Continuous Star Formation Models Research Articles

The Age Dependence of Mid-infrared Emission around Young Star Clusters

Abstract Using the star cluster catalogs from the Hubble Space Telescope program Legacy Extragalactic UV survey (LEGUS) and 8 μm images from the IRAC camera on the Spitzer Space Telescope for five galaxies within 5 Mpc, we investigate how the 8 μm dust luminosity correlates with the stellar age on the 30–50 pc scale of star-forming regions. We construct a sample of 97 regions centered at local peaks of 8 μm emission, each containing one or more young star cluster candidates from the LEGUS catalogs. We find a tight anticorrelation with a Pearson correlation coefficient of r = −0.84 ± 0.05 between the mass-normalized dust-only 8 μm luminosity and the age of stellar clusters younger than 1 Gyr; the 8 μm luminosity decreases with increasing age of the stellar population. Simple assumptions on a combination of stellar and dust emission models reproduce the observed trend. We also explore how the scatter of the observed trend depends on assumptions of stellar metallicity, polycyclic aromatic hydrocarbon (PAH) abundance, fraction of stellar light absorbed by dust, and instantaneous versus continuous star formation models. We find that variations in stellar metallicity have little effect on the scatter, while PAH abundance and the fraction of dust-absorbed light bracket the full range of the data. We also find that the trend is better explained by continuous star formation, rather than instantaneous burst models. We ascribe this result to the presence of multiple star clusters with different ages in many of the regions. Upper limits of the dust-only 8 μm emission as a function of age are provided.

N/O abundance ratios in gamma-ray burst and supernova host galaxies at z < 4. Comparison with AGN, starburst and H ii regions

The distribution of the N/O element abundance ratios calculated by the detailed modelling of different galaxy spectra at z < 4 is investigated. Supernova (SN) and long gamma-ray-burst (LGRB) host galaxies cover different redshift domains. N/O ratios in SN hosts increase due to secondary N production towards low z (0.01) accompanying the growing trend of active galaxies [active galactic nucleus (AGN), low-ionization nuclear emission-line region (LINER)]. N/O ratios in LGRB hosts decrease rapidly between z > 1 and z ∼ 0.1 following the N/H trend and reach the characteristic N/O ratios calculated for the H ii regions in local and nearby galaxies. The few short-period gamma-ray-burst (SGRB) hosts included in the galaxy sample show N/H ≤ 0.04 solar and O/H solar. They seem to continue the low bound N/H trend of SN hosts at z < 0.3. The distribution of N/O as a function of metallicity for SN and LGRB hosts is compared with star chemical evolution models. The results show that several LGRB hosts can be explained by star multibursting models when 12+log(O/H) < 8.5, while some objects follow the trend of continuous star formation models. N/O in SN hosts at log(O/H)+12 < 8.5 are not well explained by stellar chemical evolution models calculated for starburst galaxies. At 12+log(O/H) > 8.5 many different objects are nested close to O/H solar with N/O ranging between the maximum corresponding to starburst galaxies and AGN and the minimum corresponding to H ii regions and SGRB.

SpitzerObservations ofz∼ 3 Lyman Break Galaxies: Stellar Masses and Mid‐Infrared Properties

We describe the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z ~ 3, using deep mid-infrared and optical observations of the extended Groth strip, obtained with IRAC and MIPS on board Spitzer and from the ground, respectively. We focus on LBGs with detections at all four IRAC bands, in particular the 26 galaxies with IRAC 8 μm band (rest-frame K-band) detections. We use stellar population-synthesis models and probe the stellar content of these galaxies. Based on best-fit continuous star formation models, we derive estimates of the stellar mass for these LBGs. As in previous studies, we find that a fraction of LBGs have very red colors and large estimated stellar masses (M_* > 5 × 10^(10) M_⊙); the current Spitzer data allow us, for the first time, to study these massive LBGs in detail. We discuss the link between these LBGs and submillimeter-luminous galaxies, and we find that the number density of these massive LBGs at high redshift is higher than predicted by current semianalytic models of galaxy evolution.

