arXiv subjects

Sergey E. Koposov

Publications and source records attributed to Sergey E. Koposov.

At least 145 records · Page 8Linked to original sources

arXiv2016

A 10 kpc stellar substructure at the edge of the Large Magellanic Cloud: perturbed outer disk or evidence for tidal stripping?

We report the discovery of a substantial stellar overdensity in the periphery of the Large Magellanic Cloud (LMC), found using public imaging from the first year of the Dark Energy Survey. The structure appears to emanate from the edge of the outer LMC disk at a radius $\approx 13.5$ degrees due north of its centre, and stretches more than $10$ kpc towards the east. It is roughly $1.5$ kpc wide and has an integrated $V$-band luminosity of at least $M_V = -7.4$. The stellar populations in the feature are indistinguishable from those in the outer LMC disk. We attempt to quantify the geometry of the outer disk using simple planar models, and find that only a disk with mild intrinsic ellipticity can simultaneously explain the observed stellar density on the sky and the azimuthal line-of-sight distance profile. We also see possible non-planar behaviour in the outer disk that may reflect a warp and/or flare, as well as deviations that resemble a ring-like structure between $\sim9-12$ degrees from the LMC centre. Based on all these observations, we conclude that our remote, stream-like feature is likely comprised of material that has been stripped from the outskirts of the LMC disk, although we cannot rule out that it represents a transient overdensity in the disk itself. We conduct a simple $N$-body simulation to show that either type of structure could plausibly arise due to the tidal force of the Milky Way; however we also recognize that a recent close interaction between the LMC and the SMC may be the source of the stripping or perturbation. Finally, we observe evidence for extremely diffuse LMC populations extending to radii of $\sim 18.5$ kpc in the disk plane ($\approx 20$ degrees on the sky), corroborating previous spectroscopic detections at comparable distances.

astro-ph.GA↗

arXiv2015

Magellan/M2FS Spectroscopy of Tucana 2 and Grus 1

We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of $147$ stellar targets along the line of sight to the newly-discovered `ultrafaint' stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru 1). Based on simultaneous estimates of line-of-sight velocity and stellar-atmospheric parameters, we identify 8 and 7 stars as probable members of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to resolve an internal velocity dispersion of $8.6_{-2.7}^{+4.4}$ km s$^{-1}$ about a mean of $-129.1_{-3.5}^{+3.5}$ km s$^{-1}$ (solar rest frame), and to estimate a mean metallicity of [Fe/H]= $-2.23_{-0.12}^{+0.18}$. These results place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies, suggesting a dominant dark matter component with dynamical mass $2.7_{-1.3}^{+3.1}\times 10^6$ $\mathrm{M}_{\odot}$ enclosed within the central $\sim 160$ pc, and dynamical mass-to-light ratio $1900_{-900}^{+2200}$ $\mathrm{M}_{\odot}/L_{V,\odot}$. For Gru 1 we estimate a mean velocity of $-140.5_{-1.6}^{+2.4}$ km s$^{-1}$ and a mean metallicity of [Fe/H]=$-1.42_{-0.42}^{+0.55}$, but our sample does not resolve Gru 1's velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic phase space suggest that their orbits are among the most energetic within distance $\leq 300$ kpc. Moreover, their proximity to each other in this space arises naturally if both objects are trailing the Large Magellanic Cloud.

astro-ph.GA↗

arXiv2015

A Hubble Space Telescope Study of the Enigmatic Milky Way Halo Globular Cluster Crater

