Spectral Population Synthesis Research Articles

HST FUV Spectroscopy of Super Star Cluster A in the Green Pea Analog Mrk 71: Revealing the Presence of Very Massive Stars

Mrk 71 is a low-metallicity (Z = 0.16 Z ☉) starburst region in the local dwarf galaxy NGC 2366, hosting two super star clusters (SSCs A and B), and it is recognized as a Green Pea (GP) analog with SSC A responsible for the GP properties. We present STIS and FOS far-ultraviolet (FUV) spectra of the embedded SSC Mrk 71-A obtained with the Hubble Space Telescope. The STIS FUV spectrum shows the characteristic features of very massive stars (VMS; masses >100 M ⊙) and we derive an age of 1 ± 1 Myr by comparison with the Charlot & Bruzual suite of spectral population synthesis models with upper mass limits of 300 and 600 M ⊙. We compare the STIS spectrum with all known SSC spectra exhibiting VMS signatures: NGC 5253-5, R136a, NGC 3125-A1, and the z = 2.37 Sunburst cluster. We find that the cluster mass-loss rates and wind velocities, as characterized by the C iv P Cygni profiles and the He ii emission line strengths, are very similar over Z = 0.16–0.4 Z☉. This agrees with predictions that the optically thick winds of VMS will be enhanced near the Eddington limit and show little metallicity dependence. We find very strong damped Lyα absorption with N(H i) =1022.2 cm−2 associated with Mrk 71-A. We discuss the natal environment of this young SSC in terms of radiatively driven winds, catastrophic cooling, and recent models where the cluster is surrounded by highly pressurized clouds with large neutral columns.

Spatially resolved self-consistent spectral modelling of jellyfish galaxies from MUSE with FADO: trends with mass and stripping intensity

ABSTRACT We present a spatially resolved stellar population analysis of 61 jellyfish galaxies and 47 control galaxies observed with ESO/MUSE attempting to understand the general trends of the stellar populations as a function of the stripping intensity and mass. This is the public sample from the GASP programme, with 0.01 < z < 0.15 and 8.9 < log (M⋆/M⊙) < 12.0. We apply the spectral population synthesis code fado to fit self-consistently both the stellar and nebular contributions to the spectra of the sources. We present 2D morphological maps for mean stellar ages, metallicities, gas-phase oxygen abundances, and star formation rates for the galaxies with Integrated Nested Laplace Approximation (inla), which is efficient in reconstructing spatial data of extended sources. We find that ‘extreme stripping’ and ‘stripping’ galaxies are typically younger than the other types. Regarding stellar and nebular metallicities, the ‘stripping’ and ‘control passive’ galaxies are the most metal-poor. Based on the phase space for jellyfish cluster members we find trends in ages, metallicities, and abundances with different regions of the diagram. We also compute radial profiles for the same quantities. We find that both the stripping and the stellar masses seem to influence the profiles, and we see differences between various groups and distinct mass bins. The radial profiles for different mass bins present relations already shown in the literature for undisturbed galaxies, i.e. profiles of ages and metallicities tend to increase with mass. However, beyond ∼0.75 effective radius, the ages of the most massive galaxies become similar to or lower than the ages of the lower mass ones.

Lyman continuum leaker candidates among highly ionised, low-redshift dwarf galaxies selected from He II

Context. Contemporary research suggests that the reionisation of the intergalactic medium (IGM) in the early Universe was predominantly realised by star-forming (proto-)galaxies (SFGs). Due to observational constraints, our knowledge on the origins of sufficient amounts of ionising Lyman continuum (LyC) photons and the mechanisms facilitating their transport into the IGM remains sparse. Recent efforts have thus focussed on the study of local analogues to these high-redshift objects. Aims. We aim to acquire a set of very low-redshift SFGs that exhibit signs of a hard radiation field being present. A subsequent analysis of their emission line properties is intended to shed light on how the conditions prevalent in these objects compare to those predicted to be present in early SFGs that are thought to be LyC emitters (LCEs). Methods. We used archival spectroscopic SDSS DR12 data to select a sample of low-redshift He II 4686 emitters and restricted it to a set of SFGs with an emission line diagnostic sensitive to the presence of an active galactic nucleus, which serves as our only selection criterion. We performed a population spectral synthesis with FADO to reconstruct these galaxies’ star-formation histories (SFHs). Utilising the spectroscopic information at hand, we constrained the predominant ionisation mechanisms in these galaxies and inferred information on interstellar medium (ISM) conditions relevant for the escape of LyC radiation. Results. Our final sample consists of eighteen ionised, metal-poor galaxies (IMPs). These low-mass (6.2 ≤ log(M⋆/M⊙) ≤ 8.8), low-metallicity (7.54 ≤ log(O/H) + 12 ≤ 8.13) dwarf galaxies appear to be predominantly ionised by stellar sources. We find large [OIII] 5007/[OII] 3727 ratios and [SII] 6717,6731/Hα deficiencies, which provide strong indications for these galaxies to be LCEs. At least 40% of these objects are candidates for featuring cosmologically significant LyC escape fractions ≳10%. The IMPs’ SFHs exhibit strong similarities and almost all galaxies appear to contain an old (> 1 Gyr) stellar component, while also harbouring a young, two-stage (∼10 Myr and < 1 Myr) starburst, which we speculate might be related to LyC escape. Conclusions. The properties of the compact emission line galaxies presented here align well with those observed in many local LCEs. In fact, our sample may prove as an extension to the rather small catalogue of local LCEs, as the extreme ISM conditions we find are assumed to facilitate LyC leakage. Notably, all of our eighteen candidates are significantly closer (z < 0.1) than most established LCEs. If the inferred LyC photon loss is genuine, this demonstrates that selecting SFGs from He II 4686 is a powerful selection criterion in the search for LCEs.

