Anthony and Alexey would like to hold a discussion session in the next few days on the cluster stellar content. The topics proposed for discussion are:

  • Identifying cluster progenitors. How can we best identify / isolate cluster progenitors at high redshifts (i.e., how are the z=2-3 galaxies which end up in z=0 clusters in any way different from the general galaxy population back then?) What are the current observational constraints on the formation redshifts for the bulk of stars within z=0 clusters, and what are the prospects for improved constraints?

  • What observations / observational constraints can be used to link the high-z star formation with today's clusters?

  • How closely is the stellar content of today's clusters connected to the distribution of mass and star formation efficiencies at z=3? Can we develop a mapping algorithm between the two and use it to predict how f*(M) evolves with redshift?

  • How does the overall division of baryons between gas and stars evolve with redshift?

  • ICL -- constraints on the evolution of intracluster light, constraints on the total stellar mass in the ICL component from SN and direct observations

Andrey: I would add two big questions to this area:

  • What is the uncertainty (and its sources) in the estimates of stellar mass in clusters? Recent work of van Dokkum and Conroy indicates that IMF in the centers of BCGs is bottom-heavy and M/L rations may need to be revised up by a factor of 2-3. Could this be the case for a significant fraction of stellar mass in clusters? How would one go about testing this?
  • How does stellar fraction change with parent halo mass? How does this trend compare to the trend constrained for galaxies at smaller masses? At what halo mass does stellar fraction peak?

Results of our discussion on Thursday 02/03/11 (with update by Andreon on March 20th):

  • results of van Dokkum and Conroy suggest M/L ratio in I-band that is ~0.25-0.3 dex larger than that for Kroupa IMF. This is borderline consistent with Sauron results (see Fig. 17) for the most massive Es, but are not consistent with Sauron masses for smaller mass Es. It will be critical to assess whether their results extend in radius and to lower masses. It was noted by Ming that some of the systems in the van Dokkum and Conroy paper are relatively modest mass [note: the paper states all galaxies have dispersions >250 km/s].
  • However, van Dokkum & Conroy results would not change the baryon puzzle in clusters substantially. Anthony Gonzalez already uses M/L ratio close to that for Salpeter IMF (actually ~25% below, and based upon SAURON results) - this is somewhat lower than M/L for vDC IMF, but by not too much. With such M/L the baryons in clusters are still significantly below universal value within r500. Anthony et al. are working on improving M500 estimates for their clusters using XMM data. Andreon (2011) use clusters with accurately determined masses for a larger sample of clusters, and foud the same trend. Note, furthermore, that while Gonzalez et al. sample has many clusters BCG-dominated (and thus potentially affected by a selection effect toward high stellar masses at a given cluster mass), the one by Andreon (2011) is X-ray selected, and, therefore, if any, affected by an opposite selection effect (provided the total baryon fraction has a small cluster to cluster scatter).
  • It would be good to review and evaluate level of systematic error in measurements of f* and M500. A review is in Andreon (2011).
  • During the workshop it would interesting to assess how constraints on f* as a function of halo mass from halo abundance matching (e.g., Conroy & Wechsler 2009; Berhoozi et al. 2010) to direct measurements for galaxy groups and clusters (compiling the up-to-date collection of such measurements in the process). It is likely that these will not match (e.g., stellar fractions of central galaxies approach ~50% for groups, while abundance matching indicates that f* peaks at ~20-30% of universal at M~1e12 Msun), in which case we will need to understand the possible causes of differences.
  • It would be interesting to understand the physical origin of the trend of f* and fgas with M500 for groups and clusters. One way to do this is to assign M* to cluster progenitor halos at z~1.5-2 using e.g. abundance matching results for those redshifts and then combine their stellar masses, assuming no additional SF between initial redshift and z=0.
  • Given that high-mass end of stellar mass function does not evolve, but high-mass end of the halo mass function does, it means that mergers of stellar components of galaxies are highly non-additive. This can be understood if a substantial fraction of stellar mass in majoor mergers goes into ICL component (see, e.g., Conroy et al. 2007; Monaco et al. 2007). This implies that ICL fraction in clusters and groups should increase from z~1 to 0. Can this be tested observationally? Anthony argued that WFC3 can trace ICL out to 200-300 kpc in clusters to z~0.8.
  • Evolution of SFR in galaxies of different M* is being constrained now over a wide range of redshits. One of the recent studies: Karim et al. 2011

