Tuesday. Wednesday.
1312.1337
The mystery of the $\sigma$-bump -- A new signature for major mergers in early-type galaxies?
Schauer et al
The stellar velocity dispersion as a function of the galactocentric radius of an early-type galaxy can generally be well approximated by a power law sigma ~ r^beta. However, some observed dispersion profiles show a deviation from this fit at intermediate radii, usually between one and 3 R_eff, where the velocity dispersion remains constant with radius, showing a bump-like behavior ("sigma-bump"). Study a set of simulated early-type galaxies formed in major mergers. Find the sigma-bump in all of the simulated early-type galaxies, with the size and position of the bump slightly varying from galaxy to galaxy, leading to the assumption that the bump is a characteristic of the major merger formation scenario. The feature can be seen both in the intrinsic and projected stellar velocity dispersions. In contrast to shells that form during the merger event but evolve with time and finally disappear, the sigma-bump stays nearly constant with radius and is a permanent feature that is preserved until the end of the simulation. The sigma-bump is not seen in the dark matter and gas components and therefore conclude that it is a purely stellar feature of merger remnants. [no sigma-bumps with minor merger ellipticals?]
1312.1340
More than just halo mass: modelling how the red galaxy fraction depends on multi scale density in a HOD framework
Phelps, … Budavari, et al
The fraction of galaxies with red colors depends sensitively on environment, and on the way in which environment is measured. To distinguish competing theories for the quenching of star formation, a robust and complete description of environment is required, to be applied to a large sample of galaxies. The environment of galaxies can be described using the density field of neighbors on multiple scales - the multi scale density field. Use the Millennium simulation and a simple HOD prescription which describes the multi scale density field of SDSS DR7 galaxies to investigate the dependence of the fraction of red galaxies on the environment. Using a volume limited sample where there are sufficient galaxies in narrow density bins, there are more dynamic range in halo mass and density for satellite galaxies than for central galaxies. Therefore model the red fraction of central galaxies as constant while using a functional form to describe the red fraction of satellites as a function of halo mass which allows distinguishing a sharp from a gradual transition. While it is clear that the data can only be explained by a gradual transition, an analysis of the multi scale density field on different scales suggests that color segregation within the haloes is needed to explain the results. Also rule out a share transition for central galaxies, within the halo mass range sampled.
1312.1358
On the contribution of active galactic nuclei to reionization (Research Note)
Grissom, Ballantyne, Wise
The e- scattering optical depth constraints on reionization suggest that there may be other sources that contribute to the ionization of hydrogen aside from observable SF galaxies. Often the calculated value of the e- scattering optical depth, tau_es, falls below the measurements derived from observations of the CMB or an assumption about non-observable sources must be made in order to reach agreement. Here, calculate the H ionization fraction as a function of z and the tau_es from both galaxies and AGN factoring in the secondary collisional ionizations from the AGN X-ray emission. In this paper, use the most current determination of the evolving hard X-ray luminosity function and extrapolate its evolution beyond z=6. The AGN SEDs include both UV and X-ray ionization photons. To search for the largest possible effect, all AGN are assumed to have lambda_Edd=1.0 and be completely unobscured. The realists show that AGNs produce a perturbative effect on the reionization of hydrogen and remains in agreement with current constrains. Calculations find the EoR still ends at z~6 and only increases the tau_es by ~2.3% under the most optimal conditions. This can only be moderately increased by assuming a constant BH mass of M_BH=1e5 Msun. As a result, conclude that there is a need for other sources beyond observable galaxies and AGNs that contribute to the reionization of H at z>6.
1312.1694
Star formation in the cluster CLG0218.3-0510 at z=1.62 and its large-scale environment: the infrared perspective
Santos et al
This cluster is one of the most distant galaxy clusters known, with multi-wavelength data that confirms a mature galaxy population already in place. Investigate dust-obscured, SF properties in the cluster and its associated large scale environment. Galaxy sample = 693 at z=1.62 detected at 24um (mostly photo-z) includes both cluster galaxies (within r<1 Mpc projected cluster centric radius) and field galaxies, defined as the region beyond a radius of 3 Mpc. The SFRs derived from the measured IR luminosity range from 18 to 2500 Msun/yr, with a median of 55 Msun/yr, over the entire radial range (10 Mpc). The cluster brightest FIR galaxy, taken as the center of the galaxy system, is vigorously forming stars at the rate of 256 pm 70 Msun/yr, and the total cluster SFR enclosed in a circle of 1 Mpc is 1479 Msun/yr/Mpc^2. Estimate a dust extinction of about 3 mags by comparing the SFRs derived from OII luminosity with the ones computed from the 24 um fluxes. Find that in the in-falling region (1-3Mpc) is special: there is a significant decrement of passive relative to SF galaxies in this region, and the total SFR of the galaxies located in this region is lower (130 Msun/yr/Mpc^2) than anywhere in the cluster of field, regardless of their stellar mass. In a complementary approach, compute the local galaxy density, Sigma5, and find no trend between SFR and Sigma5. However, measure an excess of SF galaxies int he cluster relative to the field by a factor 1.7, that lends support to a reversal of the SF-density relation in this cluster.
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