arXiv subjects

Mario Mateo

Publications and source records attributed to Mario Mateo.

At least 73 records · Page 4Linked to original sources

arXiv2015

Evidence for a chemical enrichment coupling of globular clusters and field stars in the Fornax dSph

The globular cluster H4, located in the center of the Fornax dwarf spheroidal galaxy, is crucial for understanding the formation and chemical evolution of star clusters in low-mass galactic environments. H4 is peculiar because the cluster is significantly more metal-rich than the galaxy's other clusters, is located near the galaxy center, and may also be the youngest cluster in the galaxy. In this study, we present detailed chemical abundances derived from high-resolution (R~28000) spectroscopy of an isolated H4 member star for comparison with a sample of 22 nearby Fornax field stars. We find the H4 member to be depleted in the alpha-elements Si, Ca, and Ti with [Si/Fe]=-0.35+-0.34, [Ca/Fe]=+0.05+-0.08, and [Ti/Fe]=-0.27+-0.23, resulting in an average [alpha/Fe]=-0.19+-0.14. If this result is representative of the average cluster properties, H4 is the only known system with a low [alpha/Fe] ratio and a moderately low metallicity embedded in an intact birth environment. For the field stars we find a clear sequence, seen as an early depletion in [alpha/Fe] at low metallicities, in good agreement with previous measurements. H4 falls on top of the observed field star [alpha/Fe] sequence and clearly disagrees with the properties of Milky Way halo stars. We therefore conclude that within a galaxy, the chemical enrichment of globular clusters may be closely linked to the enrichment pattern of the field star population. The low [alpha/Fe] ratios of H4 and similar metallicity field stars in Fornax give evidence that slow chemical enrichment environments, such as dwarf galaxies, may be the original hosts of alpha-depleted clusters in the halos of the Milky Way and M31.

astro-ph.GA↗

arXiv2015

A Spectroscopic Analysis of the Galactic Globular Cluster NGC 6273 (M19)

A combined effort utilizing spectroscopy and photometry has revealed the existence of a new globular cluster class. These "anomalous" clusters, which we refer to as "iron-complex" clusters, are differentiated from normal clusters by exhibiting large (>0.10 dex) intrinsic metallicity dispersions, complex sub-giant branches, and correlated [Fe/H] and s-process enhancements. In order to further investigate this phenomenon, we have measured radial velocities and chemical abundances for red giant branch stars in the massive, but scarcely studied, globular cluster NGC 6273. The velocities and abundances were determined using high resolution (R~27,000) spectra obtained with the Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph on the Magellan-Clay 6.5m telescope at Las Campanas Observatory. We find that NGC 6273 has an average heliocentric radial velocity of +144.49 km s^-1 (sigma=9.64 km s^-1) and an extended metallicity distribution ([Fe/H]=-1.80 to -1.30) composed of at least two distinct stellar populations. Although the two dominant populations have similar [Na/Fe], [Al/Fe], and [alpha/Fe] abundance patterns, the more metal-rich stars exhibit significant [La/Fe] enhancements. The [La/Eu] data indicate that the increase in [La/Fe] is due to almost pure s-process enrichment. A third more metal-rich population with low [X/Fe] ratios may also be present. Therefore, NGC 6273 joins clusters such as omega centauri, M 2, M 22, and NGC 5286 as a new class of iron-complex clusters exhibiting complicated star formation histories.

