- ID:
- ivo://CDS.VizieR/J/AJ/154/87
- Title:
- Stellar population of the young star cluster NGC 6231
- Short Name:
- J/AJ/154/87
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- NGC 6231 is a young cluster (age ~2-7 Myr) dominating the Sco OB1 association (distance ~1.59 kpc) with ~100 O and B stars and a large pre-main-sequence stellar population. We combine a reanalysis of archival Chandra X-ray data with multiepoch near-infrared (NIR) photometry from the VISTA Variables in the Via Lactea (VVV) survey and published optical catalogs to obtain a catalog of 2148 probable cluster members. This catalog is 70% larger than previous censuses of probable cluster members in NGC 6231. It includes many low-mass stars detected in the NIR but not in the optical and some B stars without previously noted X-ray counterparts. In addition, we identify 295 NIR variables, about half of which are expected to be pre-main-sequence stars. With the more complete sample, we estimate a total population in the Chandra field of 5700-7500 cluster members down to 0.08 M_{sun}_ (assuming a universal initial mass function) with a completeness limit at 0.5 M_{sun}_. A decrease in stellar X-ray luminosities is noted relative to other younger clusters. However, within the cluster, there is little variation in the distribution of X-ray luminosities for ages less than 5 Myr. The X-ray spectral hardness for B stars may be useful for distinguishing between early-B stars with X-rays generated in stellar winds and B-star systems with X-rays from a pre-main-sequence companion (>35% of B stars). A small fraction of catalog members have unusually high X-ray median energies or reddened NIR colors, which might be explained by absorption from thick or edge-on disks or being background field stars.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/764/154
- Title:
- Stellar populations in the central 0.5pc. I.
- Short Name:
- J/ApJ/764/154
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present new high angular resolution near-infrared spectroscopic observations of the nuclear star cluster surrounding the Milky Way's central supermassive black hole. Using the integral-field spectrograph OSIRIS on Keck II behind the laser-guide-star adaptive optics system, this spectroscopic survey enables us to separate early-type (young, 4-6Myr) and late-type (old, >1Gyr) stars with a completeness of 50% down to K'=15.5mag, which corresponds to ~10M_{sun}_ for the early-type stars. This work increases the radial extent of reported OSIRIS/Keck measurements by more than a factor of three from 4" to 14" (0.16 to 0.56pc), along the projected disk of young stars. For our analysis, we implement a new method of completeness correction using a combination of star-planting simulations and Bayesian inference. We assign probabilities for the spectral type of every source detected in deep imaging down to K'=15.5mag using information from spectra, simulations, number counts, and the distribution of stars. The inferred radial surface-density profiles, {Sigma}(R){prop.to}R^-{Gamma}^, for the young stars and late-type giants are consistent with earlier results ({Gamma}_early_=0.93+/-0.09, {Gamma}_late_=0.16+/-0.07). The late-type surface-density profile is approximately flat out to the edge of the survey. While the late-type stellar luminosity function is consistent with the Galactic bulge, the completeness-corrected luminosity function of the early-type stars has significantly more young stars at faint magnitudes compared with previous surveys with similar depth. This luminosity function indicates that the corresponding mass function of the young stars is likely less top-heavy than that inferred from previous surveys.
- ID:
- ivo://CDS.VizieR/J/ApJ/795/108
- Title:
- Stellar populations in the Magellanic bridge
- Short Name:
- J/ApJ/795/108
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the discovery of a young stellar bridge that forms a continuous connection between the Magellanic Clouds. This finding is based on number density maps for stellar populations found in data gathered by OGLE-IV that fully cover over 270 deg^2^ of the sky in the Magellanic Bridge area. This is the most extensive optical survey of this region to date. We find that the young population is present mainly in the western half of the MBR, which, together with the newly discovered young population in the eastern Bridge, form a continuous stream of stars connecting both galaxies along {delta}~-73.5 deg. The young population distribution is clumped, with one of the major densities close to the SMC and the other fairly isolated and located approximately mid-way between the Clouds, which we call the OGLE island. These overdensities are well matched by H I surface density contours, although the newly found young population in the eastern Bridge is offset by ~2 deg north from the highest H I density contour. We observe a continuity of red clump stars between the Magellanic Clouds which represent an intermediate-age population. Red clump stars are present mainly in the southern and central parts of the Magellanic Bridge, below its gaseous part, and their presence is reflected by a strong deviation from the radial density profiles of the two galaxies. This may indicate either a tidal stream of stars, or that the stellar halos of the two galaxies overlap. On the other hand, we do not observe such an overlap within an intermediate-age population represented by the top of the red giant branch and the asymptotic giant branch stars. We also see only minor mixing of the old populations of the Clouds in the southern part of the Bridge, represented by the lowest part of the red giant branch.
