do protoplanetry disks make an important contribution to the NIR flux of star forming galaxies?

The most common (and best) way of deriving stellar masses from broadband SEDs is to fit the observed SED with synthetic stellar population models.  This method basically assumes that you can reliably decompose the observed SED into the contributions from stars of all ages, almost always with some simplifying assumption about the actual age distribution (viz. a parameterised star formation history and a stellar initial mass function).

Mentuch et al. have modelled a bunch of galaxies using standard stellar population models.  Their sample is drawn from the GDDS, and comprises 103 galaxies at z < 1 with strong detections in IRAC channel 4 (8 microns).  They have shown that the star forming galaxies in their sample show an excess in the NIR at 2--5 microns (upper panel of this Figure).  This Figure also shows that the excess can be very well described by inclusion of an additional component (lower panel), which they describe as 'a template PAH spectrum superposed on an 850 K greybody modified by lamba^-1 extinction'.  They argue that this additional component is most likely associated with emission from circumstellar protoplanetary disks (rather than instellar cirrus, reflection nebulae, or post AGB stars).

This is pretty cool, 'cause it potentially offers a means of probing the formation of planetary systems in high-redshift galaxies. (!!)

Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies

Mmmmmm peas!!!!!

The next journal club to be presented on the 10th of August.

Check it out at;

http://arxiv.org/abs/0907.4155

Plot: Inhomogeneity in the Supernova Remnants as Natural Explanation of the PAMELA/ATIC Observations

PAMELA anomaly could have a simple answer.

Plot: Strong [CII] emission at high redsift

With the detection of the [CII] transition this increases the potential to use this transition to search and characterise high-z sources.

A Comparison of Galaxy Merger History Observations and Predictions from Semi-Analytic Models

 This paper (http://xxx.lanl.gov/abs/0904.2365) presents some interesting results from comparing predicted merger rates and fractions from the Millennium Simulation to observations of galaxy mergers out to z~3.   

Plot: Cluster Strong Lensing in Millennium Simulation

The Problem: How do secondary matter structures and stellar mass in galaxies along the line-of-sight affect strong cluster lensing?
What they do: Use ray-tracing through the Millennium simulation to probe this contribution.
Answer: There can be considerable increases to strong-lensing optical depth and cross-sections.

160309: Evidence for hierarchical, inside-out growth

The Problem: massive but really compact, high z, quiescent galaxies have been observed - but where are their descendants?

What they do: compare the densities of a sample of these high z galaxies to nearby elliptical galaxies;

compare growth evolution from three models  

Answer:  they may be the dense cores of nearby galaxies suggesting inside-out growth

Journal Club: 20/02/2009

We continue our series on how baryons contribute to dark matter halo formation with this tasty morsel: http://arxiv.org/abs/0902.2100.

We analyse the dark matter (DM) distribution in a approx 10^12 M_sun halo extracted from a simulation consistent with the concordance cosmology, where the physics regulating the transformation of gas into stars was allowed to change producing galaxies with different morphologies. Although the DM profiles get more concentrated as baryons are collected at the centre of the haloes compared to a pure dynamical run, the total baryonic mass alone is not enough to fully predict the reaction of the DM profile. We also note that baryons affect the DM distribution even outside the central regions. Those systems where the transformation of gas into stars is regulated by Supernova (SN) feedback, so that significant disc structures are able to form, are found to have more concentrated dark matter profiles than a galaxy which has efficiently transformed most of its baryons into stars at early times. The accretion of satellites is found to be associated with an expansion of the dark matter profiles, triggered by angular momentum transfer from the incoming satellites. As the impact of SN feedback increases, the satellites get less massive and are even strongly disrupted before getting close to the main structure causing less angular momentum transfer. Our findings suggest that the response of the DM halo is driven by the history of assembly of baryons into a galaxy along their merger tree.

Journal article 6.02.09

This week's article is: A dark matter disc in three cosmological simulations of Milky Way mass galaxies. You can find it here. Its abstract looks like this:

Making robust predictions for the phase space distribution of dark matter at the solar neighbourhood is vital for dark matter direct detection experiments. To date, almost all such predictions have been based on simulations that model the dark matter alone. Here, we use three cosmological hydrodynamics simulations of bright, disc dominated galaxies to include the effects of baryonic matter self-consistently for the first time. We find that the addition of baryonic physics drastically alters the dark matter profile in the vicinity of the Solar neighbourhood. A stellar/gas disc, already in place at high redshift, causes merging satellites to be dragged preferentially towards the disc plane where they are torn apart by tides. This results in an accreted dark matter disc that contributes ~0.25 - 1.5 times the non-rotating halo density at the solar position. The dark disc, unlike dark matter streams, is an equilibrium structure that must exist in disc galaxies that form in a hierarchical cosmology. Its low rotation lag with respect to the Earth significantly boosts WIMP capture in the Earth and Sun, boosts the annual modulation signal, and leads to distinct variations in the flux as a function of recoil energy that allow the WIMP mass to be determined.

Cool? Cool.