astro-ph.SR↗

arXiv2015

Magellan/M2FS Spectroscopy of the Reticulum 2 Dwarf Spheroidal Galaxy

We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 182 stellar targets along the line of sight to the newly-discovered `ultrafaint' object Reticulum 2 (Ret 2). For 38 of these targets, the spectra are sufficient to provide simultaneous estimates of line-of-sight velocity ($v_{\rm los}$, median random error $δ_{v_{\rm los}}=1.3$ km s$^{-1}$), effective temperature ($T_{\rm eff}$, $δ_{T_{\rm eff}}=464$ K), surface gravity ($\log g$, $δ_{\rm logg}=0.54$ dex) and iron abundance ([Fe/H], $δ_{\mathrm{[Fe/H]}}=0.45$ dex). We use these results to confirm 18 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of $σ_{v_{\rm los}}=3.6_{-0.6}^{+0.9}$ km s$^{-1}$ about a mean of $\langle v_{\rm los}\rangle =64.8_{-1.0}^{+1.1}$ km s$^{-1}$ in the solar rest frame ($\sim -90.9$ km s$^{-1}$ in the Galactic rest frame), and a metallicity dispersion of $σ_{\rm [Fe/H]}=0.50_{-0.13}^{+0.17}$ dex about a mean of $\langle \mathrm{[Fe/H]} \rangle =-2.67_{-0.34}^{+0.34}$. These estimates marginalize over possible velocity and metallicity gradients, which are consistent with zero. Our results place Ret 2 on chemodynamical scaling relations followed by the Milky Way's dwarf-galactic satellites. Under assumptions of dynamic equilibrium and negligible contamination from binary stars---both of which must be checked with deeper imaging and repeat spectroscopic observations---the estimated velocity dispersion suggests a dynamical mass of $M(R_{\rm h})\approx 5R_{\rm h}σ_{v_{\rm los}}^2/(2G)=2.4_{-0.8}^{+1.3}\times 10^5$ $M_{\odot}$ enclosed within projected halflight radius $R_{\rm h}\sim 32$ pc, with mass-to-light ratio $\approx 2M(R_{\rm h})/L_{\rm V}=462_{-157}^{+264}$ in solar units.

astro-ph.GA↗

arXiv2015

Bayesian analysis of resolved stellar spectra: application to MMT/Hectochelle Observations of the Draco dwarf spheroidal

We introduce a Bayesian method for fitting faint, resolved stellar spectra in order to obtain simultaneous estimates of redshift and stellar-atmospheric parameters. We apply the method to thousands of spectra---covering 5160-5280 Angs. at resolution R~20,000---that we have acquired with the MMT/Hectochelle fibre spectrograph for red-giant and horizontal branch candidates along the line of sight to the Milky Way's dwarf spheroidal satellite in Draco. The observed stars subtend an area of ~4 deg^2, extending ~3 times beyond Draco's nominal `tidal' radius. For each spectrum we tabulate the first four moments---central value, variance, skewness and kurtosis---of posterior probability distribution functions representing estimates of the following physical parameters: line-of-sight velocity v_los, effective temperature (T_eff), surface gravity (logg) and metallicity ([Fe/H]). After rejecting low-quality measurements, we retain a new sample consisting of 2813 independent observations of 1565 unique stars, including 1879 observations for 631 stars with (as many as 13) repeat observations. Parameter estimates have median random errors of sigma_{v_los}=0.88 km/s, sigma_{T_eff}=162 K, sigma_logg=0.37 dex and sigma_[Fe/H]=0.20 dex. Our estimates of physical parameters distinguish ~470 likely Draco members from interlopers in the Galactic foreground.

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arXiv2015

Kinematic and Spatial Substructure in NGC 2264

We present an expanded kinematic study of the young cluster NGC 2264 based upon optical radial velocities measured using multi-fiber echelle spectroscopy at the 6.5 meter MMT and Magellan telescopes. We report radial velocities for 695 stars, of which approximately 407 stars are confirmed or very likely members. Our results more than double the number of members with radial velocities from F{\H u}r{é}sz et al., resulting in a much better defined kinematic relationship between the stellar population and the associated molecular gas. In particular, we find that there is a significant subset of stars that are systematically blueshifted with respect to the molecular ($^{13}$CO) gas. The detection of Lithium absorption and/or infrared excesses in this blue-shifted population suggests that at least some of these stars are cluster members; we suggest some speculative scenarios to explain their kinematics. Our results also more clearly define the redshifted population of stars in the northern end of the cluster; we suggest that the stellar and gas kinematics of this region are the result of a bubble driven by the wind from O7 star S Mon. Our results emphasize the complexity of the spatial and kinematic structure of NGC 2264, important for eventually building up a comprehensive picture of cluster formation.

