Day 614

Wednesday.

1403.6114
Variations in the initial mass function in early-type galaxies: a critical comparison between dynamical and spectroscopic results
Smith

Present a comparison between published dynamical and spectroscopic constraints on the stellar IMF in early-type galaxies, using the 34 galaxies in common between the two works.  Both studies infer an average IMF mass factor alpha (the stellar mass relative to Kroupa-IMF population of similar age and metallicity) greater than unity, i.e. both methods favor an IMF which is heavier than that of the MW, on average over the sample.  However, on a galaxy-by-galaxy basis, there is no correlation between alpha inferred from the two approaches.  Investigate how the two estimates of alpha are correlated systematically with the galaxy velocity dispersion, sigma, and with the Mg/Fe abundance ratio.  The spectroscopic method, based on the strengths of metal absorption lines, yields a correlation only with metal abundance ratios:at fixed Mg/Fe, there is no residual correlation with sigma.  The dynamical method, applied to exactly the same galaxy sample, yields the opposite result: the IMF variation correlates only with dynamics, with no residual correlation with Mg/Fe after controlling for sigma.  Hence although both methods indicate a heavy IMF on average in ellipticals, they lead to incompatible results for the systematic trends, when applied to the same set of galaxies.  Although other explanations are possible, the sense of the disagreement suggest that one (or both) of the methods has not accounted fully for the main confounding factors, i.e. element abundance ratios or DM contributions.

1403.6123
The birth of a galaxy - III. propelling reionisation with the faintest galaxies
Wise et al

...Find that low mass galaxies are just as efficient at producing ionizing photons per unit mass as atomic cooling haloes.  However, they are gradually photo-suppressed as they are engulfed in ionizing regions, giving way for larger galaxies to dominate the latter half of reionization.  …   Work suggests that a yet-to-be observed population of low-mass galaxies was responsible for starting reionization at very high z.

1403.6124
Why stellar feedback promotes disc formation in simulated galaxies
Übler et al

Study how feedback influences baryon infall onto galaxies using cosmological, zoom-in simulations of haloes with present mass Mvir=6.9e11 Msun to 1.7e12 Msun.  Starting at z=4 from identical IC, implementations of weak and strong stellar feedback produce bulge- and disc-dominated galaxies, respectively.  Strong feedback favors disc formation: (1) because conversion of gas into stars is suppressed at early times, as required by abundance matching arguments, resulting in flat SFH and higher gas fractions; (2) because 50% of the stars form in situ from recycled disc gas with angular momentum only weakly related to that of the z=0 dark haloes; (3) because late-time gas accretion is typically an order of magnitude stronger and has higher specific angular momentum, with recycled gas dominated over primordial infall; (4) because 25-30% of the total accreted gas is ejected entirely before z~1, removing primarily low angular momentum material which enriches the nearby IGM.  Most recycled gas roughly conserves its angular momentum, but material ejected for long times and to large radii can gain significant angular momentum before re-accretion.  These processes lower galaxy formation efficiency in addition to promoting disc formation.

1403.6125
The X-ray spectra of the first galaxies: 21cm signatures
Paucci et al

X-ray SED can have a significant impact on interferometric observations.  These galaxies ave ubiquitous, subu-keV thermal emission from the hot ISM, which generally dominates the soft X-ray luminosities (with energies below 1 keV, sufficiently low to significantly interact with the IGM)….IGM temperature fluctuations in the early universe would be substantially increased if the X-ray spectra of the first galaxies were dominated by the hot ISM, compared with X-ray binaries with harder spectra.  Show that the peak in the redshift evolution of the large-scale (k=0.2/Mpc) 21cm power is a robust probe of the soft-band SED of the first galaxies, and is not degenerate with their bolometric luminosities.

1403.6310
How much can we learn about the physics of inflation?
Dodelson

The recent BICEP2 measurement of B-modes in the polarization of the CMB suggests that inflation was driven by a field at an energy scale of 2e16 GeV.  Explore the potential of upcoming CMB polarization experiments to further constrain the physics underlying inflation.  If the signal is confirmed, then two sets of experiments covering larger area will shed light on inflation.  Low resolution measurements can pin down the tensor to scalar ratio at the percent level, thereby distinguishing models from one another.  A high angular resolution experiment will be necessary to measure the tilt of the tensor spectrum, testing the consistency relation that relates the tilt to the amplitude.