We present Keck/MOSFIRE observations of UV metal lines in four bright gravitationally-lensed z~6-8 galaxies behind the cluster Abell 1703. The spectrum of A1703-zd6, a highly-magnified star forming galaxy with a Lyman-alpha redshift of z=7.045, revea
ls a confident detection of the nebular CIV emission line (unresolved with FWHM < 125 km/s). UV metal lines are not detected in the three other galaxies. At z~2-3, nebular CIV emission is observed in just 1% of UV-selected galaxies. The presence of strong CIV emission in one of the small sample of galaxies targeted in this paper may indicate hard ionizing spectra are more common at z~7. The total estimated equivalent width of the CIV doublet (38 A) and CIV/Lyman-alpha flux ratio (0.3) are comparable to measurements of narrow-lined AGNs. Photoionization models show that the nebular CIV line can also be reproduced by a young stellar population, with very hot metal poor stars dominating the photon flux responsible for triply ionizing carbon. Regardless of the origin of the CIV, we show that the ionizing spectrum of A1703-zd6 is different from that of typical galaxies at z~2, producing more H ionizing photons per unit 1500A luminosity and a larger flux density at 30-50 eV. If such extreme radiation fields are typical in UV-selected systems at z>7, it would indicate that reionization-era galaxies are more efficient ionizing agents than previously thought. Alternatively, we suggest that the small sample of Lyman-alpha emitters at z>7 may trace a rare population with intense radiation fields capable of ionizing their surrounding hydrogen distribution. Additional constraints on high ionization emission lines in galaxies with and without Lyman-alpha detections will help clarify whether hard ionizing spectra are common in the reionization era.
Deep spectroscopic observations of z~6.5 galaxies have revealed a marked decline with increasing redshift in the detectability of Lyman-alpha emission. While this may offer valuable insight into the end of the reionisation process, it presents a fund
amental challenge to the detailed spectroscopic study of the many hundreds of photometrically-selected distant sources now being found via deep HST imaging, and particularly those bright sources viewed through foreground lensing clusters. In this paper we demonstrate the validity of a new way forward via the convincing detection of an alternative diagnostic line, CIII]1909, seen in spectroscopic exposures of two star forming galaxies at z=6.029 and 7.213. The former detection is based on a 3.5 hour X-shooter spectrum of a bright (J=25.2) gravitationally-lensed galaxy behind the cluster Abell 383. The latter detection is based on a 4.2 hour MOSFIRE spectra of one of the most distant spectroscopically confirmed galaxies, GN-108036, with J=25.2. Both targets were chosen for their continuum brightness and previously-known redshift (based on Lyman-alpha), ensuring that any CIII] emission would be located in a favorable portion of the near-infrared sky spectrum. We compare our CIII] and Lyman-alpha equivalent widths in the context of those found at z~2 from earlier work and discuss the motivation for using lines other than Lyman-alpha to study galaxies in the reionisation era.
We present new spectro-photometric NIR observations of 16 post-starburst galaxies especially designed to test for the presence of strong carbon features of thermally pulsing AGB (TP-AGB) stars, as predicted by recent models of stellar population synt
hesis. Selection based on clear spectroscopic optical features indicating the strong predominance of stellar populations with ages between 0.5 and 1.5 Gyr and redshift around 0.2 allows us to probe the spectral region that is most affected by the carbon features of TP-AGB stars (unaccessible from the ground for z~0 galaxies) in the evolutionary phase when their impact on the IR luminosity is maximum. Nevertheless, none of the observed galaxies display such features. Moreover the NIR fluxes relative to optical are consistent with those predicted by the original Bruzual & Charlot (2003) models, where the impact of TP-AGB stars is much lower than has been recently advocated.
We present a simple, physically-motivated model to interpret consistently the emission from galaxies at ultraviolet, optical and infrared wavelengths. We combine this model with a Bayesian method to obtain robust statistical constraints on key parame
ters describing the stellar content, star formation activity and dust content of galaxies. Our model is now publicly available via a user-friendly code package, MAGPHYS at www.iap.fr/magphys. We present an application of this model to interpret a sample of ~1400 local (z<0.5) galaxies from the H-ATLAS survey. We find that, for these galaxies, the diffuse interstellar medium, powered mainly by stars older than 10 Myr, accounts for about half the total infrared luminosity. We discuss the implications of this result to the use of star formation rate indicators based on total infrared luminosity.
The mass of super massive black holes at the centre of galaxies is tightly correlated with the mass of the galaxy bulges which host them. This observed correlation implies a mechanism of joint growth, but the precise physical processes responsible ar
e a matter of some debate. Here we report on the growth of black holes in 400 local galactic bulges which have experienced a strong burst of star formation in the past 600Myr. The black holes in our sample have typical masses of 10^6.5-10^7.5 solar masses, and the active nuclei have bolometric luminosities of order 10^42-10^44erg/s. We combine stellar continuum indices with H-alpha luminosities to measure a decay timescale of ~300Myr for the decline in star formation after a starburst. During the first 600Myr after a starburst, the black holes in our sample increase their mass by on-average 5% and the total mass of stars formed is about 1000 times the total mass accreted onto the black hole. This ratio is similar to the ratio of stellar to black hole mass observed in present-day bulges. We find that the average rate of accretion of matter onto the black hole rises steeply roughly 250Myr after the onset of the starburst. We show that our results are consistent with a simple model in which 0.5% of the mass lost by intermediate mass stars in the bulge is accreted by the black hole, but with a suppression in the efficiency of black hole growth at early times plausibly caused by supernova feedback, which is stronger at earlier times. We suggest this picture may be more generally applicable to black hole growth, and could help explain the strong correlation between bulge and black hole mass.
