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Register a new userNear-infrared integrated spectra of Galactic globular clusters: testing simple stellar population models
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We present SOAR/OSIRIS cross-dispersed NIR integrated spectra of 12 Galactic globular clusters that are employed to test Maraston (2005, M05) NIR EPS models, and to provide spectral observational constraints to calibrate future models. We measured Ew of the most prominent NIR absorption features. Optical Ew were also measured. The globular clusters Ew were compared with model predictions with ages within 4-15 Gyr, and metallicities between 1/200 and 2 Zsun. Observed integrated colours were also compared with models. The NIR integrated spectra among our sample appear qualitatively similar in most the absorption features. The M05 models can properly predict the optical Ew observed in globular clusters. Regarding the NIR, they do underestimate the strength of Mg I 1.49mum, but they can reproduce the observed Ew of Fe I 1.58mum, Si I 1.59mum, and CO 2.29mum, in about half of our sample. The remaining objects require the inclusion of intermediate-age populations. Thus, we suggest that the presence of C- and O-rich stars in models is important to reproduce the observed strengths of metallic lines. Another possibility is the lack of alpha-enhancement in the models. In the case of the optical and NIR Fe I lines, standard models and those that include blue horizontal branch stars, produce similar results. A similar trend is observed for Na I 5895A, while in the case of the G-band, the models with blue horizontal branch do describe better the observations. For most of the sample the optical to NIR colours are well described by the M05 models. In general, M05 models can provide reliable information on the NIR stellar population of galaxies, but only when Ew and colours are taken together, in other words, Ew and continuum fluxes should be simultaneously fitted. However, the results should be taken with caution, since the models tend to predict results biased towards young ages.
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We present near-infrared spectroscopic observations of massive stars in three stellar clusters located in the direction of the inner Galaxy. One of them, the Quartet, is a new discovery while the other two were previously reported as candidate clusters identified on mid-infrared Spitzer images (GLIMPSE20 and GLIMPSE13). Using medium-resolution (R=900-1320) H and K spectroscopy, we firmly establish the nature of the brightest stars in these clusters, yielding new identifications of an early WC and two Ofpe/WN9 stars in the Quartet and an early WC star in GLIMPSE20. We combine this information with the available photometric measurements from 2MASS, to estimate cluster masses, ages, and distances. The presence of several massive stars places the Quartet and GLIMPSE20 among the small sample of known Galactic stellar clusters with masses of a few 10^3 Msun, and ages from 3 to 8 Myr. We estimate a distance of about 3.5 kpc for Glimpse 20, and 6.0 kpc for Quartet. The large number of giant stars identified in GLIMPSE13 indicates that it is another massive (~ 6500 Msun) cluster, but older, with an age between 30 and 100 Myr, at a distance of about 3 kpc.
Spectrum syntheses for three elements (Mg, Na, and Eu) in high-resolution integrated light spectra of the Galactic globular clusters 47 Tuc, M3, M13, NGC 7006, and M15 are presented, along with calibration syntheses of the Solar and Arcturus spectra. Iron abundances in the target clusters are also derived from integrated light equivalent width analyses. Line profiles in the spectra of these five globular clusters are well fit after careful consideration of the atomic and molecular spectral features, providing levels of precision that are better than equivalent width analyses of the same integrated light spectra, and that are comparable to the precision in individual stellar analyses. The integrated light abundances from the 5528 and 5711 A Mg I lines, the 6154 and 6160 A Na I lines, and the 6645 A Eu II line fall within the observed ranges from individual stars; however, these integrated light abundances do not always agree with the average literature abundances. Tests with the second parameter clusters M3, M13, and NGC 7006 show that assuming an incorrect horizontal branch morphology is likely to have only a small (< 0.06 dex) effect on these Mg, Na, and Eu abundances. These tests therefore show that integrated light spectrum syntheses can be applied to unresolved globular clusters over a wide range of metallicities and horizontal branch morphologies. Such high precision in integrated light spectrum syntheses is valuable for interpreting the chemical abundances of globular cluster systems around other galaxies.
We present an extenstive literature compilation of age, metallicity, and chemical abundance pattern information for the 41 Galactic globular clusters (GGCs) studied by Schiavon et al. (2005). Our compilation constitutes a notable improvement over previous similar work, particularly in terms of chemical abundances. Its primary purpose is to enable detailed evaluations of and refinements to stellar population synthesis models designed to recover the above information for unresolved stellar systems based on their integrated spectra. However, since the Schiavon sample spans a wide range of the known GGC parameter space, our compilation may also benefit investigations related to a variety of astrophysical endeavours, such as the early formation of the Milky Way, the chemical evolution of GGCs, and stellar evolution and nucleosynthesis. For instance, we confirm with our compiled data that the GGC system has a bimodal metallicity distribution and is uniformly enhanced in the alpha-elements. When paired with the ages of our clusters, we find evidence that supports a scenario whereby the Milky Way obtained its globular clusters through two channels, in situ formation and accretion of satellite galaxies. The distributions of C, N, O, and Na abundances and the dispersions thereof per cluster corroborate the known fact that all GGCs studied so far with respect to multiple stellar populations have been found to harbour them. Finally, using data on individual stars, we also confirm that the atmospheres of stars become progressively polluted by CN(O)-processed material after they leave the main sequence and uncover evidence which suggests the alpha-elements Mg and Ca may originate from more than one nucleosynthetic production site. [abridged]
Evidence that the multiple populations (MPs) are common properties of globular clusters (GCs) is accumulated over the past decades from clusters in the Milky Way and in its satellites. This finding has revived GC research, and suggested that their formation at high redshift must have been a much-more complex phenomenon than imagined before. However, most information on MPs is limited to nearby GCs. The main limitation is that most studies on MPs rely on resolved stars, facing a major challenge to investigate the MP phenomenon in distant galaxies. Here we search for integrated colors of old GCs that are sensitive to the multiple-population phenomenon. To do this, we exploit integrated magnitudes of simulated GCs with MPs, and multi-band Hubble Space Telescope photometry of 56 Galactic GCs, where MPs are widely studied, and characterized as part of the UV Legacy Survey of Galactic GCs. We find that both integrated $C_{rm F275W,F336W,F438W}$ and $m_{rm F275W}-m_{rm F814W}$ colors strongly correlate with the iron abundance of the host GC. In second order, the pseudo two-color diagram built with these integrated colors is sensitive to the MP phenomenon. In particular, once removed the dependence from cluster metallicity, the color residuals depend on the maximum internal helium variation within GCs and on the fraction of second-generation stars. This diagram, which we define here for Galactic GCs, has the potential of detecting and characterizing MPs from integrated photometry of old GCs, thus providing the possibility to extend their investigation outside the Local Group.
We present ultraviolet (UV) integrated colors of 44 Galactic globular clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV and NUV bands. This data-base is the largest homogeneous catalog of UV colors ever published for stellar systems in our Galaxy. The proximity of GGCs makes it possible to resolve many individual stars even with the somewhat low spatial resolution of GALEX. This allows us to determine how the integrated UV colors are driven by hot stellar populations, primarily horizontal branch stars and their progeny. The UV colors are found to be correlated with various parameters commonly used to define the horizontal branch morphology. We also investigate how the UV colors vary with parameters like metallicity, age, helium abundance and concentration. We find for the first time that GCs associated with the Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs with the same metallicity. Finally, we speculate about the presence of an interesting trend, suggesting that the UV color of GCs may be correlated with the mass of the host galaxy, in the sense that more massive galaxies possess bluer clusters.
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