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

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 5-band photometry and low- and medium-resolution spectroscopy at optical rest wavelengths on a set of physical parameters characterizing the stars and the interstellar medium. Since these parameters cannot be known a priori for any galaxy sample, we assess the accuracy to which they can be retrieved 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 galaxy spectra provides valuable constraints on all physical parameters. At higher resolution, the analysis of the combined stellar and nebular emission in 12,660 SDSS star-forming galaxies using our approach yields likelihood distributions of stellar mass, gas-phase oxygen abundance, optical depth of the dust and specific star formation rate similar to those obtained in previous separate analyses of the stellar and nebular emission at the original (twice higher) SDSS spectral resolution. 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. [abridged]Comment: 24 pages, 19 figures, accepted for publication in MNRAS. Full-resolution version available from ftp://ftp.iap.fr/pub/from_users/pacifici/paper_pacifici_hr.pd

The Rise and Fall of Star Formation Histories of Blue Galaxies at Redshifts 0.2 < z < 1.4

Popular cosmological scenarios predict that galaxies form hierarchically from the merger of many progenitor, each with their own unique star formation history (SFH). We use the approach recently developed by Pacifici et al. to constrain the SFHs of 4517 blue (presumably star-forming) galaxies with spectroscopic redshifts in the range O.2 < z < 1:4 from the All-Wavelength Extended Groth Strip International Survey (AEGIS). This consists in the Bayesian analysis of the observed galaxy spectral ' energy distributions with 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 constrain the SFH of each galaxy in our sample by comparing the observed fluxes in the B, R,l and K(sub s) bands and rest-frame optical emission-line luminosities with those of one million model spectral energy distributions. We explore the dependence of the resulting SFH on galaxy stellar mass and redshift. We find that the average SFHs of high-mass galaxies rise and fall in a roughly symmetric bell-shaped manner, while those of low-mass galaxies rise progressively in time, consistent with the typically stronger activity of star formation in low-mass compared to high-mass galaxies. For galaxies of all masses, the star formation activity rises more rapidly at high than at low redshift. These findings imply that the standard approximation of exponentially declining SFHs wIdely used to interpret observed galaxy spectral energy distributions is not appropriate to constrain the physical parameters of star-forming galaxies at intermediate redshifts

AGN Feedback in SDSS-IV MaNGA: AGNs have Suppressed Central Star Formation Rates

Despite the importance of feedback from active galactic nuclei (AGNs) in models of galaxy evolution, observational constraints on the influence of AGN feedback on star formation remain weak. To this end, we have compared the star formation trends of 279 low-redshift AGN galaxies with 558 inactive control galaxies using integral field unit spectroscopy from the SDSS-IV MaNGA survey. With a Gaussian process-based methodology, we reconstruct nonparametric star formation histories in spatially resolved spaxels covering the face of each galaxy. Based on galaxy-wide star formation rates (SFRs) alone, we find no obvious signatures of AGN feedback. However, the AGN galaxies have significantly suppressed central (kiloparsec-scale) SFRs, lying up to a factor of $2$ below those of the control galaxies, providing direct observational evidence of AGN feedback suppressing star formation. The suppression of central SFRs in the AGN galaxies began in the central regions ${\sim} 6$ Gyr ago (redshift $z {\sim} 0.7$), taking place over a few gigayears. A small subset of the AGN galaxies were rapidly driven to quiescence shortly before being observed (in the last $500$ Myr), potentially indicating instances of AGN-driven feedback. More frequently, however, star formation continues in the AGN galaxies, with suppression primarily in the central regions. This is suggestive of a picture in which integrated (Gyr-timescale) AGN feedback can significantly affect central star formation, but may be inefficient in driving galaxy-wide quenching in low-redshift galaxies, instead leaving them in the green valley.Comment: 22 pages, 15 figures. Accepted for publication in Ap