- ID:
- ivo://CDS.VizieR/J/A+A/429/101
- Title:
- Stellar populations of CL 0048-2942
- Short Name:
- J/A+A/429/101
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a detailed study of the cluster CL 0048-2942, located at z~0.64, based on a photometric and spectroscopic catalogue of 54 galaxies in a 5x5arcmin^2^ region centred in that cluster. Of these, 23 galaxies were found to belong to the cluster. Based on this sample, the line-of-sight velocity dispersion of the cluster is approximately 680+/-140km/s.
- ID:
- ivo://CDS.VizieR/J/MNRAS/409/1455
- Title:
- Stellar populations of early-type galaxies
- Short Name:
- J/MNRAS/409/1455
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The influence of environment on the formation and evolution of early-type galaxies is, as yet, an unresolved issue. Constraints can be placed on models of early-type galaxy formation and evolution by examining their stellar populations as a function of environment. We present a catalogue of galaxies well suited to such an investigation. The magnitude-limited (b_J_<=19.45) sample was drawn from four clusters (Coma, A1139, A3558 and A930 at <z>=0.04) and their surrounds. The catalogue contains luminosities, redshifts, velocity dispersions and Lick line strengths for 416 galaxies, of which 245 are classified as early types. Luminosity-weighted ages, metallicities and {alpha}-element abundance ratios have been estimated for 219 of these early types. We also outline the steps necessary for measuring fully calibrated Lick indices and estimating the associated stellar population parameters using up-to-date methods and stellar population models. In a subsequent paper we perform a detailed study of the stellar populations of early-type galaxies in clusters and investigate the effects of environment.
- ID:
- ivo://CDS.VizieR/J/A+A/618/A172
- Title:
- Stellar populations of fossil galaxy groups
- Short Name:
- J/A+A/618/A172
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Fossil groups (FGs) are galaxy aggregates with an extended and luminous X-ray halo, which are dominated by a very massive early-type galaxy and lack of L* objects. FGs are indeed characterized by a large magnitude gap between their central and surrounding galaxies. This is explained by either speculating that FGs are failed groups which formed without bright satellite galaxies and did not suffer any major merger, or suggesting that FGs are very old systems which had enough time to exhaust their bright satellite galaxies through multiple major mergers. Since major mergers leave signatures in the stellar populations of the resulting galaxy, we study the stellar population parameters of the brightest central galaxies (BCGs) of FGs as a benchmark against which the formation and evolution scenarios of FGs can be compared. We present long-slit spectroscopic observations along the major, minor, and diagonal axis of NGC 6482 and NGC 7556, which are the BCGs of two nearby FGs. The measurements include spatially resolved stellar kinematics and radial profiles of line-strength indices, which we converted into stellar population parameters using single stellar-population models. NGC 6482 and NGC 7556 are very massive (M*~1*10^11.5^M_{sun}_) and large (D_25_~50kpc) galaxies. They host a centrally-concentrated stellar population, which is significantly younger and more metal rich than the rest of the galaxy. The age gradients of both galaxies are somewhat larger than those of the other FG BCGs studied so far, whereas their metallicity gradients are similarly negative and shallow. Moreover, they have negligible gradients of alpha-element abundance ratio. The measured metallicity gradients are less steep than those predicted for massive galaxies which formed monolithically and evolved without experiencing any major merger. We conclude that the observed FGs formed through major mergers rather than being failed groups missing bright satellite galaxies since the beginning.
