arXiv subjects

V. Belokurov

Publications and source records attributed to V. Belokurov.

At least 73 records · Page 4Linked to original sources

arXiv2014

The Rich Are Different: Evidence from the RAVE Survey for Stellar Radial Migration

Using the RAdial Velocity Experiment fourth data release (RAVE DR4), and a new metallicity calibration that will be also taken into account in the future RAVE DR5, we investigate the existence and the properties of super-solar metallicity stars ([M/H] > +0.1 dex) in the sample, and in particular in the Solar neighbourhood. We find that RAVE is rich in super-solar metallicity stars, and that the local metallicity distribution function declines remarkably slowly up to +0.4 dex. Our results show that the kinematics and height distributions of the super-solar metallicity stars are identical to those of the [M/H] < 0 thin-disc giants that we presume were locally manufactured. The eccentricities of the super-solar metallicity stars indicate that half of them are on a roughly circular orbit (e < 0.15), so under the assumption that the metallicity of the interstellar medium at a given radius never decreases with time, they must have increased their angular momenta by scattering at corotation resonances of spiral arms from regions far inside the Solar annulus. The likelihood that a star will migrate radially does not seem to decrease significantly with increasing amplitude of vertical oscillations within range of oscillation amplitudes encountered in the disc.

astro-ph.GA↗

arXiv2014

Discovery of $\sim$ 9,000 new RR Lyrae in the Southern Catalina Surveys

We present the results of a deep, wide-area variability survey in the Southern hemisphere, the first of its kind. As part of the Catalina Sky Surveys, the Siding Spring Survey (SSS) has covered $14,800$ square degrees in the declination range of $-75^{\circ}\leqδ\leq-15^{\circ}$. To mine the enormous SSS dataset efficiently we have developed two algorithms: Automatic Period Selection (APS) and Automatic Fourier Decomposition (AFD), which aim to sharpen the period estimation and produce robust lightcurve models. Armed with the APS and AFD outputs we classify $10,540$ ab-type RR Lyrae (RRab) stars ($\sim$90% of which are new) across the Southern sky. As well as the positional information we supply photometric metallicities, and unreddened distances. For the RRab stars in the halo, a study of the photometric metallicity distribution reveals a nearly Gaussian shape with a mean metallicity of ${\rm [Fe/H]}=-1.4$ dex and a dispersion of $0.3$ dex. A spatial study of the RRab metallicities shows no significant radial gradient in the first $\sim7$ kpc from the Galaxy center. However, further out, a small negative gradient is clearly present. This is complemented by a very obvious correlation of the mean RR Lyrae metallicity with distance above the Galactic plane, $z$. We have also carried out an initial substructure search using the discovered RRab, and present the properties of the candidates with significance greater than $2 σ$. Most prominent among these is a southern extension of the Sagittarius dwarf galaxy's stream system, reaching down to declinations $\sim -40°$.

astro-ph.GA↗

arXiv2014

Exposing Sgr tidal debris behind the Galactic disk with M giants selected in WISE$\cap$2MASS

We show that a combination of infrared photometry from WISE and 2MASS surveys can yield highly pure samples of M giant stars. We take advantage of the new WISE$\cap$2MASS M giant selection to trace the Sagittarius trailing tail behind the Galactic disk in the direction of the anti-centre. The M giant candidates selected via broad-band photometry are confirmed spectroscopically using AAOmega on the AAT in 3 fields around the extremity of the Sgr trailing tail in the Southern Galactic hemisphere. We demonstrate that at the Sgr longitude $\tilde Λ_{\odot} = 204^{\circ}$, the line-of-sight velocity of the trailing tail starts to deviate from the track of the Law & Majewski (2010) model, confirming the prediction of Belokurov et al. (2014). This discovery serves to substantiate the measurement of low differential orbital precession of the Sgr stream which in turn may imply diminished dark matter content within 100 kpc.

