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Register a new userPredicting dust extinction properties of star-forming galaxies from H-alpha/UV ratio
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Using star-forming galaxies sample in the nearby Universe (0.02<z<0.10) selected from the SDSS (DR7) and GALEX all-sky survey (GR5), we present a new empirical calibration for predicting dust extinction of galaxies from H-alpha-to-FUV flux ratio. We find that the H-alpha dust extinction (A(Ha)) derived with H-alpha/H-beta ratio (Balmer decrement) increases with increasing H-alpha/UV ratio as expected, but there remains a considerable scatter around the relation, which is largely dependent on stellar mass and/or H-alpha equivalent width (EW(Ha)). At fixed H-alpha/UV ratio, galaxies with higher stellar mass (or galaxies with lower EW(Ha)) tend to be more highly obscured by dust. We quantify this trend and establish an empirical calibration for predicting A(Ha) with a combination of H-alpha/UV ratio, stellar mass and EW(Ha), with which we can successfully reduce the systematic uncertainties accompanying the simple H-alpha/UV approach by ~15-30%. The new recipes proposed in this study will provide a convenient tool for predicting dust extinction level of galaxies particularly when Balmer decrement is not available. By comparing A(Ha) (derived with Balmer decrement) and A(UV) (derived with IR/UV luminosity ratio) for a subsample of galaxies for which AKARI FIR photometry is available, we demonstrate that more massive galaxies tend to have higher extra extinction towards the nebular regions compared to the stellar continuum light. Considering recent studies reporting smaller extra extinction towards nebular regions for high-redshift galaxies, we argue that the dust geometry within high-redshift galaxies resemble more like low-mass galaxies in the nearby Universe.
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Because the timescale of H$alpha$ emission (several tens of Myr) following star formation is significantly shorter than that of UV radiation (a few hundred Myr), the H$alpha$/UV flux ratio provides insight on the star formation histories (SFHs) of galaxies on timescales shorter than $sim100$ Myr. We present H$alpha$/UV ratios for galaxies at $z=$ 0.02--0.1 on the familiar star-forming main sequence based on the AKARI-GALEX-SDSS archive dataset. The data provide us with robust measurements of dust-corrected SFRs in both H$alpha$ and UV for 1,050 galaxies. The results show a correlation between the H$alpha$/UV ratio and the deviation from the main sequence in the sense that galaxies above/below the main sequence tend to have higher/lower H$alpha$/UV ratios. This trend increases the dispersion of the main sequence by 0.04 dex (a small fraction of the total scatter of 0.36 dex), suggesting that diversity of recent SFHs of galaxies has a direct impact on the observed main sequence scatter. We caution that the results suffer from incompleteness and a selection bias which may lead us to miss many sources with high H$alpha$/UV ratios, this could further increase the scatter from SFHs in the star-forming main sequence.
Lyman break analogues (LBAs) are a population of star-forming galaxies at low redshift (z ~ 0.2) selected in the ultraviolet (UV). These objects present higher star formation rates and lower dust extinction than other galaxies with similar masses and luminosities in the local universe. In this work we present results from a survey with the Combined Array for Research in Millimetre-wave Astronomy (CARMA) to detect CO(1-0) emission in LBAs, in order to analyse the properties of the molecular gas in these galaxies. Our results show that LBAs follow the same Schmidt-Kennicutt law as local galaxies. On the other hand, they have higher gas fractions (up to 66%) and faster gas depletion time-scales (below 1 Gyr). These characteristics render these objects more akin to high-redshift star-forming galaxies. We conclude that LBAs are a great nearby laboratory for studying the cold interstellar medium in low-metallicity, UV-luminous compact star-forming galaxies.