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arXiv2014

AGB Sodium Abundances in the Globular Cluster 47 Tucanae (NGC 104)

A recent analysis comparing the [Na/Fe] distributions of red giant branch (RGB) and asymptotic giant branch (AGB) stars in the Galactic globular cluster NGC 6752 found that the ratio of Na-poor to Na-rich stars changes from 30:70 on the RGB to 100:0 on the AGB. The surprising paucity of Na-rich stars on the AGB in NGC 6752 warrants additional investigations to determine if the failure of a significant fraction of stars to ascend the AGB is an attribute common to all globular clusters. Therefore, we present radial velocities, [Fe/H], and [Na/Fe] abundances for 35 AGB stars in the Galactic globular cluster 47 Tucanae (47 Tuc; NGC 104), and compare the AGB [Na/Fe] distribution with a similar RGB sample published previously. The abundances and velocities were derived from high resolution spectra obtained with the Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph on the Magellan-Clay 6.5m telescope. We find the average heliocentric radial velocity and [Fe/H] values to be =-18.56 km s^-1 (sigma=10.21 km s^-1) and <[Fe/H]>=-0.68 (sigma=0.08), respectively, in agreement with previous literature estimates. The average [Na/Fe] abundance is 0.12 dex lower in the 47 Tuc AGB sample compared to the RGB sample, and the ratio of Na-poor to Na-rich stars is 63:37 on the AGB and 45:55 on the RGB. However, in contrast to NGC 6752, the two 47 Tuc populations have nearly identical [Na/Fe] dispersion and interquartile range values. The data presented here suggest that only a small fraction <20% of Na-rich stars in 47 Tuc may fail to ascend the AGB. Regardless of the cause for the lower average [Na/Fe] abundance in AGB stars, we find that Na-poor stars and at least some Na-rich stars in 47 Tuc evolve through the early AGB phase. [abridged]

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arXiv2013

A Comprehensive, Wide-Field Study of Pulsating Stars in the Carina Dwarf Spheroidal Galaxy

We report the detection of 388 pulsating variable stars (and some additional miscellaneous variables) in the Carina dSph galaxy over an area covering the full visible extent of the galaxy and extending a few times beyond its photometric (King) tidal radius along the direction of its major axis. Included in this total are 340 newly discovered dwarf Cepheids which are mostly located ~2.5 magnitudes below the horizontal branch and have very short periods (<0.1 days) typical of their class and consistent with their location on the upper part of the extended main sequence of the younger populations of the galaxy. Several extra-tidal dwarf cepheids were found in our survey up to a distance of ~1 degree from the center of Carina. Our sample also includes RR Lyrae stars and anomalous Cepheids some of which were found outside the galaxy's tidal radius as well. This supports past works that suggests Carina is undergoing tidal disruption. We use the period-luminosity relationship for dwarf Cepheids to estimate a distance modulus of 20.17 +/- 0.10 mags, in very good agreement with the estimate from RR Lyrae stars. We find some important differences in the properties of the dwarf Cepheids of Carina and those in Fornax and the LMC, the only extragalactic samples of dwarf Cepheids currently known. These differences may reflect a metallicity spread, depth along the line of sight and/or, different evolutionary paths of the dwarf Cepheid stars.

astro-ph.SR↗

arXiv2012

The extensive age gradient of the Carina dwarf galaxy

The evolution of small systems such as dwarf spheroidal galaxies (dSph) is likely to have been a balance between external environmental effects and internal processes within their own relatively shallow potential wells. Assessing how strong such environmental interactions may have been is therefore an important element in understanding the baryonic evolution of dSphs and their derived dark matter distribution. Here we present results from a wide-area CTIO/MOSAIC II photometric survey of the Carina dSph, reaching down to about two magnitudes below the oldest main sequence turn-off (MSTO). This data-set enables us to trace the structure of Carina in detail out to very large distances from its center, and as a function of stellar age. We observe the presence of an extended structure made up primarily of ancient MSTO stars, at distances between 25arcmin-60arcmin from Carina's center, confirming results in the literature that Carina extends well beyond its nominal tidal radius. The large number statistics of our survey reveals features such as isophote twists and tails that had gone undetected in other previous shallower surveys. This is the first time that such unambiguous signs of tidal disruption have been found in a Milky Way "classical" dwarf other than Sagittarius. We also demonstrate the presence of a negative age gradient in Carina directly from its MSTOs, and trace it out to very large distances from the galaxy center. The signs of interaction with the Milky Way make it unclear whether the age gradient was already in place before Carina underwent tidal disruption.