astro-ph.GA↗

arXiv2014

"Skinny Milky Way, Please", says Sagittarius

Motivated by recent observations of the Sagittarius stream, we devise a rapid algorithm to generate faithful representations of the centroids of stellar tidal streams formed in a disruption of a progenitor of an arbitrary mass in an arbitrary potential. Our method works by releasing swarms of test particles at the Lagrange points around the satellite and subsequently evolving them in a combined potential of the host and the progenitor. We stress that the action of the progenitor's gravity is crucial to making streams that look almost indistinguishable from the N-body realizations, as indeed ours do. The method is tested on mock stream data in three different Milky Way potentials with increasing complexity, and is shown to deliver unbiased inference on the Galactic mass distribution out to large radii. When applied to the observations of the Sagittarius stream, our model gives a natural explanation of the stream's apocentric distances and the differential orbital precession. We, therefore, provide a new independent measurement of the Galactic mass distribution beyond 50 kpc. The Sagittarius stream model favours a light Milky Way with the mass 4.1 +/- 0.4 x 10^11 M_sun at 100 kpc, which can be extrapolated to 5.6 +/- 1.2 x 10^11 M_sun at 200 kpc. Such a low mass for the Milky Way Galaxy is in good agreement with estimates from the kinematics of halo stars and from the satellite galaxies (once Leo I is removed from the sample). It entirely removes the "Too Big To Fail Problem".

astro-ph.GA↗

arXiv2014

On Asymmetric Distributions of Satellite Galaxies

We demonstrate that the asymmetric distribution of M31 satellites cannot be produced by tides from the Milky Way as such effects are too weak. However, loosely bound associations and groups of satellites can fall into larger haloes and give rise to asymmetries. We compute the survival times for such associations. We prove that the survival time is always shortest in Keplerian potentials, and can be ~ 3 times longer in logarithmic potentials. We provide an analytical formula for the dispersal time in terms of the size and velocity dispersion of the infalling structure. We show that, if an association of ~10 dwarfs fell into the M31 halo, its present aspect would be that of an asymmetric disk of satellites. We also discuss the case of cold substructure in the Andromeda II and Ursa Minor dwarfs.

astro-ph.GA↗

arXiv2014

TriAnd and its Siblings: Satellites of Satellites in the Milky Way Halo

We explore the Triangulum-Andromeda (TriAnd) overdensity in the SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) and SEGUE (the Sloan Extension for Galactic Understanding and Exploration) spectroscopic surveys. Milky Way main sequence turn-off stars in the SPLASH survey reveal that the TriAnd overdensity and the recently discovered PAndAS stream (Martin et al. 2014) share a common heliocentric distance (D ~ 20 kpc), position on the sky, and line-of-sight velocity (V_GSR ~ 50 km/s). Similarly, A-type, giant, and main sequence turn-off stars selected from the SEGUE survey in the vicinity of the Segue 2 satellite show that TriAnd is prevalent in these fields, with a velocity and distance similar to Segue 2. The coincidence of the PAndAS stream and Segue 2 satellite in positional and velocity space to TriAnd suggests that these substructures are all associated, and may be a fossil record of group-infall onto the Milky Way halo. In this scenario, the Segue 2 satellite and PAndAS stream are "satellites of satellites", and the large, metal-rich TriAnd overdensity is the remains of the group central.

astro-ph.GA↗

arXiv2014

The Alpha-element knee of the Sagittarius Stream

We employ abundances from the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE) to study the alpha-element distribution of the stellar members of the Sagittarius stream. To test the reliability of SDSS/SEGUE abundances for the study of Sagittarius, we select high-likelihood samples tracing the different components of the Milky Way, and recover known literature alpha-element distributions. Using selection criteria based on the spatial position, radial velocity, distance and colours of individual stars, we obtain a robust sample of Sagittarius-stream stars. The alpha-element distribution of the Sagittarius stream forms a narrow sequence at intermediate metallicities with a clear turn-down, consistent with the presence of an alpha-element "knee". This is the first time that the alpha-element knee of the Sagittarius dwarf galaxy has been detected. Fitting a toy model to our data, we determine that the alpha-knee in Sagittarius takes place at [Fe/H]=-1.27pm0.05, only slightly less metal-poor than the knee in the Milky Way. This indicates that a small number of Sagittarius-like galaxies could have contributed significantly to the build-up of the Milky Way's stellar halo system at ancient times.

