arXiv subjects

D. Bizyaev

Publications and source records attributed to D. Bizyaev.

72 records · Page 4Linked to original sources

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↗

arXiv2010

Numerical Modelling of the Vertical Structure and Dark Halo Parameters in Disc Galaxies

The non-linear dynamics of bending instability and vertical structure of a galactic stellar disc embedded into a spherical halo are studied with N-body numerical modelling. Development of the bending instability in stellar galactic disc is considered as the main factor that increases the disc thickness. Correlation between the disc vertical scale height and the halo-to-disc mass ratio is predicted from the simulations. The method of assessment of the spherical-to-disc mass ratio for edge-on spiral galaxies with small bulges is considered. Modelling of eight edge-on galaxies: NGC 891, NGC 4738, NGC 5170, UGC 6080, UGC 7321, UGC 8286, UGC 9422 and UGC 9556 is performed. Parameters of stellar discs, dark haloes and bulges are estimated. The lower limit of the dark-to-luminous mass ratio in our galaxies is of the order of 1 within the limits of their stellar discs. The dark haloes dominate by mass in the galaxies with very thin stellar discs (NGC 5170, UGC 7321 and UGC 8286).

astro-ph.CO↗

arXiv2009

Optically Selected BL Lacertae Candidates from the Sloan Digital Sky Survey Data Release Seven

We present a sample of 723 optically selected BL Lac candidates from the SDSS DR7 spectroscopic database encompassing 8250 deg^2 of sky; our sample constitutes one of the largest uniform BL Lac samples yet derived. Each BL Lac candidate has a high-quality SDSS spectrum from which we determine spectroscopic redshifts for ~60% of the objects. Redshift lower limits are estimated for the remaining objects utilizing the lack of host galaxy flux contamination in their optical spectra; we find that objects lacking spectroscopic redshifts are likely at systematically higher redshifts. Approximately 80% of our BL Lac candidates match to a radio source in FIRST/NVSS, and ~40% match to a ROSAT X-ray source. The homogeneous multiwavelength coverage allows subdivision of the sample into 637 radio-loud BL Lac candidates and 86 weak-featured radio-quiet objects. The radio-loud objects broadly support the standard paradigm unifying BL Lac objects with beamed radio galaxies. We propose that the majority of the radio-quiet objects may be lower-redshift (z<2.2) analogs to high-redshift weak line quasars (i.e., AGN with unusually anemic broad emission line regions). These would constitute the largest sample of such objects, being of similar size and complementary in redshift to the samples of high-redshift weak line quasars previously discovered by the SDSS. However, some fraction of the weak-featured radio-quiet objects may instead populate a rare and extreme radio-weak tail of the much larger radio-loud BL Lac population. Serendipitous discoveries of unusual white dwarfs, high-redshift weak line quasars, and broad absorption line quasars with extreme continuum dropoffs blueward of rest-frame 2800 Angstroms are also briefly described.

astro-ph.CO↗

arXiv2009

The age of blue LSB galaxies

(Abridged). Low metallicities, large gas-to-star mass ratios, and blue colors of most low surface brightness (LSB) galaxies imply that these systems may be younger than their high surface brightness counterparts. We seek to find observational signatures that can help to constrain the age of blue LSB galaxies. We use numerical hydrodynamic modelling to study the long-term (~13 Gyr) dynamical and chemical evolution of blue LSB galaxies adopting a sporadic scenario for star formation. Our models utilize various rates of star formation and different shapes of the initial mass function (IMF). We complement hydrodynamic modelling with population synthesis modelling to produce the integrated B-V colors and Halpha equivalent widths (EW(Ha)). We find that the mean oxygen abundances, B-V colors, EW(Ha), and the radial fluctuations in the oxygen abundance, when considered altogether, can be used to constrain the age of blue LSB galaxies if some independent knowledge of the IMF is available. Our modelling strongly suggests the existence of a minimum age for blue LSB galaxies. Model B-V colors and mean oxygen abundances set a tentative minimum age at 1.5-3.0 Gyr, whereas model EW(Ha) suggest a larger value of order 5-6 Gyr. The latter value may decrease somewhat, if blue LSB galaxies host IMFs with a truncated upper mass limit. We found no firm evidence that the age of blue LSB galaxies is significantly smaller than 13 Gyr.

