How to Measure Galaxy Star Formation Histories. II. Nonparametric Models

Nonparametric star formation histories (SFHs) have long promised to be the `gold standard' for galaxy spectral energy distribution (SED) modeling as they are flexible enough to describe the full diversity of SFH shapes, whereas parametric models rule out a significant fraction of these shapes {\it a priori}. However, this flexibility is not fully constrained even with high-quality observations, making it critical to choose a well-motivated prior. Here, we use the SED-fitting code \texttt{Prospector} to explore the effect of different nonparametric priors by fitting SFHs to mock UV-IR photometry generated from a diverse set of input SFHs. First, we confirm that nonparametric SFHs recover input SFHs with less bias and return more accurate errors than do parametric SFHs. We further find that, while nonparametric SFHs robustly recover the overall shape of the input SFH, the primary determinant of the size and shape of the posterior star formation rate (SFR) as a function of time is the choice of prior, rather than the photometric noise. As a practical demonstration, we fit the UV-IR photometry of $\sim$6000 galaxies from the GAMA survey and measure inter-prior scatters in mass (0.1 dex), SFR$_{100\; \mathrm{Myr}}$ (0.8 dex), and mass-weighted ages (0.2 dex), with the bluest star-forming galaxies showing the most sensitivity. An important distinguishing characteristic for nonparametric models is the characteristic timescale for changes in SFR(t). This difference controls whether galaxies are assembled in bursts or in steady-state star formation, corresponding respectively to (feedback-dominated/accretion-dominated) models of galaxy formation and to (larger/smaller) confidence intervals derived from SED-fitting. High-quality spectroscopy has the potential to further distinguish between these proposed models of SFR(t).Comment: replacing with ApJ accepted versio

Maximising the power of deep extragalactic imaging surveys with the James Webb Space Telescope

We present a new analysis of the potential power of deep, near-infrared, imaging surveys with the James Webb Space Telescope (JWST) to improve our knowledge of galaxy evolution. In this work we properly simulate what can be achieved with realistic survey strategies, and utilise rigorous signal:noise calculations to calculate the resulting posterior constraints on the physical properties of galaxies. We explore a broad range of assumed input galaxy types (>20,000 models, including extremely dusty objects) across a wide redshift range (out to z~12), while at the same time considering a realistic mix of galaxy properties based on our current knowledge of the evolving population (as quantified through the Empirical Galaxy Generator: EGG). While our main focus is on imaging surveys with NIRCam, spanning lambda(obs) = 0.6-5.0 microns, an important goal of this work is to quantify the impact/added-value of: i) parallel imaging observations with MIRI at longer wavelengths, and ii) deeper supporting optical/UV imaging with HST (potentially prior to JWST launch) in maximising the power and robustness of a major extragalactic NIRCam survey. We show that MIRI parallel 7.7-micron imaging is of most value for better constraining the redshifts and stellar masses of the dustiest (A_V > 3) galaxies, while deep B-band imaging (reaching~28.5 AB mag) with ACS on HST is vital for determining the redshifts of the large numbers of faint/low-mass, z < 5 galaxies that will be detected in a deep JWST NIRCam survey.Comment: 19 Pages, 11 Figures, Submitted to MNRA

The evolution of the galaxy stellar mass function over the last twelve billion years from a combination of ground-based and HST surveys

