Galaxy formation and evolution

Abstract:

[abridged] Aims. The aim of this paper is to assess the impact of the interstellar medium (ISM) physics on Lyman-alpha (Lya) radiation transfer and to quantify how galaxy orientation with respect to the line of sight alters observational signatures. Methods. We compare the results of Lya radiation transfer calculations through the ISM of a couple of idealized galaxy simulations with different ISM models. Results. First, the small-scale structuration of the ISM plays a determinant role in shaping a galaxys Lya properties.The artificially warm, and hence smooth, ISM of G1 yields an escape fraction of 50 percent at the Lya line center, and produces symmetrical double-peak profiles. On the contrary, in G2, most young stars are embedded in thick star-forming clouds, and the result is a 10 times lower escape fraction. G2 also displays a stronger outflowing velocity field, which favors the escape of red-shifted photons, resulting in an asymmetric Lya line. Second, the Lya properties of G2 strongly depend on the inclination at which it is observed: From edge-on to face-on, the line goes from a double-peak profile with an equivalent width of -5 Angstrom to a 15 times more luminous red-shifted asymmetric line with EW 90 Angstrom. Conclusions. Lya radiation transfer calculations can only lead to realistic properties in simulations where galaxies are resolved into giant molecular clouds, putting these calculations out of reach of current large scale cosmological simulations. Finally, we find inclination effects to be much stronger for Lya photons than for continuum radiation. This could potentially introduce severe biases in the selection function of narrow-band Lya emitter surveys, which could indeed miss a significant fraction of the high-z galaxy population.

Abstract:

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey (NGLS) comprises an Hi-selected sample of 155 galaxies spanning all morphological types with distances less than 25Mpc. We describe the scientific goals of the survey, the sample selection and the observing strategy. We also present an atlas and analysis of the CO J=3 - 2 maps for the 47 galaxies in the NGLS which are also part of the Spitzer Infrared Nearby Galaxies Survey. We find a wide range of molecular gas mass fractions in the galaxies in this sample and explore the correlation of the far-infrared luminosity, which traces star formation, with the CO luminosity, which traces the molecular gas mass. By comparing the NGLS data with merging galaxies at low and high redshift, which have also been observed in the CO J=3 - 2 line, we show that the correlation of far-infrared and CO luminosity shows a significant trend with luminosity. This trend is consistent with a molecular gas depletion time which is more than an order of magnitude faster in the merger galaxies than in nearby normal galaxies. We also find a strong correlation of the LFIR/LCO(3-2) ratio with the atomic-to-molecular gas mass ratio. This correlation suggests that some of the far-infrared emission originates from dust associated with atomic gas and that its contribution is particularly important in galaxies where most of the gas is in the atomic phase. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.