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1.
Archival Hubble Space Telescope (HST) data have been used for the photometry of stars in blue compact dwarf (BCD) galaxies of the IZw18 system. Applying the spatial selection of stars, we have detected red giants, stars older than 1–2 Gyr, in the galaxies. These red giants have allowed the distance to IZw18 to be reliably determined for the first time: D = 13.9 ± 1.2 Mpc. The presence of old stars in the galaxies of the IZw18 system refutes the hypothesis about the observed primary star formation in these galaxies.  相似文献   

2.
We present the results of our stellar photometry and spectroscopy for the new Local Group galaxy VV124 (UGC4879) obtainedwith the 6-m BTAtelescope. The presence of a fewbright supergiants in the galaxy indicates that the current star formation process is weak. The apparent distribution of stars with different ages in VV 124 does not differ from the analogous distributions of stars in irregular galaxies, but the ratio of the numbers of young and old stars indicates that VV 124 belongs to the rare Irr/Sph type of galaxies. The old stars (red giants) form the most extended structure, a thick disk with an exponential decrease in the star number density to the edge. Definitely, the young population unresolvable in images makes a great contribution to the background emission from the central galactic regions. The presence of young stars is also confirmed by the [OIII] emission line visible in the spectra that belongs to extensive diffuse galactic regions. The mean radial velocity of several components (two bright supergiants, the unresolvable stellar population, and the diffuse gas) is υ h = −70 ± 15 km s−1 and the velocity with which VV 124 falls into the Local Group is υ LG = −12 ± 15 km s−1. We confirm the distance to the galaxy (D = 1.1 ± 0.1 Mpc) and the metallicity of red giants ([Fe/H] = −1.37) found by Kopylov et al. (2008). VV 124 is located on the periphery of the Local Group approximately at the same distance from M31 and our Galaxy and is isolated from other galaxies. The galaxy LeoA nearest to it is 0.5 Mpc away.  相似文献   

3.
The physical nature of 277 ultraviolet excess objects of the third Asiago field (22 square degrees, 15 ≦ mpg ≦ 18, U–B ≦≦ -0.2) is investigated on the basis of objective prism plates. Most of them are stars, 8% are quasar candidates and 7% compact galaxies. The proportions of different kinds of objects changes with the magnitude and U–B colour index. The surface density of quasars is estimated to be between 0.5 Q/sq. deg. and 1.0 Q/sq. deg. for the UVX A3 objects. The results of the A3 field (located in the Virgo cluster of galaxies) statistically agree with the ones of the A1 field, outside clusters of galaxies.  相似文献   

4.
The processing of the plates of the Kitab part of the FON project has been completed. In total, 1963 plates were processed. The catalog of equatorial coordinates α, δ, and B-magnitudes for 13 413268 stars and galaxies up to B ≤ 17.5 m for the epoch 1984.97 is compiled. The Epson Expression 10000XL scanner with a 1200 dpi scanning mode and a plate size of 30 × 30 cm or 13000 × 13000 pel was used to digitize astronegatives. Coordinates of stars and galaxies are obtained in the Tycho-2 catalog system and B-magnitudes are obtained in the photovoltaic system. The catalog internal accuracy for all objects is σα,δ = 0.23" and σ B = 0.15 m (for stars in the range B = 5 m …14 m , errors are σα,δ = 0.085" and σ B = 0.054 m ) for equatorial coordinates and stellar B-magnitudes, respectively. Convergence between the calculated and reference positions is σα,δ = 0.042", and convergence between photoelectric B-magnitudes is σ B = 0.16 m . Coordinate errors with respect to the UCAC-4 catalog are σα,δ = 0.26" (9892697 or 73.75% of stars and galaxies were identified).  相似文献   

5.
The hot stellar component in elliptical galaxies offers clues to both stellar evolution and galaxy evolution. Current observations suggest that extreme horizontal branch (EHB) stars dominate the far-UV emission from galaxies with the strongest "UV upturns," while post asymptotic giant branch (PAGB) stars are probably significant contributors for weaker galaxies. Spectra near the Lyman limit indicate that a rather narrow range of temperature (and hence EHB star mass) is required. However, other arguments suggest that most of the helium-burning stars in elliptical galaxies are in the red clump. The HB star mass distribution therefore appears to be strongly bimodal. Such bimodality is qualitatively reproduced by two radically different stellar population models, (those of Lee and Bressan et al., 1994), both of which require that the galaxies be very old. However, the Galactic open cluster NGC 6791 also contains EHB stars and exhibits strong bimodality, indicating that old age may not necessarily be a requirement for the UV upturn phenomenon. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

