A Proto-Galaxy Candidate at z=2.7 Discovered by its Young Stellar Population

We revisit the bimodal distribution of the galaxy population commonly seen in the local universe. Here we address the bimodality observed in galaxy properties in terms of spectral synthesis products, such as mean stellar ages and stellar masses, derived from the application of this powerful method to a volume-limited sample, with magnitude limit cutoff M_r = -20.5, containing about 50 thousand luminous galaxies from the SDSS Data Release 2. In addition, galaxies are classified according to their emission line properties in three distinct spectral classes: star-forming galaxies, with young stellar populations; passive galaxies, dominated by old stellar populations; and, hosts of active nuclei, which comprise a mix of young and old stellar populations. We show that the extremes of the distribution of some galaxy properties, essentially galaxy colours, 4000 A break index, and mean stellar ages, are associated to star-forming galaxies at one side, and passive galaxies at another. We find that the mean light-weighted stellar age of galaxies is the direct responsible for the bimodality seen in the galaxy population. The stellar mass, in this view, has an additional role since most of the star-forming galaxies present in the local universe are low-mass galaxies. Our results also give support to the existence of a 'downsizing' in galaxy formation, where massive galaxies seen nowadays have stellar populations formed at early times.

Context. Interstellar dust absorbs stellar light very efficiently, thus shaping the energy output of galaxies. Studying the impact of different stellar populations on the dust heating continues to be a challenge because it requires decoupling the relative geometry of stars and dust and also involves complex processes such as scattering and non-local dust heating. Aims. We aim to constrain the relative distribution of dust and stellar populations in the spiral galaxy M 81 and create a realistic model of the radiation field that adequately describes the observations. By investigating the dust-starlight interaction on local scales, we want to quantify the contribution of young and old stellar populations to the dust heating. We aim to standardise the setup and model selection of such inverse radiative transfer simulations so these can be used for comparable modelling of other nearby galaxies. Methods. We present a semi-automated radiative transfer modelling pipeline that implements necessary steps such as the geometric model construction and the normalisation of the components through an optimisation routine. We used the Monte Carlo radiative transfer code SKIRT to calculate a self-consistent, panchromatic model of the interstellar radiation field. By looking at different stellar populations independently, we were able to quantify to what extent different stellar age populations contribute to the heating of dust. Our method takes into account the effects of non-local heating. Results. We obtained a realistic 3D radiative transfer model of the face-on galaxy M 81. We find that only 50.2% of the dust heating can be attributed to young stellar populations (≲100 Myr). We confirm that there is a tight correlation between the specific star formation rate and the heating fraction by young stellar populations, both in sky projections and in 3D, which is also found for radiative transfer models of M 31 and M 51. Conclusions. We conclude that old stellar populations can be a major contributor to the heating of dust. In M 81, old stellar populations are the dominant heating agent in the central regions, contributing to half of the absorbed radiation. Regions of higher star formation do not correspond to the highest dust temperatures. On the contrary, it is the dominant bulge which is most efficient in heating the dust. The approach we present here can immediately be applied to other galaxies. It does contain a number of caveats, which we discuss in detail.

While JWST has observed galaxies assembling as early as $z\sim 14$, evidence of galaxies with significant old stellar populations in the Epoch of Reionization (EoR) – the descendants of these earliest galaxies – are few and far between. Bursty star-formation histories (SFHs) have been invoked to explain the detectability of the earliest UV-bright galaxies, but also to interpret galaxies showing Balmer breaks without nebular emission lines. We present the first spectroscopic evidence of a $z\sim 7.9$ galaxy, A2744-YD4, which shows a Balmer break and emission lines, indicating the presence of both a mature and young stellar population. The spectrum of A2744-YD4 shows peculiar emission line ratios suggesting a relatively low ionization parameter and high gas-phase metallicity. A median stack of galaxies with similar emission line ratios reveals a clear Balmer break in their stacked spectrum. This suggests that a mature stellar population (∼80 Myr old) has produced a chemically enriched, disrupted interstellar medium. Based on SED-fitting and comparison to simulations, we conclude that the observed young stellar population is in fact the result of a rejuvenation event following a lull in star formation lasting ∼20 Myr, making A2744-YD4 and our stack the first spectroscopic confirmation of galaxies that have rejuvenated following a mini-quenched phase. These rejuvenating galaxies appear to be in an exceptional evolutionary moment where they can be identified. Our analysis shows that a young stellar population of just ∼30 per cent of the total stellar mass would erase the Balmer break. Hence, ‘outshining’ through bursty SFHs in early galaxies is likely plaguing attempts to measure their stellar ages and masses accurately.

