Thursday, August 17, 2006

M. Hudson: "Downsizing" from the fossil record

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.

O. Gnedin: The formation of dwarf galaxies and small-scale problems of lambda-CDM

The Lambda-CDM cosmological model works very well in predicting large scale structure. It however predicts many more dwarf galaxies than are found. This is called the "missing satellite problem" and it is not just a few missing, but it should be ten times more.

The solution may be twofold: fist of all, only the more massive of satellite halos may be able to retain enough gas to form stars and thereby are seen by us. In addition, subhalos evaporate due to tidal forces, once they come close to "their" big galaxy. It has been known since long from studies of the Local Group that different types of dwarfs live at different radii from the large galaxies and there seems to be an evolutionary connection.

A new method of measuring the DM-halo of the Milky Way are hypervelocity stars that move at 500-1000 km/s with respect to us. They probably have been slingshotted by the black hole in the center of our galaxy. By following and calculating the paths of these stars, the shape of the DM-halo can be determined and according to CDM, it should be triaxial. This test will yield first results in a few years and it is a good test of predictions from lambda-cold-dark-matter cosmology, which is the widely accepted picture of our universe.

C. Conselice: Galaxy Interactions and Mergers at High Redshifts

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<1 is produced by interactions and mergers. I think this is last point is still debated and there have been contradicting results, e.g. showing that interactions dot not really increase SFR as much as one would think.

D. Elmegreen: Clumpy Galaxies in the Early Universe

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.

R. Bouwens: Galaxies Buildup in the Frist 2 Gyr

UV-luminosity functions at z=4,5,6 are presented and no evolution is found at the low mass end and the slope is steep there (-1.75). However, at the high-mass end things get brighter with time. This is the opposite of downsizing that I wrote about yesterday.

Converting this to the Madau plot means that it peaks around z=4 and declines towards 5 and 6. This is heavily debated since the dust-correction at high z is fairly uncertain.

Going even further (z-J), they found 4 candidates of z~7-8 galaxies (Bouwens & Illingworth, Nature 2006).

T. Wilkind: Massive and old galaxies at z>5

If galaxies form hierarchically, i.e. big ones form by the merging of small ones, shouldn't big galaxies then appear rather late in the history of the universe? One cound think so, but would be mistaken. Tommy repots on their finding from last year of a massive galaxy at z=6.5 that is red in color and show no ongoing star formation at all.

Is the presence of such an object that has finished forming all its stars at so early times a threat to the lambda-CDM cosmological model? This first of all depends on how many of these really exist. They look in the K-selected GOODS-south sample and find 18 candidates out of which 5 had to be discarded as being something else.

So they have 13 galaxies at z>5 with over 10^11 solar masses in stars and no ongoing star formation (although 50% are detected in 24micron). Correcting this for completeness gives a rather high number density which indeed opposes the lamba-CDM paradigm (too many as compared to existing DM-halos at that time), unless these estimates are either flawed in redshift or stellar-mass-estimates.

M. Steinmetz: Cosmic Web - Simulations

The "Cosmic Web" is the structure that arises in cosmological simulations and that is also observed: the universe is clumpy and most of the matter is in huge filametary structures that are made of and connect galaxy clusters.

In simulations the control of the dark matter is much easier than normal matter since it only interacts by gravity. With normal matter, one needs recipies for handling star formation, hydrodynamics have to be taken into account.

Very important: he shows in simulations that a merger may actually look like a disk in kinematical data and he warns the people around Genzel who find "rotating disks" at high redshift. I have to find that movie/paper on the web.

Simulated disks nowadays however seem to fit nicely wit observed ones when it comes to angular momentum. A comparison of a merger with and without AGN feedback is shown and it helps in the sense that otherwise simulated galaxies are too centrally concetrated as compared to real ones.

To test if angular momentum is induced to disks by tidal torques from the cosmic web, it is possible to check the orientations of disks. Indeed there is a correlation between the large scale structure and the orientation of disk galaxies.

The Milky-Way dark matter halo seems to rotate with around 100 km/s as derived from halo star kinematics. But I might have gotten this part wrong. :-)

L. Portinari: Cosmological formation of disk galaxies and the Tully-Fischer relation

The Tully-Fischer relation (TFR) for disk galaxies relates the absolute luminosity to its rotation speed. The angular momentum in simulations however is difficult to match with observed values and this is most probably due to the simplified treatment of baryons and thereby, again, feedback.

The question if the disk forms from a cooling flow from hot gas (at virial temperature) or by cold accretion is adressed and X-ray observations can give important clues here. Birnmoim & Dekel 2003 found less than 10% of the expected amount of hot gas, so cold accretion might be favorable. Gas does not need to be heated to virial temperature and there can be cold gas accreting along filaments.

They found an offset in the TFR for certain models but it was hard to grasp which objects they were, but one solution is claimed to be dynamical friction, i.e. the galaxy rotates slower than it should for the same luminosity.

Portinari et al. 2006 look at the evolution of the TFR from z=1 and find no significant mass evolution, while the individual objects gets to almost twice its mass.

A. Shapley: Galaxy Formation in protoclusters at high redshift

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.

M. Franx: Properties of galaxies at z=2-3

Between z=2-3 presumably many galaxies build the bulk of their stars and one has to have control over sample properties and selection effects.

