疏散星团NGC6530天区的恒星自行和成员概率

利用上海天文台的照相底片资料,确定了疏散星团NGC6530天区364颗恒星的自行和成员概率,并对有关自行测定的方法、结果和精度等问题作了较为详细的介绍和讨论。使用的底片历元差为87年,全部恒星自行中误差的均方根值为1．09mas/a。     

Stellar populations and dust in the galaxy NGC 2976, a low-luminosity member of the M81 group

Photographic UBV photometry of NGC 2976, a low-luminosity member of the central M81 group of galaxies, is presented. Young stars in the central disk determine the optical view and the classification of this Sc(pec) galaxy. It is surrounded by a halo of an old population which contains nearly all the mass and half the luminosity of the system. This halo has some properties typical of spheroidal dwarf galaxies: an exponential brightness profile, an ellipticity trend of the isophotes typical of low-mass systems, and mass and luminosity near the upper limit of typical dwarfs. In the central population I disk, star formation proceeds in dense associations scattered irregularly in a broad ringlike region of 1.2 kpc radius just inside the turnover of the rotation curve. This star formation episode may last since some 10⁸ a; it is possibly triggered by gas infall from the interstellar cloud generated during encounters between other group members in the central M81 group. A direct triggering by recent encounters is excluded since NGC 2976 is undisturbed in its outer parts.     

BeppoSAX observations of the Seyfert 2 galaxies NGC 7172 and ESO 103-G35

We investigate the X-ray spectra of the type 2 Seyfert galaxies NGC 7172 and ESO 103-G35, using BeppoSAX observations, separated by approximately one year. We find that the X-ray spectra of both NGC 7172 and ESO 103-G35 can be well fitted using a power-law model with an Fe K α emission line at 6.4 keV. We did not find any statistically significant evidence for the existence of a reflection component in the X-ray spectra of these two galaxies. The continuum flux has decreased by a factor of approximately 2 during this period, in both objects. However, the spectral index as well as the absorption column have remained constant. We find weak evidence for the decrease of the normalization of the Fe K α emission line in a similar manner to the continuum in NGC 7172. We also report tentative evidence for a broad Fe K α line in agreement with previous ASCA observations. In contrast, in the case of ESO 103-G35 the line flux does not change while its width remains unresolved.     

The Velocity Field and Kinematics of the Galaxy NGC 784 in the Hα Line

In this paper we present the H map, the 2D velocity field and the rotation curve of the galaxy NGC 784 obtained with the ByuFOSC2 scanning Fabry-Perot interferometer, attached at the prime focus of the 2.6-m telescope of Byurakan Observatory. The H image shows several HII condensations along the major axis of the galaxy. The galaxy has an asymmetric distribution of the H emission. The rotation curve is quite symmetric with a low gradient in the central part of the galaxy.     

Dynamical stability and evolution of the discs of Sc galaxies

We examine the local stability of galactic discs against axisymmetric density perturbations with special attention to the different dynamics of the stellar and gaseous components. In particular, the discs of the Milky Way and of NGC 6946 are studied. The Milky Way is shown to be stable, whereas the inner parts of NGC 6946, a typical Sc galaxy from the Kennicutt sample, are dynamically unstable. The ensuing dynamical evolution of the composite disc is studied by numerical simulations. The evolution is so fierce that the stellar disc heats up dynamically on a short time-scale to such a high degree, which seems to contradict the morphological appearance of the galaxy. The star formation rate required to cool the disc dynamically is estimated. Even if the star formation rate in NGC 6946 is at present high enough to meet this requirement, it is argued that the discs of Sc galaxies cannot sustain such a high star formation rate for extended periods.     

Possible formation scenarios for the giant H i envelope around the NGC 4490/4485 system

We present observations of H i in the nearby interacting galaxies NGC 4490 and 4485 made with the VLA in both C and D arrays. The galaxies are embedded in an extensive envelope of neutral hydrogen which is elongated in a direction approximately perpendicular to the plane of NGC 4490, with an extent of about 56 kpc. We argue that this distribution of neutral hydrogen can best be explained by a galactic-scale bipolar outflow of H  i driven by supernovae in NGC 4490. The flow from the disc appears to be reasonably well collimated and has probably persisted for approximately 6 × 10⁸ yr. The implications for galaxy evolution when such mass loss occurs are briefly discussed.     

The stellar content of a spiral arm of NGC 4321

The effects of the passage of a spiral arm through the disc of the giant Virgo Sc galaxy NGC 4321 are investigated with Hubble Space Telescope WFPC2 images in two colours. Concentrating on a portion of the southern spiral arm of NGC 4321, we have applied a new program to solve for the star formation histories in the arm and interarm regions separately. The observational uncertainties and the variable crowding across the spiral arm are taken into account using the results of artificial star tests. In the interarm regions the data are consistent with a constant star formation rate for the last 50 Myr while the stars in the arm region show a star formation rate four times larger than in the interarm regions in the last 5 Myr.     

A Study of the Star-forming Dwarf Galaxy NGC 855 with Spitzer

We present a study of the dwarf elliptical galaxy NGC 855 using the narrow-band Hα and Spitzer data. Both the Hα and Spitzer IRAC images confirm star-forming activity in the center of NGC 855. We obtained a star formation rate （SFR） of 0.022 and 0.025Mo yr^-1, respectively, from the Spitzer IRAC 8.0 μm and MIPS 24 #m emission data. The HI observation suggests that the star-forming activity might be triggered by a minor merger. We also find that there is a distinct IR emission region in 5.8 and 8.0 μm bands, located at about 10＂ away from the nucleus of NGC 855. Given the strong 8.0 μm but faint Hα emission, we expect that it is a heavily obscured star-forming region, which needs to be confirmed by further optical spectroscopic observations.