Individual Star Formation Histories Research Articles

A Census of Star Formation Histories of Massive Galaxies at 0.6 < z < 1 from Spectrophotometric Modeling Using Bagpipes and Prospector

We present individual star formation histories (SFHs) of ∼3000 massive galaxies (log(M */M ⊙) > 10.5) from the Large Early Galaxy Astrophysics Census spectroscopic survey at a lookback time of ∼7 billion yr and quantify the population trends leveraging 20 hr deep-integrated spectra of these ∼1800 star-forming and ∼1200 quiescent galaxies at 0.6 < z < 1.0. Essentially all galaxies at this epoch contain stars of age <3 Gyr, in contrast with older massive galaxies today, facilitating better recovery of previous generations of star formation at cosmic noon and earlier. We conduct spectrophotometric analysis using parametric and nonparametric Bayesian stellar population synthesis modeling tools—Bagpipes and Prospector—to constrain the median SFHs of this mass complete sample and characterize population trends. A consistent picture arises for the late-time stellar mass growth when quantified as t 50 and t 90, corresponding to the age of the Universe when galaxies formed 50% and 90% of their total stellar mass, although the two methods disagree at the earliest formation times (e.g., t 10). Our results reveal trends in both stellar mass and stellar velocity dispersion as in the local Universe—low-mass galaxies with shallower potential wells grow their stellar masses later in cosmic history compared to high-mass galaxies. Unlike local quiescent galaxies, the median duration of late-time star formation (τ SF,late = t 90–t 50) does not consistently depend on the stellar mass. This census sets a benchmark for future deep spectrophotometric studies of the more distant Universe.

Constraining delay time distribution of binary neutron star mergers from host galaxy properties

ABSTRACT Gravitational wave (GW) observatories are discovering binary neutron star mergers (BNSMs), and in at least one event we were able to track it down in multiple wavelengths of light, which allowed us to identify the host galaxy. Using a catalogue of local galaxies with inferred star formation histories and adopting a BNSM delay time distribution (DTD) model, we investigate the dependence of BNSM rate on an array of galaxy properties. Compared to the intrinsic property distribution of galaxies, that of BNSM host galaxies is skewed towards galaxies with redder colour, lower specific star formation rate, higher luminosity, and higher stellar mass, reflecting the tendency of higher BNSM rates in more massive galaxies. We introduce a formalism to efficiently make forecast on using host galaxy properties to constrain DTD models. We find comparable constraints from the dependence of BNSM occurrence distribution on galaxy colour, specific star formation rate, and stellar mass, all better than those from dependence on r-band luminosity. The tightest constraints come from using individual star formation histories of host galaxies, which reduces the uncertainties on DTD parameters by a factor of three or more. Substantially different DTD models can be differentiated with about 10 BNSM detections. To constrain DTD parameters at 10 per cent precision level requires about one hundred detections, achievable with GW observations on a decade time-scale.

Measuring the Delay Time Distribution of Binary Neutron Stars. III. Using the Individual Star Formation Histories of Gravitational-wave Event Host Galaxies in the Local Universe

Abstract In Paper I we studied the determination of the delay time distribution (DTD) of binary neutron stars (BNSs) through scaling relations between halo/stellar mass and the star formation history (SFH) of galaxies hosting gravitational-wave (GW) events in the local universe. Here we explore how a detailed reconstruction of the individual SFHs of BNS merger host galaxies can improve on the use of the scaling relations. We use galaxies from the Galaxy and Mass Assembly survey, which is mass complete at M * &gt; 109 M ⊙ in the redshift range 0.05 &lt; z &lt; 0.08. We use the reconstructed SFHs derived from the Prospector code for two distinct sets of priors (favoring continuous and bursty SFHs), and convolve those with power-law DTDs characterized by an index Γ and a minimum delay time t min. We find that with this approach host galaxies are required to constrain the DTD parameters, with the number depending on the choice of SFH prior and on the parameters of the true DTD. We further show that using only the host galaxies of BNS mergers, as opposed to the full population of potential host galaxies in the relevant cosmic volume, leads to a minor bias in the recovered DTD parameters. The required host galaxy sample size is nearly an order of magnitude smaller relative to the approach of using scaling relations, and we expect such a host galaxy sample to be collected within a decade or two, prior to the advent of third-generation GW detectors.

