Wednesday, August 16, 2006

Over and out

That's it for today. More to come tomorrow.

Time for food and some of the good czech beers.

F. Fraternali on "Gaseous haloes: linking galaxies to the IGM"

Spiral galaxies have lots of gas around them. And the space between galaxies is not totally empty either: there is the inter-galactic medium (IGM).

The speaker presents very deep radio observations of NGC891 and, not surprisingly, finds gas t large distances from the plane. They find filaments and small counter-rotating clouds. In another case (NGC2403) they see infall of gas in the position-velocity-diagram.

They model their observations assuming hydrostatic equilibrium and the things that not not fit yet, may be resolved, when the interaction of the cold gas with the hot halo gas is included.

The IGM that was in the title, was however not mentioned very much.

G.L. Granato on "Mutual Feedback between SF and nuclear activity"

More on feedback, but while the last talk was mainly about feedback from stars, there are also AGN, i.e. supermassive black holes (SMBH) in the centers of galaxies, that radiate very brightly, when mass falls into them.

There are beautiful simulations by Di Matteo and Springel from last year, where they compare the merging of two disk galaxies with and without feedback from the quasar and find it to differ significantly. Text, images and movies here.

The AGN feedback mechanism can at the same time explain the relation between the mass of the black hole and the bulge of the galaxy (because it's the feedback that shuts down both BH-growth and SF at the same time) and the quick reddening of galaxies because the decline in SF is so quick.

As in stellar feedback, it is not clear how exacly and how much of the engergy is deposited in the gas. Radiation pressure, heating and kinematical flows are the candidates.

Much SF promotes the formation of the SMBH by producing the seed black holes and also by pushing material into it. It takes around 500 Myr in Granatos models for the SMBH to form and feedback from it to kick in and have an effect on the surroundings, cleaning it from gas. This is what we see as quasars at high (but not highest) redshifts and this phase is rather short. What is left afterwards is a "red and dead" galaxy that does no longer form stars.

As main conclusion, Granato stresses the mutual link between SF and the AGN that has to be included into models. The comparisons of his models with observations were in impressive agreement, but I have no idea how many free parameters are fitted here. This was just answered in the question-session: it's around 6-7.

G. Hensler on "Feedback from Star Formation"

As mentioned before, the energy release from various sources (mainly stars and AGN) is called feedback and it can act positively, enhancing SF even more, or negatively by shutting down star formation (SF). To understand all this, one needs to understand how stars work and how the ISM behaves.

Feedback takes place at all scales, from small (e.g. HII regions) to large, for example global outflows from galaxies and galactic winds. By feedback, SF is self-regulated in the sense that little SF can trigger more SF, but once you get too much, positive turns into negative feedback, preventing further SF.

This self regulation manifests itself in the Schmitt-law (famous paper by Kennicutt), stating that SF is proportional to the surface density of the gas to the power of around 1.5. The Toomre-criterion must be fulfilled to have SF at all and this accounts for a cut-off of SF at low surface densities. This self-regulation comes automatically, even in simulations and the Schmitt-law is nicely reproduced.

The efficiency of SF, i.e. what fraction of gas is turned into stars, depends strongly on the density of the ISM and in starbursts both can be very high. One uncertainty in studying feedback is how much if the released energy (e.b. by stellar winds) is actually deposited in the ISM and how much escapes by radiation. The values for this effficiency vary between one percent and one permille.

a. van der Wel on "The IR Properties of Early-Type Galaxies at z=1"

The rest-frame K-band mass-to-light ratio has increased by a factor of three since z=1. The one for B even slightly more. Even though it's are early-type (elliptical) galaxies that are studied, there seems to be 10% of young stars (not sure if I heard that correctly) in the z=1 sample. There is however no obscured SF or contribution from AGN.

Dynamical mass estimates are fine at all redshifts, but if one does SED-fitting in the optical and the IR, the derrived masses are higher by a factor of two to three in the IR, as compared to optical. This has implications in how well we know the global stellar mass density and how it involves

C. Martin on "Galaxy Evolution with GALEX UV Surveys, Spitzer and SDSS"

This is not the same C. Martin as two talks ago. Then it was Crystal, now it's Chris. He argues that the blue part of the bimodal galaxy distribution (in the color-magnitude diagram) is not a "cloud" but a sequence that is only blown up bt the spread in age of the stellar population.

So how does a galaxy move from the blue to the red sequence? He showed models of transitional colors and mass fluxes between the two regions, but I did not really understand this.

From several samples of galaxies and by looking at the IR-excess, he finds that, as expected, the SFR in high-mass systems has declined a lot sinced z=1, however, it has remained constant over this time in low-mass systems. Again, this is evidence for downsizing.


I found my lost USB-cable at the "lost & found" counter and therefore, there are some more pictures from yesterday afternoon in the gallery.

R. Terlevich on starbursts and downsizing

How does the evolution of galaxies depend on their mass? It's known that small galaxies seem to evolve slower than large ones. This is called downsizing and may seem counter-intuitive if one thinks that galaxies form hierarchically, but it is not. It simply stems from the fact that regions of higher density (like clusters) evolve quicker than low-density regions, where mergers are much less frequent.

