arXiv subjects

Robert Zinn

Publications and source records attributed to Robert Zinn.

At least 19 recordsLinked to original sources

arXiv2016

Disentangling the Virgo Overdensity with RR Lyrae stars

We use a combination of spatial distribution and radial velocity to search for halo sub-structures in a sample of 412 RR Lyrae stars (RRLS) that covers a $\sim 525$ square degrees region of the Virgo Overdensity (VOD) and spans distances from the Sun from 4 to 75 kpc. With a friends-of-friends algorithm we identified six high significance groups of RRLS in phase space, which we associate mainly with the VOD and with the Sagittarius stream. Four other groups were also flagged as less significant overdensities. Three high significance and 3 lower significance groups have distances between $\sim 10$ and 20 kpc, which places them with the distance range attributed by others to the VOD. The largest of these is the Virgo Stellar Stream (VSS) at 19 kpc, which has 18 RRLS, a factor of 2 increase over the number known previously. While these VOD groups are distinct according to our selection cirteria, their overlap in position and distance, and, in a few cases, similarity in radial velocity are suggestive that they may not all stem separate accretion events. Even so, the VOD appears to be caused by more than one overdensity. The Sgr stream is a very obvious feature in the background of the VOD at a mean distance of 44 kpc. Two additional high significant groups were detected at distances $>40$ kpc. Their radial velocities and locations differ from the expected path of the Sgr debris in this part of the sky, and they are likely to be remnants of other accretion events.

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arXiv2015

The QUEST-La Silla AGN Variability Survey

We present the characterization and initial results from the QUEST-La Silla AGN variability survey. This is an effort to obtain well sampled optical light curves in extragalactic fields with unique multi-wavelength observations. We present photometry obtained from 2010 to 2012 in the XMM-COSMOS field, which was observed over 150 nights using the QUEST camera on the ESO-Schmidt telescope. The survey uses a broadband filter, the $Q$-band, similar to the union of the $g$ and the $r$ filters, achieving an intrinsic photometric dispersion of $0.05$ mag, and a systematic error of $0.05$ mag in the zero-point. Since some detectors of the camera show significant non-linearity, we use a linear correlation to fit the zero-points as a function of the instrumental magnitudes, thus obtaining a good correction to the non-linear behavior of these detectors. We obtain good photometry to an equivalent limiting magnitude of $r\sim 20.5$. Studying the optical variability of X-ray detected sources in the XMM-COSMOS field, we find that the survey is $\sim75-80$% complete to magnitudes $r\sim20$, and $\sim67$% complete to a magnitude $r\sim21$. The determination and parameterization of the structure function (${SF}_{norm}(τ) = A τ^γ$) of the variable sources shows that most BL AGN are characterized by $A > 0.1$ and $γ> 0.025$. It is further shown that variable NL AGN and GAL sources occupying the same parameter space in $A$ and $γ$ are very likely to correspond to obscured or low luminosity AGN. Our samples are, however, small, and we expect to revisit these results using larger samples with longer light curves obtained as part of our ongoing survey.

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arXiv2015

A Search for Intergalactic Globular Clusters in the Local Group

The whole Sloan Digital Sky Survey (SDSS, 14,555 deg^2 has been searched for intergalactic globular clusters (IGCs) in the Local Group (LG). Using optical, infrared, and ultraviolet photometric selection criteria and photometric redshifts, the 2.1x10^8 of objects in the SDSS Galaxy Catalogue were reduced to only 183,791 brighter than r_o = 19 that might be GCs. Visual examination of their SDSS images recovered 84 percent of the confirmed GCs in M31 and M33 and yielded 17 new GC candidates, 5 of them of high confidence, which we could confirm as GCs in MegaPrime images from the Canada, France, Hawaii Telescope. These 5 GCs are within M31's halo, but the other 12 candidates are not close to LG galaxies or galaxies within 3 Mpc of the LG. Even though this search covers only one-third of the sky and some GCs could have been missed, it suggests that the LG does not contain a large population of IGCs more luminous than Mv ~ -6. In the direction of the M81 Group, the search yielded five candidate GCs, probable members of that group.