Andrey 02/18/2011: follow-up to the above discussion of van Dokkum & Conroy results. Second paper by van Dokkum & Conroy came out this week on arXiv. They study high-res spectra of several globular clusters in M31 and find that spectral features from ~0.2Msun dwarfs are much weaker, even though metallicities of these globular clusters are similar to those of ellipticals they studied in the first paper. This answers the main criticism that the first paper faced - that metallicity of stars used for template spectra in their SPS models was lower than the metallicity of stellar populations in the ellipticals they studied. This second result provides strong support to their interpretation of results in their first paper that IMF in ellipticals is steep at m<1 Msun.

Possible discussion items for next time:
  • What physics in simulations can enable them to produce the stellar mass vs mass trend?
  • What are the most interesting observables going forward?
  • Star formation rates and passive galaxy fractions as a function of cluster mass and radius.(This question has been addressed in a study by Andrew Wetzel and collaborators and was presented in Andrew's talk at the workshop. The talk can be viewed online among online talks and slides can be found here: .)


Abundance matching model discussion. Abundance matching results may be in tension with cluster and group data.

A point was brought up that in abundance matching, the BCG and cD galaxies (which could be a bump in LF) may not be taken into account properly.

Cristoph Pfrommer: Giant radio galaxies - as possible probes of gas halo in groups.(see background references below).

Is there a covariance of stellar and gas fractions of groups and clusters? Not that people present knew about...

How well BCG luminosity (and stellar mass) correlates with halo mass?

It was discussed that mis-centering in optically selected cluster catalogs is an issue, both for richness-mass and for stacking on other signals.One of the sources of mis-centering error is the fact that brightest galaxies have similar luminosities and so it is often difficult to determine which galaxy is the correct BCG.

Meghan Donahue: also, about ~25% of BCGs are blue - they can be off the red sequence by ~0.4-0.5 mag.

Stefano Andreon: we (SA & Moretti, A&A submitted), we *observationally* confirm that 30 % of maxBCG clusters have a wrong center (by looking where the X-ray emission and the galaxy density peak).

Hans Boehringer has asked a question of whether simulations could reproduce the fact that in many clusters it is not obvious which galaxies is BCG.

Hans: eRosita will be able to probe group gas, at least statistically.

References on the Giant Radio Galaxies from Christoph:

1) A nice introductory paper:
Lin, Shen et al. astro-ph/1006.5452

2) An attempt to measure ambient densities, of course very model dependent!
Subrahmanyan et al. ApJ 677, 2008

3) more examples, the first one is the record holder with L_project ~ 4.7 Mpc:
Machalski et al., Astrophys.J.679:149-155,2008

Schoenmakers et al. AA 336, 445 1998

Machalski and Jamrozy AA 363, L17 2000

Mach et al. AA, 329, 431 1998

4) One of problems in addressing the number density of GRGs is that there has not been a proper survey of GRGs with well-defined selection criteria. Mostly, because 1) it is difficult to find these low surface brightness emission regions in first place and 2) one has to do an optical follow-up of the host galaxy to determine its redshift for confirmation. The following authors present a complete sample of megaparsec-sized double radio sources from a radio survey (attempting a controlled selection):
Saripalli ApJ 130, 896, 2005
Their main conclusions are "The giant radio sources with linear size exceeding 0.7 Mpc have an abundance of (215 Mpc)3 at the sensitivity of the survey. In the low-redshift universe, the survey may be suggesting the possibility that giant radio sources with relict lobes are more numerous than giant sources in which beams from the center currently energize the lobes."

A recen paper on ICL modeling: Rudick et al. 2011