We present a new determination of the galaxy stellar mass function (GSMF) over the redshift interval $0.25 \leq z \leq 3.75$, derived from a combination of ground-based and Hubble Space Telescope (HST) imaging surveys. Based on a near-IR selected galaxy sample selected over a raw survey area of 3 deg$^{2}$ and spanning $\geq 4$ dex in stellar mass, we fit the GSMF with both single and double Schechter functions, carefully accounting for Eddington bias to derive both observed and intrinsic parameter values. We find that a double Schechter function is a better fit to the GSMF at all redshifts, although the single and double Schechter function fits are statistically indistinguishable by $z=3.25$. We find no evidence for significant evolution in $M^{\star}$, with the intrinsic value consistent with $\log_{10}(M^{\star} / M_{\odot})=10.55\pm{0.1}$ over the full redshift range. Overall, our determination of the GSMF is in good agreement with recent simulation results, although differences persist at the highest stellar masses. Splitting our sample according to location on the UVJ plane, we find that the star-forming GSMF can be adequately described by a single Schechter function over the full redshift range, and has not evolved significantly since $z\simeq 2.5$. In contrast, both the normalization and functional form of the passive GSMF evolves dramatically with redshift, switching from a single to a double Schechter function at $z \leq 1.5$. As a result, we find that while passive galaxies dominate the integrated stellar-mass density at $z \leq 0.75$, they only contribute $\lesssim 10$ per cent by $z\simeq 3$. Finally, we provide a simple parameterization that provides an accurate estimate of the GSMF, both observed and intrinsic, at any redshift within the range $0 \leq z \leq 4$.Comment: 24 pages, 16 figures, accepted for publication in MNRA

On the simultaneous modelling of dust and stellar populations for interpretation of galaxy properties

The physical properties of galaxies are encoded within their spectral energy distribution and require comparison with models to be extracted. These models must contain a synthetic stellar population and, where infrared data are to be used, also consider prescriptions for energy reprocessing and re-emission by dust. While many such models have been constructed, there are few analyses of the impact of stellar population model choice on derived dust parameters, or vice versa. Here, we apply a simple framework to compare the impact of these choices, combining three commonly used stellar population synthesis models and three dust emission models. We compare fits to the ultraviolet to far-infrared spectral energy distributions of a validation sample of infrared-luminous galaxies. We find that including different physics, such as binary stellar evolution, in the stellar synthesis model can introduce biases and uncertainties in the derived parameters of the dust and stellar emission models, largely due to differences in the far-ultraviolet emission available for reprocessing. This may help to reconcile the discrepancy between the cosmic star formation rate and stellar mass density histories. Notably the inclusion of a dusty stellar birth cloud component in the dust emission model provides more flexibility in accommodating the stellar population model, as its re-emission is highly sensitive to the ultraviolet radiation field spectrum and density. Binary populations favour a longer birth cloud dissipation time-scale than is found when assuming only single star population synthesis

The vandels survey: The star-formation histories of massive quiescent galaxies at 1.0 < z < 1.3

We present a Bayesian full-spectral-fitting analysis of 75 massive (⁠M∗>1010.3M⊙⁠) UVJ-selected galaxies at redshifts of 1.0 1011M⊙

The VANDELS survey: Dust attenuation in star-forming galaxies at z=3−4\mathbf{z=3-4}

We present the results of a new study of dust attenuation at redshifts $3 < z < 4$ based on a sample of $236$ star-forming galaxies from the VANDELS spectroscopic survey. Motivated by results from the First Billion Years (FiBY) simulation project, we argue that the intrinsic spectral energy distributions (SEDs) of star-forming galaxies at these redshifts have a self-similar shape across the mass range $8.2 \leq$ log$(M_{\star}/M_{\odot}) \leq 10.6$ probed by our sample. Using FiBY data, we construct a set of intrinsic SED templates which incorporate both detailed star formation and chemical abundance histories, and a variety of stellar population synthesis (SPS) model assumptions. With this set of intrinsic SEDs, we present a novel approach for directly recovering the shape and normalization of the dust attenuation curve. We find, across all of the intrinsic templates considered, that the average attenuation curve for star-forming galaxies at $z\simeq3.5$ is similar in shape to the commonly-adopted Calzetti starburst law, with an average total-to-selective attenuation ratio of $R_{V}=4.18\pm0.29$. We show that the optical attenuation ($A_V$) versus stellar mass ($M_{\star}$) relation predicted using our method is consistent with recent ALMA observations of galaxies at $2<z<3$ in the \emph{Hubble} \emph{Ultra} \emph{Deep} \emph{Field} (HUDF), as well as empirical $A_V - M_{\star}$ relations predicted by a Calzetti-like law. Our results, combined with other literature data, suggest that the $A_V - M_{\star}$ relation does not evolve over the redshift range $0<z<5$, at least for galaxies with log$(M_{\star}/M_{\odot}) \gtrsim 9.5$. Finally, we present tentative evidence which suggests that the attenuation curve may become steeper at log$(M_{\star}/M_{\odot}) \lesssim 9.0$.Comment: 16 pages, 12 figures, accepted for publication in MNRA

