Metal?poor turnoff and subgiant field stars in the Galaxy (original) (raw)
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Basic calibrations of the photographic RGU system. IV. Metal-poor subgiant and giant stars
Astronomy & Astrophysics - ASTRON ASTROPHYS, 2000
We present a large grid of theoretical color-absolute magnitude and two-color diagrams in the RGU photometric system. The grid comprises wide ranges of stellar ages and physical parameters, and was computed using the YALE isochrones (Demarque et al. 1996) combined with the most recent version of the Basel Stellar Library (hereafter BaSeL) of color-calibrated theoretical model-atmosphere flux distributions (Lejeune et al. 2000). The present data provide a significant expansion of vital calibrations which are required for the full analysis of the new Basel three-color high-latitude field star survey of the Galaxy (Buser et al. 1998). In particular, they bring about, for the first time, the consistent determination of metallicities and metallicity-dependent absolute magnitudes for subgiant and giant stars from observed ultraviolet excesses, delta (U-G)G-R, and G-R colors, respectively.
The luminosity function of field galaxies and its evolution since z=1
Astronomy and Astrophysics, 2001
We present the B-band luminosity function and comoving space and luminosity densities for a sample of 2779 I-band selected field galaxies based on multi-color data from the CADIS survey. The sample is complete down to I815 = 22 without correction and with completeness correction extends to I815 = 23.0. By means of a new multi-color analysis the objects are classified according to their spectral energy distributions (SEDs) and their redshifts are determined with typical errors of δz ≤ 0.03. We have split our sample into four redshift bins between z = 0.1 and z = 1.04 and into three SED bins E-Sa,Sa-Sc and starbursting (emission line) galaxies. The evolution of the luminosity function is clearly differential with SED. The normalization φ * of luminosity function for the E-Sa galaxies decreases towards higher redshift, and we find evidence that the comoving galaxy space density decreases with redshift as well. In contrast, we find φ * and the comoving space density increasing with redshift for the Sa-Sc galaxies. For the starburst galaxies we find a steepening of the luminosity function at the faint end and their comoving space density increases with redshift.
Monthly Notices of the Royal Astronomical Society: Letters, 2009
Predicting the colors of Luminous Red Galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS) has been a long-standing problem. The g, r, i colors of LRGs are inconsistent with stellar population models over the redshift range 0.1 < z < 0.7. The g − r colors in the models are on average redder than the data (of the order 0.1 mag) while the r − i colors in the models are bluer (by 0.05 mag) towards low redshift. Beyond redshift 0.4, the predicted r − i color becomes instead too red, while the predicted g − r agrees with the data. We provide a solution to this problem, through a combination of new astrophysics and a fundamental change to the stellar population modeling. We find that the use of the empirical library of Pickles (1998), in place of theoretical libraries based on model atmosphere calculations, modifies the evolutionary population synthesis predicted colors exactly in the way suggested by the data, i.e., gives a redder r − i color, and a bluer g − r color, in the observed frame at z = 0.1. The reason is a lower flux in the empirical libraries, with respect to the theoretical ones, in the wavelength range 5500 − 6500Å. The discrepancy increases with decreasing effective temperature independently of gravity. This result has general implications for a variety of studies from globular clusters to high-redshift galaxies. The astrophysical part of our solution regards the composition of the stellar populations of these massive Luminous Red Galaxies. We find that on top of the previous effect one needs to consider a model in which ∼ 3% of the stellar mass is in old metal-poor stars. Other solutions such as substantial blue Horizontal Branch at high metallicity or young stellar populations can be ruled out by the data. Our new model provides a better fit to the g − r and r − i colors of LRGs and gives new insight into the formation histories of these most massive galaxies. Our model will also improve the k-and evolutionary corrections for LRGs which are critical for fully exploiting present and future galaxy surveys.
The Astrophysical Journal, 1996
In this paper, we discuss a method to conduct a quantitative study of the star formation history (SFH) of Local Group (LG) galaxies using Hubble Space Telescope (HST) data. This method has proven to be successful in the analysis of the SFH of the same kind of galaxies using ground-based observations. It is based on the comparison of observed CMDs with a set of model CMDs. The latter are computed assuming different evolutionary scenarios, and include a detailed simulation of observational effects.
