Friday, 29 August 2008

All MW satellites have the same mass?

from Strigari et al., arXiv:0808.3772

Line-of-sight velocity measurements were used to derive masses within the innermost 300pc of 18 Milky Way dwarf satellites. The above figure is self-explanatory, and surprising: over 5 orders of magnitude in luminosity, there's almost no change in the dark matter+stellar mass. They mention several possible explanations, including a sharp cutoff in star formation efficiency below this halo mass, a characteristic formation time (in CDM models) around the epoch of reionization, or dark matter temperature of >1keV (in WDM models).

Friday, 22 August 2008

Contradiction between strong lensing statistics and a feedback solution to the cusp/core problem

Title: Contradiction between strong lensing statistics and a feedback solution to the cusp/core problem
Authors: Da-Ming Chen, Stacy McGaugh

Standard cosmology has many successes on large scales, but faces some fundamental difficulties on small, galactic scales. One such difficulty is the cusp/core problem. High resolution observations of the rotation curves for dark matter dominated low surface brightness (LSB) galaxies imply that galactic dark matter halos have a density profile with a flat central core, whereas N-body structure formation simulations predict a divergent (cuspy) density profile at the center. It has been proposed that this problem can be resolved by stellar feedback driving turbulent gas motion that erases the initial cusp. However, strong gravitational lensing prefers a cuspy density profile for galactic halos. In this paper, we use the most recent high resolution observations of the rotation curves of LSB galaxies to fit the core size as a function of halo mass, and compare the resultant lensing probability to the observational results for the well defined combined sample of the Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS). The lensing probabilities based on such density profiles are too low to match the observed lensing in CLASS/JVAS. High baryon densities in the galaxies that dominate the lensing statistics can reconcile this discrepancy, but only if they steepen the mass profile rather than making it more shallow. The result is contradictory demands upon the effects of baryons on the central mass profiles of galaxies.

Friday, 15 August 2008

Constraints on high-z disk formation

figure 1 from Robertson & Bullock (arXiv:0808.1100)

The authors investigate the claim by Genzel et al. (2006) that the
z=2.4 "disk" galaxy BzK-15504 formed very rapidly and early on (with
a correspondingly rapid accretion of mass), but shows no evidence of
a merger (because its velocity asymmetry is low). Using their
simulations of gas-rich disk mergers and taking into account noise
and PSFs, the authors figure out how their merger remnants would
appear viewed through SINFONI. In all four of their simulations
(each with different initial configurations), galaxies that would be
observationally classified as "disks" appear 100-150Myr after the 
merger. The above figure shows one particular simulation that they
claim matches the kinematic properties of BzK-15504 (shown below for
comparison, from Genzel et al. 2006) remarkably well.

Thursday, 14 August 2008

Observations of the Gas Reservoir around a Star Forming Galaxy in the Early Universe

Frye et al.,

The figure shows a high-S/N spectrum of a z=4.9 starburst galaxy; this kind of S/N is only possible because of a long exposure time (14hrs on an 8m telescope) and because the flux is boosted by a factor of 10 due to gravitational lensing by a foreground cluster. The most interesting feature is the broad Gunn-Peterson trough blueward of the bright Lya emission line.

Since the optical depth in this trough is significantly larger than observed at the same wavelengths for random sight-lines through the IGM (the universe was already reionized at this redshift), the authors conclude that we are seeing direct evidence of a "cosmic web" of gas that surrounds galaxies and feeds their growth.

Another possibility is that the neutral gas is outflowing material from the galaxy itself, however the authors discount this explanation since stellar population modeling suggests that the galaxy is too young to have driven such large amounts of gas outwards. Also, typical outflow velocities are not large enough to explain the broad trough, even for outflows that are powered by AGN.

This is a neat observation, but I'm not sure how much we can infer from a single object. Unfortunately, a galaxy at this redshift has to be strongly lensed to be bright enough for this sort of analysis, so there isn't much hope of obtaining a large sample in the near future. Of course QSOs are also bright enough, but they tend to ionize most of the hydrogen in their immediate vicinity.

Friday, 8 August 2008

Size evolution from z=1 to present

figure 7 of van der Wel et al., arXiv:0808.0077

In this paper, the authors find that early-type galaxies grow larger
by a factor of ~2 from z=1 to z=0, consistent with previous studies.
This figure shows the ratio of sizes at the two redshifts from this
work and others, and compares it to the Khochfar & Silk (2006)
semianalytic model. The observed size evolution between z=1-0 is
much steeper than predicted by the model, which is based on the idea
that (cold)-gas-rich mergers at high redshift produce smaller
galaxies than the gas-poor mergers at low redshift.

Probably the most important difference between this and previous
studies is that masses here are calculated using dynamical, rather
than photometric, measurements. Thus, the mass (and hence surface
density) estimates here should be less prone to systematic effects.
Nonetheless, the offset in the size-mass and size-surface density
relations are consistent with the photometric studies. This seems to
imply that any systematic effects are small compared to the observed
size evolution.

The Millennium Simulation compared to z~2 galaxies

The Millennium Simulation compared to z~2 galaxies
Genel et al.

The authors use the Millennium Simulation to extract merger fractions and
mass accretion rates. They find that the accretion rates are sufficient to
account for the high star formation rates observed in z~2 UV-optically selected disks.

(not in figure) When following the fate of these disks and submm galaxies, they find that subsequent mergers are not frequent enough to either convert all disks into elliptical galaxies at z~0, or transform all submm galaxies to massive cluster ellipticals at z~0. They conclude that secular and internal evolution must play an important role in the evolution of these z~2 galaxies

Thursday, 7 August 2008

Disc instabilities and semi-analytic modelling of galaxy formation
E. Athanassoula

This paper points out that the method to form spheroids in semi-analytic models of galaxy formation are wrong. A criterium, based on bar instability for given disk parameters is often used in these models and either the whole disk, or a fraction of the disk than suddenly transforms into a spheroid. This is a necessary ingredient to match the observed near-IR luminosity functions.

In reality, this criterium does not hold. It was derived from 2D Nbody simulation long ago. If you take into account that the halo is non-static and that there are random motions in the disk and in the halo, then the situation is different: disks are much more stable, and form at best small pseudo-bulges. A simple creterium for bulge formation from disk instability is not easily possible, and should not be used in semi-analytic models.

Friday, 1 August 2008

On the SFR-brightest cluster relation: estimating the peak SFR in post-merger galaxies

Nate Bastian: astro-ph/0807.4687

He does Monte Carlo simulations and comparisons to observations of star cluster formation in galaxies. It appears thet there is quite a tight relation between the magnitude of the brightest young cluster present, and the current SFR, because the brightest cluster often is young (<15Myr) and the mass of the most massive cluster is determined by the number of clusters formed, which is strongly related to the SFR. In the figure, the colored lines are lines of different cluster mass functions (schechter with some M*. ). The gamma indicates the factor between star formation rate and cluster formation rate, i.e. they find that CFR = 0.08 SFR, which is sort of low. This SFR indicator agrees well with other indicators.

Using the fading of clusters, an old cluster that is very bright can indicate a burst in SFR some time ago, and its luminosity and age give an estimate of the peak SFR of that galaxy (or galaxy merger). Here cluster disruption effects need to be taken into account (which is difficult).