Full-spectral fitting techniques to characterise the stellar content of ultra diffuse galaxies

AbstractUnderstanding the peculiar properties of Ultra Diffuse Galaxies (UDGs) via spectroscopic analysis is a challenging task that is now becoming feasible. The advent of 10m-class telescopes and high sensitivity instruments is enabling the gathering of high quality spectra even for the faintest systems. In addition, advances in the modelling of stellar populations, stellar libraries, and full-spectral fitting codes are allowing the recovery of the stellar content shaping those spectra with unprecedented reliability. In this contribution we report on the extensive tests we have carried out using the inversion code STECKMAP. The similarities between the Star Formation Histories (SFH) recovered from STECKMAP (applied to high-quality spectra) and deep Colour-Magnitude diagrams fitting (resolved stars) in two Local Group dwarf galaxies (LMC and LeoA) are remarkable, demonstrating the impressive performance of STECKMAP. We exploit the capabilities of STECKMAP and perform one of the most complete and reliable characterisations of the stellar component of UDGs to date using deep spectroscopic data. We measure radial and rotation velocities, SFHs and mean population parameters, such as ages and metallicities, for a sample of five UDG candidates in the Coma cluster. From the radial velocities, we confirm the Coma membership of these galaxies. We find that their rotation properties, if detected at all, are compatible with dwarf-like galaxies. The SFHs of the UDG are dominated by old (∼ 7 Gyr), metal-poor ([M/H] ∼ -1.1) and alpha-enhanced ([Mg/Fe]∼ 0.4) populations followed by a smooth or episodic decline which halted ∼ 2 Gyr ago, possibly a sign of cluster-induced quenching. We find no obvious correlation between individual SFH shapes and any UDG morphological properties. The recovered stellar properties for UDGs are similar to those found for DDO 44, a local UDG analogue resolved into stars. We conclude that the UDGs in our sample are extended dwarfs whose properties are likely the outcome of both internal processes, such as bursty SFHs and/or high-spin haloes, as well as environmental effects within the Coma cluster.

Star Formation Histories of z ∼ 1 Galaxies in LEGA-C

Abstract Using high-resolution spectra from the VLT Large Early Galaxy Astrophysics Census (LEGA-C) program, we reconstruct the star formation histories (SFHs) of 607 galaxies at redshifts z = 0.6–1.0 and stellar masses ≳1010 M ⊙ using a custom full spectrum fitting algorithm that incorporates the emcee and FSPS packages. We show that the mass-weighted age of a galaxy correlates strongly with stellar velocity dispersion (σ *) and ongoing star formation (SF) activity, with the stellar content in higher-σ * galaxies having formed earlier and faster. The SFHs of quiescent galaxies are generally consistent with passive evolution since their main SF epoch, but a minority show clear evidence of a rejuvenation event in their recent past. The mean age of stars in galaxies that are star-forming is generally significantly younger, with SF peaking after z &lt; 1.5 for almost all star-forming galaxies in the sample: many of these still have either constant or rising SFRs on timescales &gt;100 Myr. This indicates that z &gt; 2 progenitors of z ∼ 1 star-forming galaxies are generally far less massive. Finally, despite considerable variance in the individual SFHs, we show that the current SF activity of massive galaxies (&gt;L *) at z ∼ 1 correlates with SF levels at least 3 Gyr prior: SFHs retain “memory” on a large fraction of the Hubble time. Our results illustrate a novel approach to resolve the formation phase of galaxies, and, by identifying their individual evolutionary paths, one can connect progenitors and descendants across cosmic time. This is uniquely enabled by the high-quality continuum spectroscopy provided by the LEGA-C survey.

No assembly required: mergers are mostly irrelevant for the growth of low-mass dwarf galaxies

We investigate the merger histories of isolated dwarf galaxies based on a suite of 15 high-resolution cosmological zoom-in simulations, all with masses of $M_{\rm halo} \approx 10^{10}\,{\rm M}_{\odot}$ (and M$_\star\sim10^5-10^7\,{\rm M}_{\odot}$) at $z=0$, from the Feedback in Realistic Environments (FIRE) project. The stellar populations of these dwarf galaxies at $z=0$ are formed essentially entirely "in situ": over 90$\%$ of the stellar mass is formed in the main progenitor in all but two cases, and all 15 of the galaxies have >70$\%$ of their stellar mass formed in situ. Virtually all galaxy mergers occur prior to $z\sim3$, meaning that accreted stellar populations are ancient. On average, our simulated dwarfs undergo 5 galaxy mergers in their lifetimes, with typical pre-merger galaxy mass ratios that are less than 1:10. This merger frequency is generally comparable to what has been found in dissipationless simulations when coupled with abundance matching. Two of the simulated dwarfs have a luminous satellite companion at $z=0$. These ultra-faint dwarfs lie at or below current detectability thresholds but are intriguing targets for next-generation facilities. The small contribution of accreted stars make it extremely difficult to discern the effects of mergers in the vast majority of dwarfs either photometrically or using resolved-star color-magnitude diagrams (CMDs). The important implication for near-field cosmology is that star formation histories of comparably massive galaxies derived from resolved CMDs should trace the build-up of stellar mass in one main system across cosmic time as opposed to reflecting the contributions of many individual star formation histories of merged dwarfs.