We analyze the resolved stellar populations of the faint stellar system, Crater, based on deep optical imaging taken with the Hubble Space Telescope. The HST/ACS-based color-magnitude diagram (CMD) of Crater extends $\sim$4 magnitudes below the oldest main sequence turnoff, providing excellent leverage on Crater's physical properties. Structurally, Crater has a half-light radius of $\sim$20 pc and shows no evidence for tidal distortions. Crater is well-described by a simple stellar population with an age of $\sim$7.5 Gyr, [M/H]$\sim-1.65$, a M$_{\star}\sim10^4$ M$_{\odot}$, M$_{\rm V}\sim -5.3$, located at a distance of d$\sim$ 145 kpc, with modest uncertainties in these properties due to differences in the underlying stellar evolution models. The sparse sampling of stars above the turnoff and sub-giant branch are likely to be 1.0-1.4 M$_{\odot}$ binary star systems (blue stragglers) and their evolved descendants, as opposed to intermediate age main sequence stars. Confusion of these populations highlights a substantial challenge in accurately characterizing sparsely populated stellar systems. Our analysis shows that Crater is not a dwarf galaxy, but instead is an unusually young cluster given its location in the Milky Way's very outer stellar halo. Crater is similar to SMC cluster Lindsay 38, and its position and velocity are in good agreement with observations and models of the Magellanic stream debris, suggesting it may have accreted from the Magellanic Clouds. However, its age and metallicity are also in agreement with the age-metallicity relationships of lower mass dwarf galaxies such as Leo I or Carina. Despite uncertainty over its progenitor system, Crater appears to have been incorporated into the Galaxy more recently than $z\sim1$ (8 Gyr ago), providing an important new constraint on the accretion history of the Milky Way. [abridged]

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arXiv2015

Indication of Gamma-ray Emission from the Newly Discovered Dwarf Galaxy Reticulum II

We present a search for gamma-ray emission from the direction of the newly discovered dwarf galaxy Reticulum II. Using Fermi-LAT data, we detect a signal that exceeds expected backgrounds between ~2-10 GeV and is consistent with annihilation of dark matter for particle masses less than a few x 10^2 GeV. Modeling the background as a Poisson process based on Fermi-LAT diffuse models, and taking into account trials factors, we detect emission with p-value less than 9.8 x 10^-5 (>3.7 sigma). An alternative, model-independent treatment of background reduces the significance, raising the p-value to 9.7 x 10^-3 (2.3 sigma). Even in this case, however, Reticulum II has the most significant gamma-ray signal of any known dwarf galaxy. If Reticulum II has a dark matter halo that is similar to those inferred for other nearby dwarfs, the signal is consistent with the s-wave relic abundance cross section for annihilation.

astro-ph.HE↗

arXiv2015

Kinematics and chemistry of recently discovered Reticulum 2 and Horologium 1 dwarf galaxies

Photometry alone is not sufficient to unambiguously distinguish between ultra-faint star clusters and dwarf galaxies because of their overlap in morphological properties. Here we report on VLT/GIRAFFE spectra of candidate member stars in two recently discovered ultra-faint satellites Reticulum 2 and Horologium 1, obtained as part of the ongoing Gaia-ESO Survey. We identify 18 members in Reticulum 2 and 5 in Horologium 1. We find Reticulum 2 to have a velocity dispersion of ~3.22 km/s, implying a M/L ratio of ~ 500. We have inferred stellar parameters for all candidates and we find Reticulum 2 to have a mean metallicity of [Fe/H] = -2.46+/-0.1, with an intrinsic dispersion of ~ 0.29, and is alpha-enhanced to the level of [alpha/Fe]~0.4. We conclude that Reticulum 2 is a dwarf galaxy. We also report on the serendipitous discovery of four stars in a previously unknown stellar substructure near Reticulum 2 with [Fe/H] ~ -2 and V_hel ~ 220 km/s, far from the systemic velocity of Reticulum 2. For Horologium 1 we infer a velocity dispersion of 4.9^{+2.8}_{-0.9} km/s and a consequent M/L ratio of ~ 600, leading us to conclude that Horologium 1 is also a dwarf galaxy. Horologium 1 is slightly more metal-poor than Reticulum 2 [Fe/H] = -2.76 +/- 0.1 and is similarly alpha-enhanced: [alpha/Fe] ~ 0.3. Despite a large error-bar, we also measure a significant spread of metallicities of 0.17 dex which strengthen the evidence that Horologium 1 is indeed a dwarf galaxy. The line-of-sight velocity of Reticulum 2 is offset by some 100 km/s from the prediction of the orbital velocity of the LMC, thus making its association with the Cloud uncertain. However, at the location of Horologium 1, both the backward integrated LMC's orbit and the LMC's halo are predicted to have radial velocities similar to that of the dwarf. Therefore, it is likely that Horologium 1 is or once was a member of the Magellanic Family.