astro-ph.CO↗

arXiv2009

Halo streams in the 7th SDSS data release

We have detected stellar halo streams in the solar neighborhood using data from the 7th public data release of the Sloan Digital Sky Survey (SDSS), which includes the directed stellar program SEGUE: Sloan Extension For Galactic Understanding and Exploration. In order to derive distances to each star, we used the metallicity-dependent photometric parallax relation from Ivezic et al. (2008) for which we examine and quantify the accuracy. Our final sample consists of 22,321 nearby (d < 2 kpc), metal-poor ([Fe/H] < -0.5) main-sequence stars with 6D estimates of position and space velocity. We characterize the orbits of these stars through suitable kinematic proxies for their "effective" integrals of motion, angular momentum, eccentricity, and orbital polar angle and compare the observed distribution to expectations from a smooth distribution in four [Fe/H] bins. On this basis we identify at least five significant "phase-space overdensities" of stars on very similar orbits in the solar neighborhood to which we can assign unambiguously peaked [Fe/H] distributions. Three of them have been identified previously, including the halo stream discovered by Helmi et al. (1999) at a significance level of 12.0. In addition, we find at least two new genuine halo streams, judged by their kinematics and [Fe/H], at significance levels of 2.9 and 4.8, respectively. For one stream the stars even show coherence in configuration space, matching a spatial overdensity of stars found by Juric et al. (2008) at (R,z) \approx (9.5,0.8) kpc. Our results demonstrate the practical power of our search method to detect substructure in the phase-space distribution of nearby stars without making a-priori assumptions about the detailed form of the gravitational potential.

astro-ph.GA↗

arXiv2009

The kinematics of late type stars in the solar cylinder studied with SDSS data

We study the velocity distribution of Milky Way disk stars in a kiloparsec-sized region around the Sun, based on ~ 2 million M-type stars from DR7 of SDSS, which have newly re-calibrated absolute proper motions from combining SDSS positions with the USNO-B catalogue. We estimate photometric distances to all stars, accurate to ~ 20 %, and combine them with the proper motions to derive tangential velocities for this kinematically unbiased sample of stars. Based on a statistical de-projection method we then derive the vertical profiles (to heights of Z = 800 pc above the disk plane) for the first and second moments of the three dimensional stellar velocity distribution. We find that = -7 +/- 1 km/s and = -9 +/- 1 km/s, independent of height above the mid-plane, reflecting the Sun's motion with respect to the local standard of rest. In contrast, changes distinctly from -20 +/- 2 km/s in the mid-plane to = -32 km/s at Z = 800 pc, reflecting an asymmetric drift of the stellar mean velocity that increases with height. All three components of the M-star velocity dispersion show a strong linear rise away from the mid-plane, most notably σ_{ZZ}, which grows from 18 km/s (Z = 0) to 40 km/s (at Z = 800 pc). We determine the orientation of the velocity ellipsoid, and find a significant vertex deviation of 20 to 25 degrees, which decreases only slightly to heights of Z = 800 pc. Away from the mid-plane, our sample exhibits a remarkably large tilt of the velocity ellipsoid towards the Galactic plane, which reaches 20 deg. at Z = 800 pc and which is not easily explained. Finally, we determine the ratio σ^2_{ϕϕ}/σ^2_{RR} near the mid-plane, which in the epicyclic approximation implies an almost perfectly flat rotation curve at the Solar radius.

astro-ph.GA↗

arXiv2008

Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey

We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system -- its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310-00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk.

astro-ph↗

arXiv2008

Light and Motion in SDSS Stripe 82: The Catalogues

We present a new public archive of light-motion curves in Sloan Digital Sky Survey (SDSS) Stripe 82, covering 99 deg in right ascension from RA = 20.7 h to 3.3 h and spanning 2.52 deg in declination from Dec = -1.26 to 1.26 deg, for a total sky area of ~249 sq deg. Stripe 82 has been repeatedly monitored in the u, g, r, i and z bands over a seven-year baseline. Objects are cross-matched between runs, taking into account the effects of any proper motion. The resulting catalogue contains almost 4 million light-motion curves of stellar objects and galaxies. The photometry are recalibrated to correct for varying photometric zeropoints, achieving ~20 mmag and ~30 mmag root-mean-square (RMS) accuracy down to 18 mag in the g, r, i and z bands for point sources and extended sources, respectively. The astrometry are recalibrated to correct for inherent systematic errors in the SDSS astrometric solutions, achieving ~32 mas and ~35 mas RMS accuracy down to 18 mag for point sources and extended sources, respectively. For each light-motion curve, 229 photometric and astrometric quantities are derived and stored in a higher-level catalogue. On the photometric side, these include mean exponential and PSF magnitudes along with uncertainties, RMS scatter, chi^2 per degree of freedom, various magnitude distribution percentiles, object type (stellar or galaxy), and eclipse, Stetson and Vidrih variability indices. On the astrometric side, these quantities include mean positions, proper motions as well as their uncertainties and chi^2 per degree of freedom. The here presented light-motion curve catalogue is complete down to r~21.5 and is at present the deepest large-area photometric and astrometric variability catalogue available.

astro-ph↗

arXiv2006

Modeling the star formation history in the ring galaxy Arp 10 with the help of spectral indices