The Life Cycle of Massive Red Galaxies

AbstractSamples of Extremely Red Galaxies (ERGs) have generally been seen to comprise a mix of actively star-forming galaxies with significant dust reddening and evolved, passive galaxies, at redshifts about z ≈ 1 − 2. Initial results from deep Keck spectroscopy of ERGs (Doherty et al. 2005) revealed dominant old stellar populations in 75% of our spectroscopic sample, but only 28% have spectra with no evidence of recent star formation activity, such as would be expected for a strictly passively-evolving population. This study suggests that the bulk of the ERGs are luminous, spheroidal, evolved galaxies, but undergoing intermittent activity consistent with continued growth.Through a detailed investigation of individual galaxies in our sample we aim to address various outstanding questions. What fraction of their mass is produced in ongoing star formation? Is there a characteristic mass at which star formation is abruptly truncated? What mechanism provokes a secondary burst of star formation in evolved galaxies?We fit Bruzual &amp; Charlot (2003; BC03) simple stellar population models to the broad band SEDs over a wide baseline, using a reduced χ2 minimisation, to investigate ages, stellar masses and star formation histories. The fits for the early types agree well with information in the spectra and return ages of 2–3 Gyr and masses in the range 1011–1012M⊙. The objects with recent star formation episodes are more complex. Some are fit well by continuous star formation models, accounting for the effects of dust. We are now in the process of exploring multi-population fits to investigate the effects of episodic bursts.Previous morphological studies of ERGs have revealed a diverse mix of galaxies – a combination of pure bulges, disks and a small fraction of irregular or interacting systems. We are curious to determine whether a morphological analysis produces results consistent with the spectroscopic properties of our sample. We are investigating a sub-sample of our galaxies which have HST imaging publically available. Initial results from a quantitative analysis using bulge/disk decomposition with GALFIT and GIM2D indicate that most galaxies with Early type spectra are bulge dominated. In contrast, a significant fraction of the galaxies showing spectroscopic signatures of on-going star formation on top of underlying old stellar populations appear to have a well-established classical spiral morphology, wih knots of star formation located in spiral arms around a central bulge. There is tenuous evidence (under further investigation) that at least half of the post-starbursts in our sample are barred spirals, lending support to theories relating post-starbursts to recent mergers.

How old are H ii galaxies?

Using a novel approach we have reanalysed the question of whether the extreme star-forming galaxies known as H ii galaxies are truly young or rejuvenated old systems. We first present a method of inversion that applies to any monotonic function of time describing the evolution of independent events. We show that, apart from a normalization constant, the ‘true’ time dependence can be recovered from the inversion of its probability density function. We applied the inversion method to the observed equivalent width of the Hβ [EW(Hβ)] distribution for objects in the Spectrophotometric Catalogue of H ii Galaxies compiled by Terlevich and collaborators, and found that the global history of star formation behaves more like a continuous star-formation model than an instantaneous model. On the other hand, when the inversion method is applied to samples within a restricted metallicity range, we find that their history of star formation is much closer to what the instantaneous model predicts. Our main conclusion is that, globally, the evolution of H ii galaxies seems consistent with a succession of short starbursts separated by quiescent periods and that, while the emission lines trace the properties of the present burst, the underlying stellar continuum traces the whole star-formation history of the galaxy. Thus, observables like the EW(Hβ) that combine an emission line flux, i.e. a parameter pertaining to the present burst, with the continuum flux, i.e. a parameter that traces the whole history of star formation, should not be used alone to characterize the present burst.

Chemical evolution of starburst galaxies: How does star formation proceed?

We compute chemical evolution models to constrain the mode and the history of star formation in starburst galaxies as a whole, i.e. over a large range of mass and metallicity. To this end, we investigate the origin of the dispersion observed in the evolution of both nitrogen-to-oxygen abundance ratio and galaxy luminosity as a function of metallicity for a large sample of starburst galaxies. We nd that the variation of the star formation eciency, in the framework of continuous star formation models, produces a scatter equivalent to that observed in the N/O versus O/H diagram for low-mass Hii galaxies only. However, continous star formation models are unable to reproduce i) the scatter observed for massive starburst and UV-selected galaxies in the N/O versus O/H relation, and ii) the scatter in the MB versus O/H scaling relation observed for the whole sample of starburst galaxies. The dispersion associated with the distribution of N/O as a function of metallicity, for both low-mass and massive galaxies, is well explained in the framework of bursting star formation models. It is interpreted as a consequence of the time-delay between the ejection of nitrogen and that of oxygen into the ISM. These models also reproduce the spread observed in the luminosity-metallicity relation. Metal-rich spiral galaxies dier from metal- poor ones by a higher star formation eciency and starburst frequency. Low-mass galaxies experienced a few bursts of star formation whereas massive spiral galaxies experienced numerous and extended powerful starbursts. Finally, we conrm previous claims (Contini et al. 2002) that UV-selected galaxies are observed at a special stage in their evolution. Their low N/O abundance ratios with respect to other starburst galaxies is well explained if they have just undergone a powerful starburst that enriched their ISM in oxygen.