6.
The absolute proper motions of about 275 million stars from the Kharkov XPM catalog have been obtained by comparing their positions in the 2MASS and USNO-A2.0 catalogs with an epoch difference of about 45 yr for northern-hemisphere stars and about 17 yr for southern-hemisphere stars. The zero point of the system of absolute proper motions has been determined using 1.45 million galaxies. The equatorial components of the residual rotation vector of the ICRS/UCAC2 coordinate system relative to the system of extragalactic sources have been determined by comparing the XPM and UCAC2 stellar proper motions: ω x,y,z = (−0.06, 0.17, −0.84) ± (0.15, 0.14, 0.14) mas yr−1. These parameters have been calculated using about 1 million faintest UCAC2 stars with magnitudes R UCAC2 > 16 m and J > 14 m . 7, for which the color and magnitude equation effects are negligible.  相似文献   

7.
The catalog of equatorial coordinates α and δ and B-magnitudes of stars has been created at the Main Astronomical Observatory, National Academy of Sciences of Ukraine (MAO NASU), for the circumpolar region (from 58° to 90°) of the Northern Sky Survey (FON) project within the work on the rational use of resources accumulated in the JDA (Joint Digital Archive) of the Ukrainian Virtual Observatory (UkrVO). The total number of processed plates is 477. The plates were digitized with the using Microtek ScanMaker 9800XL TMA and Epson Expression 10000XL scanners (scanning mode was 1200 dpi, the linear size of plates was 30 × 30 cm or 13000 × 13000 px). The catalog includes 1 975 967 stars and galaxies with B of up to 16.5 m as of the epoch of 1985.28. The coordinates of stars and galaxies were obtained in the Tycho-2 reference system and B-magnitudes were obtained in the system of photoelectric standards. The internal errors of the catalog for all the objects are σαδ = 0.23′′ and σ B = 0.12 m , and those for stars of the B range from 8 m –14 m , 0.11′′ and 0.06 m , respectively. The convergence between the calculated and reference positions is σαδ = 0.06′′ (for 171124 stars from Tycho-2), and that between the photoelectric stellar B-magnitudes is σ B = 0.15m (for 5130 stars). The external error from the comparison with UCAC-4 are σαδ = 0.33′′ (1928367 stars and galaxies have been cross identified).  相似文献   

8.
Accurate surface photometry has been obtained inJ andK for 12 giant elliptical galaxies. Ellipses have been fitted, to obtain luminosity, ellipticity, and major axis position angle profiles. The results have been combined with visual profiles from CCD observations. We find that elliptical galaxies become bluer inV - K on the average by 0.3 (mag/sq xr e) going outward from the nucleus. Radial colour gradients inV - K correlate strongly with those inU - V andB - V. By modelling, using theoretical isochrones, it is found that the observed colour gradients can be explained by a gradient in metallicity, if in addition the proportion of hot stars rises very rapidly with decreasing metallicity.  相似文献   

9.
《New Astronomy Reviews》2000,44(4-6):283-285
The statistical investigation of integrated characteristics of the blue compact galaxies from the Second Byurakan survey has been carried out. The tight correlation between the luminosities in the far-infrared range and radio range (radio-continuum at 21 cm) has been revealed. It implies the common origin of this radiation in star forming regions. The current star formation rates have been derived both from the luminosity in the far-infrared range (SFRFIR) and from the luminosity in the radio range (SFR21 cm). The Salpeter shape of the initial mass function (IMF) and low-mass cut-off of 0.1 M⊙ in IMF has been assumed for the SFRFIR calculation. We derive the SFR21 cm assuming that radio free–free emission at λ=21 cm is thermal emission of gas ionised by massive stars. The SFR21 cm is shown to be about 3.5 times as large as SFRFIR for our sample of galaxies. These star formation rates are proposed to be considered as lower and upper limits of true galaxy star formation rates.  相似文献   

10.
We have identified 22 galaxies with photometric redshifts zph=5–7 in the northern and southern Hubble Space Telescope deep fields. An analysis of the images of these objects shows that they are asymmetric and very compact (～1 kpc) structures with high surface brightness and absolute magnitudes of MB≈?20m. The average spectral energy distribution for these galaxies agrees with the distributions for galaxies with active star formation. The star formation rate in galaxies with zph=5–7 was estimated from their luminosity at λ=1500 Å to be ~30 M⊙yr?1. The spatial density of these objects is close to the current spatial density of bright galaxies. All the above properties of the distant galaxies considered are very similar to those of the so-called Lyman break galaxies (LBGs) with z ～ 3–4. The similarity between the objects considered and LBGs suggests that at z ～6, we observe the progenitors of present-day galaxies that form duringmergers of protogalactic objects and that undergo intense starbursts.  相似文献   

