Friday, 27 March 2009

High z Low Z Galaxies

From Salzer et al. (arXiv:0903.3948). Caption reads: Luminosity-metallicity relation for 1300+ low-z KISS galaxies (z < 0.095; small dots) and the 13 [O III]-detected star-forming
KISS galaxies (red squares). The solid line is a linear fit to the low-z galaxies, while the lower dashed line has the same slope but fits the higher-z galaxies with an offset of -1.1 dex.

These galaxies have redshifts in the range 0.29-0.42. Such luminous galaxies at such low metallicities pose problems for galaxy evolution models, as the evolution in metallicity at these redshifts is expected to be mild.

Dissecting the fundamental plane

In, Graves et al. dissect the red sequence. They use low redshift red sequence galaxies from the spectroscopic sample. They bin in central stellar velocity dispersion, effective radius and residual central surface brightness. Here they show three panels, the fundamental plane (FP) midplane, and stuff with lower central SB, and higher SB. Color indicates the mean luminosity weighted age of the stellar population. The diagonal dashed lines indicate constant dynamical mass. The near vertical structure in the color scale indicates that the mean age is a much stronger function of veloicty dispersion than of effective radius and/or dynamical mass. This implies that the velocity dispersion is a better indicator of stellar age, and thus that stelar velocity dispersion and dynamical mass should not be considered 'the same'.

This result is consistent with merger siulations in which the effective radius depends mainly on orbital parameters of the merging galaxies.

Galaxies at high surface brightness are younger, implying that the thickness of the FP is real (i.e. has a physical origin).

Thursday, 19 March 2009

An SMG with A_V>~5

One of the brightest sub-mm galaxies (SMGs) in the GOODS-N field is completely undetected in deep optical/NIR imaging.  Guessing that this object might be part of a known protocluster at z=4.05, Daddi et al. have used the IRAM Plateau de Bure Interferometer to search for CO emission in the expected wavelength range.  This figure shows that they found it.

This is quite a remarkable object: the sub-mm emission corresponds to a star formation rate of order 1000 Msun/yr, and the SED suggests stellar mass of roughly 10^11 Msun and A_V in the range 5-7.5.  That's a lot of extinction.

Friday, 13 March 2009

Inside-out galaxy growth?

Figure 1 from Bezanson et al.,

The authors investigate possible mechanisms for the size and surface
density evolution from z=2.3 to the present. The above figure shows
average stellar density profiles of ellipticals at z=0 (colored
lines, representing different masses) and at high redshift (grey
shaded region). At small radii the profiles match pretty well, but
farther out they diverge rather drastically. This suggests that the
compact high-redshift quiescent galaxies may be the cores of massive
ellipticals in the local universe, growing from the inside out via
minor mergers. (later in the paper the authors show that major
mergers can't produce the observed evolution in the mass-size relation).

Friday, 6 March 2009

Evolution of Lyman alpha halos around HzRG

Zirm, Dey, Dickinson, Norman
ApJ 2009, 694, L31

Summary: The authors take STIS spectra of 5 z=1 HzRGs. 4 show extended Lyman alpha emission. The Lyman alpha halos that surround these z=1 HzRGs are smaller, and less luminous than those that surround z>2 HzRGs. The authors claim this is due to evolution of these massive galaxies. The z=1 halos are not ionized by shock heating of the infalling gas (as in the high-z halos), but rather by the AGN and star formation.

Weak velocity dispersion evolution

from Cenarro & Trujillo,

We know massive quiescent galaxies evolve strongly in size and
surface density from z=2 to the present. But what about in velocity
dispersion? Here the authors use a stacked spectrum of 13 early-type
galaxies at z~1.6 (total exposure time of 480 hours) from the GMASS
survey. By fitting templates to this stacked spectrum, they derive a
velocity dispersion of 240 km/s at z=1.6, in contrast to the local
value of about 180 km/s. This (shown in the bottom panel) is much
weaker than the size evolution over the same interval (top panel),
but agrees well with models of the sigma evolution via merging
(Hopkins et al 2008; shaded region) and disagrees with the AGN
feedback scenario of Fan et al. (2008; solid line). The authors
attribute the weak evolution to the changing role of dark matter in
galaxies' potential wells: at high redshift the central potential is
dominated by baryons, and at low redshifts it's dominated by dark

Upper Limit on Dimming of Cosmological Sources by Intergalactic Grey Dust from the Soft X-ray Background

From Dijkstra and Loeb (

Abstract reads:
"Active Galactic Nuclei (AGN) produce a dominant fraction (~80%) of the Soft X-ray background (SXB) at photon energies 0.5 < E < 2 keV. If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray halos around AGN. Taking account of known galaxies and galaxy clusters, only a fraction F_halo <10% of the SXB can be in the form of diffuse X-ray halos around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains (with radii in the range a=0.2-2.0 mum) to a level tau_GD<0.15(F_halo/10%) to a redshift z~1. Our results are only weakly dependent on the grain size distribution or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state. "

The different lines are different models for the dust distribution.