arXiv subjects

V. Belokurov

Publications and source records attributed to V. Belokurov.

At least 91 records · Page 5Linked to original sources

arXiv2012

The stretching of Hercules

We present VLT/FORS2 spectroscopy of candidate blue horizontal branch (BHB) stars in the vicinity of the Hercules ultrafaint dwarf galaxy. We identify eight convincing Hercules BHB members, and a further five stars with similar systemic velocities to that of Hercules, but ~ 0.5 kpc from the centre of the galaxy along its major axis. It is likely that these stars once belonged to Hercules, but have been tidally stripped and are now unbound. We emphasise the usefulness of looking for any gradient in the systemic velocity of this stretched system, which would further support our interpretation of the origin of its elongated and distended morphology.

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arXiv2012

Broken degeneracies: the rotation curve and velocity anisotropy of the Milky Way halo

We use distant Blue Horizontal Branch stars with Galactocentric distances 16 < r/kpc < 48 as kinematic tracers of the Milky Way dark halo. We model the tracer density as an oblate, power-law embedded within a spherical power-law potential. Using a distribution function method, we estimate the overall power-law potential and the velocity anisotropy of the halo tracers. We measure the slope of the potential to be gamma ~ 0.4 and the overall mass within 50 kpc is ~ 4 x 10^11 M_sol. The tracer velocity anisotropy is radially biased with beta ~ 0.5, which is in good agreement with local solar neighbourhood studies. Our results provide an accurate outer circular velocity profile for the Milky Way and suggest a relatively high concentration dark matter halo (c_vir ~ 20).

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arXiv2012

The Sagittarius Streams in the Southern Galactic Hemisphere

The structure of the Sagittarius stream in the Southern Galactic hemisphere is analysed with the Sloan Digital Sky Survey Data Release 8. Parallel to the Sagittarius tidal track, but ~ 10deg away, there is another fainter and more metal-poor stream. We provide evidence that the two streams follow similar distance gradients but have distinct morphological properties and stellar populations. The brighter stream is broader, contains more metal-rich stars and has a richer colour-magnitude diagram with multiple turn-offs and a prominent red clump as compared to the fainter stream. Based on the structural properties and the stellar population mix, the stream configuration is similar to the Northern "bifurcation". In the region of the South Galactic Cap, there is overlapping tidal debris from the Cetus Stream, which crosses the Sagittarius stream. Using both photometric and spectroscopic data, we show that the blue straggler population belongs mainly to Sagittarius and the blue horizontal branch stars belong mainly to the Cetus stream in this confused location in the halo.

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arXiv2012

The radial distribution of galaxies in groups and clusters

We present a new catalogue of 55,121 groups and clusters centred on Luminous Red Galaxies from SDSS DR7 in the redshift range 0.15 ~2.6) that is approximately a factor of two lower than that of the dark matter (as predicted by N-body cosmological simulations) and nearly independent of halo mass. Interestingly, in spite of the difference in shape between the galaxy and dark matter radial distributions, both exhibit a high degree of self-similarity. A self-consistent comparison to several recent semi-analytic models of galaxy formation indicates that: (1) beyond ~0.3 r_500 current models are able to reproduce both the shape and normalisation of the satellite profiles; and (2) within ~0.3 r_500 the predicted profiles are sensitive to the details of the satellite-BCG merger timescale calculation. The former is a direct result of the models being tuned to match the global galaxy luminosity function combined with the assumption that the satellite galaxies do not suffer significant tidal stripping, even though their surrounding dark matter haloes can be removed through this process. Combining our results with measurements of the intracluster light should provide a way to inform theoretical models on the efficacy of the tidal stripping and merging processes.

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arXiv2012

Elliptical galaxy masses out to five effective radii: the realm of dark matter

We estimate the masses of elliptical galaxies out to five effective radii using planetary nebulae and globular clusters as tracers. A sample of 15 elliptical galaxies with a broad variation in mass is compiled from the literature. A distribution function-maximum likelihood analysis is used to estimate the overall potential slope, normalisation and velocity anisotropy of the tracers. We assume power-law profiles for the potential and tracer density and a constant velocity anisotropy. The derived potential power-law indices lie in between the isothermal and Keplerian regime and vary with mass: there is tentative evidence that the less massive galaxies have steeper potential profiles than the more massive galaxies. We use stellar mass-to-light ratios appropriate for either a Chabrier/KTG (Kroupa, Tout & Gilmore) or Salpeter initial mass function to disentangle the stellar and dark matter components. The fraction of dark matter within five effective radii increases with mass, in agreement with several other studies. We employ simple models to show that a combination of star formation efficiency and baryon extent are able to account for this trend. These models are in good agreement with both our measurements out to five effective radii and recent SLACS measurements within one effective radii when a universal Chabrier/KTG initial mass function is adopted.