(Abridged) We assemble a sample of 3258 low-redshift galaxies from the SDSS DR6 with complementary photometric observations by GALEX, 2MASS and IRAS at far-ultraviolet and infrared wavelengths. We use a recent, simple but physically motivated model t
o interpret the observed spectral energy distributions of the galaxies in this sample in terms of statistical constraints on physical parameters describing the star formation history and dust content. The focus on a subsample of 1658 galaxies with highest S/N observations enables us to investigate most clearly several strong correlations between various derived physical properties of galaxies. We find that the typical dust mass of a star-forming correlates remarkably well with the star formation rate (SFR). We also find that the dust-to-stellar mass ratio, the ratio of dust mass to star formation rate and the fraction of dust luminosity contributed by the diffuse interstellar medium all correlate strongly with specific SFR. A comparison with recent models of chemical and dust evolution of galaxies suggests that these correlations could arise, at least in part, from an evolutionary sequence. As galaxies form stars, their ISM becomes enriched in dust, while the drop in gas supply makes the specific SFR decrease. Interestingly, as a result, a young, actively star-forming galaxy with low dust-to-gas ratio may still be highly dusty because it contains large amounts of interstellar gas. This may be important for the interpretation of the infrared emission from young, gas-rich star-forming galaxies at high redshift. Our study provides a useful local reference for future statistical studies of the star formation and dust properties of galaxies at high redshifts.
We present a simple, largely empirical but physically motivated model, which is designed to interpret consistently multi-wavelength observations from large samples of galaxies in terms of physical parameters, such as star formation rate, stellar mass
and dust content. Our model is both simple and versatile enough to allow the derivation of statistical constraints on the star formation histories and dust contents of large samples of galaxies using a wide range of ultraviolet, optical and infrared observations. We illustrate this by deriving median-likelihood estimates of a set of physical parameters describing the stellar and dust contents of local star-forming galaxies from the Spitzer Infrared Nearby Galaxy Sample (SINGS) and from a newly-matched sample of SDSS galaxies observed with GALEX, 2MASS, and IRAS. The model reproduces well the observed spectral energy distributions of these galaxies across the entire wavelength range from the far-ultraviolet to the far-infrared. We find important correlations between the physical parameters of galaxies which are useful to investigate the star formation activity and dust properties of galaxies. Our model can be straightforwardly applied to interpret observed ultraviolet-to-infrared spectral energy distributions (SEDs) from any galaxy sample.
We present optical and IR integrated colours and SBF magnitudes, computed from stellar population synthesis models that include emission from the dusty envelopes surrounding TP-AGB stars undergoing mass-loss. We explore the effects of varying the mas
s-loss rate by one order of magnitude around the fiducial value, modifying accordingly both the stellar parameters and the output spectra of the TP-AGB stars plus their dusty envelopes. The models are single burst, and range in age from a few Myr to 14 Gyr, and in metallicity between $Z$ = 0.0001 and $Z$ = 0.07; they combine new calculations for the evolution of stars in the TP-AGB phase, with star plus envelope SEDs produced with the radiative transfer code DUSTY. We compare these models to optical and near-IR data of single AGB stars and Magellanic star clusters. This comparison validates the current understanding of the role of mass-loss in determining stellar parameters and spectra in the TP-AGB. However, neither broad-band colours nor SBF measurements in the optical or the near-IR can discern global changes in the mass-loss rate of a stellar population. We predict that mid-IR SBF measurements can pick out such changes, and actually resolve whether a relation between metallicity and mass-loss exists.
We measure star formation rates of ~50,000 optically-selected galaxies in the local universe (z~0.1), spanning a range from gas-rich dwarfs to massive ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (UV) and SDSS (optical) photometry
to a library of population synthesis models that include dust attenuation. For star-forming galaxies, our UV-based SFRs compare remarkably well with those derived from SDSS H alpha. Deviations from perfect agreement between these two methods are due to differences in the dust attenuation estimates. In contrast to H alpha, UV provides reliable SFRs for galaxies with weak or no H alpha emission, and where H alpha is contaminated with an emission from an AGN. We use full-SED SFRs to calibrate a simple prescription that uses GALEX UV magnitudes to produce good SFRs for normal star-forming galaxies. The specific SFR is considered as a function of stellar mass for (1) star-forming galaxies with no AGN, (2) those hosting an AGN, and for (3) galaxies without H alpha emission. We find that the three have distinct star formation histories, with AGN lying intermediate between the star-forming and the quiescent galaxies. Normal star forming galaxies (without an AGN) lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. Weak AGN, while also massive, have lower SFR, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent (red sequence) galaxies via strong and weak AGN. We confirm that some galaxies with no H alpha emission show signs of SF in the UV. We derive a UV-based cosmic SFR density at z=0.1 with smaller total error than previous measurements (abridged).