astro-ph.CO↗

arXiv2011

Uncloaking globular clusters in the inner Galaxy

Extensive photometric studies of the globular clusters located towards the center of the Milky Way have been historically neglected. The presence of patchy differential reddening in front of these clusters has proven to be a significant obstacle to their detailed study. We present here a well-defined and reasonably homogeneous photometric database for 25 of the brightest Galactic globular clusters located in the direction of the inner Galaxy. These data were obtained in the B, V, and I bands using the Magellan 6.5m telescope and the Hubble Space Telescope. A new technique is extensively used in this paper to map the differential reddening in the individual cluster fields, and to produce cleaner, dereddened color-magnitude diagrams for all the clusters in the database. Subsequent papers will detail the astrophysical analysis of the cluster populations, and the properties of the obscuring material along the clusters' lines of sight.

astro-ph.GA↗

arXiv2011

Massive black holes in stellar systems: 'quiescent' accretion and luminosity

Only a small fraction of local galaxies harbor an accreting black hole, classified as an active galactic nucleus (AGN). However, many stellar systems are plausibly expected to host black holes, from globular clusters to nuclear star clusters, to massive galaxies. The mere presence of stars in the vicinity of a black hole provides a source of fuel via mass loss of evolved stars. In this paper we assess the expected luminosities of black holes embedded in stellar systems of different sizes and properties, spanning a large range of masses. We model the distribution of stars and derive the amount of gas available to a central black hole through a geometrical model. We estimate the luminosity of the black holes under simple, but physically grounded, assumptions on the accretion flow. Finally we discuss the detectability of 'quiescent' black holes in the local Universe.

astro-ph.GA↗

arXiv2011

Mapping differential reddening in the inner Galactic globular cluster system

A serious limitation in the study of many globular clusters -- especially those located near the Galactic Center -- has been the existence of large and differential extinction by foreground dust. In a series of papers we intend to map the differential extinction and remove its effects, using a new dereddening technique, in a sample of clusters in the direction of the inner Galaxy, observed using the Magellan 6.5m telescope and the Hubble Space Telescope. These observations and their analysis will let us produce high quality color-magnitude diagrams of these poorly studied clusters that will allow us to determine these clusters' relative ages, distances and chemistry and to address important questions about the formation and the evolution of the inner Galaxy. We also intend to use the maps of the differential extinction to sample and characterize the interstellar medium along the numerous low latitude lines of sight where the clusters in our sample lie. In this first paper we describe in detail our dereddening method along with the powerful statistics tools that allow us to apply it, and we show the kind of results that we can expect, applying the method to M62, one of the clusters in our sample. The width of the main sequence and lower red giant branch narrows by a factor of 2 after applying our dereddening technique, which will significantly help to constrain the age, distance, and metallicity of the cluster.

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arXiv2010

Comparing the Dark Matter Halos of Spiral, Low Surface Brightness and Dwarf Spheroidal Galaxies

We consider dark masses measured from kinematic tracers at discrete radii in galaxies for which baryonic contributions to overall potentials are either subtracted or negligible. Recent work indicates that rotation curves due to dark matter (DM) halos at intermediate radii in spiral galaxies are remarkably similar, with a mean rotation curve given by $\log_{10}[V_{c,\mathrm{DM}}/(\mathrm{km s^{-1}})]=1.47_{-0.19}^{+0.15}+0.5\log_{10}[r/\mathrm{kpc}]$. Independent studies show that while estimates of the dark mass of a given dwarf spheroidal (dSph) galaxy are robust only near the half-light radius, data from the Milky Way's (MW's) dSph satellites are consistent with a narrow range of mass profiles. Here we combine published constraints on the dark halo masses of spirals and dSphs and include available measurements of low surface brightness galaxies for additional comparison. We find that most measured MW dSphs lie on the extrapolation of the mean rotation curve due to DM in spirals. The union of MW-dSph and spiral data appears to follow a mass-radius relation of the form $M_{\mathrm{DM}}(r)/M_{\odot}=200_{-120}^{+200}(r/\mathrm{pc})^2$, or equivalently a constant acceleration $g_{\mathrm{DM}}=3_{-2}^{+3}\times 10^{-9}\mathrm{cm s^{-2}}$, spanning $0.02\la r \la 75$ kpc. Evaluation at specific radii immediately generates two results from the recent literature: a common mass for MW dSphs at fixed radius and a constant DM central surface density for galaxies ranging from MW dSphs to spirals. However, recent kinematic measurements indicate that M31's dSph satellites are systematically less massive than MW dSphs of similar size. Such deviations from what is otherwise a surprisingly uniform halo relation presumably hold clues to individual formation and evolutionary histories.