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arXiv2011

The Milky Way stellar halo out to 40 kpc : Squashed, broken but smooth

We introduce a new maximum likelihood method to model the density profile of Blue Horizontal Branch and Blue Straggler stars and apply it to the Sloan Digital Sky Survey Data Release 8 (DR8) photometric catalogue. There are a large number (20,000) of these tracers available over an impressive 14,000 square degrees in both Northern and Southern Galactic hemispheres, and they provide a robust measurement of the shape of the Milky Way stellar halo. After masking out stars in the vicinity of the Virgo Overdensity and the Sagittarius stream, the data are consistent with a smooth, oblate stellar halo with a density that follows a broken power-law. The best fitting model has an inner slope = 2.3 and an outer slope = 4.6, together with a break radius occurring at 27 kpc and a constant halo flattening (that is, ratio of minor axis to major axis) of q = 0.6. Although a broken power-law describes the density fall-off most adequately, it is also well fit by an Einasto profile. There is no strong evidence for variations in flattening with radius, or for triaxiality of the stellar halo.

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arXiv2011

Mismatch and Misalignment: Dark Haloes and Satellites of Disc Galaxies

(Abridged) We study the phase-space distribution of satellite galaxies associated with late-type galaxies in the GIMIC suite of simulations. GIMIC consists of re-simulations of 5 cosmologically representative regions from the Millennium simulation, which have higher resolution and incorporate baryonic physics. Whilst the disc of the galaxy is well aligned with the inner regions (r ~ 0.1r_200) of the dark matter halo, both in shape and angular momentum, there can be substantial misalignments at larger radii (r ~ r_200). Misalignments of > 45 deg are seen in ~ 30% of our sample. We find that the satellite population aligns with the shape (and angular momentum) of the outer dark matter halo. However, the alignment with the galaxy is weak owing to the mismatch between the disc and dark matter halo. Roughly 20% of the satellite systems with ten bright galaxies within r_200 exhibit a polar spatial alignment with respect to the galaxy --- an orientation reminiscent of the classical satellites of the Milky Way. We find that a small fraction (~10%) of satellite systems show evidence for rotational support which we attribute to group infall. There is a bias towards satellites on prograde orbits relative to the spin of the dark matter halo (and to a lesser extent with the angular momentum of the disc). This preference towards co-rotation is stronger in the inner regions of the halo where the most massive satellites accreted at relatively early times are located. We attribute the anisotropic spatial distribution and angular momentum bias of the satellites at z=0 to their directional accretion along the major axes of the dark matter halo. The satellite galaxies have been accreted relatively recently compared to the dark matter mass and have experienced less phase-mixing and relaxation --- the memory of their accretion history can remain intact to z=0.

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arXiv2011

Accurate Stellar Kinematics at Faint Magnitudes: application to the Bootes~I dwarf spheroidal galaxy

We develop, implement and characterise an enhanced data reduction approach which delivers precise, accurate, radial velocities from moderate resolution spectroscopy with the fibre-fed VLT/FLAMES+GIRAFFE facility. This facility, with appropriate care, delivers radial velocities adequate to resolve the intrinsic velocity dispersions of the very faint dSph dwarf galaxies. Importantly, repeated measurements let us reliably calibrate our individual velocity errors ($0.2 \leq δ_V\leq 5$ km s$^{-1}$) and directly detect stars with variable radial velocities. We show, by application to the Bootes-1 dwarf spheroidal, that the intrinsic velocity dispersion of this system is significantly below 6.5\,km/s reported by previous studies. Our data favor a two-population model of Bootes-1, consisting of a majority `cold' stellar component, with velocity dispersion $2.4^{+0.9}_{-0.5}$\,km/s, and a minority `hot' stellar component, with velocity dispersion $\sim 9$\,km/s, although we can not completely rule out a single component distribution with velocity dispersion $4.6^{0.8}_{-0.6}$\,km/s. We speculate this complex velocity distribution actually reflects the distribution of velocity anisotropy in Bootes-1, which is a measure of its formation processes.