astro-ph.GA↗

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

Beasts of the Southern Wild: Discovery of nine Ultra Faint satellites in the vicinity of the Magellanic Clouds

We have used the publicly released Dark Energy Survey data to hunt for new satellites of the Milky Way in the Southern hemisphere. Our search yielded a large number of promising candidates. In this paper, we announce the discovery of 9 new unambiguous ultra-faint objects, whose authenticity can be established with the DES data alone. Based on the morphological properties, three of the new satellites are dwarf galaxies, one of which is located at the very outskirts of the Milky Way, at a distance of 380 kpc. The remaining 6 objects have sizes and luminosities comparable to the Segue~1 satellite and can not be classified straightforwardly without follow-up spectroscopic observations. The satellites we have discovered cluster around the LMC and the SMC. We show that such spatial distribution is unlikely under the assumption of isotropy, and, therefore, conclude that at least some of the new satellites must have been associated with the Magellanic Clouds in the past.

astro-ph.GA↗

arXiv2014

Balancing mass and momentum in the Local Group

In the rest frame of the Local Group (LG), the total momentum of the Milky Way (MW) and Andromeda (M31) should balance to zero. We use this fact to constrain new solutions for the solar motion with respect to the LG centre-of-mass, the total mass of the LG, and the individual masses of M31 and the MW. Using the set of remote LG galaxies at $>350$ kpc from the MW and M31, we find that the solar motion has amplitude $V_{\odot}=299\pm 15 {\rm ~km~s^{-1}}$ in a direction pointing toward galactic longitude $l_{\odot}=98.4^{\circ}\pm 3.6^{\circ}$ and galactic latitude $b_{\odot}=-5.9^{\circ}\pm 3.0^{\circ}$. The velocities of M31 and the MW in this rest frame give a direct measurement of their mass ratio, for which we find $\log_{10} (M_{\rm M31}/M_{\rm MW})=0.36 \pm 0.29$. We combine these measurements with the virial theorem to estimate the total mass within the LG as $M_{\rm LG}=(2.5\pm 0.4)\times 10^{12}~{\rm M}_{\odot}$. Our value for $M_{\rm LG}$ is consistent with the sum of literature values for $M_{\rm MW}$ and $M_{\rm M31}$. This suggests that the mass of the LG is almost entirely located within the two largest galaxies rather than being dispersed on larger scales or in a background medium. The outskirts of the LG are seemingly rather empty. Combining our measurement for $M_{\rm LG}$ and the mass ratio, we estimate the individual masses of the MW and M31 to be $M_{\rm MW}=(0.8\pm 0.5)\times 10^{12}~{\rm M}_{\odot}$ and $M_{\rm M31}=(1.7\pm 0.3)\times 10^{12}~{\rm M}_{\odot}$, respectively. Our analysis favours M31 being more massive than the MW by a factor of $\sim$2.3, and the uncertainties allow only a small probability (9.8%) that the MW is more massive. This is consistent with other properties such as the maximum rotational velocities, total stellar content, and numbers of globular clusters and dwarf satellites, which all suggest that $M_{\rm M31}/M_{\rm MW}>1$.

astro-ph.GA↗

arXiv2014

GS-TEC: the Gaia Spectrophotometry Transient Events Classifier

We present an algorithm for classifying the nearby transient objects detected by the Gaia satellite. The algorithm will use the low-resolution spectra from the blue and red spectro-photometers on board of the satellite. Taking a Bayesian approach we model the spectra using the newly constructed reference spectral library and literature-driven priors. We find that for magnitudes brighter than 19 in Gaia $G$ magnitude, around 75\% of the transients will be robustly classified. The efficiency of the algorithm for SNe type I is higher than 80\% for magnitudes $G\leq$18, dropping to approximately 60\% at magnitude $G$=19. For SNe type II, the efficiency varies from 75 to 60\% for $G\leq$18, falling to 50\% at $G$=19. The purity of our classifier is around 95\% for SNe type I for all magnitudes. For SNe type II it is over 90\% for objects with $G \leq$19. GS-TEC also estimates the redshifts with errors of $σ_z \le$ 0.01 and epochs with uncertainties $σ_t \simeq$ 13 and 32 days for type SNe I and SNe II respectively. GS-TEC has been designed to be used on partially calibrated Gaia data. However, the concept could be extended to other kinds of low resolution spectra classification for ongoing surveys.