We use long-slit spectra obtained with the 6-m telescope BTA (Russian Academy of Sciences) to investigate the history of star formation in the peculiar ring galaxy Arp 10. The radial distributions of quasi-Lick spectral indices are calculated from the observed spectra and are compared with the model spectra, the latter are generated using the Starburst99 population synthesis code. Our model includes an outward propagating density wave which triggers star formation in the gas disk, and an old stellar disk. The metallicity of both old and young stellar populations is a function of radius. We show that a mix of the young and old populations is required to explain the radial distribution of the spectral indices. The density wave propagates outward with a moderate speed (of order 40 km/s), and the metallicities of both young and old populations decrease with the radius. The model indices corresponding to the alpha-elements require somewhat higher metallicities as compared to the Fe-peak elements. We acknowledge partial support from grant RFBR 04-02-16518.

astro-ph↗

arXiv2006

Structural Parameters of Stellar Disks in Edge-on Galaxies from 2MASS Images

We analyze the J, H, and Ks 2MASS images of 140 late-type edge-on galaxies selected from the RFGC catalog (which contains flat galaxies with major-to-minor axis ratio a/b > 7). The NIR scalelengths (h) and scaleheights (z0) of the stellar disks are determined for all selected galaxies. The mean relative ratios of the scaleheights of their stellar disks are 1.00:0.91:0.86 in J:H:Ks bands, respectively. We infer that the scaleheight determined from the Ks-band images is, on average, 13% larger than the extinction-free scaleheight. This difference is much larger if the scaleheights were found from the optical-band images. The relative thickness (z0/h) of the stellar disks correlates well with their deprojected central surface brightness obtained from the 2MASS images. This project was partially supported by grant RFBR 04-02-16518.

astro-ph↗

arXiv2005

Photometric observations of Grid Giant Star Survey candidates

We compare variability of radial velocity (RV) and brightness for a sample of red giants which were selected as candidates for the Grid Giant Star Survey (GGSS). The purpose of the survey is to select astrometrically stable stars as a reference frame for the future Space Interferometry Mission. We incorporate photometric data taken by ROTSE-I and being taken by ROTSE-III robotic telescopes. Typically 2/3 of all GGSS stars to the north of DEC=-35 were observed 76 and 8 times by ROTSE-I and III respectively in white-light filters. It is shown that photometric stability at the 0.02 mag level cannot be used for a pre-selection of RV-stable stars, and the most important tool remains high-resolution spectroscopy. We deeply thank the ROTSE collaboration providing us observing time at the ROTSE-IIIb telescope. We gratefully acknowledge NASA grant NRA-99-04-OSS-058.

astro-ph↗

arXiv2005

The detection of non-thermal radio continuum spokes and the study of star formation in the Cartwheel

New sensitive Very Large Array 20 cm continuum observations of the Cartwheel, the prototypical collisional ring galaxy, were carried out with the principal aim of tracing supernova remnants that are expected to lie in the wake of the expanding ring and in the ring itself. We detect predominantly non-thermal radio continuum emission from regions associated with 13 ring HII complexes. The emission interior to the ring is confined to structures that resemble spokes of the wheel. The spokes start near bright HII complexes, and extend to around 6 arcsec (4 kpc) inward in the direction of the geometrical center of the ring. There is no apparent positional coincidence between the radio continuum and optical spokes. Radial distribution of intensity along the spokes suggests that the past star formation rate (SFR) in the Cartwheel was much lower than the current SFR. New Halpha observations were used to revise the current SFR in the Cartwheel. The revised value is 18 Msun/yr, which is a factor of 4 lower than the value reported previously, but is in good agreement with the SFR estimated from far infrared luminosity. About 30% of the observed 20 cm continuum non-thermal emission seems to originate in processes that are not related to star formation. Revised SFR in the Cartwheel is comparable to that in the rest of the ring galaxies.

astro-ph↗

arXiv2003

The Early Optical Afterglow of GRB 030418 and Progenitor Mass Loss

The ROTSE-IIIa telescope and the SSO-40 inch telescope, both located at Siding Spring Observatory, imaged the early time afterglow of GRB 030418. In this report we present observations of the early afterglow, first detected by the ROTSE-IIIa telescope 211 s after the start of the burst, and only 76 s after the end of the gamma-ray activity. We detect optical emission that rises for ~600 s, slowly varies around R=17.3 mag for ~1400 s, and then fades as a power law of index alpha=-1.36. Additionally, the ROTSE-IIIb telescope, located at McDonald Observatory, imaged the early time afterglow of GRB 030723. The behavior of this light curve was qualitatively similar to that of GRB 030418, but two magnitudes dimmer. These two afterglows are dissimilar to other afterglows such as GRB 990123 and GRB 021211. We investigate whether the early afterglow can be attributed to a synchrotron break in a cooling synchrotron spectrum as it passes through the optical band, but find this model is unable to accurately describe the early light curve. We present a simple model for gamma-ray burst emission emerging from a wind medium surrounding a massive progenitor star. This model provides an effective description of the data, and suggests that the rise of the afterglow can be ascribed to extinction in the local circumburst environment. In this interpretation, these events provide further evidence for the connection between gamma-ray bursts and the collapse of massive stars.