Exploring the genesis of spiral galaxies

Context. A tantalizing enigma in extragalactic astronomy concerns the chronology and driving mechanisms of the build-up of late-type galaxies (LTGs). The standard scenario envisages two formation routes, with classical bulges (CBs) assembling first in a quick and violent quasi-monolithic episode followed by gradual disk assembly, and pseudo-bulges (PBs) progressively forming over gigayear-long timescales through gentle gas inflow from the disk and in situ star formation. The expectation from this antagonistic rationale is the segregation of present-day LTG bulges into two evolutionary distinct groups, which is in sharp contrast with recent observations. Aims. The present study aims for a thorough investigation of the star formation history (SFH) of LTGs with its ultimate goal being to outline a coherent framework for the formation and evolution of spiral galaxies and their main stellar components. Methods. Using population spectral synthesis models, we analyse the spatially resolved SFH of bulges and disks of 135 LTGs from the CALIFA survey, covering the relevant range in LTG mass. Complementarily, characteristic physical properties of bulges and disks, such as mean colours, mass- and light-weighted stellar age and metallicity, and EW(Hα), were contrasted with predictions from evolutionary synthesis models, by adopting exponentially declining SFHs with e-folding times τ between 0.1 and 20 Gyr. Results. Analysis of the SFH of roughly half a million spaxels consistently reveals that the main physical and evolutionary properties of both bulges and disks are continuously distributed across present-day total stellar mass ℳ⋆, T. The τ in spiral galaxies with log(ℳ⋆, T) > 10 increases from the centre to the periphery, suggesting that these systems grow in an inside-out fashion. Quite importantly, the radial gradient of τ in an individual galaxy increases with increasing ℳ⋆, T, which is consistent with a high bulge-to-disk age contrast in high-mass spirals, while lower-mass LTGs display roughly the same τ throughout their entire radial extent, with intermediate mass galaxies in between. Predictions obtained through evolutionary synthesis are overall consistent with observed properties. Finally, bulges and disks of higher mass galaxies exhibit shorter formation timescales as compared to their lower mass counterparts. Conclusions. Collectively, the obtained results evince a coherent and unified picture for the formation and evolution of LTGs, in which PBs and CBs denote extremities of a continuous mass sequence. Our findings are consistent with the framework where bulges are assembled jointly with their parent disks by gradual inside-out growth, at a pace that is regulated by the depth of the galactic potential. This postulate is further supported by the fact that the revealed correlations are entirely devoid of a bimodality, as it would be expected if CBs and PBs were to emerge from two distinct formation routes.

An investigation of the star-forming main sequence considering the nebular continuum emission at low-z