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arXiv2010

Rotation of Halo Populations in the Milky Way and M31

We search for signs of rotation in the subsystems of the Milky Way and M31 that are defined by their satellite galaxies, their globular cluster populations, and their BHB stars. A set of simple distribution functions are introduced to describe anisotropic and rotating stellar populations embedded in dark haloes of approximate Navarro-Frenk-White form. The BHB stars in the Milky Way halo exhibit a dichotomy between a prograde rotating, comparatively metal-rich component ([Fe/H] > -2) and a retrograde rotating, comparatively metal-poor ([Fe/H] < -2) component. The prograde metal-rich population may be associated with the accretion of a massive satellite (~ 10^9 solar masses). The metal-poor population may characterise the primordial stellar halo and the net retrograde rotation could then reflect an underestimate in our adopted local standard of rest circular velocity Theta_0. If Theta_0 is ~ 240 km/s then the metal-poor component has no rotation and there is a net prograde rotation signal of ~45 km/s in the metal-rich component. There is reasonable evidence that the Milky Way globular cluster and satellite galaxy systems are rotating with ~50 km/s and ~ 40 km/s respectively. Furthermore, a stronger signal is found for the satellite galaxies when the angular momentum vector of the satellites is inclined with respect to the normal of the disc. The dwarf spheroidal satellites of M31 exhibit prograde rotation relative to the M31 disc with ~ 40 km/s. We postulate that this group of dwarf spheroidals may share a common origin. We also find strong evidence for systemic rotation in the globular clusters of M31 particularly for the most metal-rich.

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arXiv2010

The Tidal Tails of the Ultra-Faint Globular Cluster Palomar 1

Using the Optimal Filter Technique applied to Sloan Digital Sky Survey photometry, we have found extended tails stretching about 1 degree (or several tens of half-light radii) from either side of the ultra-faint globular cluster Palomar 1. The tails contain roughly as many stars as does the cluster itself. Using deeper Hubble Space Telescope data, we see that the isophotes twist in a chacteristic S-shape on moving outwards from the cluster centre to the tails. We argue that the main mechanism forming the tails may be relaxation driven evaporation and that Pal 1 may have been accreted from a now disrupted dwarf galaxy ~500 Myr ago.

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arXiv2010

Chemical Enrichment in the Faintest Galaxies: the Carbon and Iron Abundance Spreads in the Boötes I Dwarf Spheroidal Galaxy and the Segue 1 System

We present an AAOmega spectroscopic study of red giant stars in Bootes I, which is an ultra-faint dwarf galaxy, and Segue 1, suggested to be either an extremely low-luminosity dwarf galaxy or a star cluster. Our focus is quantifying the mean abundance and abundance dispersion in iron and carbon, and searching for distant radial-velocity members, in these systems. The primary conclusion of our investigation is that the spread of carbon abundance in both Bootes I and Segue 1 is large. For Bootes I, 4 of our 16 velocity members have [C/H] < ~-3.1, while 2 have [C/H] > ~-2.3, suggesting a range of Delta[C/H] ~ 0.8. For Segue 1 there exists a range Delta[C/H] ~ 1.0, including our discovery of a star with [Fe/H] = -3.5 and [C/Fe] = +2.3, which is a radial velocity member at a distance of 4 half-light radii from the system center. The accompanying ranges in iron abundance are Delta[Fe/H] ~ 1.6 for both Bootes I and Segue 1. For [Fe/H] < -3.0, the Galaxy's dwarf galaxy satellites exhibit a dependence of [C/Fe] on [Fe/H] which is very similar to that observed in its halo populations. We find [C/Fe] ~ 0.3 for stars in the dwarf systems that we believe are the counterpart of the Spite et al. (2005) ``unmixed'' giants of the Galactic halo and for which they report [C/Fe] ~ 0.2, and which presumably represents the natal relative abundance of carbon for material with [Fe/H] = -3.0 to -4.0. We confirm the correlation between luminosity and both mean metallicity and abundance dispersion in the Galaxy's dwarf satellites, which extends to at least as faint as Mv = -5. The very low mean metallicity of Segue 1, and the high carbon dispersion in Bootes I, consistent with inhomogeneous chemical evolution in near zero-abundance gas, suggest these ultra-faint systems could be surviving examples of the very first bound systems.