astro-ph.IM↗

arXiv2014

Touching The Void: A Striking Drop in Stellar Halo Density Beyond 50 kpc

We use A-type stars selected from Sloan Digital Sky Survey data release 9 photometry to measure the outer slope of the Milky Way stellar halo density profile beyond 50 kpc. A likelihood-based analysis is employed that models the ugr photometry distribution of blue horizontal branch (BHB) and blue straggler (BS) stars. In the magnitude range, 18.5 < g < 20.5, these stellar populations span a heliocentric distance range of: 10 kpc < D_BS < 75 kpc, 40 kpc < D_BHB < 100 kpc. Contributions from contaminants, such as QSOs, and the effect of photometric uncertainties, are also included in our modeling procedure. We find evidence for a very steep outer halo profile, with power-law index alpha ~ 6 beyond Galactocentric radii r=50 kpc, and even steeper slopes favored (alpha ~ 6-10) at larger radii. This result holds true when stars belonging to known overdensities, such as the Sagittarius stream, are included or excluded. We show that, by comparison to numerical simulations, stellar halos with shallower slopes at large distances tend to have more recent accretion activity. Thus, it is likely that the Milky Way has undergone a relatively quiet accretion history over the past several Gyr. Our measurement of the outer stellar halo profile may have important implications for dynamical mass models of the Milky Way, where the tracer density profile is strongly degenerate with total mass-estimates.

astro-ph.GA↗

arXiv2014

Strong RR Lyrae excess in the Hercules-Aquila Cloud

We map the large-scale sub-structure in the Galactic stellar halo using accurate 3D positions of ~14,000 RR Lyrae reported by the Catalina Sky Survey. In the heliocentric distance range of 10-25 kpc, in the region of the sky approximately bounded by 30° < l < 55° and -45° < b < -25°, there appears to be a strong excess of RRab stars. This overdensity, peaking at 18 kpc, is most likely associated with the so-called Hercules-Aquila Cloud, previously detected using Main Sequence tracers at similar distances in the Sloan Digital Sky Survey data. Our analysis of the period-amplitude distribution of RR Lyrae in this region indicates that the HAC is dominated by the Oosterhoff I type population. By comparing the measured RR Lyrae number density to models of a smooth stellar halo, we estimate the significance of the observed excess and provide an updated estimate of the total luminosity of the Cloud's progenitor.

astro-ph.GA↗

arXiv2013

Sagittarius stream 3-d kinematics from SDSS Stripe 82

Using multi-epoch observations of the Stripe 82 region done by Sloan Digital Sky Survey, we measure precise statistical proper motions of the stars in the Sagittarius stellar stream. The multi-band photometry and SDSS radial velocities allow us to efficiently select Sgr members and thus enhance the proper motion precision to ~0.1 mas/yr. We measure separately the proper motion of a photometrically selected sample of the main sequence turn-off stars, as well as of a spectroscopically selected Sgr giants. The data allow us to determine the proper motion separately for the two Sgr streams in the South found in Koposov et al.(2012). Together with the precise velocities from SDSS, our proper motion provide exquisite constraints of the 3-D motions of the stars in the Sgr streams.

astro-ph.GA↗

arXiv2012

Photometric study of open clusters Berkeley 96, Berkeley 97, King 12, NGC 7261, NGC 7296, and NGC 7788

The BVRcIc CCD photometry in the fields of six open clusters toward the Perseus spiral arm is presented. These data along with JHKs magnitudes taken from 2MASS catalog, have been used to determine cluster's ages, distances and color excesses. In addition, the gaps in mass function of Be 97, King 12 and NGC 7788 clusters have been revealed in mass intervals of 1.3-1.5, 1.4-1.6 and 1.5-1.7 solar masses, respectively.