Context. Galaxy evolution has been studied by interpreting the spectral energy distribution of galaxies using spectral synthesis codes. This method has been crucial in discovering different pillars of modern galaxy evolution theories. However, this analysis was mostly carried out using spectral synthesis codes that are purely stellar, that is, they assume that the nebular contribution to the total continuum is negligible. The code FADO is the first publicly available population spectral synthesis tool that treats the contribution from ionised gas to the observed emission self-consistently. This is expected to have a particularly strong effect in star-forming (SF) galaxies. Aims. We study the impact of the nebular contribution on the determination of the star formation rate (SFR), stellar mass, and consequent effect on the star-forming main sequence (SFMS) at low redshift. Methods. We applied FADO to the spectral database of the SDSS to derive the physical properties of galaxies. As a comparison, we used the data in the MPA-JHU catalogue, which contains the properties of SDSS galaxies derived without the nebular contribution. We selected a sample of SF galaxies with Hα and Hβ flux measurements, and we corrected the fluxes for the nebular extinction through the Balmer decrement. We then calculated the Hα luminosity to estimate the SFR. Then, by combining the stellar mass and SFR estimates from FADO and MPA-JHU, the SFMS was obtained. Results. The Hα flux estimates are similar between FADO and MPA-JHU. Because the Hα flux was used as tracer of the SFR, FADO and MPA-JHU agree in their SFR. The stellar mass estimates are slightly higher for FADO than for MPA-JHU on average. However, considering the uncertainties, the differences are negligible. With similar SFR and stellar mass estimates, the derived SFMS is also similar between FADO and MPA-JHU. Conclusions. Our results show that for SDSS normal SF galaxies, the additional modelling of the nebular contribution does not affect the retrieved fluxes and consequentially also does not influence SFR estimators based on the extinction-corrected Hα luminosity. For the stellar masses, the results point to the same conclusion. These results are a consequence of the fact that the vast majority of normal SF galaxies in the SDSS have a low nebular contribution. However, the obtained agreement might only hold for local SF galaxies, but higher-redshift galaxies might show different physical properties when FADO is used. This would then be an effect of the expected increased nebular contribution.

Self-consistent population spectral synthesis with FADO

Context. The field of galaxy evolution will make a great leap forward in the next decade as a consequence of the huge effort by the scientific community in multi-object spectroscopic facilities. Various future surveys will enormously increase the number of available galaxy spectra, providing new insights into unexplored areas of research. To maximise the impact of such incoming data, the analysis methods must also step up, extracting reliable information from the available spectra. It is therefore urgent to refine and test reliable analysis tools that are able to infer the properties of a galaxy from medium- or high-resolution spectra. Aims. In this paper we aim to investigate the limits and the reliability of different spectral synthesis methods in the estimation of the mean stellar age and metallicity. These two quantities are fundamental to determine the assembly history of a galaxy by providing key insights into its star formation history. The main question this work aims to address is which signal-to-noise ratios (S/N) are needed to reliably determine the mean stellar age and metallicity from a galaxy spectrum and how this depends on the tool used to model the spectra. Methods. To address this question we built a set of realistic simulated spectra containing stellar and nebular emission, reproducing the evolution of a galaxy in two limiting cases: a constant star formation rate and an exponentially declining star formation with a single initial burst. We degraded the synthetic spectra built from these two star formation histories (SFHs) to different S/N and analysed with three widely used spectral synthesis codes, namely FADO, STECKMAP, and STARLIGHT, assuming similar fitting set-ups and the same base of spectral templates. Results. For S/N ≤ 5 all the three tools show a large diversity in the results. The FADO and STARLIGHT tools find median differences in the light-weighted mean stellar age of ∼0.1 dex, while STECKMAP shows a higher value of ∼0.2 dex. For S/N > 50 the median differences in FADO are ∼0.03 dex (∼7%), a factor 3 and 4 lower than the 0.08 dex (∼20%) and 0.11 dex (∼30%) obtained from STARLIGHT and STECKMAP, respectively. Detailed investigations of the best-fit spectrum for galaxies with overestimated mass-weighted quantities point towards the inability of purely stellar models to fit the observed spectral energy distribution around the Balmer jump. Conclusions. Our results imply that when a galaxy enters a phase of high specific star formation rate (sSFR) the neglect of the nebular continuum emission in the fitting process has a strong impact on the estimation of its SFH when purely stellar fitting codes are used, even in presence of high S/N spectra. The median value of these differences are of the order of 7% (FADO), 20% (STARLIGHT), and 30% (STECKMAP) for light-weighted quantities, and 20% (FADO), 60% (STARLIGHT), and 20% (STECKMAP) for mass-weighted quantities. More specifically, for a continuous SFH both STECKMAP and STARLIGHT overestimate the stellar age by > 2 dex within the first ∼100 Myr even for high S/N spectra. This bias, which stems from the neglect of nebular continuum emission, obviously implies a severe overestimation of the mass-to-light ratio and stellar mass. But even in the presence of a mild contribution from nebular continuum, there is still the possibility to misinterpret the data as a consequence of the poor quality of the observations. Our work underlines once more the importance of a self-consistent treatment of nebular emission, as implemented in FADO, which, according to our analysis, is the only viable route towards a reliable determination of the assembly of any high-sSFR galaxy at high and low redshift.