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arXiv2010

The OGLE View of Microlensing towards the Magellanic Clouds. II. OGLE-II SMC data

The primary goal of this paper is to provide the evidence that can either prove or falsify the hypothesis that dark matter in the Galactic halo can clump into stellar-mass compact objects. If such objects existed, they would act as lenses to external sources in the Magellanic Clouds, giving rise to an observable effect of microlensing. We present the results of our search for such events, based on the data from the second phase of the OGLE survey (1996-2000) towards the SMC. The data set we used is comprised of 2.1 million monitored sources distributed over an area of 2.4 square degrees. We found only one microlensing event candidate, however its poor quality light curve limited our discussion on the exact distance to the lensing object. Given a single event, taking the blending (crowding of stars) into account for the detection efficiency simulations, and deriving the HST-corrected number of monitored stars, the microlensing optical depth is tau=(1.55+-1.55)10e-7. This result is consistent with the expected SMC self-lensing signal, with no need of introducing dark matter microlenses. Rejecting the unconvincing event leads to the upper limit on the fraction of dark matter in the form of MACHOs to f<20 per cent for deflectors' masses around 0.4 Msun and f<11 per cent for masses between 0.003 and 0.2 Msun (95 per cent confidence limit). Our result indicates that the Milky Way's dark matter is unlikely to be clumpy and form compact objects in the sub-solar-mass range.

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arXiv2010

Big fish, small fish: Two New Ultra-Faint Satellites of the Milky Way

We report the discovery of two new Milky Way satellites in the neighboring constellations of Pisces and Pegasus identified in data from the Sloan Digital Sky Survey. Pisces II, an ultra-faint dwarf galaxy lies at the distance of ~180 kpc, some 15 degrees away from the recently detected Pisces I. Segue 3, an ultra-faint star cluster lies at the distance of 16 kpc. We use deep follow-up imaging obtained with the 4-m Mayall telescope at Kitt Peak National Observatory to derive their structural parameters. Pisces II has a half-light radius of ~60 pc, while Segue 3 is twenty times smaller at only 3pc.

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arXiv2010

Re-Assembling the Sagittarius Dwarf Galaxy

What is the mass of the progenitor of the Sagittarius (Sgr) dwarf galaxy? Here, we reassemble the stellar debris using SDSS and 2MASS data to find the total luminosity and likely mass. We find that the luminosity is in the range 9.6-13.2 x10^7 solar luminosities or M_V ~ -15.1 - 15.5, with 70% of the light residing in the debris streams. The progenitor is somewhat fainter than the present-day Small Magellanic Cloud, and comparable in brightness to the M31 dwarf spheroidals NGC 147 and NGC 185. Using cosmologically motivated models, we estimate that the mass of Sgr's dark matter halo prior to tidal disruption was ~10^10 solar masses.

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arXiv2009

A photometric and spectroscopic study of the new dwarf spheroidal galaxy in Hercules

Our aim is to provide as clean and as complete a sample as possible of red giant branch stars that are members of the Hercules dSph galaxy. With this sample we explore the velocity dispersion and the metallicity of the system. Stromgren photometry and multi-fibre spectroscopy are combined to provide information about the evolutionary state of the stars (via the Stromgren c_1 index) and their radial velocities. Based on this information we have selected a clean sample of red giant branch stars, and show that foreground contamination by Milky Way dwarf stars can greatly distort the results. Our final sample consists of 28 red giant branch stars in the Hercules dSph galaxy. Based on these stars we find a mean photometric metallicity of -2.35 dex which is consistent with previous studies. We find evidence for an abundance spread. Using those stars for which we have determined radial velocities we find a systemic velocity of 45.2 km/s with a dispersion of 3.72 km/s, this is lower than values found in the literature. Furthermore we identify the horizontal branch and estimate the mean magnitude of the horizontal branch of the Hercules dSph galaxy to be V_0=21.17, which corresponds to a distance of 147 kpc. We have shown that a proper cleaning of the sample results in a smaller value for the velocity dispersion of the system. This has implications for galaxy properties derived from such velocity dispersions.