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arXiv2014

A comprehensive view of the Virgo Stellar Stream

Radial velocities and metallicities have been measured for 82 RR Lyrae identified by the QUEST survey in the direction of the Virgo constellation. Distributed over 90 sq. deg. of the sky, they lie from 4 to 23 kpc from the Sun. Using an algorithm for finding groups in phase space and modeling the smooth halo component in the region, we identified the 5 most significant RRLS groups. We have examined the SEKBO and the Catalina catalog of RRLS (Prior et al. 2009, and Drake et al. 2013), as well as the bright QUEST RRLS sample (Vivas et al. in prep.), the catalogs of Blue Horizontal Branch stars compiled by Sirko et al (2004) and Brown et al (2008, 2010) and the catalog of Red Giant stars from the Spaghetti survey, for stars that may be related to the QUEST RRLS groups. The most significant group of RRLS is the Virgo Stellar Stream (VSS, Duffau et al 2006), group A, which is composed of at least 10 RRLS and 3 BHB stars. It has a mean distance of 19.6 kpc and a mean radial velocity Vgsr = 128 km/s, as estimated from its RRLS members. With the revised velocities reported here, there is no longer an offset in velocity between the RRLS in the VSS and the prominent peak in the velocities of main-sequence turnoff stars reported by Newberg et al (2007) in the same direction and at a similar distance (S297+63-20.5). The location in phase space of two other groups (F,H) suggests a possible connection with the VSS, which cannot be discarded at this point, although the turnoff colors of the VSS and group H, as identified from Newberg et al. (2007), suggest they might be composed of different populations. Two more groups (B,D), are found at mean distances of 19 and 5.7 kpc, and mean radial velocities of -94 and 32 km/s. None of our groups seems to relate to Sgr streams. The excess of stars observed in Virgo appears to be composed of several halo substructures along the same line of sight.

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arXiv2013

More Remote Globular Clusters in the Outer Halo of M31

We searched the Sloan Digital Sky Survey for outer halo globular clusters (GCs) around M31. Our search of non-stellar objects, within the limits 0.3 < (g-i)o < 1.5 and 14.0 < ro < 19.0 concentrated in some remote areas of the extended halo, to a maximum projected distance of 240 kpc, for a total of approximately 200 square degrees. Another roughly 50 square degrees, approximately 5-75 kpc from M31, were surveyed as test areas. In these areas, we identified 39 GCs and 2 GC candidates, 84% of the previously known GCs (93% of the "classical GCs" and 40% of the "halo extended clusters", on the cluster classification scheme of Huxor et al.). For the entire survey, we visually inspected 78,516 objects for morphological evidence of cluster status, and we identified 18 new clusters, and 75 candidate clusters. The new clusters include 15 classical globulars and three clusters of lower density. Six of the clusters reside in the remote areas of the outer halo, beyond projected distances of 100 kpc. Previously, only MGC1 was found beyond this limit at 117 kpc. The farthest cluster discovered in this survey lies at a projected radius of 158 kpc from M31, assuming that the M31 distance is 780 kpc.

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arXiv2012

La Silla-QUEST Variability Survey in the Southern Hemisphere

We describe the La Silla-QUEST (LSQ) Variability Survey. LSQ is a dedicated wide-field synoptic survey in the Southern Hemisphere, focussing on the discovery and study of transients ranging from low redshift (z < 0.1) SN Ia, Tidal Disruption events, RR Lyræ variables, CVs, Quasars, TNOs and others. The survey utilizes the 1.0-m Schmidt Telescope of the European Southern Observatory at La Silla, Chile, with the large-area QUEST camera, a mosaic of 112 CCDs with field of view of 9.6 square degrees. The LSQ Survey was commissioned in 2009, and is now regularly covering ~1000 square deg per night with a repeat cadence of hours to days. The data are currently processed on a daily basis. We present here a first look at the photometric capabilities of LSQ and we discuss some of the most interesting recent transient detections.