The VANDELS survey: Dust attenuation in star-forming galaxies at z=3−4\mathbf{z=3-4}

We present the results of a new study of dust attenuation at redshifts $3 < z < 4$ based on a sample of $236$ star-forming galaxies from the VANDELS spectroscopic survey. Motivated by results from the First Billion Years (FiBY) simulation project, we argue that the intrinsic spectral energy distributions (SEDs) of star-forming galaxies at these redshifts have a self-similar shape across the mass range $8.2 \leq$ log$(M_{\star}/M_{\odot}) \leq 10.6$ probed by our sample. Using FiBY data, we construct a set of intrinsic SED templates which incorporate both detailed star formation and chemical abundance histories, and a variety of stellar population synthesis (SPS) model assumptions. With this set of intrinsic SEDs, we present a novel approach for directly recovering the shape and normalization of the dust attenuation curve. We find, across all of the intrinsic templates considered, that the average attenuation curve for star-forming galaxies at $z\simeq3.5$ is similar in shape to the commonly-adopted Calzetti starburst law, with an average total-to-selective attenuation ratio of $R_{V}=4.18\pm0.29$. We show that the optical attenuation ($A_V$) versus stellar mass ($M_{\star}$) relation predicted using our method is consistent with recent ALMA observations of galaxies at $2<z<3$ in the \emph{Hubble} \emph{Ultra} \emph{Deep} \emph{Field} (HUDF), as well as empirical $A_V - M_{\star}$ relations predicted by a Calzetti-like law. Our results, combined with other literature data, suggest that the $A_V - M_{\star}$ relation does not evolve over the redshift range $0<z<5$, at least for galaxies with log$(M_{\star}/M_{\odot}) \gtrsim 9.5$. Finally, we present tentative evidence which suggests that the attenuation curve may become steeper at log$(M_{\star}/M_{\odot}) \lesssim 9.0$.Comment: 16 pages, 12 figures, accepted for publication in MNRA

The connection between stellar mass, age and quenching timescale in massive quiescent galaxies at z≃1z \simeq 1

We present a spectro-photometric study of a mass-complete sample of quiescent galaxies at $1.0 < z < 1.3$ with $\mathrm{log_{10}}(M_{\star}/\mathrm{M_{\odot}}) \geq 10.3$ drawn from the VANDELS survey, exploring the relationship between stellar mass, age and star-formation history. Within our sample of 114 galaxies, we derive a stellar-mass vs stellar-age relation with a slope of $1.20^{+0.28}_{-0.27}$ Gyr per decade in stellar mass. When combined with recent literature results, we find evidence that the slope of this relation remains consistent over the redshift interval $0<z<4$. The galaxies within the VANDELS quiescent display a wide range of star-formation histories, with a mean star-formation timescale of $1.5\pm{0.1}$ Gyr and a mean quenching timescale of $1.4\pm{0.1}$ Gyr. We also find a large scatter in the quenching timescales of the VANDELS quiescent galaxies, in agreement with previous evidence that galaxies at $z \sim 1$ cease star formation via multiple mechanisms. We then focus on the oldest galaxies in our sample, finding that the number density of galaxies that quenched before $z = 3$ with stellar masses $\mathrm{log_{10}}(M_{\star}/\mathrm{M_{\odot}}) \geq 10.6$ is $1.12_{-0.72}^{+1.47} \times 10^{-5} \ \mathrm{Mpc}^{-3}$. Although uncertain, this estimate is in good agreement with the latest observational results at $3<z<4$, tentatively suggesting that neither rejuvenation nor merger events are playing a major role in the evolution of the oldest massive quiescent galaxies within the redshift interval $1<z<3$.Comment: Accepted for publication in MNRAS, 11 pages, 6 figure