astro-ph.GA↗

arXiv2014

The Catalina Surveys Periodic Variable Star Catalog

We present ~47,000 periodic variables found during the analysis of 5.4 million variable star candidates within a 20,000 square degree region covered by the Catalina Surveys Data Release-1 (CSDR1). Combining these variables with type-ab RR Lyrae from our previous work, we produce an on-line catalog containing periods, amplitudes, and classifications for ~61,000 periodic variables. By cross-matching these variables with those from prior surveys, we find that > 90% of the ~8,000 known periodic variables in the survey region are recovered. For these sources we find excellent agreement between our catalog and prior values of luminosity, period and amplitude, as well as classification. We investigate the rate of confusion between objects classified as contact binaries and type-c RR Lyrae (RRc's) based on periods, colours, amplitudes, metalicities, radial velocities and surface gravities. We find that no more than few percent of these variables in these classes are misidentified. By deriving distances for this clean sample of ~5,500 RRc's, we trace the path of the Sagittarius tidal streams within the Galactic halo. Selecting 146 outer-halo RRc's with SDSS radial velocities, we confirm the presence of a coherent halo structure that is inconsistent with current N-body simulations of the Sagittarius tidal stream. We also find numerous long-period variables that are very likely associated within the Sagittarius tidal streams system. Based on the examination of 31,000 contact binary light curves we find evidence for two subgroups exhibiting irregular lightcurves. One subgroup presents significant variations in mean brightness that are likely due to chromospheric activity. The other subgroup shows stable modulations over more than a thousand days and thereby provides evidence that the O'Connell effect is not due to stellar spots.

astro-ph.SR↗

arXiv2014

Discovery of a cold stellar stream in the ATLAS DR1 data

We report the discovery of a narrow stellar stream crossing the constellations of Sculptor and Fornax in the Southern celestial hemisphere. The portion of the stream detected in the Data Release 1 photometry of the ATLAS survey is at least 12 degrees long, while its width is $\approx$ 0.25 deg. The Color Magnitude Diagram of this halo sub-structure is consistent with a metal-poor [Fe/H] $\lesssim -1.4$ stellar population located at a heliocentric distance of 20 $\pm$ 2 kpc. There are three globular clusters that could tentatively be associated with the stream: NGC 7006, NGC 7078 (M15) and Pyxis, but NGC 7006 and 7078 seem to have proper motions incompatible with the stream orbit.

astro-ph.GA↗

arXiv2014

ATLAS lifts the Cup: Discovery of a New Milky Way satellite in Crater

We announce the discovery of a new Galactic companion found in data from the ESO VST ATLAS survey, and followed up with deep imaging on the 4m William Herschel Telescope. The satellite is located in the constellation of Crater (the Cup) at a distance of $\sim$ 170 kpc. Its half-light radius is $r_h=30$ pc and its luminosity is $M_V=-5.5$. The bulk of its stellar population is old and metal-poor. We would probably have classified the newly discovered satellite as an extended globular cluster were it not for the presence of a handful of Blue Loop stars and a sparsely populated Red Clump. The existence of the core helium burning population implies that star-formation occurred in Crater perhaps as recently as 400 Myr ago. No globular cluster has ever accomplished the feat of prolonging its star-formation by several Gyrs. Therefore, if our hypothesis that the blue bright stars in Crater are Blue Loop giants is correct, the new satellite should be classified as a dwarf galaxy with unusual properties. Note that only ten degrees to the North of Crater, two ultra-faint galaxies Leo IV and V orbit the Galaxy at approximately the same distance. This hints that all three satellites may once have been closely associated before falling together into the Milky Way halo.

astro-ph.GA↗

arXiv2013

Precession of the Sagittarius stream

Using a variety of stellar tracers -- blue horizontal branch stars, main-sequence turn-off stars and red giants -- we follow the path of the Sagittarius (Sgr) stream across the sky in Sloan Digital Sky Survey data. Our study presents new Sgr debris detections, accurate distances and line-of-sight velocities that together help to shed new light on the puzzle of the Sgr tails. For both the leading and the trailing tail, we trace the points of their maximal extent, or apo-centric distances, and find that they lie at $R^L$ = 47.8 $\pm$ 0.5 kpc and $R^T$ = 102.5 $\pm$ 2.5 kpc respectively. The angular difference between the apo-centres is 93.2 $\pm$ 3.5 deg, which is smaller than predicted for logarithmic haloes. Such differential orbital precession can be made consistent with models of the Milky Way in which the dark matter density falls more quickly with radius. However, currently, no existing Sgr disruption simulation can explain the entirety of the observational data. Based on its position and radial velocity, we show that the unusually large globular cluster NGC 2419 can be associated with the Sgr trailing stream. We measure the precession of the orbital plane of the Sgr debris in the Milky Way potential and show that, surprisingly, Sgr debris in the primary (brighter) tails evolves differently to the secondary (fainter) tails, both in the North and the South.

