1,356 research outputs found
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.Comment: Accepted for publication in ApJ. 11 pages 6 figure
Automated search for galactic star clusters in large multiband surveys: I. Discovery of 15 new open clusters in the Galactic anticenter region
Aims: According to some estimations, there are as many as 100000 open
clusters in the Galaxy, but less than 2000 of them have been discovered,
measured, and cataloged. We plan to undertake data mining of multiwavelength
surveys to find new star clusters. Methods: We have developed a new method to
search automatically for star clusters in very large stellar catalogs, which is
based on convolution with density functions. We have applied this method to a
subset of the Two Micron All Sky Survey catalog toward the Galactic anticenter.
We also developed a method to verify whether detected stellar groups are real
star clusters, which tests whether the stars that form the spatial density peak
also fall onto a single isochrone in the color-magnitude diagram. By fitting an
isochrone to the data, we estimate at the same time the main physical
parameters of a cluster: age, distance, color excess. Results: For the present
paper, we carried out a detailed analysis of 88 overdensity peaks detected in a
field of degrees near the Galactic anticenter. From this analysis,
15 overdensities were confirmed to be new open clusters and the physical and
structural parameters were determined for 12 of them; 10 of them were
previously suspected to be open clusters by Kronberger (2006) and Froebrich
(2007). The properties were also determined for 13 yet-unstudied known open
clusters, thus almost tripling the sample of open clusters with studied
parameters in the anticenter. The parameters determined with this method showed
a good agreement with published data for a set of well-known clusters.Comment: accepted to A&
Search for and investigation of new stellar clusters using the data from huge stellar catalogues
We present new automatic methods of search for star clusters using the data
available in new huge stellar catalogues. Using 2MASS catalogue we have
discovered over ten new open clusters in the region of Galaxy anticenter and
determined their physical parameters.Comment: Proceedings of the 79th Annual Scientific Meeting of the
Astronomische Gesellschaft, Cologne(Germany), September 200
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.Comment: Accepted for publication in ApJ
Automated search for star clusters in large multiband surveys: II. Discovery and investigation of open clusters in the Galactic plane
Automated search for star clusters in J,H,K_s data from 2MASS catalog has
been performed using the method developed by Koposov et. al (2008). We have
found and verified 153 new clusters in the interval of the galactic latitude
-24 < b < 24 degrees. Color excesses E(B-V), distance moduli and ages were
determined for 130 new and 14 yet-unstudied known clusters. In this paper, we
publish a catalog of coordinates, diameters, and main parameters of all the
clusters under study. A special web-site available at http://ocl.sai.msu.ru has
been developed to facilitate dissemination and scientific usage of the results.Comment: 9 pages, 3 tables, 5 figures, accepted to Astronomy Letter
Ages and abundances in large-scale stellar disks of nearby S0 galaxies
By undertaking deep long-slit spectroscopy with the focal reducer SCORPIO of
the Russian 6m telescope, we studied stellar population properties and their
variation with radius in 15 nearby S0 galaxies sampling a wide range of
luminosities and environments. For the large-scale stellar disks of S0s, we
have measured SSP-equivalent metallicities ranging from the solar one down to
[Z/H]=-0.4 - -0.7, rather high magnesium-to-iron ratios, [Mg/Fe] > +0.2, and
mostly old SSP-equivalent ages. Nine of 15 (60%) galaxies have large-scale
stellar disks older than 10 Gyr, and among those we find all the galaxies which
reside in denser environments. The isolated galaxies may have intermediate-age
stellar disks which are 7-9 Gyr old. Only two galaxies of our sample, NGC 4111
and NGC 7332, reveal SSP-equivalent ages of their disks of 2-3 Gyrs. Just these
two young disks appear to be thin, while the other, older disks have scale
heights typical for thick stellar disks. The stellar populations in the bulges
at radii of 0.5r_eff are on the contrary more metal-rich than the solar
metallicity, with the ages homogeneously distributed between 2 and 15 Gyr,
being almost always younger than the disks. We conclude that S0 galaxies could
not form in groups at z=0.4 as is thought now; a new scenario of the general
evolution of disk galaxies is proposed instead.Comment: Accepted to the MNRA
Exposure to Community Violence, Psychopathology, and Personality Traits in Russian Youth
Previous research with the US inner-city youth demonstrated the hazardous effects of community violence exposure. It remains unclear, however,
whether these findings are generalizable to other cultures and populations. Furthermore, the role of factors influencing the processing of traumatic events
such as personality has not been investigated. Two groups of Russian adolescents (community youth (N = 546) and male delinquents (N = 352)) completed
questionnaires assessing their exposure to community violence, conduct problems, internalizing psychopathology and personality. The study demonstrates
that the relationships between exposure to violence and psychopathology are similar across different populations within the same culture (community youth
and juvenile delinquents), suggesting similar mechanisms behind this phenomenon. The patterns of these relationships were also similar for boys and girls,
suggesting similarities in the mechanisms across gender. Hence, the effects of community violence exposure are generalizable to other cultures outside the US.
The associations between personality traits and specific types of behaviors also tend to be similar across different populations. Higher levels of novelty seeking
were related to more severe problem behaviors and to higher levels of witnessing and victimization, whereas higher levels of harm avoidance were related to higher
levels of depression and posttraumatic stress
Leo V: A Companion of a Companion of the Milky Way Galaxy
We report the discovery of a new Milky Way dwarf spheroidal galaxy in the
constellation of Leo identified in data from the Sloan Digital Sky Survey. Leo
V lies at a distance of about 180 kpc, and is separated by about 3 degrees from
another recent discovery, Leo IV. We present follow-up imaging from the Isaac
Newton Telescope and spectroscopy from the Hectochelle fiber spectrograph at
the Multiple Mirror Telescope. Leo V's heliocentric velocity is 173.4 km/s,
which is offset by about 40 km/s from that of Leo IV. A simple interpretation
of the kinematic data is that both objects may lie on the same stream, though
the implied orbit is only modestly eccentric (e = 0.2)Comment: Submitted to ApJ (Letters
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