FADO: A novel self-consistency spectral population synthesis tool for the exploration of galaxy evolution at high redshift

AbstractDespite significant progress over the past decades, all state-of-the-art population synthesis (PS) codes suffer from deficiencies limiting their potential of gaining sharp insights into the star formation history (SFH) and Chemical Enrichment History (CEH) of star-forming galaxies, i.e. the neglect of nebular continuum and, the lack of a mechanism to ensure consistency between the best-fitting SFH and the observed nebular characteristics (ONC; Balmer-lines, Balmer/Paschen jumps). These introduce biases in their recovered physical properties (stellar mass M* and sSFR). FADO is a novel self-consistent PS code employing genetic optimization, publicly available (http://www.spectralsynthesis.org), capable of identifying the SFH & CEH that reproduce the ONC of a galaxy, alleviating degeneracies in the spectral fits. The current version of FADO (v1.b) uses standard BPT emission-line ratios for the classification of low redshift (z) galaxies. Whereas this permits a reliable distinction between star-forming, Composite, Seyfert and LINERs, it is inapplicable to many intermediate-z galaxies. We present an adaptation of FADO (version v1.c) to classify higher z galaxies employing the “Blue Diagram” (e.g., Lamareille 2010) for which the most prominent blue emission-lines (< [OIII]5007Å) are observable while the Hα and [NII] are inaccessible. FADO v1.c was applied to synthetic spectra simulating the evolution of galaxies formed at higher-z with different SFHs. FADO can recover the physical and evolutionary properties of galaxies, such as M* and mean age/metallicity, with an accuracy significantly better than purely-stellar codes. An outline of FADO v1.c and applications to local and intermediate-z galaxies will be presented.

Self-consistent population spectral synthesis with FADO

Context.Spectral population synthesis (PS) is a fundamental tool in extragalactic research that aims to decipher the assembly history of galaxies from their spectral energy distribution (SED). Whereas this technique has led to key insights into galaxy evolution in recent decades, star formation histories (SFHs) inferred therefrom have been plagued by considerable uncertainties stemming from inherent degeneracies and the fact that until recently allPScodes were restricted to purely stellar fits, neglecting the essential contribution of nebular emission (ne). With the advent ofFADO(Fitting Analysis using Differential evolution Optimisation), the now possible self-consistent modelling of stellar andneopens new routes to the exploration of galaxy SFHs.Aims.The main goal of this study is to quantitatively explore the accuracy to whichFADOcan recover physical and evolutionary properties of galaxies and compare its output with that from purely stellarPScodes.Methods.FADOandSTARLIGHTwere applied to synthetic SEDs that track the spectral evolution of stars and gas in extinction-free mock galaxies of solar metallicity that form their stellar mass (M⋆) according to different parametric SFHs. Spectral fits were computed for two different set-ups that approximate the spectral range of SDSS and CALIFA (V500) data, using up to seven libraries of simple stellar population spectra in the 0.005–2.5Z⊙metallicity range.Results.Our analysis indicates thatFADOcan recover the key physical and evolutionary properties of galaxies, such asM⋆and mass- and light-weighted mean age and metallicity, with an accuracy better than 0.2 dex. This is the case even in phases of strongly elevated specific star formation rate (sSFR) and thus with considerablenecontamination (EW(Hα) > 103Å). Likewise, population vectors fromFADOadequately recover the mass fraction of stars younger than 10 Myr and older than 1 Gyr (M⋆<10Myr/M⋆totalandM⋆>1Gyr/M⋆total, respectively) and reproduce with a high fidelity the observed Hαluminosity. As forSTARLIGHT, our analysis documents a moderately good agreement with theoretical values only for evolutionary phases for whichnedrops to low levels (EW(Hα) ≤ 60 Å) which, depending on the assumed SFH, correspond to an age between ∼0.1 Gyr and 2–4 Gyr. However, fits withSTARLIGHTduring phases of high sSFR severely overestimate bothM⋆and the mass-weighted stellar age, whereas strongly underestimate the light-weighted age and metallicity. Furthermore, our analysis suggests a subtle tendency ofSTARLIGHTto favour a bi-modal SFH, as well a slightly overestimatedM⋆<10Myr/M⋆total, regardless of galaxy age. Whereas the amplitude of these biases can be reduced, depending on the specifics of the fitting procedure (e.g. accuracy and completeness of flagging emission lines, omission of the Balmer and Paschen jump from the fit), they persist even in the idealised case of a line-free SED comprising only stellar and nebular continuum emission.Conclusions.The insights from this study suggest that the neglect of nebular continuum emission inSTARLIGHTand similar purely stellarPScodes could systematically impactM⋆and SFH estimates for star-forming galaxies. We argue that these biases can be relevant in the study of a range of topics in extragalactic research, including the redshift-dependent slope of the star formation (SF) main sequence, the SF frosting hypothesis, and the regulatory role of supermassive black holes on the global SFH of galaxies.