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arXiv2009

A Universal Mass Profile for Dwarf Spheroidal Galaxies?

(abridged) We apply the Jeans equation to estimate masses for eight of the brightest dSph galaxies. For Fornax we obtain a model-independent constraint on the maximum-circular velocity, Vmax=18_{-3}^{+5} km/s. Although we obtain only lower-limits of Vmax > 10 km/s for the remaining dSphs, we find that in all cases the enclosed mass at the projected half-light radius is well constrained and robust to a wide range of halo models and velocity anisotropies. We derive a simple analytic formula that estimates M(rhalf) accurately with respect to results from the full Jeans analysis. Applying this formula to the entire population of Local Group dSphs, we demonstrate a correlation such that M(rhalf)\propto rhalf^{1.4\pm 0.4}, or in terms of the mean density interior to the half-light radius, \propto rhalf^{-1.6\pm 0.4}. This relation is driven by the fact that the dSph data exhibit a correlation between global velocity dispersion and half-light radius. We argue that tidal forces are unlikely to have introduced this relation, but tides may have increased the scatter and/or altered the slope. While the data are well described by mass profiles ranging over a factor of < 2 in normalization (Vmax ~ 10-20 km/s), we consider the hypothesis that all dSphs are embedded within a "universal" dark matter halo. We show that in addition to the power law M\propto r^{1.4}, viable candidates include a cuspy "NFW" halo with Vmax ~ 15 km/s and scale radius r_0 ~ 800 pc, as well as a cored halo with Vmax ~ 13 km/s and r_0 ~ 150 pc. Finally, assuming that their measured velocity dispersions accurately reflect their masses, the smallest dSphs now allow us to resolve dSph densities at radii as small as a few tens of pc. At these small scales we find mean densities as large as <ρ> ~ 5 Msun/pc^3 (200 GeV/cm^3).

astro-ph.CO↗

arXiv2009

Mapping the Galactic Halo VIII: Quantifying substructure

We have measured the amount of kinematic substructure in the Galactic halo using the final data set from the Spaghetti project, a pencil-beam high latitude sky survey. Our sample contains 101 photometrically selected and spectroscopically confirmed giants with accurate distance, radial velocity and metallicity information. We have developed a new clustering estimator: the "4distance" measure, which when applied to our data set leads to the identification of 1 group and 7 pairs of clumped stars. The group, with 6 members, can confidently be matched to tidal debris of the Sagittarius dwarf galaxy. Two pairs match the properties of known Virgo structures. Using models of the disruption of Sagittarius in Galactic potentials with different degrees of dark halo flattening, we show that this favors a spherical or prolate halo shape, as demonstrated by Newberg et al. (2007) using SDSS data. One additional pair can be linked to older Sagittarius debris. We find that 20% of the stars in the Spaghetti data set are in substructures. From comparison with random data sets we derive a very conservative lower limit of 10% to the amount of substructure in the halo. However, comparison to numerical simulations shows that our results are also consistent with a halo entirely built up from disrupted satellites, provided the dominating features are relatively broad due to early merging or relatively heavy progenitor satellites.

