New cosmic ruler

A comparison of the correlation lengths on red galaxies with blue can provide a new cosmic ruler.  Clustering of old, red galaxies reflects the density fluctuations in the early universe.  Assuming the length scales associated with this clustering are constant everywhere and evolve slowly with time, the average size or correlation length of the clusters can be calibrated against local galaxies and used as a cosmic ruler.  Longo finds that the correlation length averaged over many clusters is nearly constant and can be measured to ~2% out to z ~ 0.5  (http://au.arxiv.org/obs/0810.4066).

the importance of mass selection

This is from a zCOSMOS paper (Silverman et al., arXiv:0810.3653), looking at the interrelationship between star formation and AGN activity. The two panels are showing the difference between selecting your sample by luminosity (left) and mass (right). Just focus on the black points, which show the AGN fraction as a function of restframe (U-V) color.

Because there is a relationship between color and mass-to-light ratio, whereby star forming galaxies are both bluer and brighter for a given mass, if you select by luminosity you will include a bunch of lower mass blue galaxies. In other words, the average mass you're probing will be color-dependent. In this case, because AGN activity is mass-dependent (see their Figure 4 if you wanna), that translates to an artificial color-dependence of the AGN fraction.

So, you can see that the AGN fraction is roughly constant for all blue galaxies, provided you select by mass. But if you looked in a luminosity-selected sample, you might think that AGN lie preferentially in green galaxies -- ie less actively star forming ones -- and so that maybe AGN only switched on after SF slowed down. But you'd be wrong. (Maybe.)

An Enigmatic HI Cloud

As a result of a resurgence in Galactic HI astronomy, a HI Cloud with a few peculiar properties was detected (arXiv:0810.3846v1).  In particular, the cloud is found to have a) a large velocity gradient b) an elliptical distribution c) filamentary structure and d) no correlation with emission in any other waveband. These properties combined suggest an interesting environment for this enigmatic HI Cloud - dark galaxy or expanding circumsteller shell??...

Presence of HVB's indicating the possibility of a second BH orbiting Sg A

It is believed that a second BH could be orbiting Sg A, and the detection of these HVB's (Hyper Velocity Binary) could provide the required evidence for the presence of a second BH.

Can have a look at this article, http://arxiv.org/abs/0810.3848

Journal Club 24th Oct 2008

This week's paper will be
'Hypercompact Stellar Systems Around Recoiling Supermassive Black Holes'

http://lanl.arxiv.org/abs/0809.5046v2

Abstract:
A supermassive black hole ejected from the center of a galaxy by gravitational wave recoil carries a retinue of bound stars - a "hypercompact stellar system" (HCSS). The numbers and properties of HCSSs contain information about the merger histories of galaxies, the late evolution of binary black holes, and the distribution of gravitational-wave kicks. We relate the structural properties of HCSSs to the properties of their host galaxies, in two regimes: collisional, i.e. short nuclear relaxation times; and collisionless, i.e. long nuclear relaxtion times. HCSSs are expected to be similar in size and luminosity to globular clusters but in extreme cases their stellar mass can approach that of UCDs. They differ from all known classes of compact stellar system in having very high internal velocity dispersions. We show that the kick velocity is encoded in the velocity dispersion of the bound stars. Given a large enough sample of HCSSs, the distribution of gravitational wave kicks can therefore be empirically determined.We combine a hierarchical merger algorithm with stellar population models to compute the rate of production of HCSSs over time and the probability of observing HCSSs in the local universe as a function of their apparent magnitude, color, size and velocity dispersion, under two assumptions about the star formation history prior to the kick. We predict that roughly 100 HCSSs should be detectable within 2 Mpc of the center of the Virgo cluster and that many of these should be bright enough that their high internal velocity dispersions could be measured with reasonable exposure times.

Galaxy Bimodality in 3D

The optical color-magnitude relation of galaxies is known to follow a bimodal distribution separating red and blue galaxies.  Mignoli et. al. (arXiv:0810.2245v1) used spectral, photometric and morphological classification to see if this bimodality extends to other galaxy optical properties.  Figure 5 shows the composite spectra of the three main galaxy classes found from spectral classification of 507 medium resolution spectra.

The three types of measurements were then used to define a classification cube which resulted in 85% concordant classification of galaxies being either quiescent,red,bulge-dominated or star-forming,blue,disk-dominated.

Star formation around supermassive black holes

The presence of massive young stars very close (0.1 pc) to the Galactic centre challenges star formation theories: stars should not be able to form this close to the supermassive black hole, due to tidal shearing of the molecular cloud, and they should not be able to travel from elsewhere to the Galactic centre to still be young when they get there. Bonnell and Rice (arXiv:0810.2723) have done some numerical simulations showing the infall of a giant molecular cloud that interacts with the black hole. They show that part of the cloud becomes bound to the black hole and forms an eccentric disk which fragments to form stars (as shown in above figure). This process can explain the observed rings of massive young stars at the Galactic centre.

Kochfar & Silk on dry mergers

This week i'll present Kochfar & Silk, 'Dry Mergers: A crucial test for galaxy formation'
http://arxiv.org/abs/0809.1734

Abstract:
We investigate the role that dry mergers play in the build-up of massive galaxies within the cold dark matter paradigm. Implementing an empirical shut-off mass scale for star formation, we find a nearly constant dry merger rate of $ \sim 6 \times 10^{-5}$ Mpc$^{-3}$ Gyr$^{-1}$ at $z \leq 1$ and a steep decline at larger z. Less than half of these mergers are between two galaxies that are morphologically classified as early-types, and the other half is mostly between an early-type and late-type galaxy. Latter are prime candidates for the origin of tidal features around red elliptical galaxies. The introduction of a transition mass scale for star formation has a strong impact on the evolution of galaxies, allowing them only to grow by mergers above a characteristic mass scale of $M_{*,c} \sim 6.3 \times 10^{10}$ M$_{\odot}$. As a consequence of this transition, we find that around $M_{*,c}$, the fraction of 1:1 mergers is enhanced with respect to unequal mass major mergers. This suggest that it is possible to detect the existence of a transition mass scale by measuring the relative contribution of equal mass mergers to unequal mass mergers as a function of galaxy mass. The evolution of the high-mass end of the luminosity function is mainly driven by dry mergers at low z. We however find that only $10% -20%$ of galaxies more massive than $M_{*,c}$ experience dry major mergers within their last Gyr at $z \le 1$, and conclude that dry mergers do not significantly change the mass function at the high-mass end.

And if that's not exciting enough, there'll be donuts!

Sunyaev-Zel'dovich Effect as Galaxy Cluster Probe

The Sunyaev-Zel'dovich Effect occurs when CMB photons travel through the hot gas surrounding a galaxy cluster, and are scattered to higher energy levels by the electrons therein. This effect has, for the first time, led to the detection of new galaxy clusters by looking for the signatures of this effect in a cosmological survey! Staniszewski et. al. deserve your praise and admiration.

all MW halos have the same mass!

All the satelites of our galaxy have aroundabout the same mass enclosed within a fixed radius (see Strigari et al., arXiv:0808.3772). Here, the authors have taken a N-body sim + SAM to see whether or not this falls out of current models. And indeed it does! The black points show their model results; the red points are from Strigari et al.Another interesting sidenote: of the 2000ish subhalos that the authors tracked, only 51 became fully fledged satelite galaxies. This is still twice as many as are observed, but they reckon that optical selection effects can account for this. (Which you can kinda see from the plot.) This is essentially because they have completely supressed gas cooling in subhaloes with virial temperatures below 10^4 K.