Terlevich reconstructs the evolution of H-beta equivalent width from observations and finds a large discrepancy with models which he attributes to the fact that the continuum contains not only the newly formed stars, but also the old stars and thereby the whole star formation history.

Generally, he also finds that more luminous galaxies are older and that more metal-rich ones are older, too. No surprise here, but the correlation seems to be quite tight.

C. Martin on "Starburst Feedback"

With "feedback" all kinds of energy input is meant. Feedback mainly comes from supernovae and from active galactic nuclei (AGN), that can significantly dirsturb the interstellar medium of a galaxy and produce outflows.

Again, we talk about ULIRGS and the ones which have the U for "ultra-luminous" are indeed all mergers. Blue-shifted absorption lines show that these outflows really exist and move at several hundred km/s. Very interesting is one example where they find the wind over the whole galaxy, but it still rotates, which it should not. She explains this by the wind being driven by the whole disk, but I do not see how that should work right now.

These global winds can be driven both by a starburst and AGN, but the actual mass in the wind is weakly constrained. In addition, she claims that there is little evidence that AGN can provide enough energy to shut down star formation by removing the gas, a mechanism that is becoming more and more popular.

C. Cesarsky on "The role of LIRGS in galaxu evolution"

I shortly mentioned (U)LIRGS being associated with mergers before. They are very bright in the near and far-infrared and dust emission plays a major role here. Indeed, they must have been much more usual in the earlier universe, since the IR-background which is weak in the local universe, but reaches or even succeeds the optical background radiation at higher redshifts.

By counting galaxies in different wavelengths and at different z, one can determine how the galaxy population has changed over cosmic time. A strong evolution is found and LIRGS are found to be frequent enough to account for all of the IR-background radiation.

Not all of them are irregular: 36% of LIRGS are instead found to be disk-like galaxies. At the rate that LIRGS are forming stars, they would double their mass (in stars) in less than 1 Gyr. But they only do this temporarily (for about 100 Myr) in the LIRG phase. This phase can reoccur and each time about 5-10% of the stellar mass is added.

Catherine Cesarsky, by the way, is the director general of ESO.


I just boldly claimed (last post) that communicating with the public is important and maybe even this blog is little contribution to that, or at least I would like to think so. Nevertheless, I am well aware that what I write here may sometimes be cryptic to non-astronomers. I am afraid this is unavoidable, because there is no way to write as much and at the same time write the background to make it publicly understandable. Writing popular science articles is hard work and I have little experience with it.

Anyway, I'd like to hear your feedback on how you perceive this blog, so please drop a comment.


Leaving S237, I just had a quick look into S236 about "near-earth objects" and the speaker talked about "rotational fission". There is much more interesting science in those rocks that fly around in our solar system than one could guess, but these topics are probably as far from my field as you can get inside astronomy.

After that, I wanted to have a look into the special session on Astronomical Facilities of the Next Decade. This was held in a too little room and people were already standing by the entrance. I think the speaker talked about the Square Kilometer Array (SKA), but I immediately left again. You always hear things about the future main telescopes from several sources anyway and there is little you cannot find out from the corresponding websites.

I went on to a session about Communicating Astronomy with the Public which is an immensely important topic in my opinion. There were only 20 people present and I just bumped into the questions of the last talk before lunch. It must have been about some software that faciliates the creation of pretty images. The speaker was questioned from several attendants, why this plugin-software works with (expensive and proprietary) photoshop only and not with the free (and open source) alternative GIMP. Since their software will be open-source, too, they were confident, that some funding for a port to GIMP will be found. I liked that this point was stressed so strongly and I think it is fair to say that the astronomical community is well aware of open source and using it heavily.

Triggered Star Formation

I just went over to another hall, where the S237 on "Triggered Star Formation in a turbulent ISM" takes place. As far as I can see, this mainly contains studies in the Milky way and the Magellanic Clound, where one can get much more detailed information than in distant galaxies.

Righ tnow, it is H. Beuther speaking on "Physics and Chemistry of hot molecular cores". Observations of the well-known Orion nebula are presented and sub-mm data seems to show that there is no massive compact source in the hot core itself. Molecular lines are studied here, Methane seems to be important, and I suddenly realise, that although this field should be closely related to galaxy evolution, I have very little clue what is going on here.

I think I'll continue wandering...

J. Palous on the "Star-gas cycle in galaxies"

Stars form from gas. Big stars explode and throw out gas, thereby enriching the gas with heavy elements. The gas forms new stars. If you have many stars, they can blow winds out of galaxies.

I admit, I did not listen very carefully to this talk. :-)

M. Dopita on "Quiescent Star Formation throughout Cosmic Time"

Starbursts have been mentioned in previous postings here. But is there also a significant amount of quiet, non-bursting SF going on? If not today, has it been existing at earlier times?

The first stars that formed in the early universe have probably been very massive and quite different from todays stars, mainly because heavy elemets are needed to cool more efficiently and this is not in place before the first stars. The IR-background and also the neutrino flux are possible means of getting a hold on this remote epoch.

