Relative merits of different types of rest-frame optical observations to constrain galaxy physical parameters

Abstract

We present a new approach to constrain galaxy physical parameters from the combined interpretation of stellar and nebular emission in wide ranges of observations. This approach relies on the Bayesian analysis of any type of galaxy spectral energy distribution using a comprehensive library of synthetic spectra assembled using state‐of‐the‐art models of star formation and chemical enrichment histories, stellar population synthesis, nebular emission and attenuation by dust. We focus on the constraints set by five‐band ugriz photometry and low‐ and medium‐resolution spectroscopy at rest wavelengths λ= 3600–7400 Å on a few physical parameters of galaxies: the observer‐frame absolute r‐band stellar mass‐to‐light ratio, M*/Lr; the fraction of a current galaxy stellar mass formed during the last 2.5 Gyr, fSFH; the specific star formation rate, ψS; the gas‐phase oxygen abundance, 12 + log(O/H); the total effective V‐band absorption optical depth of the dust, ; and the fraction of this arising from dust in the ambient interstellar medium, μ. Since these parameters cannot be known a priori for any galaxy sample, we assess the accuracy to which they can be retrieved from observations by simulating ‘pseudo‐observations’ using models with known parameters. Assuming that these models are good approximations of true galaxies, we find that the combined analysis of stellar and nebular emission in low‐resolution [50 Å full width at half‐maximum (FWHM)] galaxy spectra provides valuable constraints on all physical parameters. The typical uncertainties in high‐quality spectra are about 0.13 dex for M*/Lr, 0.23 for fSFH, 0.24 dex for ψS, 0.28 for 12 + log(O/H), 0.64 for and 0.16 for μ. The uncertainties in 12 + log(O/H) and tighten by about 20 per cent for galaxies with detectable emission lines and by another 45 per cent when the spectral resolution is increased to 5 Å FWHM. At this spectral resolution, the analysis of the combined stellar and nebular emission in the high‐quality spectra of 12 660 Sloan Digital Sky Survey (SDSS) star‐forming galaxies using our approach yields likelihood distributions of M★, 12 + log(O/H), and ψS similar to those obtained in previous separate analyses of the stellar and nebular emission at the original (twice higher) SDSS spectral resolution. Meanwhile, rest‐frame ugriz photometry provides competitive constraints on M*/Lr. We show that the constraints derived on galaxy physical parameters from these different types of observations depend sensitively on signal‐to‐noise ratio. Our approach can be extended to the analysis of any type of observation across the wavelength range covered by spectral evolution models.

firefly (Fitting IteRativEly For Likelihood analYsis): a full spectral fitting code

“…Here we adopt the Calzetti law as in Calzetti et al (2000), using a range of values of E(B − V) from 0.0 to 1.0. This law has been shown to be generally applicable to good accuracy to many different types of galaxies such as those from the SDSS surveys used in this work (Chevallard et al (2013), Pacifici et al (2012)). We assume that a dust screen is uniform across the whole galaxy and thus is applied to the SSP combinations equally.…”

Encoding of the infrared excess in theNUVrKcolor diagram for star-forming galaxies

“…To confirm the validity of this approach, and to explain the physical origin of the observed trends, we appeal to a library of galaxy SEDs computed with the BC03 spectral evolution model. We follow the approach of Pacifici et al (2012) and extract a set of star formation and chemical enrichment histories from the semi-analytic post-treatment of De Lucia & Blaizot (2007) of the Millennium cosmological simulation (Springel et al 2005). The star formation and chemical enrichment histories computed in this way reproduce the mean properties of nearby SDSS galaxies.…”

“…Thus, they span only limited ranges in sSFR, around 10 −10 yr −1 , and in the fraction of the current stellar mass formed in the last 2.5 Gyr at z = 0. Following Pacifici et al (2012), to account for the broader range of spectral properties of the galaxies in our sample with respect to the SDSS, we re-draw the evolutionary stage at which a galaxy is looked at in the library of star formation and chemical enrichment histories (we do this uniformly in redshift between 0.2 and 1.5). We also resample the current (i.e., averaged over the last 30 Myr) SFR from a Gaussian distribution centered on log (sSFR) = −9.1, with a dispersion of 0.6.…”

Ultraviolet to infrared emission ofz> 1 galaxies: Can we derive reliable star formation rates and stellar masses?

“…We refer to Conroy & Gunn (2010) for a comparison of the most popular population synthesis models in the framework of SED modelling. Numerous papers have been published to explore the reliability of SED fitting techniques, most of which are based on artificial catalogues of galaxies drawn from semianalytical models or hydrodynamical codes (Wuyts et al 2009;Lee et al 2009;Pforr et al 2012;Pacifici et al 2012;Mitchell et al 2013). These studies are all based on UV-optical and NIR data, but do not include IR emission.…”