- ID:
- ivo://CDS.VizieR/J/AJ/158/14
- Title:
- Stellar populations of globular clusters using APOGEE
- Short Name:
- J/AJ/158/14
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We investigate aluminum abundance variations in the stellar populations of globular clusters using both literature measurements of sodium and aluminum and APOGEE measurements of nitrogen and aluminum abundances. For the latter, we show that the Payne is the most suitable of the five available abundance pipelines for our purposes. Our combined sample of 42 globular clusters spans approximately 2 dex in [Fe/H] and 1.5 dex in logM_GC_/M_{sun}_. We find no fewer than five globular clusters with significant internal variations in nitrogen and/or sodium with little to no corresponding variation in aluminum, and that the minimum present-day cluster mass for aluminum enrichment in metal-rich systems is logM_GC_/M_{sun}_~4.50+2.17([Fe/H]+1.30). We demonstrate that the slopes of the [Al/Fe] versus [Na/Fe] and [Al/Fe] versus [N/Fe] relations for stars without field-like abundances are approximately log-linearly dependent on both the metallicity and the stellar mass of the globular clusters. In contrast, the relationship between [Na/Fe] and [N/Fe] shows no evidence of such dependencies. This suggests that there were (at least) two classes of non-supernova chemical polluters that were common in the early universe, and that their relative contributions within globular clusters somehow scaled with the metallicity and mass of globular clusters. The first of these classes is predominantly responsible for the CNO and NeNa abundance variations, and likewise the second for the MgAl abundance variations. Particularly striking examples of this dichotomy include 47 Tuc and M4. As an auxiliary finding, we argue that abundance variations among Terzan 5 stars are consistent with it being a normal globular cluster.
- ID:
- ivo://CDS.VizieR/J/A+A/649/A93
- Title:
- Stellar populations of NGC 3311
- Short Name:
- J/A+A/649/A93
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Massive early-type galaxies are believed to be the end result of an extended mass accretion history. The stars formed in situ, very early during the initial phase of the assembly, might have originated from an extremely intense and rapid burst of star formation. These stars may still be found within the cores of such galaxies at z=0, depending on their accretion and merger histories. We wish to investigate the presence of a surviving high-z compact progenitor component in the brightest galaxy of the Hydra~I cluster, NGC~3311, by mapping its 2D kinematics and stellar population out to 2 effective radii. Our goal is to understand the formation of its several structural components and trace their mass assembly back in time. We combine MUSE observations, a customized and extended version of the state-of-the-art EMILES single stellar population models, and a newly developed parametric fully Bayesian framework to model the observed spectra using full-spectrum fitting. We present 2D maps, as well as radial profiles, of the stellar velocity dispersion, age, total metallicity, {alpha}-element, sodium abundance ([Na/Fe]), and the initial mass function slope. All properties have significant gradients, confirming the existence of multiple structural components, also including a "blue spot" characterized by younger and metal-richer stars. We find that the component dominating the light budget of NGC 3311 within R<2.0kpc, is the surviving z=0 analogue of a high-z compact core, according to the definition of Pulsoni et al. (2021). This concentrated structure has a relatively small velocity dispersion (sigma~180km/s), is very old (ages~11Gyr), metal-rich ([Z/H]~0.2 and [Na/Fe]~0.4), and has a bottom-heavy IMF (with slope Gamma_b_~2.4). In the outer region, instead, the line-of-sight velocity distribution becomes increasingly broader, and the stars have younger age. They are also metal and sodium poorer but {alpha}-element richer. The low-mass end of the IMF slope becomes Chabrier-like with increasing galactocentric distances. The existence of multiple structural components in NGC 3311 from photometry, kinematics, and stellar populations confirms the predictions from the two-phase formation scenario for NGC 3311, according to which a first very short, high-z star formation episode formed a compact stellar structure in its core, which then grew in size by the extended mass assembly of relatively massive satellites. Interestingly, the outer stellar population has an overabundant [alpha/Fe], most likely because NGC 3311, located at the center of the galaxy cluster, accreted stars from rapidly quenched satellites.