astro-ph.GA↗

arXiv2013

The similarity of the stellar mass fractions of galaxy groups and clusters

We employ a large sample of 20171 optically-selected groups and clusters at 0.15 < z < 0.4 in the SDSS to investigate how the stacked stellar mass fraction varies across a wide range of total mass, $M_{500}$. Our study improves upon previous observational studies in a number of important ways, including having a much larger sample size, an explicit inclusion of the intracluster light (ICL) component, and a thorough examination of the accuracy of our total mass estimates via comparisons to simulations and weak lensing observations. We find that the stellar mass fraction depends only weakly on total mass and that the contribution of ICL to the total stellar mass fraction is significant (typically 20-40 per cent). Both of these findings are in excellent accordance with the predictions of cosmological simulations. Under the assumption of a Chabrier (Salpeter) IMF, the derived star formation efficiency ($f_{star}$/$f_{b}$, where $f_b=Ω_b/Ω_m$) is relatively low at 8 per cent (14 per cent) and is consistent with the global star formation efficiency of semi-analytic models that reproduce the galaxy stellar mass function. When our measured stellar mass fractions are combined with the observed relation between hot gas mass fraction and total mass from X-ray observations, our results imply that galaxy groups have significantly lower baryon fractions than massive clusters. Ejection of gas due to energetic AGN feedback (most likely at high redshift) provides a plausible mechanism for explaining the trends we observe.

astro-ph.CO↗

arXiv2013

Are group- and cluster-scale dark matter halos over-concentrated?

We investigate the relationship between the halo mass, M_200, and concentration, c, for a sample of 26 group- and cluster-scale strong gravitational lenses. In contrast with previous results, we find that these systems are only ~ 0.1 dex more over-concentrated than similar-mass halos from dark matter simulations; the concentration of a halo with M_200 = 10^14 M_sun is log c = 0.78\pm0.05, while simulations of halos with this mass at similar redshifts (z ~ 0.4) predict log c ~ 0.56 - 0.71. We also find that we are unable to make informative inference on the slope of the M_200-c relation in spite of our large sample size; we note that the steep slopes found in previous studies tend to follow the slope in the covariance between M_200 and c, indicating that these results may be measuring the scatter in the data rather than the intrinsic signal. Furthermore, we conclude that our inability to constrain the M_200-c slope is due to a limited range of halo masses, as determined by explicitly modelling our halo mass distribution, and we suggest that other studies may be producing biased results by using an incorrect distribution for their halo masses.

astro-ph.CO↗

arXiv2013

Triaxial Cosmological Haloes and the Disc of Satellites

We construct simple triaxial generalisations of Navarro-Frenk-White haloes. The models have elementary gravitational potentials, together with a density that is cusped like 1/r at small radii and falls off like 1/r^3 at large radii. The ellipticity varies with radius in a manner that can be tailored to the user's specification. The closed periodic orbits in the planes perpendicular to the short and long axes of the model are well-described by epicyclic theory, and can be used as building blocks for long-lived discs. As an application, we carry out the simulations of thin discs of satellites in triaxial dark halo potentials. This is motivated by the recent claims of an extended, thin disc of satellites around the M31 galaxy with a vertical rms scatter of ~12 kpc and a radial extent of ~ 300 kpc (Ibata et al. 2013). We show that a thin satellite disc can persist over cosmological times if and only if it lies in the planes perpendicular to the long or short axis of a triaxial halo, or in the equatorial or polar planes of a spheroidal halo. In any other orientation, then the disc thickness doubles on ~5 Gyr timescales and so must have been born with an implausibly small vertical scaleheight.

astro-ph.CO↗

arXiv2013

The dark matter halo concentration and stellar initial mass function of a CASSOWARY group

We exploit the group environment of the CAmbridge Sloan Survey Of Wide ARcs in the skY (CASSOWARY) z=0.3 lens J2158+0257 to measure the group dynamical mass as a complement to the central dynamical and lensing mass constraints. Follow-up spectroscopy of candidate group members is performed using VLT/FORS2. From the resulting N=21 confirmed members we measure the group dynamical mass by calibrating an analytic tracer mass estimator with cosmological simulations. The luminosity weighted line-of-sight velocity dispersion and the Einstein radius of the lens are used as mass probes in the inner regions of the galaxy. Combining these three observational probes allows us to independently constrain the mass and concentration of the dark matter halo, in addition to the total stellar mass of the central galaxy. We find a dark matter halo in remarkably good agreement with simulations (log_10 M_200/M_sun = 14.2 +/- 0.2, c_200 = 4.4 (+1.6, -1.4)) and a stellar mass-to-light ratio which favors a Salpeter initial mass function ((M/L)* = 5.7 +/- 1.2). Our measurement of a normal halo concentration suggests that there is no discrepancy between simulations and observations at the group mass scale. This is in contrast to the cluster mass scale for which a number of studies have claimed over-concentrated halos. While the halo mass is robustly determined, and the halo concentration is not significantly affected by systematics, the resulting stellar mass-to-light ratio is sensitive to the choice of stellar parameters, such as density profile and velocity anisotropy.