A Panchromatic Study of Massive Stars in the Extremely Metal-Poor Local Group Dwarf Galaxy Leo A
arXiv (Cornell University), 2022
We characterize massive stars (M > 8 M e) in the nearby (D ∼ 0.8 Mpc) extremely metal-poor (Z ∼ 5% Z e) galaxy Leo A using Hubble Space Telescope ultraviolet (UV), optical, and near-infrared (NIR) imaging along with Keck/ Low-Resolution Imaging Spectrograph and MMT/Binospec optical spectroscopy for 18 main-sequence OB stars. We find that: (a) 12 of our 18 stars show emission lines, despite not being associated with an H II region, suggestive of stellar activity (e.g., mass loss, accretion, binary star interaction), which is consistent with previous predictions of enhanced activity at low metallicity; (b) six are Be stars, which are the first to be spectroscopically studied at such low metallicity-these Be stars have unusual panchromatic SEDs; (c) for stars well fit by the TLUSTY nonlocal thermodynamic equilibrium models, the photometric and spectroscopic values of T log eff () and g log() agree to within ∼0.01 dex and ∼0.18 dex, respectively, indicating that near-UV/optical/NIR imaging can be used to reliably characterize massive (M ∼ 8-30 M e) main-sequence star properties relative to optical spectroscopy; (d) the properties of the most-massive stars in H II regions are consistent with constraints from previous nebular emission line studies; and (e) 13 stars with M > 8M e are >40 pc from a known star cluster or H II region. Our sample comprises ∼50% of all known massive stars at Z 10% Z e with derived stellar parameters, high-quality optical spectra, and panchromatic photometry.
The Optical and Near-Infrared Properties of Galaxies. I. Luminosity and Stellar Mass Functions
The Astrophysical …, 2003
We use a large sample of galaxies from the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) to calculate galaxy luminosity and stellar mass functions in the local Universe. We estimate corrections for passband shifting and galaxy evolution, as well as present-day stellar mass-to-light (M/L) ratios, by fitting the optical-near-infrared galaxy data with simple models. Accounting for the 8% galaxy overdensity in the SDSS early data release region, the optical and near-infrared luminosity functions we construct for this sample agree with most recent literature optical and near-infrared determinations within the uncertainties. We argue that 2MASS is biased against low surface brightness galaxies, and use SDSS plus our knowledge of stellar populations to estimate the 'true' K-band luminosity function. This has a steeper faint end slope and a slightly higher overall luminosity density than the direct estimate. Furthermore, assuming a universally-applicable stellar initial mass function (IMF), we find good agreement between the stellar mass function we derive from the 2MASS/SDSS data and that derived by Cole et al. (2001; MNRAS, 326, 255). The faint end slope slope for the stellar mass function is steeper than −1.1, reflecting the low stellar M/L ratios characteristic of low-mass galaxies. We estimate an upper limit to the stellar mass density in the local Universe Ω * h = 2.0 ± 0.6 × 10 −3 by assuming an IMF as rich in low-mass stars as allowed by observations of galaxy dynamics in the local Universe. The stellar mass density may be lower than this value if a different IMF with fewer low-mass stars is assumed. Finally, we examine typedependence in the optical and near-infrared luminosity functions and the stellar mass function. In agreement with previous work, we find that the characteristic luminosity or mass of early-type galaxies is larger than for later types, and the faint end slope is steeper for later types than for earlier types. Accounting for typing uncertainties, we estimate that at least half, and perhaps as much as 3/4, of the stellar mass in the Universe is in early-type galaxies. As an aid to workers in the field, we present in an appendix the relationship between model stellar M/L ratios and colors in SDSS/2MASS passbands, an updated discussion of near-infrared stellar M/L ratio estimates, and the volume-corrected distribution of g and K-band stellar M/L ratios as a function of stellar mass.