- ID:
- ivo://CDS.VizieR/J/A+A/589/A139
- Title:
- Stellar populations of NGC 3311
- Short Name:
- J/A+A/589/A139
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The history of the mass assembly of brightest cluster galaxies may be studied by mapping the stellar populations at large radial distances from the galaxy centre, where the dynamical times are long and preserve the chemodynamical signatures of the accretion events. To provide extended and robust measurements of the stellar population parameters in NGC 3311, the cD galaxy at the centre of the Hydra I cluster, and out to three effective radius. We wish to characterise the processes that drove the build up of the stellar light at all these radii. We obtained the spectra from several regions in NGC 3311 covering an area of ~3arcmin^2^ in the wavelength range 4800<lambda({AA})<5800, using the FORS2 spectrograph at the Very Large Telescope in the MXU mode. We measured the equivalent width of seven absorption-features defined in the Lick/IDS system, which were modelled by single stellar populations, to provide luminosity-weighted ages, metallicities, and alpha element abundances. The trends in the Lick indices and the distribution of the stellar population parameters indicate that the stars of NGC3311 may be divided in two radial regimes, one within and the another beyond one effective radius, R_e_=8.4kpc, similar to the distinction between the inner galaxy and the external halo derived from the NGC3311 velocity dispersion profile. The inner galaxy (R<R_e_) is old (age ~14Gyr), has negative metallicity gradients and positive alpha element gradients. The external halo is also very old, but has a negative age gradient. The metal and element abundances of the external halo both have a large scatter, indicating that stars from a variety of satellites with different masses have been accreted. The region in the extended halo associated with the off-centred envelope at 0<P.A.({deg})<90 (Arnaboldi et al., 2012A&A...545A..37A) has higher metallicity with respect to the symmetric external halo. The different stellar populations in the inner galaxy and extended halo reflect the dominance of in situ stars in the former and the accreted origin for the large majority of the stars in the latter. The low value of the velocity dispersion in the inner galaxy indicates that its stars are bound to the galaxy's gravitational potential, and the abundances and gradients suggest that the inner galaxy is formed in an outside-in scenario of merging gas-rich lumps, reminiscent of the first phase of galaxy formation. The external halo has a higher velocity dispersion, it is dynamically hotter than the galaxy and its stars are gravitationally driven by the cluster's gravitational potential. The stars in the external halo were removed from their parent galaxies, either disks with truncated star formation, or the outer regions of early-type galaxies. Late mass accretion at large radii is now coming from the tidal stripping of stars from dwarfs and S0 galaxies. These results provide supporting evidence for the recent theoretical models of formation of massive ellipticals as a two-phase process.
- ID:
- ivo://CDS.VizieR/J/A+A/631/A156
- Title:
- Stellar populations of quiescent galaxies
- Short Name:
- J/A+A/631/A156
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Our aim is to determine the distribution of stellar population parameters (extinction, age, metallicity, and star formation rates) of quiescent galaxies within the rest-frame stellar mass-colour diagrams and UVJ colour-colour diagrams corrected for extinction up to z~1. These novel diagrams reduce the contamination in samples of quiescent galaxies owing to dust-reddened galaxies, and they provide useful constraints on stellar population parameters only using rest-frame colours and stellar mass. We set constraints on the stellar population parameters of quiescent galaxies combining the ALHAMBRA multi-filter photo-spectra with our fitting code for spectral energy distribution, MUlti-Filter FITting (MUFFIT), making use of composite stellar population models based on two independent sets of simple stellar population (SSP) models. The extinction obtained by MUFFIT allowed us to remove dusty star-forming (DSF) galaxies from the sample of red UVJ galaxies. The distributions of stellar population parameters across these rest-frame diagrams are revealed after the dust correction and are fitted by LOESS, a bi-dimensional and locally weighted regression method, to reduce uncertainty effects. Quiescent galaxy samples defined via classical UVJ diagrams are typically contaminated by a 20% fraction of DSF galaxies. A significant part of the galaxies in the green valley are actually obscured star-forming galaxies (30-65%). Consequently, the transition of galaxies from the blue cloud to the red sequence, and hence the related mechanisms for quenching, seems to be much more efficient and faster than previously reported. The rest-frame stellar mass-colour and UVJ colour-colour diagrams are useful for constraining the age, metallicity, extinction, and star formation rate of quiescent galaxies by only their redshift, rest-frame colours, and/or stellar mass. Dust correction plays an important role in understanding how quiescent galaxies are distributed in these diagrams and is key to performing a pure selection of quiescent galaxies via intrinsic colours.