Stellar population studies of early-type galaxies (ETGs) based on their optical stellar continuum suggest that these are quiescent systems. However, emission lines and ultraviolet photometry reveal a diverse population. We use a new version of the starlight spectral synthesis code and state-of-the-art stellar population models to simultaneously fit Sloan Digital Sky Survey spectra and Galaxy Evolution Explorer photometry for a sample of 3453 galaxies at z < 0.1 with near ultraviolet (NUV) − r > 5 that are classified as elliptical by Galaxy Zoo. We reproduce far ultraviolet (FUV) magnitudes for 80 per cent of UV upturn galaxies selected using criteria from the literature, suggesting that additional stellar population ingredients such as binaries and extreme horizontal branch stars may have a limited contribution to the UV upturn. The addition of ultraviolet data leads to a broadening of the distributions of mean stellar ages, metallicities, and attenuation. Stellar populations younger than $1\,$ Gyr are required to reproduce the ultraviolet emission in 17 per cent of our sample. These systems represent 43 per cent of the sample at 5 < NUV − r < 5.5 and span the same stellar mass range as other ETGs in our sample. ETGs with young stellar components have larger H α equivalent widths (WH α) and larger dust attenuation. Emission line ratios and WH α indicate that the ionizing source in these systems is a mixture of young and old stellar populations. Their young stellar populations are metal-poor, especially for high-mass galaxies, indicating recent star formation associated with rejuvenation events triggered by external processes, such as minor mergers.

CONTEXT. Stellar populations are the building blocks of galaxies including the Milky Way. The majority, if not all extragalactic studies are entangled with the use of stellar population models given the unresolved nature of their observation. Extragalactic systems contain multiple stellar populations with complex star formation histories. However, their study is mainly based upon the principles of simple stellar populations (SSP). Hence, it is critical to examine the validity of SSP models. AIMS. This work aims to empirically test the validity of SSP models. This is done by comparing SSP models against observations of spatially resolved young stellar population in the determination of its physical properties, i.e. age and metallicity. METHODS. Integral field spectroscopy of a young stellar cluster in the Milky Way, NGC 3603, is used to study the properties of the cluster both as a resolved and unresolved stellar population. The unresolved stellar population is analysed using the H$\alpha$ equivalent width as an age indicator, and the ratio of strong emission lines to infer metallicity. In addition, spectral energy distribution (SED) fitting using STARLIGHT, is used to infer these properties from the integrated spectrum. Independently, the resolved stellar population is analysed using the color-magnitude diagram (CMD) for age and metallicity determination. As the SSP model represents the unresolved stellar population, the derived age and metallicity are put to test whether they agree with those derived from resolved stars. RESULTS. The age and metallicity estimate of NGC 3603 derived from integrated spectroscopy are confirmed to be within the range of those derived from the CMD of the resolved stellar population, including other estimates found in the literature. The result from this pilot study supports the reliability of SSP models for studying unresolved young stellar populations.

Post-starburst galaxies, or E+A galaxies, are characterized by optical spectra showing strong Balmer absorption lines, indicating a young stellar population, and little or no emission lines, implying no active star formation. These galaxies are interpreted as a transitional population between star-forming, disk-dominated galaxies and spheroidal quiescent, non-star forming galaxies. Here, we present single dish HI 21-cm emission line measurements of a sample of eleven of these galaxies at redshifts z<0.05. We detect H I emission in six of the E+A galaxies. In combination with earlier studies, the total number of E+A galaxies with measured cold gas components is now eleven. Roughly half of the E+As studied so far have detectable HI. The gas fractions of these galaxies, measured with respect to their stellar mass, are between 1 and 10 percent and are at the high end of the gas fractions measured in gas-bearing early type galaxies and typically lower than seen in late-type galaxies with comparable stellar masses. This finding is consistent with the idea that E+As are currently evolving from the blue cloud to the red sequence. However, the question of why the star formation has ceased in these galaxies while a significant gas reservoir is still present can only be answered by higher spatial resolution observations of the cold gas.