The luminosity-metallicity relation in the local Universe from the 2dF Galaxy Redshift Survey
Monthly Notices of the Royal Astronomical Society, 2004
We investigate the Luminosity -Metallicity (L − Z) relation in the local universe (0 < z < 0.15) using spectra of 6 387 star-forming galaxies extracted from the 2dF Galaxy Redshift Survey. This sample is by far the largest to date used to perform such a study. We distinguish star-forming galaxies from AGNs using "standard" diagnostic diagrams to build a homogeneous sample of starburst galaxies for the L − Z study. We propose new diagnostic diagrams using "blue" emission lines ([O II] λ3727, [O III] λ5007, and Hβ) only to discriminate starbursts from AGNs in intermediate-redshift (z > 0.3) galaxies. Oxygen-to-hydrogen (O/H) abundance ratios are estimated using the "strong-line" method, which relates the strength of following bright emission lines [O II] λ3727, [O III] λ5007, and Hβ (parameters R 23 and O 32 ) to O/H. We used the [N II] λ6584/Hα emission-line ratio as "secondary" abundance indicator to break the degeneracy between O/H and R 23 . We confirm the existence of the luminositymetallicity relation over a large range of abundances (∼ 2 dex) and luminosities (∼ 9 magnitudes). We find a linear relation between the gas-phase oxygen abundance and both the "raw" and extinction-corrected absolute B-band magnitude with a rms of ∼ 0.27. A similar relation, with nearly the same scatter, is found in the R band. This relation is in good agreement with the one derived by using the KISS data. However, our L − Z relation is much steeper than previous determinations using samples of "normal" irregular and spiral galaxies. This difference seems to be primarily due to the choice of the galaxy sample used to investigate the L − Z relation rather than any systematic error affecting the O/H determination. We anticipate that this luminosity -metallicity relation will be used as the local "reference" for future studies of the evolution with cosmic time of fundamental galaxy scaling relations.
The Astrophysical Journal, 2006
We explore the effects of stellar population models on estimating star formation histories, ages and masses of high redshift galaxies. The focus is on the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) phase of stellar evolution, whose treatment is a source of major discrepancy among different evolutionary population synthesis. In particular, besides the models usually adopted in the literature, we use models (by Maraston 2005), in which the contribution of the TP-AGB phase is calibrated with local stellar populations and is the dominant source of bolometric and near-IR energy for stellar populations in the age range 0.2 to 2 Gyr. These models also have an underlying different treatment of convective overshooting and Red Giant Branch stars. For our experiment we use a sample of high-z (1.4 ∼ < z ∼ < 2.5) galaxies in the Hubble Ultra Deep Field held to be mostly in passive evolution, with low-resolution UV-spectroscopy and spectroscopic redshifts from GRAPES, and Spitzer IRAC and MIPS photometry from the Great Observatories Origins Deep Survey. We choose these galaxies because their mid-UV spectra exhibit features typical of A-or F-type stars, therefore TP-AGB stars ought to be expected in post-Main Sequence. We find that indeed the TP-AGB phase plays a key role in the interpretation of Spitzer data for high-z galaxies, when the rest-frame near-IR is sampled. When fitting without dust reddening, the models with the empirically-calibrated TP-AGB phase always reproduce better the observed spectral energy distributions (SEDs), in terms of a considerably smaller χ 2 . Allowing for dust reddening improves the fits with literature models in some cases. In both cases, the results from Maraston models imply younger ages by factors up to 6 and lower stellar masses (by ∼ 60% on average). The observed strengths of the Mg UV spectral feature compare better to the predicted ones in the case of the Maraston models, implying a better overall consistency of SED fitting. Finally, we find that photometric redshifts improve significantly using these models on the SEDs extending over the IRAC bands. These results are primarily the consequence of the treatment of the TP-AGB phase in the Maraston models, which produces models with redder rest-frame optical to near-IR colors. This work provides the first direct evidence of TP-AGB stars in the primeval Universe.
The Astrophysical Journal, 2014
A new set of very high signal-to-noise (S/N > 100/1), medium-resolution (R ∼ 3000) optical spectra have been obtained for 302 of the candidate "weak-metal" stars selected by Bidelman & MacConnell. We use these data to calibrate the recently developed generalization of the Sloan Extension for Galactic Exploration and Understanding and Exploration (SEGUE) Stellar Parameter Pipeline, and obtain estimates of the atmospheric parameters (T eff , log g, and [Fe/H]) for these non-Sloan Digital Sky Survey/SEGUE data; we also obtain estimates of [C/Fe]. The new abundance measurements are shown to be consistent with available high-resolution spectroscopic determinations, and represent a substantial improvement over the accuracies obtained from the previous photometric estimates reported in Paper I of this series. The apparent offset in the photometric abundances of the giants in this sample noted by several authors is confirmed by our new spectroscopy; no such effect is found for the dwarfs. The presence of a metal-weak thick-disk (MWTD) population is clearly supported by these new abundance data. Some 25% of the stars with metallicities −1.8 < [Fe/H] −0.8 exhibit orbital eccentricities e < 0.4, yet are clearly separated from members of the inner-halo population with similar metallicities by their location in a Lindblad energy versus angular momentum diagram. A comparison is made with recent results for a similar-size sample of Radial Velocity Experiment stars from Ruchti et al. We conclude, based on both of these samples, that the MWTD is real, and must be accounted for in discussions of the formation and evolution of the disk system of the Milky Way.