(abridged) In this work we have a closer look at the gas content or fraction and the associated star formation rate in main sequence and starburst galaxies at z=0 and z~1-2 by applying an analytical model of galactic clumpy gas disks to samples of local spiral galaxies, ULIRGs, submillimeter (smm), and high-z starforming galaxies. The model gas and dust temperatures are determined by the heating and cooling equilibrium. Dense clouds are heated by turbulent mechanical and cosmic ray heating. The molecular abundances of individual gas clouds are determined by a detailed chemical network involving the cloud lifetime, density, and temperature. Molecular line emission is calculated with an escape probability formalism. The model calculates simultaneously the total gas mass, HI/H_2 mass, the gas velocity dispersion, IR luminosity, IR spectral energy distribution, CO spectral line energy distribution (SLED), HCN(1-0), and HCO+(1-0) emission of a galaxy given its size, integrated star formation rate, stellar mass radial profile, rotation curve, and Toomre Q parameter. The model reproduces the observed CO luminosities and SLEDs of all sample galaxies within the model uncertainties (~0.3 dex). Whereas the CO emission is robust against the variation of model parameters, the HCN and HCO+ emission is sensitive to the chemistry of the interstellar medium. The CO and HCN mass-to-light conversion factors including CO-dark H_2 are given and compared to the values found in the literature. Both, the HCN and HCO+ emission trace the dense molecular gas to a factor of ~2 for the local spiral galaxies, ULIRGs and smm-galaxies. About 80% of the molecular line emission of compact starburst galaxies originates in non-selfgravitating gas clouds. The integrated Kennicutt-Schmidt law has a slope of ~1 for the local spirals, ULIRGs, and smm-galaxies, whereas the slope is 1.7 for high-z starforming galaxies.
Multi-wavelength, optical to IR/sub-mm observations of 5 strongly lensed galaxies identified by the Herschel Lensing Survey, plus two well-studied lensed galaxies, MS1512-cB58 and the Cosmic Eye, for which we also provide updated Herschel measurements, are used to determine the physical properties of z~1.5-3 star-forming galaxies close to or below the detection limits of blank fields. We constrain their stellar and dust content, determine star formation rates and histories, dust attenuation and extinction laws, and other related properties. We perform SED-fits of the full photometry of each object as well for the optical and infrared parts separately, exploring various parameters, including nebular emission. The IR observations and emission line measurements, where available, are used a posteriori constraints on the models. Besides the various stellar population models we explore, we use the observed IR/UV ratio to estimate the extinction and create energy conserving models, that constrain most accurately the physical properties of our sources. Our sample has a median lensing-corrected IR luminosity ~ 3e11 Lsun, stellar masses between 2e9 and 2e11 Msun, and IR/UV luminosity ratios spanning a wide range. The dust masses of our galaxies are in the range 2 to 17e7 Msun, extending previous studies at the same redshift down to lower masses. We do not find any particular trend of the dust temperature Tdust with IR luminosity, suggesting an overall warmer dust regime at our redshift regardless of luminosity. Lensing enables us to study the detailed physical properties of individual IR-detected z~1.5-3 galaxies up to a factor ~10 fainter than achieved with deep blank field observations. We demonstrate that multi-wavelength observations combining stellar and dust emission can constrain star formation histories and extinction laws of star-forming galaxies.
We present clustering analyses of identically-selected star-forming galaxies in 3 narrow redshift slices (at z=0.8, z=1.47 and z=2.23), from HiZELS, a deep, near-infrared narrow-band survey. The HiZELS samples span the peak in the cosmic star-formation rate density, identifying typical star-forming galaxies at each epoch. Narrow-band samples have well-defined redshift distributions and are therefore ideal for clustering analyses. We quantify the clustering of the three samples, and of H-alpha luminosity-selected subsamples, initially using simple power law fits to the two-point correlation function. We extend this work to link the evolution of star-forming galaxies and their host dark matter halos over cosmic time using sophisticated dark matter halo models. We find that the clustering strength, r0, and the bias of galaxy populations relative to the clustering of dark matter increase linearly with H-alpha luminosity (and, by implication, star-formation rate) at all three redshifts, as do the host dark matter halo masses of the HiZELS galaxies. The typical galaxies in our samples are star-forming centrals, residing in halos of mass M_halo ~ a few times 10^12M_solar. We find a remarkably tight redshift-independent relation between the H-alpha luminosity scaled by the characteristic luminosity, L(H-alpha)/L(H-alpha)*(z), and the minimum host dark matter halo mass of central galaxies. This reveals that the dark matter halo environment is a strong driver of galaxy star-formation rate and therefore of the evolution of the star-formation rate density in the Universe.
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