z~2: An Epoch of Disk Assembly

We explore the evolution of the internal gas kinematics of star-forming galaxies from the peak of cosmic star-formation at $z\sim2$ to today. Measurements of galaxy rotation velocity $V_{rot}$, which quantify ordered motions, and gas velocity dispersion $\sigma_g$, which quantify disordered motions, are adopted from the DEEP2 and SIGMA surveys. This sample covers a continuous baseline in redshift from $z=2.5$ to $z=0.1$, spanning 10 Gyrs. At low redshift, nearly all sufficiently massive star-forming galaxies are rotationally supported ($V_{rot}>\sigma_g$). By $z=2$, the percentage of galaxies with rotational support has declined to 50$\%$ at low stellar mass ($10^{9}-10^{10}\,M_{\odot}$) and 70$\%$ at high stellar mass ($10^{10}-10^{11}M_{\odot}$). For $V_{rot}\,>\,3\,\sigma_g$, the percentage drops below 35$\%$ for all masses. From $z\,=\,2$ to now, galaxies exhibit remarkably smooth kinematic evolution on average. All galaxies tend towards rotational support with time, and it is reached earlier in higher mass systems. This is mostly due to an average decline in $\sigma_g$ by a factor of 3 since a redshift of 2, which is independent of mass. Over the same time period, $V_{rot}$ increases by a factor of 1.5 for low mass systems, but does not evolve for high mass systems. These trends in $V_{rot}$ and $\sigma_g$ with time are at a fixed stellar mass and should not be interpreted as evolutionary tracks for galaxy populations. When galaxy populations are linked in time with abundance matching, not only does $\sigma_g$ decline with time as before, but $V_{rot}$ strongly increases with time for all galaxy masses. This enhances the evolution in $V_{rot}/\sigma_g$. These results indicate that $z\,=\,2$ is a period of disk assembly, during which the strong rotational support present in today's massive disk galaxies is only just beginning to emerge.Comment: 12 pages, 8 figures, submitted to Ap

Recent stellar mass assembly of low-mass star-forming galaxies at redshifts 0.3 < z < 0.9

The epoch when low-mass star forming galaxies (LMSFGs) form the bulk of their stellar mass is uncertain. While some models predict an early formation, others favor a delayed scenario until later ages of the universe. We present constraints on the star formation histories (SFHs) of a sample of LMSFGs obtained through the analysis of their spectral energy distributions using a novel approach that (1) consistently combines photometric (broadband) and spectroscopic (equivalent widths of emission lines) data, and (2) uses physically motivated SFHs with non-uniform variations of the star formation rate (SFR) as a function of time. The sample includes 31 spectroscopically confirmed LMSFGs (7.3 < log M*/Msun < 8.0) at 0.3 < z_spec < 0.9 in the Extended-Chandra Deep Field-South field (E-CDF-S). Among them, 24 were selected with photometric stellar mass log M*/Msun < 8.0, 0.3 < z_phot < 1.0, and NB816 < 26 AB mag; the remaining 7 were selected as blue compact dwarfs (BCDs) within the same photometric redshift and magnitude ranges. We also study a secondary sample of 43 more massive spectroscopically confirmed galaxies (8.0 < log M*/Msun < 9.1), selected with the same criteria. The SFRs and stellar masses derived for both samples place our targets on the standard main sequence of star forming galaxies. The median SFH of LMSFGs at intermediate redshifts appears to form 90% of the median stellar mass inferred for the sample in the 0.5-1.8 Gyr immediately preceding the observation. These results suggest a recent stellar mass assembly for LMSFGs, consistent with the cosmological downsizing trends. We find similar median SFH timescales for the more massive secondary sample.Comment: 7 pages, 3 figures, 2 tables; ApJ, in pres

AGN Emission Line Diagnostics and the Mass-Metallicity Relation up to Redshift z~2: the Impact of Selection Effects and Evolution

Emission line diagnostic diagrams probing the ionization sources in galaxies, such as the Baldwin-Phillips-Terlevich (BPT) diagram, have been used extensively to distinguish AGN from purely star-forming galaxies. Yet, they remain poorly understood at higher redshifts. We shed light on this issue with an empirical approach based on a z~0 reference sample built from ~300,000 SDSS galaxies, from which we mimic selection effects due to typical emission line detection limits at higher redshift. We combine this low-redshift reference sample with a simple prescription for luminosity evolution of the global galaxy population to predict the loci of high-redshift galaxies on the BPT and Mass-Excitation (MEx) diagnostic diagrams. The predicted bivariate distributions agree remarkably well with direct observations of galaxies out to z~1.5, including the observed stellar mass-metallicity (MZ) relation evolution. As a result, we infer that high-redshift star-forming galaxies are consistent with having "normal" ISM properties out to z~1.5, after accounting for selection effects and line luminosity evolution. Namely, their optical line ratios and gas-phase metallicities are comparable to that of low-redshift galaxies with equivalent emission-line luminosities. In contrast, AGN narrow-line regions may show a shift toward lower metallicities at higher redshift. While a physical evolution of the ISM conditions is not ruled out for purely star-forming galaxies, and may be more important starting at z>2, we find that reliably quantifying this evolution is hindered by selections effects. The recipes provided here may serve as a basis for future studies toward this goal. Code to predict the loci of galaxies on the BPT and MEx diagnostic diagrams, and the MZ relation as a function of emission line luminosity limits, is made publicly available.Comment: Main article: 15 pages, 7 figures; Appendix: 13 pages, 11 figures. Revisions: Paper now accepted for publication in the Astrophysical Journal (same scientific content as previous arXiv version). IDL routines to make empirical predictions on the BPT, MEx, and M-Z plane are now released at https://sites.google.com/site/agndiagnostics/home/me