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arXiv2012

The QUEST RR Lyrae Survey: III. The Low Galactic Latitude Catalogue

We present results for the QUEST RR Lyrae Survey at low galactic latitude, conducted entirely with observations obtained with the QUEST mosaic camera and the 1.0/1.5m Jürgen Stock Schmidt telescope at the National Observatory of Venezuela. The survey spans an area of 476 sq. deg on the sky, with multi-epoch observations in the V, R and I photometric bands for 6.5x10^6 stars in the galactic latitude range 30<= b(deg) <=+25, in a direction close to the Galactic Anticenter 190<= l(deg)<= 230. The variability survey has a typical number of 30 observations per object in V and I and ~25 in R, with up to ~120-150 epochs in V and I and up to ~100 in R in the best sampled regions. The completeness magnitudes of the survey are V=R=18.5 mag, and I=18.0 mag. We identified 211 RR Lyrae stars, 160 bona fide stars of type ab and 51 candidates of type c, ours being the first deep RR Lyrae survey conducted at low galactic latitude.The completeness of the RR Lyrae survey was estimated in >95 per cent and ~85 per cent for RRab and RRc stars respectively. Photometric metallicities were computed based on the light curves and individual extinctions calculated from minimum light colours for each RRab star. Distances were obtained with typical errors ~7 per cent. The RR Lyrae survey simultaneously spans a large range of heliocentric distances 0.5<= R_hel(kpc)<=40 and heights above the plane -15<=z(kpc)<=+20, with well known completeness across the survey area, making it an ideal set for studying the struc ture of the Galactic thick disk.

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arXiv2009

No Excess of RR Lyrae Stars in the Canis Major Overdensity

Our multi-epoch survey of ~20 sq. deg. of the Canis Major overdensity has detected only 10 RR Lyrae stars (RRLS). We show that this number is consistent with the number expected from the Galactic halo and thick disk populations alone, leaving no excess that can be attributed to the dwarf spheroidal (dSph) galaxy that some authors have proposed as the origin of the CMa overdensity. If this galaxy resembles the dSph satellites of the Milky Way and of M31 and has the putative Mv~-14.5, our survey should have detected several tens of RRLS. Even if Mv<-12, the expected excess is >10, which is not observed. Either the old stellar population of this galaxy has unique properties or, as others have argued before, the CMa overdensity is produced by the thin and thick disk and spiral arm populations of the Milky Way and not by a collision with a dSph satellite galaxy.

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arXiv2008

Spectroscopy of Bright QUEST RR Lyrae Stars: Velocity Substructures toward Virgo

Using a sample of 43 bright (V<16.1, distance <13 kpc) RR Lyrae stars (RRLS) from the QUEST survey with spectroscopic radial velocities and metallicities, we find that several separate halo substructures contribute to the Virgo overdensity (VOD). While there is little evidence for halo substructure in the spatial distribution of these stars, their distribution in radial velocity reveals two moving groups. These results are reinforced when the sample is combined with a sample of blue horizontal branch stars that were identified in the SDSS, and the combined sample provides evidence for one additional moving group. These groups correspond to peaks in the radial velocity distribution of a sample of F type main-sequence stars that was recently observed in the same directon by SEGUE, although in one case the RRLS and F star groups may not lie at the same distance. One of the new substructures has a very narrow range in metallicity, which is more consistent with it being the debris from a destroyed globular cluster than from a dwarf galaxy. A small concentration of stars have radial velocities that are similar to the Virgo Stellar Stream (VSS) that was identified previously in a fainter sample of RRLS. Our results suggest that this feature extends to distances as short as ~12 kpc from its previous detection at ~19 kpc. None of the new groups and only one star in the sample have velocities that are consistent with membership in the leading tidal stream from the Sagittarius Dwarf Spheroidal Galaxy, which some authors have suggested is the origin of the VOD.