Aims. Within the framework of the DustPedia project we investigate the properties of cosmic dust and its interaction with stellar radiation (originating from different stellar populations) for 814 galaxies in the nearby Universe, all observed by the Herschel Space Observatory. Methods. We take advantage of the widely used fitting code CIGALE, properly adapted to include the state-of-the-art dust model THEMIS. For comparison purposes, an estimation of the dust properties is provided by approximating the emission at far-infrared and sub-millimeter wavelengths with a modified blackbody. Using the DustPedia photometry we determine the physical properties of the galaxies, such as the dust and stellar mass, the star-formation rate, the bolometric luminosity, the unattenuated and the absorbed by dust stellar light, for both the old (&gt; 200 Myr) and young (≤200 Myr) stellar populations. Results. We show how the mass of stars, dust, and atomic gas, as well as the star-formation rate and the dust temperature vary between galaxies of different morphologies and provide recipes to estimate these parameters given their Hubble stage (T). We find a mild correlation between the mass fraction of the small a-C(:H) grains with the specific star-formation rate. On average, young stars are very efficient in heating the dust, with absorption fractions reaching as high as ∼77% of the total unattenuated luminosity of this population. On the other hand, the maximum absorption fraction of old stars is ∼24%. Dust heating in early-type galaxies is mainly due to old stars, up to a level of ∼90%. Young stars progressively contribute more for “typical” spiral galaxies and they become the dominant source of dust heating for Sm-type and irregular galaxies, with ∼60% of their luminosity contributing to that purpose. Finally, we find a strong correlation of the dust heating fraction by young stars with morphology and the specific star-formation rate.

Context.Studying the stellar kinematics of galaxies is a key tool in the reconstruction of their evolution. However, the current measurements of the stellar kinematics are complicated by several factors, including dust extinction and the presence of multiple stellar populations.Aims.We use integral field spectroscopic data of four galaxies from the Time Inference with MUSE in Extragalactic Rings (TIMER) survey to explore and compare the kinematics measured in different spectral regions that are sensitive to distinct stellar populations.Methods.We derive the line-of-sight velocity and velocity dispersion of both a young (≲2 Gyr) and an old stellar population from the spectral regions around the Hβline and the Ca II Triplet. In addition, we determine colour excess, mean age, and metallicity.Results.We report a correlation of the colour excess with the difference in the kinematic parameters of the Hβline and the Ca II Triplet range, which are dominated by young and old stellar populations, respectively. Young stellar populations, located primarily in nuclear rings, have higher velocity dispersions than old ones. These differences in the rings are typically ∼10 km s−1in velocity dispersion but can have a mean value as high as ∼24 km s−1in the most extreme case. Trends with age exist in the nuclear rings but are less significant than those with dust extinction. We report different degrees of correlation for these trends among the galaxies in the sample, which are related to the size of the Voronoi bins in their rings. No clear trends for the line-of-sight velocity differences are observed. The absence of these trends can be explained as a consequence of the Hβline masking process during the kinematic extraction, as confirmed by dedicated simulations.Conclusions.Our study demonstrates that kinematic differences caused by different stellar populations can be identified in the central regions of nearby galaxies, even from intermediate resolution spectroscopy. This opens the door to future detailed chemo-kinematic studies of galaxies, but also serves as a warning against deriving kinematics from full-spectrum fitting across very wide wavelength ranges when intense star formation is taking place.