astro-ph.GA↗

arXiv2012

Hyper-luminous Reddened Broad-Line Quasars at z~2 from the VISTA Hemisphere Survey and WISE All Sky Survey

We present the first sample of spectroscopically confirmed heavily reddened broad-line quasars selected using the new near infra-red VISTA Hemisphere Survey and WISE All-Sky Survey. Observations of four candidates with (J-K)>2.5 and K<=16.5 over ~180 sq deg, leads to confirmation that two are highly dust-reddened broad-line Type 1 quasars at z~2. The typical dust extinctions are AV ~ 2-2.5 mags. We measure black-hole masses of ~10^9 M0 and extinction corrected bolometric luminosities of ~10^47 erg/s, making these among the brightest Type 1 quasars currently known. Despite this, these quasars lie well below the detection limits of wide-field optical surveys like the SDSS with iAB > 22. We also present WISE photometry at 3--22um, for our full sample of spectroscopically confirmed reddened quasars including those selected from the UKIDSS Large Area Survey (Banerji et al. 2012a). We demonstrate that the rest-frame infrared SEDs of these reddened quasars are similar to UV-luminous Type 1 quasars with significant hot dust emission and starburst quasar hosts like Mrk231. The average 12um flux density of our reddened quasars is similar to that of the recently discovered HyLIRG WISE1814+3412 (z=2.452) at similar redshifts, with two of our reddened quasars also having comparable 22um flux densities to this extreme HyLIRG. These optically faint, heavily reddened broad-line quasars are therefore among the most mid infrared luminous galaxies at z~2, now being discovered using WISE.

astro-ph.CO↗

arXiv2012

The coupling between the core/cusp and missing satellite problems

We calculate the energy that baryons must inject in cold dark matter (CDM) haloes in order to remove centrally-divergent DM cusps on scales relevant to observations of dwarf spheroidal galaxies (dSphs). We estimate that the CDM haloes often associated with the Milky Way's dSphs (M_vir/M_\odot \sim 10^{9-10}) require ΔE/erg \sim 10^{53-55} in order to form cores on scales comparable to the luminous size of these galaxies. While supernova type II (SNeII) explosions can in principle generate this energy, the actual contribution is limited by the low star formation efficiency implied by the abundance of luminous satellites. Considering that CDM's well-known `core/cusp' and `missing satellite' problems place opposing demands on star formation efficiencies, existing observational evidences for large cores in the most luminous dSphs require that CDM models invoke some combination of the following: (i) efficient (of order unity) coupling of SNeII energy into dark matter particles, (ii) star formation histories peaking at unexpectedly high redshifts (z>6), (iii) a top-heavy stellar IMF, and/or (iv) substantial satellite disruption or other stochastic effects to ease the substructure abundance constraints. Our models show that the tension between CDM problems on small scales would increase if cored DM profiles were to be found in fainter dwarves.

astro-ph.GA↗

arXiv2012

A statistical method for measuring the Galactic potential and testing gravity with cold tidal streams

We introduce the Minimum Entropy Method, a simple statistical technique for constraining the Milky Way gravitational potential and simultaneously testing different gravity theories directly from 6D phase-space surveys and without adopting dynamical models. We demonstrate that orbital energy distributions that are separable (i.e. independent of position) have an associated entropy that increases under wrong assumptions about the gravitational potential and/or gravity theory. Of known objects, `cold' tidal streams from low-mass progenitors follow orbital distributions that most nearly satisfy the condition of separability. Although the orbits of tidally stripped stars are perturbed by the progenitor's self-gravity, systematic variations of the energy distribution can be quantified in terms of the cross-entropy of individual tails, giving further sensitivity to theoretical biases in the host potential. The feasibility of using the Minimum Entropy Method to test a wide range of gravity theories is illustrated by evolving restricted N-body models in a Newtonian potential and examining the changes in entropy introduced by Dirac, MONDian and f(R) gravity modifications.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2011

Quantifying Kinematic Substructure in the Milky Way's Stellar Halo

We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative "close pair distribution" (CPD) as a statistic in the 4-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock-observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at $\rm r_{gc} < 20$ kpc.

astro-ph.GA↗