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arXiv2007

The Chemical Enrichment History of the Large Magellanic Cloud

Ca II triplet spectroscopy has been used to derive stellar metallicities for individual stars in four LMC fields situated at galactocentric distances of 3\arcdeg, 5\arcdeg, 6\arcdeg\@ and 8\arcdeg\@ to the north of the Bar. Observed metallicity distributions show a well defined peak, with a tail toward low metallicities. The mean metallicity remains constant until 6\arcdeg\@ ([Fe/H]$\sim$-0.5 dex), while for the outermost field, at 8\arcdeg, the mean metallicity is substantially lower than in the rest of the disk ([Fe/H]$\sim$-0.8 dex). The combination of spectroscopy with deep CCD photometry has allowed us to break the RGB age--metallicity degeneracy and compute the ages for the objects observed spectroscopically. The obtained age--metallicity relationships for our four fields are statistically indistinguishable. We conclude that the lower mean metallicity in the outermost field is a consequence of it having a lower fraction of intermediate-age stars, which are more metal-rich than the older stars. The disk age--metallicity relationship is similar to that for clusters. However, the lack of objects with ages between 3 and 10 Gyr is not observed in the field population. Finally, we used data from the literature to derive consistently the age--metallicity relationship of the bar. Simple chemical evolution models have been used to reproduce the observed age--metallicity relationships with the purpose of investigating which mechanism has participated in the evolution of the disk and bar. We find that while the disk age--metallicity relationship is well reproduced by close-box models or models with a small degree of outflow, that of the bar is only reproduced by models with combination of infall and outflow.

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arXiv2007

The Infrared Ca II triplet as metallicity indicator

From observations of almost 500 RGB stars in 29 Galactic open and globular clusters, we have investigated the behaviour of the infrared Ca II triplet (8498, 8542 and 8662 Å) in the age range 13$\leq$Age/Gyr$\leq$0.25 and the metallicity range $-2.2\leq$ [Fe/H] $\leq$+0.47. These are the widest ranges of ages and metallicities in which the behaviour of the Ca II triplet lines has been investigated in a homogeneous way. We report the first empirical study of the variation of the CaII triplet lines strength, for given metallicities, with respect to luminosity. We find that the sequence defined by each cluster in the Luminosity-$Σ$Ca plane is not exactly linear. However, when only stars in a small magnitude interval are observed, the sequences can be considered as linear. We have studied the the Ca II triplet lines on three metallicities scales. While a linear correlation between the reduced equivalent width ($W'_V$ or $W'_I$) versus metallicity is found in the \citet{cg97} and \citet{ki03} scales, a second order term needs to be added when the \citet{zw84} scale is adopted. We investigate the role of age from the wide range of ages covered by our sample. We find that age has a weak influence on the final relationship. Finally, the relationship derived here is used to estimate the metallicities of three poorly studied open clusters: Berkeley 39, Trumpler 5 and Collinder 110. For the latter, the metallicity derived here is the first spectroscopic estimate available.

