Monday, August 14, 2006

M. Tosi on IZw18

The full title of the last of today's talk is Izw18, or the Picture of Dorian Gray: the more you watch it, the older it gets. This is again about the object mentioned in the last talk (and blog entry) and discusses the case of the best candidate of a "young galaxy" in the local universe, and thereby being a counterpart of what has happened earlier in the universe.

A possible explanation of the low metallicity of the interstellar gas is that winds from supernovae blow out the metal-enriched gas and thus rid IZw18 of the metals it has produced.

There are AGB-stars detected in Izw18 (partly by my supervisor G. Östlin) and therefore the age must be at least 500 Myr. In addition, other groups (Momany 2005) have found an RGB and thus lifting the age to 2 Gyr.

Tentative results by Tosi calibrate the distance to IZw18 more accurately, thereby fixing the y-axis of the HR-diagram and determining if it is AGB or RGB stars one has found. It seems that indeed stars with an age of 1.7Gyr or older have been found.

D. Kunth: "Are the most metal-poor galaxies young?"

This is interesting for myself since it relates to the galaxies I work with, Blue Compact Galaxies (BCGs). "Young" means that it forms its first generation of stars. Only two nearby galaxies with metallicity of around a 50th of the solar value are known: IZw18 and SBS0335-052, two BCGs.

But even if we trust the metallicity determinations, it remains the question if metallicity acts as an "arrow of time". Especially, a young galaxy does not need to have low metallicity since the gas may be preenriched during the denser early phases of the universe, where also SF was stronger.

In the luminosity-metallicity relation (more luminous galaxies are usually more metal-rich) the objets mentioned above fall out of the line, since the strong short burst of SF only boosts the luminosity, while the metallicity stays the same for a longer time, because intermediate-mass stars have not yet had time to release their metals.

The claim of these galaxies being young galaxies has been challenged many times and indeed, a weak old stellar population can be found in most of the galaxies, presumed to be "young".

E. Skillman on "Age-metallicity relations in Dwarfs"

How does the metallicity of stars in dwarf galaxies behave with their age? Simply measuring the age and the metallicity does sound straight forward, but is difficult to carry out. It is expected that stars produce metals and enrich the galaxy in metals, but does the expected enrichment from our knowledge of stars match with the observed values?

Calibrations from the LMC (Cole et al. 2005) show a quick increase in Fe/H in the beginning, but not big evolution during the last 5 Gyr. This of course depends on the star formation history and in the dwarf-spheroidal galaxy Leo I (or was it II?) which shows significant recent SF, the metallicity only went up after this. Enrichment that follows star formation is also found in local group dwarf-irregular galaxies.

Talking about the gas content is difficult, since the history of gas infall is not determinable, except by assuming that gas infall is correlated with the star formation rate.

C. Maraston on "Stellar population Models"

Stellar population models are made up from models of individual stars by summing up the spectra of a distribution of stars of the same age and doing this at different timesteps, taking into account the evolution and lifetime of each type of stars.

This means that different types of stars dominate the light at different times, e.g. it is RGB stars that account for the bulk of light in old populations. To get this modelling right, it has to be calibrated against globular clusters, the stars in which are known to have formed together at some point. An alternative to this approach is using the fuel consumption theorem.

Newer models that take more physics (delayed RGB-onset) into account make the agreement with the spectra of high-z better and arrive at younger ages and smaller masses for these objects. Still, the age determinations are very insecure in some cases.

G. Gilmore about "Population Models"

Gilmore reminds us that although previous speakers have talked about dry mergers and anti-hierarchical evolution, this certainly is not true for less massive systems. In addition, the secular evolution of bulges (pseudo-bulges) may not be the whole picture, since the Milky-way's (MW) pseudo-bulge has mainly old stars, as derived by their alpha-over-Fe ratio.

He goes on to talk about the main stellar populations: PopI in the thin disk, an intermediate Pop1.5 in the thick disk, PopII in the bulge and a not-yet detected PopIII of near-primordial stars. The thick disk is surprisingly homogeneous in element ratios which indicates a coherent build-up over a rather short timescale.

There is significant structure in the outer parts of the MW and recently several more streams are found around our galaxy, that origin from disrupted dwarf galaxies that are about to be "eaten up" and the model predictions for the different stellar populations are do not really match the observations.

R. de Grijs: "From nuclear clusters to halo globulars"

Star clusters as basic building blocks of galaxies. Most of SF is believed to happen in dense clusters. The initial mass function of stars is also believed to depend on the density of the environment.

Young clusters show a power-law distribution while old globulars show a gaussian, how come? He an collaborators did work on intermediate-age clusters in M82 and found a turnover at lower masses. So very many clusters must dissolve on a very short timescale in a very compact region - or the initial distribution was not a power-law.

Review Paper: de Grijs et al 2003 MNRAS 343, 1285
Newer stuff: Ma, de Grijs, et al. 2006

E. Taylor on "Star formation and the non-existence of dark galaxies

Do all DM halos have galaxies or can you somehow prevent star formation? Some galaxies form very little stars, but have much gas around them. HIPASS is a complete census of all neutral hydrogen over the sky and it found no dark galaxies.

But why do all galaxies form stars? Cooling is the key here. To cool further down than 10^4K you need H2 -cooling to get the temperature low enough to have disk-instabilities an initiate star formation. In their simulations they set up thngs to favor dark galaxies, but all but one of their models does not form stars and there is a minimal star-formation-rate (SFR) which in turn keeps the HI-could stable by heating it up again. This means star formation is self-regulated and it again boils down to the Schmitt-law of SF.

But the major point against dark galaxies still is the non-detection in HIPASS, where about 80 detections were expected.

W. Ho about "the relation between galaxy propteries and the dark matter halo"

Galaxies residein halos of dark matter (DM) which form from initial density fluctuations. They are believed to well understood since they only interact gravitationally and can be simulated. But how do galaxies form in DM halos? Does every DM halo have a galaxy? How many galaxies are there in large halos as in galaxy clusters?
How does the mass function of MD halos translate into the luminosity function of galaxies? They do not really match well, but the "conversion", i.e. the conditional luminosity function (CLF) can be well constrained by theory and observations. Clustering in dependance of mass also comes in here. He assumes this CLF to have a Schechter-form and one outcome is that the luminosity per halo mass is not linear at all.
Once having the CLF you can use it to derive other properties. For example that the most light comes from halos of round 10^12 solar masses, or that the halo-distribution of L-star galaxies can have a quite wide distribution in halo mass.

E. Ensellem on "Spheroids ages, knematics and BH-relation

He talked about a lot more but just this one point: The feedback from AGN is a nice way to shut-down star formation and to explain the BH-Bulge-mass relation. It goes also into explaining the bimodality of galaxies, but the physical motivation for this mechanism may not well be well-grounded yet.

Monday morning talks

This mornings talks were about scaling relations of galaxies and the formation of bulges and disks. SOme interesting points were raised, e.g. that the bimodality of galaxy distributions in a color-magnitude diagramm can rather be seen as a bimodality in Bulge-to-Dike ratio than in galaxy type itself. Should we finally abandon the Hubble "tuning-fork" of classification? (Talk by S. Driver)

Rather than catching up with the past talks, I will try some "live-blogging" from now on.


So, we have arrived in Prague and stay 5 minutes by foot from the conference center which is quite a big thing. Many astronomical events happen simultaneously, but I mainly attend the Symposium 235 about Galaxy evolution. The lecture ahll is not the biggest one available, but easily fits the roughly 250 attendants. Have a look at the hall and the view from here over Prag at my first pictures. More to come...