The authors and collaborators select galaxies in rest-frame optical which means deep NIR-imaging with VLT in that case (MUSYC-survey). They place a mass limit at 10^11 solar masses. At this massive end, the red galaxies dominate the population (77%) already at that time.

But these red galaxies are not "dead", but still show significant dusty star formation. In the U-V over V-J diagram, a large part of the population lies below the local population.

Clustering correlation length correlates with J-K color (Quadri et al. 2006) which means that redder galaxies ar emore clustered.

F. Walter: The first galaxies and AGN

Quasars are not found at very high redshift. The record holder has been at z=6.4 for quite an amount of time and at this redshift, the universe was about 870 Myr old.

A fun fact with redshifts is that you observe different wavelengths in objects when use use a certain wavelenth for observations. Because there is a large peak of emission from dust in the mid- and far-infrared, this gets shifted into the mm-rane at high redshifts and thereby, an object that is much further away may not appear fainter than a closer one at all.

Molecular emission has been detected to redshifts over 6 as well and of course only the highest concentrations can be detected at these large distances. One can get hold of the ionisations state of the IGM via the proximity effect, which presumably is due to a large ionised sphere (formation time=10^7 yr * neutral gas fraction) and lets emission a little blueward of the lyman-limit at the quasars redshift escape.

S. Silich: Super-massive star clusters: from superwinds to cooling catastrophe to the injected gas reprocessing

How quickly does the gas that is heated by the SSC cool? The larger the cluster the more radiative cooling (I did not understand why) and eventually there is a regime of "catastrophic cooling" where no equilibrium solution exists any more.

Studies in M82 which blows a huge bipolar wind perpendicular to the disk (pretty picture) show that .... I missed it because I fetched the link to the picture. :-)

G. Tenoio Tagle: On the negative feedback from SSC

Instead of acting positively, feedback cann be negative in the sense that SF is suppressed by the strong winds that go out from SSCs. The speaker shows simuations of the wind and radiation that makes its way through a region of dense clouds. Wherever there is a gap and the wind can break out, it blows a bubble of hot gas out of the galaxy.

P. Kroupa: Cluster Formation and Dissolution

In last talk it was mentioned that 90% of all clusters in the Antennae get disrupted immediately and onlt the big ones will survive anyway, because it's only them who have a deep enouhg own potential well to retain the stars.

The speaker starts with cluster formation and talk about the simulations by Bonnell et al. which I just recently had included in my talk for a course back home. The efficiency in cluster formation is below 40%, which means that more than 60% of the gas in the cluster volume does not go into stars. When this gas is removed by radiation from the massive stars, the cluster suddenly loses this mass, becomes super-virial and therefore the cluster stars move quickly to larger radii. The cluster not only evaporates, it "pops".

The speaker argues that the "popping" of clusters might even account for the thick disk of galaxies, but I dont't really believe that, because, if I remember correctly, it is more likely that it is the infall of dwarf galaxies that build up the thick disk. It was just commented on this in the question session: the velocity dispersion of a smal thing is always smaller than that of the ISM itself.

B. Whitmore: The life and death of star clusters

Stars generally form in clusters and in a starburst, there can be super-star-clusters (SSCs) that make the ones still forming in our Milky Way and also 30 Doradus in the LMC look rather pathetic.

The speaker talks about the clusters in the Antennae and by age dating them one can trace the different encounters of the two galaxies, during each of which SF is triggered. Maybe surprisingly, the kinematics over a cluster region seems to be smoothly rotating with small velocity dispersions. So one does not need high velocity "smashing in" of stuff, the increase of pressure seems to be enough.

Looking at different regions, it can be seen that around a massive older cluster, newer ones are found, the formation of which was triggered by the wind from the first one. This depends very much on density: if the wind has "free way" on one side, not much will happen there, but if it runs into some dense material, this gets compressed and can form new star clusters. This is what I meant yesterday by "positive feedback".

A major point that I should have mentioned is that what makes clusters so interesting to study, is that one can be sure that all the stars inside formed at the same time. So it is possible to just treat the as single objects and determine ages and other properties for each of them. And of course they are much brighter than a single star and can be studied at larger distances.

The IMF of SC is a power-law over many scales which means that low mass clusters are much more frequent than big ones. The speaker argues that there is no special "burst-mode" of star formation, but that the really big SSCs are just the tip of the whole IMF and it is only statistics that make this tip populated only in regions where you have a lot of star formation.

Taking Picture

Im sitting in S237 and someone just tries to motivate people to go the big hall during coffee break to take a picture of the S237 attendants in from of a bis IAU logo.

This will never work. :-)
Coffee breaks are sacred to people.

Social Events

There is a whole schedule of social events that are organized around the meeting, mainly it's sightseeing. I do not know when people find the time for this, but I would guess the ones during the week-end are the most popular.

I don't think, I'll join one of these and I am not even sure if I will make it to the concert on Friday evening since I have to move to a new place. You might have heard of the Hospitality Club, anyway, that's how I'm going to stay over the week-end before I fly back home on Monday.

I somehow doubt that someone will show up and take over this blog to report from the second half of the meeting next week, so I think this blog will close already tomorrow...


This morning, S237 on "Triggered Star Formation" seems to be more interesting than S235 on galaxy evolution. On the one hand, it's a pity that potentially interesting sessions overlap. On the other hand, choice from a larger variety surely isn't bad.

The talks in S237 will be about star clusters.