Impact of an AGN featureless continuum on estimation of stellar population properties

The effect of the featureless power-law (PL) continuum of an active galactic nucleus (AGN) on the estimation of physical properties of galaxies with optical population spectral synthesis (PSS) remains largely unknown. With this in mind, we fit synthetic galaxy spectra representing a wide range of galaxy star formation histories (SFHs) and including distinct PL contributions of the form $F_{\nu} \propto \nu^{-\alpha}$ with the PSS code STARLIGHT to study to which extent various inferred quantities (e.g. stellar mass, mean age, and mean metallicity) match the input. The synthetic spectral energy distributions (SEDs) computed with our evolutionary spectral synthesis code include an AGN PL component with $0.5 \leq \alpha \leq 2$ and a fractional contribution $0.2 \leq x_{\mathrm{AGN}} \leq 0.8$ to the monochromatic flux at 4020 \AA. At the empirical AGN detection threshold $x_{\mathrm{AGN}}\simeq 0.26$ that we previously inferred in a pilot study on this subject, our results show that the neglect of a PL component in spectral fitting can lead to an overestimation by $\sim$2 dex in stellar mass and by up to $\sim$1 and $\sim$4 dex in the light- and mass-weighted mean stellar age, respectively, whereas the light- and mass-weighted mean stellar metallicity are underestimated by up to $\sim$0.3 and $\sim$0.6 dex, respectively. Other fitting set-ups including either a single PL or multiple PLs in the base reveal, on average, much lower unsystematic uncertainties of the order of those typically found when fitting purely stellar SEDs with stellar templates, however, reaching locally up to $\sim$1, 3 and 0.4 dex in mass, age and metallicity, respectively. Our results underscore the importance of an accurate modelling of the AGN spectral contribution in PSS fits as a minimum requirement for the recovery of the physical and evolutionary properties of stellar populations in active galaxies.

Fitting Analysis using Differential evolution Optimization (FADO):

The goal of population spectral synthesis (PSS) is to decipher from the spectrum of a galaxy the mass, age and metallicity of its constituent stellar populations. This technique has been established as a fundamental tool in extragalactic research. It has been extensively applied to large spectroscopic data sets, notably the SDSS, leading to important insights into the galaxy assembly history. However, despite significant improvements over the past decade, all current PSS codes suffer from two major deficiencies that inhibit us from gaining sharp insights into the star-formation history (SFH) of galaxies and potentially introduce substantial biases in studies of their physical properties (e.g., stellar mass, mass-weighted stellar age and specific star formation rate). These are i) the neglect of nebular emission in spectral fits, consequently, ii) the lack of a mechanism that ensures consistency between the best-fitting SFH and the observed nebular emission characteristics of a star-forming (SF) galaxy. In this article, we present FADO (Fitting Analysis using Differential evolution Optimization): a conceptually novel, publicly available PSS tool with the distinctive capability of permitting identification of the SFH that reproduces the observed nebular characteristics of a SF galaxy. This so-far unique self-consistency concept allows us to significantly alleviate degeneracies in current spectral synthesis. The innovative character of FADO is further augmented by its mathematical foundation: FADO is the first PSS code employing genetic differential evolution optimization. This, in conjunction with other unique elements in its mathematical concept (e.g., optimization of the spectral library using artificial intelligence, convergence test, quasi-parallelization) results in key improvements with respect to computational efficiency and uniqueness of the best-fitting SFHs.

RemoveYoung: A tool for the removal of the young stellar component in galaxies within an adjustable age cutoff

The optical morphology of galaxies holds the cumulative record of their assembly history, and techniques for its quantitative characterization offer a promising avenue toward understanding galaxy formation and evolution. However, the morphology of star-forming galaxies is generally dictated by the youngest stellar component, which can readily overshine faint structural/morphological features in the older underlying stellar background (e.g., relics from recent minor mergers) that could hold important insights into the galaxy build-up process. Stripping off galaxy images from the emission from stellar populations younger than an adjustable age cutoff tcut can, therefore, provide a valuable tool in extragalactic research. RemoveYoung (RY), a publicly available tool that is presented here, exploits the combined power of integral field spectroscopy (IFS) and spectral population synthesis (SPS) toward this goal. Two-dimensional (2D) post-processing of SPS models to IFS data cubes with RY permits computation of the spectral energy, surface brightness, and stellar surface density distribution of stellar populations older than a user-defined tcut. This suggests a variety of applications of star-forming galaxies, such as interacting or merging galaxy pairs and lower mass starburst galaxies near and far; these include blue compact and tidal dwarf galaxies.