Star Formation in the Field and Clusters of NGC 5253

We investigate the star formation history of both the bright star clusters and the diffuse "field star" population in the dwarf starburst galaxy NGC 5253 using long-slit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph. The slit covers a physical area of 370 pc × 1.6 pc and includes eight apparent clusters and several intercluster regions of diffuse light that we take to be the field. The diffuse light spectrum lacks the strong O-star wind features that are clearly visible in spectra of the brightest clusters. This discrepancy provides compelling evidence that the diffuse light is not reflected light from nearby clusters but originates in a UV-bright field star population, and it raises the issue of whether the star formation process may be operating differently in the field than in the clusters. We compare our spectra to STARBURST99 evolutionary synthesis models that incorporate a new low-metallicity (~ Z☉) atlas of O-star spectra. The clusters are well fitted by instantaneous burst models with a Salpeter initial mass function (IMF) extending up to 100 M☉, and we derive ages for them ranging from 1 to 8 Myr. Reasonable fits to the field spectrum are obtained by continuous star formation models with either an upper mass cutoff of ~30 M☉ or an IMF slope steeper than Salpeter (α ~ 3.5). We favor a scenario that accounts for the paucity of O stars in the field without requiring the field to have a different IMF than the clusters: stellar clusters form continuously and then dissolve on ~10 Myr timescales and disperse their remaining stars into the field. We consider the probable contribution of an O-star-deficient field population to the spatially unresolved spectra of high-redshift galaxies.

Synthetic Spectra of H Balmer and He i Absorption Lines. II. Evolutionary Synthesis Models for Starburst and Poststarburst Galaxies

We present evolutionary stellar population synthesis models to predict the spectrum of a single-metallicity stellar population, with a spectral sampling of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which are optimized for galaxies with active star formation, synthesize the profiles of the hydrogen Balmer series (Hβ, Hγ, Hδ, H8, H9, H10, H11, H12, and H13) and the neutral helium absorption lines (He I λ4922, He I λ4471, He I λ4388, He I λ4144, He I λ4121, He I λ4026, He I λ4009, and He I λ3819) for a burst with an age ranging from 106 to 109 yr, and different assumptions about the stellar initial mass function (IMF). Continuous star formation models lasting for 1 Gyr are also presented. The input stellar library includes non-LTE absorption profiles for stars hotter than 25,000 K and LTE profiles for lower temperatures. The temperature and gravity coverage is 4000 K ≤ Teff ≤ 50,000 K and 0.0 ≤ log g ≤ 5.0, respectively. The metallicity is solar. It is found that the Balmer and He I line profiles are sensitive to the age, except during the first 4 Myr of the evolution, when the equivalent widths of these lines are constant. At these early stages of the evolution, the profiles of the lines are also sensitive to the IMF. However, strong H Balmer and He I lines are predicted even when the low-mass cutoff of the IMF is as high as 10 M☉. The equivalent widths of the Balmer lines range from 2 to 16 A and those of the He I lines from 0.2 to 1.2 A. During the nebular phase (cluster younger than about 10 Myr), Hβ ranges from 2 to 5 A and He I λ4471 ranges between 0.5 and 1.2 A. The strength of the lines is maximum when the cluster is a few hundred (for the Balmer lines) and a few tens (for the He I lines) of Myr old. In the continuous star formation scenario, the strength of the Balmer and He I lines increases monotonically with time until 500 and 100 Myr, respectively. However, the lines are weaker than in the burst models owing to the dilution of the Balmer and He I lines by the contribution from very massive stars. The high spectral resolution of the profiles is useful to reproduce the absorption wings observed in regions of recent star formation and to estimate the effect of the underlying absorption on the nebular emission lines. The strength of the nebular emission Balmer and He I lines compared with the stellar absorption components indicates that Hδ and the higher order terms of the Balmer series and He I are dominated by the stellar absorption component if an instantaneous burst is older than 5 Myr. Some of the He I lines (e.g., He I λ3819 and He I λ4922) are more favorable than others (e.g., He I λ4471) for the detection of stellar features in the presence of nebular emission. We estimate that the correction to the He I λ4471 nebular emission line due to the stellar absorption is between 5% and 25%, if the nebular emission has equivalent width between 10 and 2.5 A (corresponding to a burst age between 1 and 3 Myr). The models can be used to date starburst and poststarburst galaxies until 1 Gyr. They have been tested on data for clusters in the LMC, the super-star cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous E+A galaxy. The full data set is available for retrieval at our websites or on request from the authors.