11.
We present CCD VIC photometry in a field 1.9 − 1.3 square arcmin and positioned about 3 arcmin north-east of the centre of M82. More than 100 stars have been identified and measured above the photometric limits V = 23m.5, I = 22m. Most of the objects fainter than V ≈︁ 20m should be members of M82 as expected from the statistics of foreground stars. Starlike as well as 2–3 somewhat diffuse objects - candidates of globular clusters - preferentially occupy yellow colours in the colour-magnitude diagram; extreme blue and red objects are missing. The colour-magnitude diagram indicates continuing star formation in this part of the disk. The absence of blue stars is caused by reddening; no convincing explanation is found for the missing red supergiants. Young stars are distributed throughout the whole field, the youngest being concentrated in associations at the SW edge of the field. To cope with the very irregular background created by the galaxy' s main body and with the low contrast of many of the stars against this background, a modified photometric method is introduced in which the background is estimated using the intensities in a ring drawn very tightly around the star in the wings of the point spread function.  相似文献   

12.
We present here rigorous analytical solutions for the Boltzmann-Poisson equation concerning the distribution of stars above the galactic plane. The number density of stars is considered to follow a behaviour n(m,0) ∼H(m - m0)m−x, wherem is the mass of a star andx an arbitrary exponent greater than 2 and also the velocity dispersion of the stars is assumed to behave as < v2(m)> ∼ m−θ the exponent θ being arbitrary and positive. It is shown that an analytic expression can be found for the gravitational field Kz, in terms of confluent hypergeometric functions, the limiting trends being Kz∼z for z →0, while Kz → constant for z → infinity. We also study the behaviour of < |z(m)|2>,i.e. the dispersion of the distance from the galactic disc for the stars of massm. It is seen that the quantity < |z(m)|2>∼ mt-θ, for m→ t, while it departs significantly from this harmonic oscillator behaviour for stars of lighter masses. It is suggested that observation of < |z(m)|2> can be used as a probe to findx and hence obtain information about the mass spectrum.  相似文献   

13.
The results of photometry and polarimetry of the R Coronae Borealis type stars and other interesting objects are given. The observation of the former objects are obtained at the light maximum or at a brightness lower by 2-3 mag. The polarization of R CrB stars at light maximum has interstellar origin. The absolute magnitude of V 854 Cen is estimated to Msvw = −3m, and for Y Mus it is Mv > −3m.7. ρ Cas has a variable polarization and is probably a giant (Mv ≅ 0m) rather than a supergiant. Many early stars in its surroundings are photometrically and polarimetrically variable. The protoplanetary star BD −0°3679 has a polarization with the Rayleigh component.  相似文献   

14.
Results are presented from CCD BVRI-observations of 145 comparison stars in the neighborhoods of 14 Seyfert galaxies, 3 quasars, and 5 BL Lac objects with right ascensions of 12 to 24 hours. The magnitudes of the observed stars ranged from V=11 to V=17. The typical photometric error for stars brighter V=14 is 0m.01. The B, V, Rc, and Ic magnitudes of most of these stars were not known previously. 14′ x 14′ finding charts are provided. These results can be used for differential photometry of the nuclei of active galaxies in the B, V, Rc, and Ic bands.__________Translated from Astrofizika, Vol. 48, No. 3, pp. 365–379 (August 2005).  相似文献   

15.
The catalog of equatorial coordinates α and δ and B-magnitudes of stars of the northern sky (from–4° to +90°) was created as a part of the FON project at the Main Astronomical Observatory of the National Academy of Sciences of Ukraine. The data accumulated in the Joint Digital Archive of the Ukrainian Virtual Observatory were used. The total number of processed plates is 2260. Astronegatives were digitized with Microtek ScanMaker 9800XL TMA and Epson Expression 10000XL scanners in the 1200 dpi scanning mode. The majority of plates have a size of 30 × 30 cm (13000 × 13000 pixels). The catalog contains the data on 19451751 stars and galaxies with B ≤ 16.5m at the epoch of 1988.1. The coordinates of stars and galaxies were obtained in the Tycho-2 reference system, and B-magnitudes were determined in the system of photoelectric standards. The mean internal errors of the catalog are σαδ = 0.23″ and σB = 0.14m for all objects or σαδ = 0.10″ and σB = 0.07m for stars in the B = 7m–14m range. The convergence between the catalog and Tycho-2 is characterized by the following values: 0.06′′ and 0.15m. The mean-square difference in coordinates from the catalog and from UCAC-4 is σαδ = 0.30′′ (18 742 932 objects, or 96.36% of stars and galaxies, were cross-identified).  相似文献   