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arXiv2009

The ACS Nearby Galaxy Survey Treasury

The ACS Nearby Galaxy Survey Treasury (ANGST) is a systematic survey to establish a legacy of uniform multi-color photometry of resolved stars for a volume-limited sample of nearby galaxies (D<4 Mpc). The survey volume encompasses 69 galaxies in diverse environments, including close pairs, small & large groups, filaments, and truly isolated regions. The galaxies include a nearly complete range of morphological types spanning a factor of ~10^4 in luminosity and star formation rate. The survey data consists of images taken with ACS on HST, supplemented with archival data and new WFPC2 imaging taken after the failure of ACS. Survey images include wide field tilings covering the full radial extent of each galaxy, and single deep pointings in uncrowded regions of the most massive galaxies in the volume. The new wide field imaging in ANGST reaches median 50% completenesses of m_F475W=28.0 mag, m_F606W=27.3 mag, and m_F814W=27.3 mag, several magnitudes below the tip of the red giant branch (TRGB). The deep fields reach magnitudes sufficient to fully resolve the structure in the red clump. The resulting photometric catalogs are publicly accessible and contain over 34 million photometric measurements of >14 million stars. In this paper we present the details of the sample selection, imaging, data reduction, and the resulting photometric catalogs, along with an analysis of the photometric uncertainties (systematic and random), for both the ACS and WFPC2 imaging. We also present uniformly derived relative distances measured from the apparent magnitude of the TRGB.

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arXiv2009

Streaming motion in Leo I

Whether a dwarf spheroidal galaxy is in equilibrium or being tidally disrupted by the Milky Way is an important question for the study of its dark matter content and distribution. This question is investigated using 328 recent observations from the dwarf spheroidal Leo I. For Leo I, tidal disruption is detected, at least for stars sufficiently far from the center, but the effect appears to be quite modest. Statistical tools include isotonic and split point estimators, asymptotic theory, and resampling methods.

stat.AP↗

arXiv2009

Leo V: Spectroscopy of a Distant and Disturbed Satellite

We present a spectroscopic study of Leo V, a recently discovered satellite of the Milky Way (MW). From stellar spectra obtained with the MMT/Hectochelle spectrograph we identify seven likely members of Leo V. Five cluster near the Leo V center (R < 3 arcmin) and have velocity dispersion 2.4_{-1.4}^{+2.4} km/s. The other two likely members lie near each other but far from the center (R~13 arcmin ~ 700 pc) and inflate the global velocity dispersion to 3.7_{-1.4}^{+2.3} km/s. Assuming the five central members are bound, we obtain a dynamical mass of M=3.3_{-2.5}^{+9.1} x 10^5M_{sun} (M/L_V=75_{-58}^{+230}[M/L_V]_{sun}). From the stacked spectrum of the five central members we estimate a mean metallicity of [Fe/H]=-2.0\pm 0.2 dex. Thus with respect to dwarf spheroidals of similar luminosity, Leo V is slightly less massive and slightly more metal-rich. Since we resolve the central velocity dispersion only marginally, we do not rule out the possibility that Leo V is a diffuse star cluster devoid of dark matter. The wide separation of its two outer members implies Leo V is losing mass; however, its large distance (D ~ 180 kpc) is difficult to reconcile with MW tidal stripping unless the orbit is very radial.

astro-ph.CO↗

arXiv2008

Clean Kinematic Samples in Dwarf Spheroidals: An Algorithm for Evaluating Membership and Estimating Distribution Parameters When Contamination is Present

(abridged) We develop an algorithm for estimating parameters of a distribution sampled with contamination, employing a statistical technique known as ``expectation maximization'' (EM). Given models for both member and contaminant populations, the EM algorithm iteratively evaluates the membership probability of each discrete data point, then uses those probabilities to update parameter estimates for member and contaminant distributions. The EM approach has wide applicability to the analysis of astronomical data. Here we tailor an EM algorithm to operate on spectroscopic samples obtained with the Michigan-MIKE Fiber System (MMFS) as part of our Magellan survey of stellar radial velocities in nearby dwarf spheroidal (dSph) galaxies. These samples are presented in a companion paper and contain discrete measurements of line-of-sight velocity, projected position, and Mg index for ~1000 - 2500 stars per dSph, including some fraction of contamination by foreground Milky Way stars. The EM algorithm quantifies both dSph and contaminant distributions, returning maximum-likelihood estimates of the means and variances, as well as the probability that each star is a dSph member. Applied to our MMFS data, the EM algorithm identifies more than 5000 probable dSph members. We test the performance of the EM algorithm on simulated data sets that represent a range of sample size, level of contamination, and amount of overlap between dSph and contaminant velocity distributions. The simulations establish that for samples ranging from large (N ~3000) to small (N~30), the EM algorithm distinguishes members from contaminants and returns accurate parameter estimates much more reliably than conventional methods of contaminant removal (e.g., sigma clipping).

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