Baryon budget in groups and clusters of galaxies

This (http://arxiv.org/abs/0705.1726) is a very careful study of baryon budget in massive halos (M_500>5.e13 M_sun). They find that the total baryon fraction (including ICM gas, BCG+ICL+stars from other galaxies) in halos > 5.e13 M_sun is only slightly smaller than the universal baryon fraction (f_b=0.17; their Fig. 2). Thus clusters represent a fair sample of baryons in the universe, considering that the small deficit from the universal value is within systematic uncertainties, and that they only go out to R_500 and not R_200. Moreover, the fractional contribution of stars to total baryon budget increases with decreasing halo mass, such that (mass of stars+ICM)/(mass of stars+ICM+dark matter) = universal baryon fraction. Their other finding is that the ICL+BCG contribution can be dominated by the ICL (which can account for 80% of the ICL+BCG light); thus it is important to account for the diffuse ICL. The fractional (ICL+BCG)/(ICL+BCG+other galaxies) contribution decreases with radius in all halos (Fig. 5). The volume integrated ICL+BCG fraction of total stellar light decreases with increasing halo mass (Fig. 4); thus ICL is more important in groups than in clusters. This paper (http://arxiv.org/abs/0911.2230; Fig. 4) shows the baryon fraction going to much lower halo masses. The baryon fraction decreases quite rapidly with the decreasing halo mass for masses smaller than ~ 5.e13 Msun (this is the "missing baryon problem").

Cluster Richness as a measure of cluster mass

Because of sensitivity and wide sky coverage (e.g., SDSS) galaxy clusters are most easily detected optically as an overdensity in galaxies on the scale of ~ Mpc. Here (http://arxiv.org/abs/0709.1159) is a good paper that goes through the details of calibrating richness (N_200, number of early type galaxies larger than a threshold luminosity) with mass measured from weak lensing shear profiles. They bin their ~ 13,000 clusters using 12 bins in N_200 (total luminosity of elliptical galaxies can also be used) and for each bin they obtain average weak lensing shear profiles which they convert into matter density profile and enclosed mass profile and obtain R_200. They find a rather tight correlation between N_200 and M_200, and argue that richness can be used as a good mass proxy (Fig. 11). They have to account for several mass contributions, e.g., the mass profile of the central bright galaxy, miscentered halo component (if cluster DM is not centered on the central bright galaxy) , neighboring halos, etc. They find that NFW is a good fit to the DM density profile. The concentration-mass relation they obtain is also consistent with simulations. They find that their best fit BCG mass is correlated with M_200 (Fig. 14). Its still a puzzle then why the SZ signal predicted by M_200-N_200 relation is larger than what is observed!

Universal pressure profile for galaxy clusters

The utility of galaxy clusters for precision cosmology (in particular to constrain $\sigma_8$ and dark energy eq. of state) depends on our ability to determine their mass precisely from observations. Systematic scatter (and bias) in mass measurement needs to be reduced to better than 10%. This paper (http://arxiv.org/abs/0910.1234) uses the representative (a fair sample reflecting their real abundance) of  REXCESS clusters, and fits a generalized NFW profile to observational pressure profiles out to R_500. The scatter in pressure is much smaller at each radius than density, temperature, or entropy (something already emphasized by simulations). Pressure profile has smallest scatter at R_500 because inner parts are affected by cooling and outer by unvirialized clumps. Since SZ effect probes the pressure, this implies that Y_SZ can be more tightly correlated with M_500 than Y_X, and can improve cosmological constraints. Moreover, since SZ signal is not dimmed at high redshifts, SZ can be quite powerful in constraining dark energy and departures from non-gaussianities in the primordial power spectrum.

Planck SZ vs. optical scaling relations

Many observables (e.g., Y_X, Y_SZ, N_200) are correlated with the halo mass. This enables obtaining mass function of halos at different redshifts, which is a sensitive probe of dark energy. However, different observables are biased differently. Thus it is necessary to cross-caliberate different observables. This paper

(http://arxiv.org/abs/1101.2027) shows that optical richness (which is calibrated against weak-lensing mass measurements) based prediction for Y_SZ is >~ twice larger than what is measured by Planck (Fig. 2). Y_SZ measurements are consistent with predictions of X-ray models applied to the same sample. Richness (number of elliptical galaxies > a cut-off luminosity within a fraction of virial radius) must have a large scatter for a fixed profile of the ICM. This is a puzzle that must be resolved if richness can be used for precision cosmology!

Physics blog

I'd write more about my Physics readings, a drastic change of flavor. This will help me write the gist of what I am reading and help as a quick reference. I aim to read at least a paper or two everyday.

Law vs Ethics & Govt. vs Anna

Its very encouraging to see the kind of momentum that Anna Hazare has gathered for his fight against corruption in India. He does not have a big political base, but his voice resonates with millions of Indians (especially young Indians; I am proud of you!) . Government has tried to suppress him but popular support and his will were too strong to yield. Some people in the government have argued that Anna's approach is unconstitutional. Although technically correct, their argument is lame. Laws change with time as the society evolves. In old times (and even now in some societies) it was fine to stone people to death, but with time as the society evolved the laws (and constitutions) changed. Similarly during the British Raj it was ok (by law) to ban newspapers which encouraged people to demand for independence. People were arrested for peacefully protesting against the illegal and unethical government. It was justified by law! So laws are not written in stone, they change with time, with changes in society and popular aspirations (at least this is how it should be in a functional democracy). Thus, although it is technically unconstitutional to introduce/modify/question bills outside the parliament, it is very reasonable to have an open and broad discussion with the citizenry at large on such a critical issues as corruption. I hope the government does not go into the technicalities, but understands the significance of this issue to the common man. If it doesn't, I am pretty positive, it will later regret its decision. Common citizens' participation on this is even more important because a large fraction of the members of the parliament (MPs) are tainted in one way or another; we, the common people, are afraid that the cartel of MPs, irrespective of their affiliations, will not write a strong law that will rein them in. Anna we are with you!

Future of Education?

I think so! See Sal Khan's TED talk:

http://www.ted.com/talks/salman_khan_let_s_use_video_to_reinvent_education.html

and

http://www.khanacademy.org/

In a nutshell he is suggesting that internet technology can be used to make good quality lectures and tests available for everyone. Anyone, even those without educational opportunities -- he gives the example of a child laborer from Calcutta -- can come home and go through these at his/her leisure. He points that a lot of kids cannot keep up with the pace of the traditional classroom and lag behind as a result, and if the basics are not strong rest of the material will just go over the top. Online learning can help students master the basics at their own pace and then only after they are comfortable (which the online tests are very good at measuring) are the advised to advanced to the next level.

The role of a teacher is not diminished in this setting. Since it is straightforward to track the learning of students -- thanks to the software tools -- the teacher can spend time with their students on their weaknesses. The advanced students can tutor their friends stuck at topics they have mastered. The whole classroom experience becomes much more productive than the traditional way. The learning experience becomes much more collaborative, as it is in the real world. e.g., if someone wants to open a business he/she does not just read books on this and take tests. He/she talks to friends who are business-owners, he/she surveys the web, and so on. Why shouldn't we aim for education that helps everyone in a real sense! The negative impact of grades, classes, exams, etc. are much less deleterious in this system and they learn what they should in a productive way.

I hope teachers -- especially those from areas with limited access to good quality teachers -- try these online resources to make the learning experience more rewarding and productive.