H. B. Ann

Measures of galaxy environment – I. What is ‘environment’?

The influence of a galaxy’s environment on its evolution has been studied and compared extensively in the literature, although differing techniques are often used to define environment. Most methods fall into two broad groups: those that use nearest neighbours to probe the underlying density field and those that use fixed apertures. The differences between the two inhibit a clean comparison between analyses and leave open the possibility that, even with the same data, different properties are actually being measured. In this work we apply twenty published environment definitions to a common mock galaxy catalogue constrained to look like the local Universe. We find that nearest neighbour-based measures best probe the internal densities of high-mass haloes, while at low masses the inter-halo separation dominates and acts to smooth out local density variations. The resulting correlation also shows that nearest neighbour galaxy environment is largely independent of dark matter halo mass. Conversely, aperture-based methods that probe super-halo scales accurately identify high-density regions corresponding to high mass haloes. Both methods show how galaxies in dense environments tend to be redder, with the exception of the largest apertures, but these are the strongest at recovering the background dark matter environment. We also warn against using photometric redshifts to define environment in all but the densest regions. When considering environment there are two regimes: the ‘local environment’ internal to a halo best measured with nearest neighbour and ‘large-scale environment’ external to a halo best measured with apertures. This leads to the conclusion that there is no universal environment measure and the most suitable method depends on the scale being probed.

Galactic satellite systems: radial distribution and environment dependence of galaxy morphology

We have studied the radial distribution of the early (E/SO) and late (S/Irr) types of satellites around bright host galaxies. We made a volume-limited sample of 4986 satellites brighter than M r = -18.0 associated with 2254 hosts brighter than M r = -19.0 from the Sloan Digital Sky Survey Data Release 5 sample. The morphology of satellites is determined by an automated morphology classifier, but the host galaxies are visually classified. We found segregation of satellite morphology as a function of the projected distance from the host galaxy. The amplitude and shape of the early-type satellite fraction profile are found to depend on the host luminosity. This is the morphology-radius/density relation at the galactic scale. There is a strong tendency for morphology conformity between the host galaxy and its satellites. The early-type fraction of satellites hosted by early-type galaxies is systematically larger than that of late-type hosts, and is a strong function of the distance from the host galaxies. Fainter satellites are more vulnerable to the morphology transformation effects of hosts. Dependence of satellite morphology on the large-scale background density was detected. The fraction of early-type satellites increases in high-density regions for both early- and late-type hosts. It is argued that the conformity in morphology of galactic satellite system is mainly originated by the hydrodynamical and radiative effects of hosts on satellites.

BOAO Photometric Survey of Galactic Open Clusters. II. Physical Parameters of 12 Open Clusters

We have initiated a long-term project, the BOAO photometric survey of open clusters, to enlarge our understanding of Galactic structure using UBVI CCD photometry of open clusters, which have been little studied before. This is the second paper of the project, in which we present the photometry of 12 open clusters. We have determined the cluster parameters by fitting the Padua isochrones to the color-magnitude diagrams of the clusters. All the clusters except for Be 60 and NGC 1348 are found to be intermediate-age to old (0.2–4.0 Gyrs) open clusters with a mean metallicity of [Fe/H] ≈ 0.0.

Search for short-period variable stars in the open cluster NGC 2301

Time-series CCD photometric observations of an intermediate age open cluster NGC 2301 were per- formed to search for variable stars, particularly Dor stars and Sct stars. We also carried outUBVI photometry to study physical properties of variable stars in the cluster. We derived observational parameters of the cluster from UBVI photometric results. The best t of the empirical ZAMS in a color-color diagram gives a reddening value of E(B V )=0 :05 0:03. We also estimated a distance modulus of (V MV )0 =9 :6 0:1 and an age of log t =8 :4 0:1, from the ZAMS and isochrone tting in a color-magnitude diagram. Using V time-series data, we carefully examined light variations of 923 stars fainter than 10: m 0, and discovered nine short-period variable stars; two Dor candidates, ve eclipsing binaries, one Slowly Pulsating B star and one magnetic Chemically Peculiar variable candidate. Two Dor candidates with relatively short periods of around 0.3 days are located in a redder region (about 0: m 1i nB V color) than the red edge of Sct instability strip in the color-magnitude diagram.

Deep and wide photometry of two open clusters NGC 1245 and NGC 2506: dynamical evolution and halo

We studied the structure of two old open clusters, NGC 1245 and NGC 2506, from a wide and deep VI photometry data acquired using the CFH12K CCD camera at CFHT. We devised a new method for assigning cluster membership probability to individual stars using both spatial positions and positions in the colour-magnitude diagram. From analyses of the luminosity functions at several cluster-centric radii and the radial surface density profiles derived from stars with different luminosity ranges, we found that the two clusters are dynamically relaxed to drive significant mass segregation and evaporation of some fraction of low-mass stars. There seems to be a signature of tidal tail in NGC 1245 but the signal is too low to be confirmed.

Search for Variable Stars in the Open Cluster NGC 2539

We present UBVIabsolute and V-band time-series CCD photometric results for the intermediate-age open cluster NGC 2539. From the empirical zero-age main sequence and theoretical isochrone fitting in color-color and color-magnitude diagrams, we estimated the physical parameters of this cluster as follows: the color excess of E(B − V) = 0.06 ± 0.03, the distance modulus of (V − MV)0 = 10.2 ± 0.1 and the age of logt = 8. 8( ∼630 Myr). By carefully examining the time-series images, we discovered seven new variable stars in the observed cluster field. Considering the light curves, periods and positions on the color-magnitude diagram, we classified them as five eclipsing binary stars, one δ Scuti star and one γ Doradus candidate. The γ Doradus candidate might not be a cluster member because it is located redder by about ∆(B −V) = 0.2 than the cool edge of γ Doradus instability strip.