16.
We have observed the largest optical spectra sample of 97 blue compact galaxies. Stellar population properties of 74 star-forming BCGs of them were derived by comparing the equivalent widths of strong absorption features and continuum colors, using a method of empirical population synthesis based on star cluster sample. The results indicate that blue compact galaxies are typically age-composite stellar system, the continuum flux fractions at 5870Å due to old stellar components and young stellar components are both important for most of the galaxies. The stellar populations of blue compact galaxies present a variety of characteristics, and the contribution from different age and metallicity components is different. The star formation episodes are usually short, some galaxies maybe undergoing their first global episode of star formation, while for the most sample galaxies, older stars contribute to at most half the optical emission. Our results suggest that BCGs are old galaxies, in which star formation occurs in short intense burst separated by long quiescent phases.  相似文献   

17.
G01 New evidence for a connection between massive black holes and ULX G02 Long‐Term Evolution of Massive Black Hole Binaries G03 NBODY Meets Stellar Population Synthesis G04 N‐body modelling of real globular star clusters G05 Fokker‐Planck rotating models of globular clusters with black hole G06 Observational Manifestation of chaos in spiral galaxies: quantitative analysis and qualitative explanation G07 GRAPE Clusters: Beyond the Million‐Body Problem G08 Orbital decay of star clusters and Massive Black Holes in cuspy galactic nuclei G09 An Edge‐on Disk Galaxy Catalog G10 Complexes of open clusters in the Solar neighborhood G11 Search for and investigation of new stellar clusters using the data from huge stellar catalogues G12 Computing 2D images of 3D galactic disk models G13 Outer Pseudoring in the Galaxy G14 Where are tidal‐dwarf galaxies? G15 Ultra compact dwarf galaxies in nearby clusters G16 Impact of an Accretion Disk on the Structure of a stellar cluster in active galactic nuclei G17 Order and Chaos in the edge‐on profiles of disk galaxies G18 On the stability of OB‐star configurations in the Orion Nebula cluster G19 Older stars captured in young star clusters by cloud collapse G20 General features of the population of open clusters within 1 kpc from the Sun G21 Unstable modes in thin stellar disks G22 From Newton to Einstein – Dynamics of N‐body systems G23 On the relation between the maximum stellar mass and the star cluster mass  相似文献   

18.
Observations of nearby star-forming regions suggest that the initial mass function is deficient in low-mass stars when the ambient star formation rate is high. We investigate the consequences of a truncated initial mass function on the photometric evolution of starburst galaxies using new models of stellar population synthesis. A major prediction with respect to the case of a standard initial mass function is that the spectra of the galaxies become unusually red when the turnoff mass reaches the cutoff of the initial mass function. This red phase can last for more than 109 yr if the lower cutoff is less than 2M . The amplitude of the reddening depends only weakly on the lower cutoff but increases with the fraction of the galaxy mass involved in the starburst. Such unusually red (V - K > 3.5) post-starburst galaxies can be distinguished from galaxies reddened by dust or with abnormally high metallicities by their unusually strong 4000 Å break and the presence of stellar absorption features typical of late-type giants.  相似文献   

19.
Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for an isolated pair of interacting galaxies, IC 1727 and NGC 672, and constructed their Hertzsprung-Russell diagrams. The galaxy IC 1727 exhibits a strong asymmetry in the apparent distribution of young stars. The distribution of old stars is smoother and less asymmetric. In the galaxy NGC672, there is no noticeable asymmetry in the distribution of stars of different ages. Based on the TRGB method, we have determined an accurate distance to each galaxy for the first time: D = 7.14 ± 0.10 Mpc for IC 1727 and D = 7.22 ± 0.10 Mpc for NGC 672, confirming that these galaxies are closely spaced. The luminosity functions of red supergiants and peripheral AGB stars in both galaxies have positionally coincident local maxima, suggesting the simultaneous enhancement of star formation in the two galaxies occurred in the intervals 20–30 and 450–700 Myr ago. The results obtained point to an enhancement of star formation processes in the interacting galaxies.  相似文献   

20.
We use three indicators of massive star formation, Hα, FIR and non-thermal radio luminosities, to compare estimates of the star formation rate (SFR) for a sample of 34 spiral galaxies. To adjust the SFR values obtained from these indicators, we considered the slope, α, and/or the upper mass limit M up of the initial mass function (IMF) as free parameters. The best agreement between the indicators is found for M up≈ 60-100 M⊙ and α ≈–3.1 at the high-mass end of the IMF (M>10 M⊙.Parallelwith the SFR we also estimated the FIR excess X FIR, defined as the fraction of the observed FIR not directly related to young massive stars. X FIR is found to be well correlated with types of spiral galaxies and their colours (B-V): the redder a galaxy, the higher its FIR excess. We conclude that for any parameters of the IMF the observed FIR flux of early-type spiral galaxies needs an additional source of energy apart from massive star radiation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

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