain redshift and only thick disks are found.

Clumpy galaxies seem to be more frequent at high z and it is basically the large star forming regions that are seen there. These clumps should dissolve and could build up a normal spirals. Indeed the “clump clusters” share several properties, altough they are less massive. The scale height of “clump chains” is found to be 1kpc, which could be connected to forming a thick disk.

A usual problem here is that one looks with a fixed set of filters (opitcal in this case), but due to the redshift, one looks at different wavelenths inside the galaxy. In this case, one admittedly only sees the regions that actively form stars and a much smoother underlying population of older stars would not be seen.

Thousands of UV-selected galaxies at z>1.5 with spectroscopic confirmation from Keck. 25% contain AGN. From clustering length (4 Mpc) the DM halo mass is derived to roughly 10^11.5-12 solar masses and these objects are presumably the progenitors of nowadays ellipticals (by following halo-evolution in simulations).

The highest X-ray detected cluster is at z=1.45. The speaker and collaborators find protoclusters at z>2 also from UV and measure/find the overdenities (factor 7) in a redshift subslice. The galaxies there have double stellar mass than the ones outside the cluster. They find the morphologies not to fall on the Hubble sequence, but I wonder if they took into account that even normal galaxies look very different in different wavelenth, especially in rest-frame UV which the HST images were made in, if I got it right.