Weak Galactic halo-Fornax dSph connection from RR Lyrae stars

For the first time accurate pulsation properties of the ancient variable stars of the Fornax dwarf spheroidal galaxy (dSph) are discussed in the broad context of galaxy formation and evolution. Homogeneous multi-band $BVI$ optical photometry of spanning {\it twenty} years has allowed us to identify and characterize more than 1400 RR Lyrae stars (RRLs) in this galaxy. Roughly 70\% are new discoveries. We investigate the period-amplitude distribution and find that Fornax shows a lack of High Amplitude (A$_V\gsim$0.75 mag) Short Period fundamental-mode RRLs (P$\lsim$0.48 d, HASPs). These objects occur in stellar populations more metal-rich than [Fe/H]$\sim$-1.5 and they are common in the Galactic halo (Halo) and in globulars. This evidence suggests that old (age older than 10 Gyr) Fornax stars are relatively metal-poor. A detailed statistical analysis of the role of the present-day Fornax dSph in reproducing the Halo period distribution shows that it can account for only a few to 20\% of the Halo when combined with RRLs in massive dwarf galaxies (Sagittarius dSph, Large Magellanic Cloud). This finding indicates that Fornax-like systems played a minor role in building up the Halo when compared with massive dwarfs. We also discuss the occurrence of HASPs in connection with the luminosity and the early chemical composition of nearby dwarf galaxies. We find that, independently of their individual star formation histories, bright (M$_V\lsim$-13.5 mag) galaxies have HASPs, whereas faint ones (M$_V\gsim$-11 mag) do not. Interestingly enough, Fornax belongs to a luminosity range (--11$<$M$_V<$--13.5 mag) in which the occurrence of HASPs appears to be correlated with the early star formation and chemical enrichment of the host galaxy.

TIMING THE EVOLUTION OF QUIESCENT AND STAR-FORMING LOCAL GALAXIES

ABSTRACT Constraining the star formation histories (SFHs) of individual galaxies is crucial for understanding the mechanisms that regulate their evolution. Here, we combine multi-wavelength (ultraviolet, optical, and infrared) measurements of a very large sample of galaxies (∼230,000) at z &lt; 0.16, with physically motivated models of galaxy spectral energy distributions to extract constraints on galaxy physical parameters (such as stellar mass and star formation rate) as well as individual SFHs. In particular, we set constraints on the timescales in which galaxies form a certain percentage of their total stellar mass (namely, 10%, 50%, and 90%). The large statistics allows us to average such measurements over different populations of galaxies (quiescent and star-forming) and in narrow ranges of stellar mass. As in the downsizing scenario, we confirm that low-mass galaxies have more extended SFHs than high-mass galaxies. We also find that at the same observed stellar mass, galaxies that are now quiescent evolve more rapidly than galaxies that are currently still forming stars. This suggests that stellar mass is not the only driver of galaxy evolution, but plays along with other factors such as merger events and other environmental effects.

The origin of S0s in clusters: evidence from the bulge and disc star formation histories

The individual star formation histories of bulges and discs of lenticular (S0) galaxies can provide information on the processes involved in the quenching of their star formation and subsequent transformation from spirals. In order to study this transformation in dense environments, we have decomposed long-slit spectroscopic observations of a sample of 21 S0s from the Virgo Cluster to produce one-dimensional spectra representing purely the bulge and disc light for each galaxy. Analysis of the Lick indices within these spectra reveals that the bulges contain consistently younger and more metal-rich stellar populations than their surrounding discs, implying that the final episode of star formation within S0s occurs in their central regions. Analysis of the $\alpha$-element abundances in these components further presents a picture in which the final episode of star formation in the bulge is fueled using gas that has previously been chemically enriched in the disc, indicating the sequence of events in the transformation of these galaxies. Systems in which star formation in the disk was spread over a longer period contain bulges in which the final episode of star formation occurred more recently, as one might expect for an approximately coeval population in which the transformation from spiral to S0 occurred at different times. With data of this quality and the new analysis method deployed here, we can begin to describe this process in a quantitative manner for the first time.