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arXiv2009

Kinematics of SDSS subdwarfs: Structure and substructure of the Milky Way halo

We construct a new sample of ~1700 solar neighbourhood halo subdwarfs from the Sloan Digital Sky Survey, selected using a reduced proper motion diagram. Radial velocities come from the SDSS spectra and proper motions from the light-motion curve catalogue of Bramich et al. (2008). Using a photometric parallax relation to estimate distances gives us the full phase-space coordinates. Typical velocity errors are in the range 30-50 km/s. This halo sample is one of the largest constructed to-date and the disc contamination is at a level of < 1 per cent. This enables us to calculate the halo velocity dispersion to excellent accuracy. We find that the velocity dispersion tensor is aligned in spherical polar coordinates and that (sigma_r, sigma_phi, sigma_theta) = (143 \pm 2, 82 \pm 2, 77 \pm 2) km/s. The stellar halo exhibits no net rotation, although the distribution of v_phi shows tentative evidence for asymmetry. The kinematics are consistent with a mildly flattened stellar density falling with distance like r^{-3.75}. Using the full phase-space coordinates, we look for signs of kinematic substructure in the stellar halo. We find evidence for four discrete overdensities localised in angular momentum and suggest that they may be possible accretion remnants. The most prominent is the solar neighbourhood stream previously identified by Helmi et al. (1999), but the remaining three are new. One of these overdensities is potentially associated with a group of four globular clusters (NGC5466, NGC6934, M2 and M13) and raises the possibility that these could have been accreted as part of a much larger progenitor.

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arXiv2009

The Origin of Segue 1

We apply the optimal filter technique to Sloan Digital Sky Survey photometry around Segue 1 and find that the outer parts of the cluster are distorted. There is strong evidence for ~ 1 degree elongations of extra-tidal stars, extending both eastwards and southwestwards of the cluster. The extensions have similar differential Hess diagrams to Segue 1. A Kolmogorov-Smirnov test suggests a high probability that both come from the same parent distribution. The location of Segue 1 is close to crossings of the tidal wraps of the Sagittarius stream. By extracting blue horizontal branch stars from Sloan's spectral database, two kinematic features are isolated and identified with different wraps of the Sagittarius stream. We show that Segue 1 is moving with a velocity that is close to one of the wraps. At this location, we estimate that there are enough Sagittarius stars, indistinguishable from Segue 1 stars, to inflate the velocity dispersion and hence the mass-to-light ratio. All the available evidence is consistent with the interpretation that Segue 1 is a star cluster, originally from the Sagittarius galaxy, and now dissolving in the Milky Way.

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arXiv2009

Substructure revealed by RR Lyraes in SDSS Stripe 82

We present an analysis of the substructure revealed by 407 RR Lyraes in Sloan Digital Sky Survey (SDSS) Stripe 82. Period estimates are determined to high accuracy using a string-length method. A subset of 178 RR Lyraes with spectrally derived metallicities are employed to derive metallicity-period-amplitude relations, which are then used to find metallicities and distances for the entire sample. The RR Lyraes lie between 5 and 115 kpc from the Galactic center. They are divided into subsets of 316 RRab types and 91 RRc types based on their period, colour and metallicity. The density distribution is not smooth, but dominated by clumps and substructure. Samples of 55 and 237 RR Lyraes associated with the Sagittarius Stream and the Hercules-Aquila Cloud respectively are identified. Hence, ~ 70 % of the RR Lyraes in Stripe 82 belong to known substructure. There is a sharp break in the density distribution at Galactocentric radii of 40 kpc, reflecting the fact that the dominant substructure in Stripe 82 - the Hercules-Aquila Cloud and the Sagittarius Stream - lies within 40 kpc. In fact, almost 60 % of all the RR Lyraes in Stripe 82 are associated with the Hercules-Aquila Cloud alone, which emphasises its pre-eminence. Additionally, evidence of a new and distant substructure - the Pisces Overdensity - is found, consisting of 28 faint RR Lyraes centered on Galactic coordinates (80 deg, -55 deg) and with distances of ~ 80 kpc. The total stellar mass in the Pisces Overdensity is ~10000 solar masses and its metallicity is [Fe/H] ~ -1.5.

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