New far ultraviolet imaging of the galaxy NGC 205 is presented, which shows the emission is significantly offset ( $\sim5^{\prime\prime}$ NW) from the optical and infrared centers of the galaxy. Spectral energy distribution (SED) modelling is applied to investigate the spatial dependence of the star formation history (SFH) of NGC 205, using data from far ultraviolet to far infrared. The SED model includes young and old stellar populations, gas emission, dust emission and dust absorption. The old stellar population has a total mass of $1.1\times10^8$ M $_{\odot}$ whereas the young population has a much smaller total mass of 3 200 M $_{\odot}$ . The best forms of SFH for old and young stars are found to be exponentially declining bursts with start times $t_0$ yr ago and e-folding times $\tau$ yr. The old stellar population has uniform $t_0$ = 9.5 Gyr, with $\tau$ decreasing with radius from 1 Gyr to 500 Myr. The young stellar population has $t_0$ = 900 Myr and $\tau$ = 800 Myr, both uniform across NGC 205. The young and old stellar mass surface densities are exponential in radius with scale lengths of 40 and 110 pc, respectively. The dust heating has a $\sim$ 40% contribution from young stars and $\sim$ 60% from old stars.

We use long-slit spectroscopic observations of the sample of E+A galaxies described by Zabludoff et al. to constrain the nature of the progenitors and remnants of the E+A phase of galaxy evolution. We measure spatially resolved kinematic properties of the young (1 Gyr) and old (few Gyr) stellar populations. The young stellar populations are more centrally concentrated than the older populations, but they are not confined to the galaxy core (radius 1 kpc). The kinematics of the old stellar population place 16 of 20 of our E+As on a trend parallel to the Faber-Jackson relation that is offset by ~0.6 mag in R. Eighteen of 20 E+As have v/σ < 1. As the young stars in these systems evolve, the luminosity offset will disappear, and the remnants will be pressure-supported systems that lie on the Faber-Jackson relation. Although Zabludoff et al. spectroscopically selected the most extreme E+A galaxies in the local volume, the sample is kinematically diverse: velocity dispersions range from 30 km s-1 to ~200 km s-1 over a luminosity range of MR = -19 to -22 + 5 log h. Combining these results with an estimate of the number of galaxies that experience an E+A phase, we conclude that the E+A phase of galaxy evolution is important in the development of a large fraction of spheroid-dominated galaxies over a wide range of luminosities and masses. Our kinematic observations, together with evidence that E+As have recently evolved from a vigorous star-forming phase to a quiescent phase (e.g., Couch & Sharples; Caldwell et al.) and that many have tidal features consistent with disklike progenitors (Zabludoff et al.), indicate that these galaxies are undergoing a transformation from gas-rich, star-forming, rotationally supported, disk-dominated galaxies into gas-poor, quiescent, pressure-supported, spheroid-dominated galaxies.

We present high-quality, wide spectral coverage long-slit optical spectra for 12 powerful radio sources at low and intermediate redshifts (z < 0.7) that show evidence for a substantial ultraviolet (UV) excess. These data were taken using the William Herschel Telescope and the ESO Very Large Telescope with the aim of determining the detailed properties of the young stellar populations (YSPs) in the host galaxies as part of a larger project to investigate evolutionary scenarios for the active galactic nucleus (AGN) host galaxies. The results of our spectral synthesis model fits to the spectra highlight the importance of taking into account AGN-related components (emission lines, nebular continuum, scattered light) and reddening of the stellar populations in studies of this type. It is also clear that careful examination of the fits to the spectra, as well consideration of auxiliary polarimetric and imaging data, are required to avoid degeneracies in the model solutions. In three out of the 12 sources in our sample we find evidence for broad permitted line components, and a combination of AGN-related continuum components and an old (12.5 Gyr) stellar population provides an adequate fit to the data. However, for the remaining nine sources we find strong evidence for YSPs. In contrast to some recent studies that suggest relatively old post-starburst ages for the YSPs in radio galaxies (0.3–2.5 Gyr), we deduce a wide range of ages for the YSPs in our sample objects (0.02–1.5 Gyr), with ∼50 per cent of the sample showing evidence for young YSP ages (≲0.1 Gyr) in their nuclear regions. The nuclear YSPs are often significantly reddened [0.2 < E(B−V) < 1.4] and make up a substantial fraction (∼1–35 per cent) of the total stellar mass in the regions sampled by the spectroscopic slits. Moreover, in all the cases in which we have sufficient spatial resolution we find that the UV excess is extended across the full measurable extent of the galaxy (typically 5–30 kpc), suggesting galaxy-wide starbursts. The implications for photometric and spectroscopic studies of active galaxies are discussed.