Semi-empirical AGN detection threshold in spectral synthesis studies of Lyman-continuum-leaking early-type galaxies

Various lines of evidence suggest that the cores of a large portion of early-type galaxies (ETGs) are virtually evacuated of warm ionised gas. This implies that the Lyman-continuum (LyC) radiation produced by an assumed active galactic nucleus (AGN) can escape from the nuclei of these systems without being locally reprocessed into nebular emission, which would prevent their reliable spectroscopic classification as Seyfert galaxies with standard diagnostic emission-line ratios. The spectral energy distribution (SED) of these ETGs would then lack nebular emission and be essentially composed of an old stellar component and the featureless power-law (PL) continuum from the AGN. A question that arises in this context is whether the AGN component can be detected with current spectral population synthesis in the optical, specifically, whether these techniques effectively place an AGN detection threshold in LyC-leaking galaxies. To quantitatively address this question, we took a combined approach that involves spectral fitting with STARLIGHT of synthetic SEDs composed of stellar emission that characterises a 10 Gyr old ETG and an AGN power-law component that contributes a fraction $0\leq x_{\mathrm{AGN}} < 1$ of the monochromatic luminosity at $\lambda_0=$ 4020 \AA. In addition to a set of fits for PL distributions $F_{\nu} \propto \nu^{-\alpha}$ with the canonical $\alpha=1.5$, we used a base of multiple PLs with $0.5 \leq \alpha \leq 2$ for a grid of synthetic SEDs with a signal-to-noise ratio of 5-$10^3$. Our analysis indicates an effective AGN detection threshold at $x_{\mathrm{AGN}}\simeq 0.26$, which suggests that a considerable fraction of ETGs hosting significant accretion-powered nuclear activity may be missing in the AGN demographics.

STELLAR KINEMATICS OFz∼ 2 GALAXIES AND THE INSIDE-OUT GROWTH OF QUIESCENT GALAXIES,

Using stellar kinematics measurements, we investigate the growth of massive, quiescent galaxies from z~2 to today. We present X-Shooter spectra from the UV to NIR and dynamical mass measurements of 5 quiescent massive (>10^11 Msun) galaxies at z~2. This triples the sample of z>1.5 galaxies with well constrained (dsigma <100 km/s) velocity dispersion measurements. From spectral population synthesis modeling we find that these galaxies have stellar ages that range from 0.5-2 Gyr, with no signs of ongoing star formation. We measure velocity dispersions (290-450 km/s) from absorption lines and find that they are 1.6-2.1 times higher than those of galaxies in the SDSS at the same mass. Sizes are measured using GALFIT from HST-WFC3 H160 and UDS K-band images. The dynamical masses correspond well to the SED based stellar masses, with dynamical masses that are ~15% higher. We find that M_*/M_dyn may decrease slightly with time, which could reflect the increase of the dark matter fraction within an increasing effective radius. We combine different stellar kinematic studies from the literature, and examine the structural evolution from z~2 to z~0: we confirm that at fixed dynamical mass, the effective radius increases by a factor of ~2.8, and the velocity dispersion decreases by a factor of ~1.7. The mass density within one effective radius decreases by a factor of ~20, while within a fixed physical radius (1 kpc) it decreases only mildly (~2). When we allow for an evolving mass limit by selecting a population of galaxies at fixed number density, a stronger size growth with time is found (~4), velocity dispersion decreases by a factor of ~1.4, and interestingly, the mass density within 1 kpc is consistent with no evolution. This finding suggests that massive quiescent galaxies at ~2 grow inside-out, consistent with the expectations from minor mergers.

THE CENTRAL MOLECULAR GAS STRUCTURE IN LINERS WITH LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI: EVIDENCE FOR GRADUAL DISAPPEARANCE OF THE TORUS

The growth of galaxies is one of the key problems in understanding the structure and evolution of the universe and its constituents. Galaxies can grow their stellar mass by accretion of halo or intergalactic gas clouds, or by merging with smaller or similar mass galaxies. The gas available translates into a rate of star formation, which controls the generation of metals in the universe. The spatially resolved history of their stellar mass assembly has not been obtained so far for any given galaxy beyond the Local Group. Here we demonstrate how massive galaxies grow their stellar mass inside-out. We report the results from the analysis of the first 105 galaxies of the largest to date three-dimensional spectroscopic survey of galaxies in the local universe (CALIFA). We apply the fossil record method of stellar population spectral synthesis to recover the spatially and time resolved star formation history of each galaxy. We show, for the first time, that the signal of downsizing is spatially preserved, with both inner and outer regions growing faster for more massive galaxies. Further, we show that the relative growth rate of the spheroidal component, nucleus and inner galaxy, that happened 5-7 Gyr ago, shows a maximum at a critical stellar mass ~10^10 Msun. We also find that galaxies less massive than ~10^10 Msun show a transition to outside-in growth, thus connecting with results from resolved studies of the growth of low mass galaxies.