astro-ph↗

arXiv2003

ROTSE-III Observations of the Early Afterglow From GRB 030329

Using two identical telescopes at widely separated longitudes, the ROTSE-III network observed decaying emission from the remarkably bright afterglow of GRB 030329. In this report we present observations covering 56% of the period from 1.5-47 hours after the burst. We find that the light curve is piecewise consistent with a powerlaw decay. When the ROTSE-III data are combined with data reported by other groups, there is evidence for five breaks within the first 20 hours after the burst. Between two of those breaks, observations from 15.9-17.1 h after the burst at 1-s time resolution with McDonald Observatory's 2.1-m telescope reveal no evidence for fluctuations or deviations from a simple power law. Multiple breaks may indicate complex structure in the jet. There are also two unambiguous episodes at 23 and 45 hours after the burst where the intensity becomes consistent with a constant for several hours, perhaps indicating multiple injections of energy into the GRB/afterglow system.

astro-ph↗

arXiv2003

The galactic disk thickness and the mass of the spherical component

We present results of numerical modeling made for the galactic stellar disk embedded in the spherical halo. The non-linear dynamics of bending instabilities developed in the disk is studied. The axisymmetrical bending mode is considered as a main factor increasing the disk thickness. The model dependence between the disk thickness and the halo to disk mass ratio $M_h/M_d$ is inferred. The modeling results enable us to estimate the dark halo mass for several edge-on galaxies. The obtained values of $M_h/M_d$ for the galaxies considered are of order 1 - 2.

astro-ph↗

arXiv2003

Propagation velocities of gas rings in collisional ring galaxies

The propagation velocity of the first gas ring in collisional ring galaxies, i.e. the velocity at which the maximum in the radial gas density profile propagates radially in the galactic disk, is usually inferred from the radial expansion velocity of gas in the first ring. Our numerical hydrodynamics modeling of ring galaxy formation however shows that the maximum radial expansion velocity of gas in the first ring ($v_{gas}$) is invariably below the propagation velocity of the first gas ring itself ($v_{ring}$). Modeling of the Cartwheel galaxy indicates that the outer ring is currently propagating at $v_{ring} \approx$ 100 km/s, while the maximum radial expansion velocity of gas in the outer ring is currently $v_{gas} \approx$ 65 km/s. Modeling of the radial B-V/V-K color gradients of the Cartwheel ring galaxy also indicates that the outer ring is propagating at $v_{ring} \ge $ 90 km/s. We show that a combined effect of inclination, finite thickness, and warping of the Cartwheel's disk might be responsible for the lack of angular difference in the peak positions found for the azimuthally averaged $Hα$, K and B surface brightness profiles of the Cartwheel's outer ring. Indeed, the radial $Hα$ surface brightness profiles obtained along the Cartwheel's major axis, where effects of inclination and finite thickness are minimized, do peak exterior to those at K- and B-bands. The angular difference in peak positions implies $v_{ring}$ = 110 km/s, which is in agreement with the model predictions. We briefly discuss the utility of radio continuum emission and spectral line equivalent widths for determining the propagation velocity of gas rings in collisional ring galaxies.

astro-ph↗

arXiv2000

Photometric deprojection of edge-on galaxies

Photometric deprojection is used to determine the stellardisk and bulge parameters for several edgeon galaxies from the FGC catalog. The assumption that the galaxies of our sample belonging to the fourth (i.e., lowest) surfacebrightness class in the FGC are edgeon, lowsurfacebrightness (LSB) galaxies is considered.

astro-ph↗

arXiv2000

Combined color indexes and photometric structure of galaxies NGC 834 and NGC 1134

We present the results of BVRI photometry of two galaxies with active star formation: NGC 834 and NGC 1134. Combined color index Q_{BVI} was used to investigate the photometrical structure of the galaxies. Index Q_{BVI} is not affected by internal extinction and is sensitive to the presence of blue stars. Ring-like region with active star formation at 15" from the center reveals itself in the Q_{BVI} map of NGC 834. Three-arm spiral structure is well-seen on the Q_{BVI} map of NGC 1134. We propose to use the combined indexes Q_{BVI} and similarly defined indices as a tracers of Star Formation activity and structure of dusty galaxies.

astro-ph↗