Context. Studying metal-poor galaxies is crucial for understanding physical mechanisms that drive the formation and evolution of galaxies, such as internal dynamics, star formation history, and chemical enrichment. Most of the observational studies on dwarf galaxies employ integral field spectroscopy to investigate gas physics in the entire body of galaxies. However, these past studies have not investigated the detailed spatially resolved properties of individual extragalactic H II regions. Aims. We study the only known H II region in the Sagittarius dwarf irregular galaxy, a metal-poor galaxy of the local Universe, using integral field unit (IFU) VIMOS/VLT and long-slit EFOSC2/NTT archival data. We explore the spatially resolved gaseous structure by using optical emission lines, to (i) provide insights into the physical processes that are shaping the evolution of this H II region, and (ii) relate these mechanisms to the metal-poor, gas-phase component in extragalactic H II regions. Methods. We probe optical emission line structures of the H II region, fully covered within the 27″×27″ field of view of VIMOS. The oxygen abundances were estimated by applying the Te-sensitive method, by using the auroral [O III]λ4363 emission line detectable at S/N &gt; 3 integrating the spectral fibres of the data cube. Results. From the emission line maps, the O++ emission is concentrated towards the centre, in comparison to the low-ionised species such as O+ and H+. The Hβ maps reveal that the H II shows two prominent clumps, showing a biconic-like shape aligned along the same axis. Radial flux-density profiles reveal that those clumps are similar in terms of size (∼8″) and flux distribution in Hβ and [O III]λ5007. Comparing stellar populations from HST photometry in the gaseous structure, we find that old stellar populations (&gt;1 Gyr) are uniformly distributed across the H II region, whereas the young stellar populations (⪅700 Myr) are found closer to the edges of the Hβ clumps and distributed in filamentary configurations. We estimate Te = 17 683±1254 K for the gaseous structure. The Te-based oxygen abundance of the SagDIG H II region is 12+log(O/H) = 7.23±0.04, which is in agreement with empirical estimations of the literature, and is also in line with the low-mass end of the mass-metallicity relation (MZR). Considering corrections on Te fluctuations, we estimate 12+log(O/H) = 7.50±0.08. Conclusions. The stratified composition of the H II region is a signature that this gaseous structure is expanding. This feature, together with SagDIG falling in the low-mass end of the MZR, suggests that the evolution of this H II region is sustained by ionisation from massive stars, stellar winds, and supernovae explosions expanding the gas structure. The filamentary configuration of young stars is likely produced by the interaction between atomic and ionised gas, in line with many galactic H II regions and those found in the Large Magellanic Cloud. If this proposed scenario is confirmed with multi-wavelength data and data cubes with better spectral coverage and spatial resolution, it could imply that H II regions in metal-poor dwarf galaxies are subject to the same physics as H II regions in the Milky Way.

We study the relations between stellar mass, star formation history, size and internal structure for a complete sample of 122 808 galaxies drawn from the Sloan Digital Sky Survey. We show that low-redshift galaxies divide into two distinct families at a stellar mass of 3 × 1010 M⊙. Lower-mass galaxies have young stellar populations, low surface mass densities and the low concentrations typical of discs. Their star formation histories are more strongly correlated with surface mass density than with stellar mass. A significant fraction of the lowest-mass galaxies in our sample have experienced recent starbursts. At given stellar mass, the sizes of low-mass galaxies are lognormally distributed with dispersion δ (ln R50) ∼ 0.5, in excellent agreement with the idea that they form with little angular momentum loss through cooling and condensation in a gravitationally dominant dark matter halo. Their median stellar surface mass density scales with stellar mass as µ* ∝ M0.54*, suggesting that the stellar mass of a disc galaxy is proportional to the three halves power of its halo mass. All of this suggests that the efficiency of the conversion of baryons into stars in low-mass galaxies increases in proportion to halo mass, perhaps as a result of supernova feedback processes. At stellar masses above 3 × 1010 M⊙, there is a rapidly increasing fraction of galaxies with old stellar populations, high surface mass densities and the high concentrations typical of bulges. In this regime, the size distribution remains lognormal, but its dispersion decreases rapidly with increasing mass and the median stellar mass surface density is approximately constant. This suggests that the star formation efficiency decreases in the highest-mass haloes, and that little star formation occurs in massive galaxies after they have assembled.