astro-ph.CO↗

arXiv2012

Broken and unbroken: the Milky Way and M31 stellar haloes

We use the Bullock & Johnston suite of simulations to study the density profiles of L*-type galaxy stellar haloes. Observations of the Milky Way and M31 stellar haloes show contrasting results: the Milky Way has a `broken' profile, where the density falls off more rapidly beyond ~ 25 kpc, while M31 has a smooth profile out to 100 kpc with no obvious break. Simulated stellar haloes, built solely by the accretion of dwarf galaxies, also exhibit this behavior: some haloes have breaks, while others don't. The presence or absence of a break in the stellar halo profile can be related to the accretion history of the galaxy. We find that a break radius is strongly related to the build up of stars at apocentres. We relate these findings to observations, and find that the `break' in the Milky Way density profile is likely associated with a relatively early (~ 6-9 Gyr ago) and massive accretion event. In contrast, the absence of a break in the M31 stellar halo profile suggests that its accreted satellites have a wide range of apocentres. Hence, it is likely that M31 has had a much more prolonged accretion history than the Milky Way.

astro-ph.GA↗

arXiv2012

Probing the Outer Galactic halo with RR Lyrae from the Catalina Surveys

We present the analysis of 12227 type-ab RR Lyrae found among the 200 million public lightcurves in the Catalina Surveys Data Release 1 (CSDR1). These stars span the largest volume of the Milky Way ever surveyed with RR Lyrae, covering ~20,000 square degrees of the sky (0 < RA < 360, -22 < Dec < 65 deg) to heliocentric distances of up to 60kpc. Each of the RR Lyrae are observed between 60 and 419 times over a six-year period. Using period finding and Fourier fitting techniques we determine periods and apparent magnitudes for each source. We find that the periods at generally accurate to sigma = 0.002% by comparison with 2842 previously known RR Lyrae and 100 RR Lyrae observed in overlapping survey fields. We photometrically calibrate the light curves using 445 Landolt standard stars and show that the resulting magnitudes are accurate to ~0.05 mags using SDSS data for ~1000 blue horizontal branch stars and 7788 of the RR Lyrae. By combining Catalina photometry with SDSS spectroscopy, we analyze the radial velocity and metallicity distributions for > 1500 of the RR Lyrae. Using the accurate distances derived for the RR Lyrae, we show the paths of the Sagittarius tidal streams crossing the sky at heliocentric distances from 20 to 60 kpc. By selecting samples of Galactic halo RR Lyrae, we compare their velocity, metallicity, and distance with predictions from a recent detailed N-body model of the Sagittarius system. We find that there are some significant differences between the distances and structures predicted and our observations.

astro-ph.GA↗

arXiv2012

The cold veil of the Milky Way stellar halo

We build a sample of distant (D > 80 kpc) stellar halo stars with measured radial velocities. Faint ($20 < g <22$) candidate blue horizontal branch (BHB) stars were selected using the deep, but wide, multi-epoch Sloan Digital Sky Survey photometry. Follow-up spectroscopy for these A-type stars was performed using the VLT-FORS2 instrument. We classify stars according to their Balmer line profiles, and find 7 are bona fide BHB stars and 31 are blue stragglers (BS). Owing to the magnitude range of our sample, even the intrinsically fainter BS stars can reach out to D ~ 90 kpc. We complement this sample of A-type stars with intrinsically brighter, intermediate-age, asymptotic giant branch stars. A set of 4 distant cool carbon stars is compiled from the literature and we perform spectroscopic follow-up on a further 4 N-type carbon stars using the WHT-ISIS instrument. Altogether, this provides us with the largest sample to date of individual star tracers out to r ~ 150 kpc. We find that the radial velocity dispersion of these tracers falls rapidly at large distances and is surprisingly cold (sigma_r ~ 50-60 km/s) between 100-150 kpc. Relating the measured radial velocities to the mass of the Milky Way requires knowledge of the (unknown) tracer density profile and anisotropy at these distances. Nonetheless, by assuming the stellar halo stars between 50-150 kpc have a moderate density fall-off (with power-law slope alpha < 5) and are on radial orbits (sigma^2_t/sigma^2_r < 1), we infer that the mass within 150 kpc is less than 10^12 M_solar and suggest it probably lies in the range (5-10) x 10^11 M_solar. We discuss the implications of such a low mass for the Milky Way.

astro-ph.GA↗