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arXiv2007

Whiting 1: the youngest globular cluster associated with the Sgr dSph

Recently, Carraro (2005) drew attention to the remarkable star cluster Whiting 1 by showing that it lies about 40 kpc from the Sun and is therefore unquestionably a member of the Galactic halo (b=-60.6 deg.). Its CMD indicated that Whiting 1 is very young (5 Gyrs) for a globular cluster. It is very likely that Whiting 1 originated in a dwarf galaxy that has since been disrupted by the Milky Way. Deep CCD photometry in the BVI pass-bands obtained with the VLT is used to improve the quality of the CMD and to determine the cluster's luminosity function and surface density profile. High-resolution spectrograms obtained with Magellan are used to measure the cluster's radial velocity and to place limits on its possible metallicity. The measurements of distance and radial velocity are used to test the cluster's membership in the stellar streams from the Sgr dSph. From our CMD of Whiting 1, we derive new estimates for the cluster's age (6.5 Gyrs), metallicity ([Fe/H]=-0.65), and distance (29.4 kpc). From echelle spectrograms of three stars, we obtain -130.6 km/s for the cluster's radial velocity and show from measurements of two infra-red CaII lines that the [Fe/H] of the cluster probably lies in the range -1.1 to -0.4. We demonstrate that the position of Whiting 1 on the sky, its distance from the Sun, and its radial velocity are identical to within the errors of both the theoretical predictions of the trailing stream of stars from the Sgr dSph galaxy and the previous observations of the M giant stars that delineate the streams. With the addition of Whiting 1, there is now strong evidence that 6 globular clusters formed within the Sgr dSph.

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arXiv2007

Searching for RR Lyrae stars in the Canis Major Overdensity

The Canis Major overdensity (CMa) was initially proposed to be the remnant of a tidally disrupting dSph galaxy. Since its nature is still subject of debate, the goal of the present work was to conduct a large-scale RR Lyrae survey in CMa, in order to see if there is an overdensity of these stars. The survey spans a total area of ~34 sq. deg. with observations in V and R filters, made with the 1.0m Jurgen Stock Schmidt telescope at the National Astronomical Observatory of Venezuela. Current results in a subregion, including spectroscopic observations, show that the small number of RR Lyrae stars found can be accounted for by the halo and thick disk components of our Galaxy.

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arXiv2006

The QUEST RR Lyrae Survey II: The Halo Overdensities in the First Catalog

The first catalog of the RR Lyrae stars (RRLS) in the Galactic halo by the QUEST Survey has been searched for significant overdensities that may be debris from disrupted dwarf galaxies or globular clusters. Away from the major overdensities, the distribution of these stars is adequately fit by a smooth halo model, in which the flattening of the halo decreases with increasing galactocentric distance (Preston et al 1991). This model was used to estimate the ``background'' of RRLS on which the halo overdensities are overlaid. A procedure was developed for recognizing groups of stars that constitute significant overdensities with respect to this background. To test this procedure, a Monte Carlo routine was used to make artificial RRLS surveys that follow the smooth halo model, but with Poisson distributed noise in the numbers of RRLS and, within limits, random variations in the positions and magnitudes of the artificial stars. The artificial surveys created by this routine were examined for significant groups in exactly the same way as the QUEST survey. These calculations provided estimates of the frequencies with which random fluctuations produce significant groups. In the QUEST survey, there are six significant overdensities that contain six or more stars and several smaller ones. The small ones and possibly one or two of the larger ones may be artifacts of statistical fluctuations, and they need to be confirmed by measurements of radial velocity and/or proper motion. The most prominent groups are the northern stream from the Sagittarius dwarf spheroidal galaxy and a large group in Virgo. Two other groups lie in the direction of the Monoceros stream and at approximately the right distance for membership. Another group is related to the globular cluster Palomar 5.

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arXiv2005

Spectroscopy of QUEST RR Lyrae Variables: the new Virgo Stellar Stream

Eighteen RR Lyrae variables (RRLs) that lie in the "$12\fh 4$ clump" identified by the QUEST survey have been observed spectroscopically to measure their radial velocities and metal abundances. Ten blue horizontal branch (BHB) stars identified by the Sloan Digital Sky Survey (SDSS) were added to this sample. Six of the 9 stars in the densest region of the clump have a mean radial velocity in the galactic rest frame ($V_{\rm gsr}$) of 99.8 and $σ$ = 17.3 ${\rm km s}^{-1}$, which is slightly smaller than the average error of the measurements. The whole sample contains 8 RRLs and 5 BHB stars that have values of $V_{\rm gsr}$ suggesting membership in this stream. For 7 of these RRLs, the measurements of [Fe/H], which have an internal precision of 0.08 dex, yield $<{\rm [Fe/H]}> = -1.86$ and $σ$ = 0.40. These values suggest that the stream is a tidally disrupted dwarf spheroidal galaxy of low luminosity. Photometry from the database of the SDSS indicates that this stream covers at least 106 deg$^2$ of the sky in the constellation Virgo. The name Virgo Stellar Stream is suggested.

