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Register a new userExtinction in Star-Forming Disk Galaxies from Inclination-Dependent Composite Spectra
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Extinction in galaxies affects their observed properties. In scenarios describing the distribution of dust and stars in individual disk galaxies, the amplitude of the extinction can be modulated by the inclination of the galaxies. In this work we investigate the inclination dependency in composite spectra of star-forming disk galaxies from the Sloan Digital Sky Survey Data Release 5. In a volume-limited sample within a redshift range 0.065-0.075 and a r-band Petrosian absolute magnitude range -19.5 to -22 which exhibits a flat distribution of inclination, the inclined relative to face-on extinction in the stellar continuum is found empirically to increase with inclination in the g, r, and i bands. Within the central 0.5 intrinsic half-light radius of the galaxies, the g-band relative extinction in the stellar continuum for the highly-inclined objects (axis ratio b/a = 0.1) is 1.2 mag, agreeing with previous studies. The extinction curve of the disk galaxies is given in the restframe wavelengths 3700-8000 angstrom, identified with major optical emission and absorption lines in diagnostics. The Balmer decrement remains constant with inclination, suggesting a different kind of dust configuration and/or reddening mechanism in the HII region from that in the stellar continuum. One factor is shown to be the presence of spatially non-uniform interstellar extinction, presumably caused by clumped dust in the vicinity of the HII region.
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We analyze the absolute magnitude (M_r) and color (u-r) of low redshift (z<0.06) galaxies in the Sloan Digital Sky Survey Data Release 6. galaxies with nearly exponential profiles (Sloan parameter fracDeV < 0.1) fall on the blue sequence of the color - magnitude diagram; if, in addition, these exponential galaxies have M_r < -19, they show a dependence of u-r color on apparent axis ratio q expected for a dusty disk galaxy. By fitting luminosity functions for exponential galaxies with different values of q, we find that the dimming is well described by the relation Delta M_r = 1.27 (log q)^2, rather than the Delta M = C log q law that is frequently assumed. When the absolute magnitudes of bright exponential galaxies are corrected to their face-on value, M_r^f = M_r - Delta M_r, the average u-r color is linearly dependent on M_r^f for a given value of q. Nearly face-on exponential galaxies (q > 0.9) have a shallow dependence of mean u-r color on M_r^f (0.096 magnitudes redder for every magnitude brighter). By comparison, nearly edge-on exponential galaxies (q < 0.3) are 0.265 magnitudes redder for every magnitude brighter. When the dimming law Delta M = 1.27 (log q)^2 is used to create an inclination-corrected sample of bright exponential galaxies, their apparent shapes are confirmed to be consistent with a distribution of mildly non-circular disks, with median short-to-long axis ratio gamma = 0.22 and median disk ellipticity epsilon = 0.08.
We develop and implement an inclination-dependent attenuation prescription for spectral energy distribution (SED) fitting and study its impact on derived star-formation histories. We apply our prescription within the SED fitting code Lightning to a clean sample of 82, z = 0.21-1.35 disk-dominated galaxies in the Great Observatories Origins Deep Survey (GOODS) North and South fields. To compare our inclination-dependent attenuation prescription with more traditional fitting prescriptions, we also fit the SEDs with the inclination-independent Calzetti et al. (2000) attenuation curve. From this comparison, we find that fits to a subset of 58, z < 0.7 galaxies in our sample, utilizing the Calzetti et al. (2000) prescription, recover similar trends with inclination as the inclination-dependent fits for the FUV-band attenuation and recent star-formation rates. However, we find a difference between prescriptions in the optical attenuation (AV) that is strongly correlated with inclination (p-value < 10^-10). For more face-on galaxies, with i < 50 deg, (edge-on, i = 90 deg), the average derived AV is 0.30 +/- 0.10 magnitudes lower (0.55 +/- 0.15 magnitudes higher) for the inclination-dependent model compared to traditional methods. Further, the ratio of stellar masses between prescriptions also has a significant (p-value < 10^-2) trend with inclination. For i = 0-65 deg, stellar masses are systematically consistent between fits, with log(Mstar_inc/Mstar_Calzetti) = -0.05 +/- 0.03 dex, and scatter of 0.11 dex. However, for i = 80-90 deg, derived stellar masses are lower for the Calzetti et al. (2000) fits by an average factor of 0.17 +/- 0.02 dex, and scatter of 0.13 dex. Therefore, these results suggest that SED fitting assuming the Calzetti et al. (2000) attenuation law potentially underestimates stellar masses in highly inclined disk-dominated 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.
The radio continuum spectra of 14 star-forming galaxies are investigated by fitting nonthermal (synchrotron) and thermal (free-free) radiation laws. The underlying radio continuum measurements cover a frequency range of ~325 MHz to 24.5 GHz (32 GHz in case of M82). It turns out that most of these synchrotron spectra are not simple power-laws, but are best represented by a low-frequency spectrum with a mean slope alpha_nth = 0.59 +/- 0.20 (S_nu ~ nu^-alpha), and by a break or an exponential decline in the frequency range of 1 - 12 GHz. Simple power-laws or mildly curved synchrotron spectra lead to unrealistically low thermal flux densities, and/or to strong deviations from the expected optically thin free-free spectra with slope alpha_th = 0.10 in the fits. The break or cutoff energies are in the range of 1.5 - 7 GeV. We briefly discuss the possible origin of such a cutoff or break. If the low-frequency spectra obtained here reflect the injection spectrum of cosmic-ray electrons, they comply with the mean spectral index of Galactic supernova remnants. A comparison of the fitted thermal flux densities with the (foreground-corrected) Halpha fluxes yields the extinction, which increases with metallicity. The fraction of thermal emission is higher than believed hitherto, especially at high frequencies, and is highest in the dwarf galaxies of our sample, which we interpret in terms of a lack of containment in these low-mass systems, or a time effect caused by a very young starburst.
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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