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Register a new userStellar Populations and the Star Formation Histories of LSB Galaxies: III. Stellar Population Models
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Authors
James Schombert
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A series of population models are designed to explore the star formation history of gas-rich, low surface brightness (LSB) galaxies. LSB galaxies are unique in having properties of very blue colors, low H$alpha$ emission and high gas fractions that indicated a history of constant star formation (versus the declining star formation models used for most spirals and irregulars). The model simulations use an evolving multi-metallicity composite population that follows a chemical enrichment scheme based on Milky Way observations. Color and time sensitive stellar evolution components (i.e., BHB, TP-AGB and blue straggler stars) are included, and model colors are extended into the Spitzer wavelength regions for comparison to new observations. In general, LSB galaxies are well matched to the constant star formation scenario with the variation in color explained by a fourfold increase/decrease in star formation over the last 0.5 Gyrs (i.e., weak bursts). Early-type spirals, from the S$^4$G sample, are better fit by a declining star formation model where star formation has decreased by 40% in the last 12 Gyrs.
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Surface photometry at 3.6$mu$m is presented for 61 low surface brightness (LSB) galaxies ($mu_o < 19$ 3.6$mu$m mag arcsecs$^{-2}$). The sample covers a range of luminosity from $-$11 to $-$22 in $M_{3.6}$ and size from 1 to 25 kpc. The morphologies in the mid-IR are comparable to those in the optical with 3.6$mu$m imaging reaches similar surface brightness depth as ground-based optical imaging. A majority of the resulting surface brightness profiles are single exponential in shape with very few displaying upward or downward breaks. The mean $V-3.6$ color of LSB is 2.3 with a standard deviation of 0.5. Color-magnitude and two color diagrams are well matched to models of constant star formation, where the spread in color is due to small changes in the star formation rate (SFR) over the last 0.5 Gyrs as also suggested by the specific star formation rate measured by H$alpha$.
The luminosities, colors and Halpha emission for 429 HII regions in 54 LSB galaxies are presented. While the number of HII regions per galaxy is lower in LSB galaxies compared to star-forming irregulars and spirals, there is no indication that the size or luminosity function of HII regions differs from other galaxy types. The lower number of HII regions per galaxy is consistent with their lower total star formation rates. The fraction of total $L_{Halpha}$ contributed by HII regions varies from 10 to 90% in LSB galaxies (the rest of the H$alpha$ emission being associated with a diffuse component) with no correlation with galaxy stellar or gas mass. Bright HII regions have bluer colors, similar to the trend in spirals; their number and luminosities are consistent with the hypothesis that they are produced by the same HII luminosity function as spirals. Comparison with stellar population models indicates that the brightest HII regions in LSB galaxies range in cluster mass from a few $10^3 M_{sun}$ (e.g., $rho$ Oph) to globular cluster sized systems (e.g., 30 Dor) and that their ages are consistent with clusters from 2 to 15 Myrs old. The faintest HII regions are comparable to those in the LMC powered by a single O or B star. Thus, star formation in LSB galaxies covers the full range of stellar cluster mass.
We present new HST WFC3 near-IR observations of the CMDs in two LSB galaxies, F575-3 and F615-1, notable for having no current star formation based on a lack of H$alpha$ emission. Key features of the near-IR CMDs are resolved, such as the red giant branch (RGB), the asymptotic giant branch (AGB) region and the top of the blue main sequence (bMS). F575-3 has the bluest RGB of any CMD in the literature, indicating an extremely low mean metallicity. F615-1 has unusually wide RGB and AGB sequences suggesting multiple episodes of star formation from metal-poor gas, possibly infalling material. Both galaxies have an unusual population of stars to the red of the RGB and lower in luminosity than typical AGB stars. These stars have normal optical colors but abnormal near-IR colors. We suggest that this population of stars might be analogous to local peculiar stars like Be stars with strong near-IR excesses owing to a surrounding disk of hot gas.
The majority of spiral and elliptical galaxies in the Universe host very dense and compact stellar systems at their centres known as nuclear star clusters (NSCs). In this work we study the stellar populations and star formation histories (SFH) of the NSCs of six nearby galaxies with stellar masses ranging between $2$ and $8times10^9~{rm M_{odot}}$ (four late-type spirals and two early-types) with high resolution spectroscopy. Our observations are taken with the X-Shooter spectrograph at the VLT. We make use of an empirical simple stellar population (SSP) model grid to fit composite stellar populations to the data and recover the SFHs of the nuclei. We find that the nuclei of all late-type galaxies experienced a prolonged SFH, while the NSCs of the two early-types are consistent with SSPs. The NSCs in the late-type galaxies sample appear to have formed a significant fraction of their stellar mass already more than $10$ Gyr ago, while the NSCs in the two early-type galaxies are surprisingly younger. Stars younger than $100$ Myr are present in at least two nuclei: NGC 247 and NGC 7793, with some evidence for young star formation in NGC 300s NSC. The NSCs of the spirals NGC 247 and NGC 300 are consistent with prolonged in situ star formation with a gradual metallicity enrichment from $sim-1.5$ dex more than $10$ Gyr ago, reaching super-Solar values few hundred Myr ago. NGC 3621 appears to be very metal rich already in the early Universe and NGC 7793 presents us with a very complex SFH, likely dominated by merging of various massive star clusters coming from different environments.
We present the stellar population content of early-type galaxies from the Atlas3D survey. Using spectra integrated within apertures covering up to one effective radius, we apply two methods: one based on measuring line-strength indices and applying single stellar population (SSP) models to derive SSP-equivalent values of stellar age, metallicity, and alpha enhancement; and one based on spectral fitting to derive non-parametric star-formation histories, mass-weighted average values of age, metallicity, and half-mass formation timescales. Using homogeneously derived effective radii and dynamically-determined galaxy masses, we present the distribution of stellar population parameters on the Mass Plane (M_JAM, Sigma_e, R_maj), showing that at fixed mass, compact early-type galaxies are on average older, more metal-rich, and more alpha-enhanced than their larger counterparts. From non-parametric star-formation histories, we find that the duration of star formation is systematically more extended in lower mass objects. Assuming that our sample represents most of the stellar content of todays local Universe, approximately 50% of all stars formed within the first 2 Gyr following the big bang. Most of these stars reside today in the most massive galaxies (>10^10.5 M_sun), which themselves formed 90% of their stars by z~2. The lower-mass objects, in contrast, have formed barely half their stars in this time interval. Stellar population properties are independent of environment over two orders of magnitude in local density, varying only with galaxy mass. In the highest-density regions of our volume (dominated by the Virgo cluster), galaxies are older, alpha-enhanced and have shorter star-formation histories with respect to lower density regions.
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