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arXiv2005

RR Lyrae search and stellar populations study in Canis Major: Preliminary Results

We present preliminary results of a RR Lyrae star search and stellar populations study performed in the Canis Major overdensity, spanning an area of 8.35sq deg. The observations were made in R and V bands, with the QUEST camera installed in the 1m Jurgen Stock Telescope, at the Venezuela National Observatory. The resulting Hess diagram shows a possible, but weak, red giant branch and no obvious horizontal branch, red clump or main sequence turnoff. After a multi--epoch photometric search, 6 RR Lyrae stars were confirmed with further observations obtained at the 1.0 and 1.3m telescopes of the SMARTS consortium at CTIO. Of these confirmed RR Lyrae stars, 5 have heliocentric distances between 5 and 7 kpc. Confirmation of their physsical association with the Canis Major system awaits for a study of their radial velocities.

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arXiv2004

VLT Spectroscopy of RR Lyrae Stars in the Sagittarius Tidal Stream

Sixteen RR Lyrae variables from the QUEST survey that lie in the leading arm of the tidal stream from the Sagittarius dSph galaxy have been observed spectroscopically to measure their radial velocities and metal abundances. The systemic velocities of 14 stars, which were determined by fitting a standard velocity curve to the individual measurements, have a sharply peaked distribution with a mean of 33 km/s and a standard deviation of only 25 km/s. The [Fe/H] distribution of these stars has a mean of -1.76 and a standard deviation of 0.22. These measurements are in good agreement with previous ones from smaller samples of stars. The mean metallicity is consistent with the age-metallicity relation that is observed in the main body of the Sgr dSph galaxy. The radial velocities and the distances from the Sun of these stars are compared with recent numerical simulations of the Sgr streams that assume different shapes for the dark matter halo. Models that assume a oblate halo do not fit the data as well as ones that assume a spherical or a prolate distribution. However, none of the fits are completely satisfactory. Every model fails to reproduce the long extent of the stream in right ascension (36 degr) that is seen in the region covered by the QUEST survey. Further modeling is required to see if this and the other mismatches between theory and observation can be removed by judicial choices for the model parameters or instead rule out a class of models.

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arXiv2004

Mapping Tidal Streams around Galactic Globular Clusters

The surroundings of globular clusters in the outer halo are one of the best places to look for the remnants of ancient dwarf satellites. The classical Searle & Zinn (1978) scenario of the formation of the Milky Way, in which the halo globular clusters formed in larger dwarf galaxies, can nowadays be considered the local manifestation of the hierarchical galaxy formation. It is now well known that some Galactic globular clusters are actually associated with the tidal stellar stream resulting from the accretion of dwarf satellites, thereby confirming that this process plays an important role in the building of the Milky Way globular cluster system. Furthermore, several recent studies have suggested that the two most luminous Galactic globular clusters (M54, associated with Sagittarius and Omega Centauri) might be the nuclei of dwarf galaxies. Extending previous suggestions, van den Bergh (2000) has argued that the ``young'' globulars located in the outer halo might be the nuclei of extinct dwarf galaxies and suggests that a search for evidence of this may prove worthwhile. If these clusters formed within larger stellar systems, we would expect to find them surrounded by a stellar population that should be revealed by deep photometry over a wide field. In this paper, we present the first (most interesting) preliminary results from a long term project designed to search for tidal debris around remote halo globular clusters, debris that may be the vestiges of the larger systems in which these clusters formed.

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