As deep spectroscopic campaigns extend to higher redshifts and lower stellar masses, the interpretation of galaxy spectra depends increasingly upon models for very young stellar populations. Here we present new HST/COS ultraviolet spectroscopy of seven nearby (&amp;lt;120 Mpc) star-forming regions hosting very young stellar populations (∼4–20 Myr) with optical Wolf–Rayet stellar wind signatures, ideal laboratories in which to benchmark these stellar models. We detect nebular C iii] in all seven, but at equivalent widths uniformly &amp;lt;10 Å. This suggests that even for very young stellar populations, the highest equivalent width C iii] emission at ≥15 Å is reserved for inefficiently cooled gas at metallicities at or below that of the SMC. The spectra also reveal strong C iv P-Cygni profiles and broad He ii emission formed in the winds of massive stars, including some of the most prominent He ii stellar wind lines ever detected in integrated spectra. We find that the latest stellar population synthesis prescriptions with improved treatment of massive stars nearly reproduce the entire range of stellar He ii wind strengths observed here. However, we find that these models cannot simultaneously match the strongest wind features alongside the optical nebular line constraints. This discrepancy can be naturally explained by an overabundance of very massive stars produced by a high incidence of binary mass transfer and mergers occurring on short ≲10 Myr time-scales, suggesting these processes may be crucial for understanding systems dominated by young stars both nearby and in the early Universe.

The great amount of data observed in recent years coupled with modelling using evolutionary synthesis codes (BPASS, COELHO, GALAXEV, GALEV, MILES, PÉGASE, etc. . .) to compute Single Stellar Populations (SSPs) and the availability of fast and ingenious spectral synthesis codes such as starlight, ULySS and VESPA, have significantly shed light on our knowledge about the formation and evolution of galaxies. However, there are still open issues concerning the stellar populations in nearby galaxies, particularly those harbouring Active Galactic Nuclei (AGN): can stellar populations mimic nuclear activity, leading to a misclassification based on optical emission line ratios (Stasińska et al. 2008)? We have applied the starlight code (Cid Fernandes et al. 2005) to a well studied sample of nearby galaxies' nuclear spectra (r &lt; ~ 200 pc), observed with the Hale 5 m telescope at Palomar Observatory in two different regions: ~ 4230-5110 Å and ~ 6210-6860 Å (Ho et al. 1995), with spectral resolutions of approximately 4 Å, and 2.5 Å. The aim is to properly derive the star-formation history (SFH), mean stellar age and metallicity and total stellar mass. Our results show that the star-formation history of Seyfert galaxies are very heterogeneous, i.e. these are composed of young, intermediate and old stellar populations, while the SFH of Low-Ionization Nuclear Emission-Line Regions (LINERs) are basically composed of old stellar populations. The absence of young stars in LINERs indicates that these are not responsible for the observed low-ionization emission lines. Furthermore, although a significant fraction of AGN spectra require a featureless continuum in their Spectral Energy Distribution (SED) modelling, this is not an indicative of the presence of an AGN, instead the continuum may simulate the presence of young stellar populations. The main objective of this research is to complement the study of spectroscopic parameters from 486 galaxies analyzed by Ho et al. (1995) that are public available in the VizieR catalog (Ho et al. 1997, 2009) and provide information about their stellar population content by means of the starlight. The base of Simple Stellar Populations used here was taken from Bruzual &amp; Charlot (2003) and spans 25 ages (from 1 Myr to 18 Gyr) and 6 metallicities (Z = 0.005, 0.02, 0.2, 0.4, 1 &amp; 2.5 Z⊙).