WFPC2 Observations of Star Clusters in the Magellanic Clouds. II. The Oldest Star Clusters in the Small Magellanic Cloud

We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F450W (~B) and F555W (~V) of the intermediate-age populous star clusters NGC 121, NGC 339, NGC 361, NGC 416, and Kron 3 in the Small Magellanic Cloud. We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these seven star clusters in order to improve our understanding of the formation chronology of the SMC. We analyzed the V versus B-V and MV versus (B-V)0 color-magnitude diagrams of these populous Small Magellanic Cloud star clusters using a variety of techniques and determined their ages, metallicities, and reddenings. These new data enable us to improve the age-metallicity relation of star clusters in the Small Magellanic Cloud. In particular, we find that a closed-box continuous star formation model does not reproduce the age-metallicity relation adequately. However, a theoretical model punctuated by bursts of star formation is in better agreement with the observational data presented herein.

Spectroscopy of Luminous Infrared Galaxies at 2 Microns. III. Analysis for Galaxies with log (LIR/L☉) ≳ 11.2

We have obtained spectra across the K window for the first large sample of luminous galaxies selected from the IRAS survey. This paper contains the principal analysis of the 43 systems in our sample with luminosities of 11.2 log (LIR/L?) 11.9. The spectra themselves were presented in a companion paper by Goldader et al. (Paper II). The Br? luminosities are proportional to LIR, at levels similar to those of star-forming regions. This strongly suggests that star formation accounts for the bulk of the energy production in these objects, in general agreement with previous studies. Good agreement is found for the continuous star formation models of Leitherer & Heckman with upper mass cutoffs well below 100 -->?. The models accommodate a range in starburst ages of ~107 to 109 yr. Instantaneous starburst models fit the data but imply an unrealistically short range of ages for the entire sample. It is difficult to avoid concluding that the initial mass functions are deficient in stars of less than ~1 -->?. Strong emission lines from molecular hydrogen are detected. The H2 v = 1-0 S(1) line luminosities are proportional to LIR; the correlation extends through the ultraluminous infrared galaxies. The H2 emission in the galaxies tends to be more spatially extended than the Br? emission. Measured values and upper limits for the ratios of the various H2 lines visible in our spectra indicate that the H2 seen in emission at 2 ?m is consistent with being shock excited. However, other mechanisms, operating at sufficiently high densities that the H2 energy levels are thermalized by collisions, cannot be excluded. Based on energy considerations, we suggest that the shocks are due to supernova remnants expanding into the interstellar medium. The frequency of Type II supernovae necessary to account for the H2 line emission agrees with frequencies deduced from the starburst models and the radio/far-infrared correlation. However, there remain a number of galaxies that cannot be made to fit this model. A decade after its discovery, a universal explanation of the strong H2 emission in luminous infrared galaxies continues to elude us. No previously unrecognized broad-line active nuclei were discovered in our survey; either they are weak or absent or the true optical depths at 2 ?m are much higher than indicated by conventional extinction measures. However, there are clear differences between the K-band properties of galaxies that contain broad-line active nuclei and those that do not. The differences seem to be due to the presence of strong nonstellar continuum emission coming from the active nuclei themselves. With the addition of the 13 ultraluminous galaxies with log (LIR/L?) 12 from Goldader et al. (Paper I), the number of systems observed in this program totals 56. We have incorporated these ultraluminous galaxies in some parts of the analysis to examine properties across the entire luminosity range of our sample.

Optical-IR Spectral Energy Distribution of the Protogalaxy Candidate MS 1512−cB58

The spectral energy distribution of the protogalaxy candidate MS 1512-cB58 at z = 2.72 is presented. Photometry in seven bands ranging from g to K' (1300-6000 A rest wavelength) are fitted with population synthesis models. The data confirm a very young age for this galaxy in agreement with ages estimated from preliminary C IV λ1550 P Cygni profile modeling. Single-burst models with ages greater than about 20 Myr can be discarded at the 99% confidence level, and continuous star formation models with ages greater than about 35 Myr can be discarded at the 95% confidence level. The spectral energy distribution is most consistent with a continuous star formation model of about 10-20 Myr, with reddening of E(B - V) ~ 0.3. No evidence for an older population of stars is seen, but the possibility of an older population with as much as 90% of the galaxy mass cannot be ruled out. We discuss the possible ramifications of a nonstandard IMF and gravitational lensing on the galaxy's age and mass.