Different tracers of SF measure different things, e.g. UV-continuum (GALEX) picks up older SF-regions in galaxies than the emission lines (H-alpha, [OIII]), that trace the young HII-regions (2-5Myr). Again, the attenuation from dust cannot be stressed enough, especially in the densest regions.

At low redshift, the different SF-tracers seem to agree quite well. But it is known that SF has declined by an order of magnitude since z=1 and conditions have been quite different and local calibrations are not good any more. The specific SFR peaks between z=6 and z=1 and that is the time when galaxies are mainly assembled. High-z sub-mm galaxies are real global starbursts with extreme SF-rates of several thousand solar masses per year. These objects are on average 100 times more luminous than nowadays galaxies.

The question raised in the beginning was not overly much addressed, but one of the main conclusions is that SF was all but quiescent during cosmic history.

Making Choices

So called "Joint Discussions" and "Special Sessions" outside and in paralled to the large symposia start today, but it is not before tomorrow, that I (and many others, I suppose) will have a hard time to choose, what to listen to.

The organisers and chairmen have done a good job so far to keep the schedule. This is essential if one wants to switch between lecture halls and events without losing time. If you want, you can have a look into the Progam Book on the web, which is the same that we got here as paper copies on arrival, together with loads of material in a shoulder/laptop-bag.

About this blog

To answer the question that came up: I wrote earlier that this blog is temporary and of course it is in the sense that the meeting only happends during this and next week. There will be no more postings after the meeting, but there is no reason to dispublish this site. It can sit here as long as Blogger exists...

I also want to apologize for language or spelling errors in the texts. The talks are coming at fast rate and I just type away. I also have not yet found the time to answer questions that were raised in the comments - but I will eventually.

L. Tacconi on "Spatially resolved kinematics of z=2-3 galaxies"

When studying distant galaxies, their spectra are shifted redward. For example, the strong optical emission line H-alpha is shifted to the infrared when observing at reedshift of 2. Emission lines can be used to measure the movement of the gas in galaxies with the help of the Doppler-effect. If you now have an instrument that measures this at each point in a galaxy, i.e. if you have a spectrum at each pixel of your "image", you can study "velocity fields" of the kind that I also have on my poster.

SINFONI at the VLT is such an instrument, working in the IR. The speaker presents very nice kinematical data of these far-away (z=2) objects and it is impressive how fast progress is going in this field. They find the objects to rotate and having disturbed morphologies. This may be not really surprising. A paper by Tacconi and collaborators (Genzel et al. 2006) that will be published in Nature tomorrow, will show the finding of a very interesting case with very high-resolution data.

V. Charmandaris on the IR properties of Galaxies

Starbursts, that is episodal strong star formation, happens in regions wiith a lot of dust. It plays an important role in the whole process of star formation, but one of its properties is: it blocks light.

Well, optical light is obscured but by observing infrared wavelengths, one can look through the dust. The speaker reminds us of the beautiful studies in the Antennae galaxy, where the regions that are dark and invisible in an optical image are the brightes in the infrared.

IR-luminosity is nowadays quite well calibrated as an absolute measure of the star-formation-rate (SFR) and has become more popular (than for example the H-alpha-flux) since the upcome of many IR-instruments during the last decade. In the mid- and far-infrared, the dust istself stars to glow due to (almost) black-body emission and measuring the amount and the temperature of the dust in these wavelengths also contributes to the understanding of what is going on. As usual, it is all information together that gives the most complete picture.

(U)LIRGS are (ultra-)luminous-infrared-galaxies and you can guess why they are called this way. Arp220 is the most close-by of these rare objects and it is very well studied. ULIRGS are most likely to be a short phase during a merger event of two large galaxies when a lot of stars are formed.

Day Three

It is the third day and S235 on Galaxy Evolution continues.

A. Ferguson on "The Structure of Galaxies at Faint Levels"

Why does one want to look at the hard-to-study faint outskirts of galaxies? The stars in the halo of galaxies and tidal tails from dwarf galaxies give clues about how galaxies are built up. It is quite certain that large galaxies form hierarchically from smaller ones and since new star formation in the center makes relics harder to study, it is in the outer parts of the galaxies where one can look for evidence.

The amount and structure of tidal debris depends on the cosmological structrue, so modelling this with different parameters and comparing it to observations, can yield another test of lambda-CDM cosmology.

The problem is that this stuff is really faint and we are not talking about many stars. Only at surface brightness levels below 30 to 35 mag/arcsec^2, it shows up clearly, and these levels are rarely studied. The speaker presents a survey of the surroundings of M31 which shows a large stream of stars stretching to very large radii. While tidal streams indicate rather recent accretion events, but the stars coming from galaxies that have fallen in long ago have settled smothly into the halo.

Let me also mention that a collegue from Uppsala, Erik Zackrisson, studies the outskirts of Blue Compact Galaxies and finds a red excess in the near-infrared colors that is very hard to explain. Many explanations for this have been ruled out and what seems to be remaining is a bottom-heavy IMF, i.e. an unusually high number of small stars compared to big ones.