ON THE DETECTION OF IONIZING RADIATION ARISING FROM STAR-FORMING GALAXIES AT REDSHIFTz∼ 3-4: LOOKING FOR ANALOGS OF “STELLAR RE-IONIZERS”

We use the spatially-resolved, multi-band photometry in the GOODS South field acquired by the CANDELS project to constrain the nature of candidate Lyman continuum (LyC) emitters at redshift z~3.7 identified using ultra-deep imaging below the Lyman limit (1-sigma limit of ~30 AB in a 2" diameter aperture). In 18 candidates, out of a sample of 19 with flux detected at >3-sigma level, the light centroid of the candidate LyC emission is offset from that of the LBG by up to 1.5". We fit the SED of the LyC candidates to spectral population synthesis models to measure photometric redshifts and the stellar population parameters. We also discuss the differences in the UV colors between the LBG and the LyC candidates, and how to estimate the escape fraction of ionizing radiation (f_esc) in cases, like in most of our galaxies, where the LyC emission is spatially offset from the host galaxy. In all but one case we conclude that the candidate LyC emission is most likely due to lower redshift interlopers. Based on these findings, we argue that the majority of similar measurements reported in the literature need further investigation before it can be firmly concluded that LyC emission is detected. Our only surviving LyC candidate is a LBG at z=3.795, which shows the bluest (B-V) color among LBGs at similar redshift, a stellar mass of M~2 x 10^9 Msun, weak interstellar absorption lines and a flat UV spectral slope with no Lya in emission. We estimate its f_esc to be in the range 25%-100%, depending on the dust and intergalactic attenuation.

Star cluster formation and evolution in Mrk 930: properties of a metal-poor starburst★

We present a HST multiband analysis of the large population of star clusters in the blue compact galaxy (BCG) Mrk 930. We have reconstructed the spectral energy distributions of the star clusters and estimated age, mass, and extinction for a representative sample. We observe a very young cluster population with 70% of the systems formed less than 10 Myr ago. The peak in the star cluster age distribution at 4 Myr is corroborated by the presence of Wolf-Rayet spectral features, and by the observed optical and IR lines ratios [OIII]/H_beta and [NeIII]/[NeII]. The recovered extinction in these very young clusters shows large variations, with a decrease at older ages. It is likely that our analysis is limited to the optically brightest objects (i.e. systems only partially embedded in their natal cocoons). We map the extinction across the galaxy using low-resolution spectra and the H_alpha/H_beta ratio, as obtained from ground-based narrow band imaging. We find that the mean optical extinction derived in the starburst regions is close to the averaged value observed in the clusters, but locally, do not trace the more extinguished clusters. Previous HST studies of BCGs have revealed a population of young and extremely red super star clusters. We detect a considerable fraction of clusters affected by a red excess also in Mrk 930. The nature of the red excess, which turns up at near-IR wavelengths remains unknown. We compare the cluster and the star formation history, the latter derived from the fit of spectral population synthesis models to the spectra. We find a general agreement between the two independently estimated quantities. Using the cluster properties we perform a study of the host environmental properties. We find that the cluster formation efficiency is significantly higher, suggesting a key role of the environment for the formation of these massive objects.[Abridged]

Spectral population synthesis including massive binaries

We have constructed a new code to produce synthetic spectra of stellar populations that includes massive binaries. We have tested this code against the broadband colours of unresolved young massive stellar clusters in nearby galaxies, the equivalent widths of the Red and Blue Wolf-Rayet bumps in star-forming SDSS galaxies and the UV and optical spectra of the star forming regions Tol-A and B in NGC5398. In each case we find a good agreement between our models and observations. We find that in general binary populations are bluer and have fewer red supergiants, and thus significantly less flux in the I-band and at longer wavelengths, than single star populations. Also we find that Wolf-Rayet stars occur over a wider range of ages up to 10^7 years in a stellar population including binaries, increasing the UV flux and Wolf-Rayet spectral features at later times. In addition we find that nebula emission contributes significantly to these observed properties and must be considered when comparing stellar models with observations of unresolved stellar populations. We conclude that incorporation of massive stellar binaries can improve the agreement between observations and synthetic spectral synthesis codes, particularly for systems with young stellar populations.