Satellites in the Local Group and Other Nearby Groups

In recent years the census of known satellites in our own Local Group and in nearby galaxy groups has increased substantially due to sensitive wide-area surveys. In the Local Group these surveys have more than doubled its known galaxy content and extended the galaxy luminosity function to very faint total magnitudes. Deep ground-based imaging and spectroscopic observations as well as high-resolution imaging with the Hubble Space Telescope have revolutionized our understanding of the chemical evolution and star formation histories of the satellites. We often find long-lasting star formation episodes with low star formation efficiencies. There is evidence for localized, stochastic enrichment, and recent searches are now beginning to uncover even extremely metal-deficient stars. In many satellites evidence for two or more distinct stellar subpopulations is found. Differing fractions of old populations have been detected in all satellites studied in sufficient detail so far. Kinematic measurements support a picture in which satellites are dark-matter dominated, although recent results indicate that the proposed common mass scale for dwarf spheroidal galaxies may not hold for very low-mass satellites. When considering satellite ensembles, we find global morphology-distance and gas-content - distance relations in all groups studied thus far, but individual star formation histories also strongly depend on a given satellite's intrinsic properties.

Tracing the sites of obscured star formation in the Antennae galaxies withHerschel-PACS

FIR imaging of interacting galaxies allows locating even hidden sites of star formation and measuring of the relative strength of nuclear and extra-nuclear star formation. We want to resolve the star-forming sites in the nearby system of the Antennae. Thanks to the unprecedented sharpness and depth of the PACS camera onboard ESA's Herschel Space Observatory, it is possible for the first time to achieve a complete assessment of individual star-forming knots in the FIR with scan maps at 70, 100, and 160 um. We used clump extraction photometry and SED diagnostics to derive the properties related to star-forming activity. The PACS 70, 100, and 160 um maps trace the knotty structure of the most recent star formation along an arc between the two nuclei in the overlap area. The resolution of the starburst knots and additional multi-wavelength data allow their individual star formation history and state to be analysed. In particular, the brightest knot in the mid-infrared (K1), east of the southern nucleus, exhibits the highest activity by far in terms of dust heating and star formation rate, efficiency, and density. With only 2 kpc in diameter, this area has a 10-1000 um luminosity, which is as high as that of our Milky Way. It shows the highest deficiency in radio emission in the radio-to-FIR luminosity ratio and a lack of X-ray emission, classifying it as a very young complex. The brightest 100 and 160 um emission region (K2), which is close to the collision front and consists of 3 knots, also shows a high star formation density and efficiency and lack of X-ray emission in its most obscured part, but an excess in the radio-to-FIR luminosity ratio. This suggests a young stage, too, but different conditions in its interstellar medium. Our results provide important checkpoints for numerical simulations of interacting galaxies when modelling the star formation and stellar feedback.

Galaxy And Mass Assembly (GAMA)

AbstractThe GAMA survey aims to deliver 250,000 optical spectra (3–7 Å resolution) over 250 sq. degrees to spectroscopic limits of rAB &lt; 19.8 and KAB &lt; 17.0 mag. Complementary imaging will be provided by GALEX, VST, UKIRT, VISTA, HERSCHEL and ASKAP to comparable flux levels leading to a definitive multi-wavelength galaxy database. The data will be used to study all aspects of cosmic structures on 1kpc to 1Mpc scales spanning all environments and out to a redshift limit of z ≈ 0.4. Key science drivers include the measurement of: the halo mass function via group velocity dispersions; the stellar, HI, and baryonic mass functions; galaxy component mass-size relations; the recent merger and star-formation rates by mass, types and environment. Detailed modeling of the spectra, broad SEDs, and spatial distributions should provide individual star formation histories, ages, bulge-disc decompositions and stellar bulge, stellar disc, dust disc, neutral HI gas and total dynamical masses for a significant subset of the sample (~ 100k) spanning both the giant and dwarf galaxy populations. The survey commenced March 2008 with 50k spectra obtained in 21 clear nights using the Anglo Australian Observatory's new multi-fibre-fed bench-mounted dual-beam spectroscopic system (AAΩ).