Stellar population gradients in Seyfert 2 galaxies: northern sample

We use high signal-to-noise ratio long-slit spectra in the λλ3600-4700 range of the 20 brightest northern Seyfert 2 galaxies to study the variation of the stellar population properties as a function of distance from the nucleus. In order to characterize the stellar population and other continuum sources (e.g. featureless continuum, FC) we have measured the equivalent width, W, of six absorption features, four continuum colours and their radial variations, and performed spectral population synthesis as a function of distance from the nucleus. About half of the sample has Ca II K and G band W values smaller at the nucleus than at 1 kpc from it, owing to a younger population and/or FC. The stellar population synthesis shows that, while at the nucleus, 75 per cent of the galaxies present contribution >20 per cent of ages ≤100 Myr and/or of an FC, this proportion decreases to 45 per cent at 3 kpc. In particular, 55 per cent of the galaxies have a contribution >10 per cent of the 3-Myr/FC component (a degenerate component in which one cannot separate what is caused by an FC or by a 3-Myr stellar population) at the nucleus, but only 25 per cent of them have this contribution at 3 kpc. As a reference, the stellar population of 10 non-Seyfert galaxies, spanning the Hubble types of the Seyfert (from S0 to Sc) was also studied. A comparison between the stellar population of the Seyferts and that of the non-Seyferts shows systematic differences: the contribution of ages younger than 1 Gyr is in most cases larger in the Seyfert galaxies than in non-Seyferts, not only at the nucleus but up to 1 kpc from it.

Stellar population gradients in normal and active galaxies

We use high signal-to-noise ratio long-slit spectra in the λλ3500–7000-A range of three early-type (Hubble type S0) normal, two low-ionization nuclear emission region (LINER) and three Seyfert 2 galaxies to make a comparative study of the variation of the stellar population properties as a function of distance from the nucleus. In order to characterize the stellar population and other continuum sources (e.g. featureless continuum) we have measured equivalent widths (Ws) of six absorption features, continuum colours and their radial variations, and performed spectral population synthesis as a function of distance from the nucleus. The Ws for the normal galaxies are strongest at the nucleus and decrease outwards. The two LINER galaxies show a similar behaviour, but with the nuclear Ws in the blue (λ<4500 A)≈10–20 per cent smaller. Both normal galaxies and LINERs present a redder nucleus than their surroundings. The three Seyfert 2 galaxies present the opposite behaviour, with most Ws in the blue being smaller at the nucleus, which is also bluer than outside. By combining the results of the present work with those of a previous study, we conclude that, in a sample of 12 Seyfert 2s of Hubble type S0, 10 (≈80 per cent) present nuclear Ws smaller than those of normal galaxies, while at ≈1 kpc from the nucleus this number decreases to 4 (≈30 per cent). Sampling regions of ≈100–200 pc, we conclude that the normal galaxies have nuclear stellar populations dominated by the components with an age of 10 Gyr and metallicity solar or above. The contribution of this component decreases, while that of 1 Gyr increases, with increasing distance from the nucleus. The LINERs present a similar behaviour but have a ≈10–20 per cent larger contribution of the 1-Gyr component at the nucleus. For the Seyfert 2 galaxies, there are additional components: in the three cases studied here, there is a significant contribution from a 100-Myr-old population at the nucleus plus featureless continuum in two cases and also of a 3-Myr-old component in Mrk463E. Outside the nucleus, the stellar population is similar to that of normal galaxies in ESO138-G1, but it has an excess of old metal-poor components in NGC7743 and of young components in Mrk463E, when compared with normal galaxies. The reddening of the continuum is lowest in the normal galaxy nuclei [E(B−V)i≈0.00] and highest in the Seyfert [E(B−V)i≈0.20] nuclei.

Population synthesis of H II galaxies

We study the stellar population of galaxies with active star formation, determining ages of the stellar components by means of spectral population synthesis of their absorption spectra. The data consist of optical spectra of 185 nearby (z0.075) emission-line galaxies. They are mostly H ii galaxies, but we also include some starbursts and Seyfert 2s, for comparison purposes. They were grouped into 19 high signal-to-noise ratio template spectra, according to their continuum distribution, absorption- and emission-line characteristics. The templates were then synthesized with a star cluster spectral base. The synthesis results indicate that H ii galaxies are typically age-composite stellar systems, presenting important contributions from generations up to as old as 500 Myr. We detect a significant contribution of populations with ages older than 1 Gyr in two groups of H ii galaxies. The age distributions of stellar populations among starbursts can vary considerably despite similarities in the emission-line spectra. In the case of Seyfert 2 groups we obtain important contributions from the old population, consistent with a bulge. From the diversity of star formation histories, we conclude that typical H ii galaxies in the local Universe are not systems presently forming their first stellar generation.