Showing papers by "A. Boselli published in 1997"

Near infrared surface photometry of late-type Virgo cluster galaxies

Abstract: Near Infrared (K 0 band) surface photometry has been obtained for 102 (88 late-type) Virgo cluster galaxies. A subset of 20 galaxies was also imaged in the H band. Magnitudes and diameters within the 21.5 and 22.0 mag arcsec 2 isophote, concentration indices and total H and K 0 magnitudes are derived. Basic statistical proper- ties of a complete sample of spiral galaxies spanning the range 6:3

Near infrared H and $K^{\prime}$ surface photometry of three Tully-Fisher calibrators: NGC 2366, NGC 2403 and NGC 4236

Abstract: Near Infrared H and K 0 surface photometry of the three nearby galaxies NGC 2366, NGC 2403 and NGC 4236, used as local calibrators of the Tully-Fisher relation, was obtained using the 256 2 NICMOS3 IR array MAGIC attached to the 2:2 m telescope of Calar Alto. The present measurements are compared with the aper- ture photometry available in the literature. Surface bright- ness proles and integrated magnitudes are given. 1

Mid-IR colors and star formation in Virgo and Coma galaxies.

Abstract: We present observations of the global emission of a sample of spiral and irregular galaxies in the Virgo cluster and the Coma supercluster. These observations have been made with the camera ISOCAM on board the Infrared Space Observatory, through two broad filters centered respectively at 6.75 and 15.0 μm. The corresponding fluxes are corrected for the stellar contribution in order to obtain the pure interstellar contribution; the stellar contribution is estimated from the near–infrared fluxes using as template some S0 and elliptical galaxies observed by chance in the ISO fields. The fluxes at 6.75 and 15.0 μm are then normalized to the stellar mass of the galaxy, and compared to the normalized far–UV fluxes at 2000 Å which are good indicators of the star–formation rate (SFR). We find that, while the mid–IR fluxes are proportional to the SFR when it is not too large, the galaxies with a high SFR have relatively less emission at 6.75 and 15μm. It appears that the carriers of the Unidentified Infrared Bands which are responsible for almost all the emission at 6.75 μm and for a part of the emission at 15 μm have different properties in these galaxies or are partly destructed by the UV field. At 15 μm, there is a contribution of very small, three–dimensional grains in galaxies with a high SFR.

Internal extinction and gas column density in spiral galaxies

Abstract: The method for the extinction correction based on the gas column density is tested. A positive correlation is found between the total gas column density and the extinction esti- matedusingamodelbasedonenergyconservation('frequency- converter model'). Assuming a Solar Neighbourhood optical- depth-to-gas ratio, extinction is predicted from the gas column density using a radiative transfer model which adopts a 'Sand- wich' configuration for the stellar and dust distributions, and which takes also into account the effect of scattering. This pre- dictionagreeswellwiththeextinctionestimatedfromtheenergy conservation consideration. This indicates that (1) the method for the extinction correction based on the gas column density is a robust one; (2) in the inner part of galaxy disks where most of the extinction occurs, the dust-to-gas ratio is on average about the same as the Solar Neighbourhood value. Ourresultsalsosuggestthatdustgrainsassociatedwithboth gas phases (HI and H2) participate in the extinction, with a rela- tive importance depending on the abundance of the gas phase in theinnerdisk.Formostofthegalaxiesinoursampletheextinc- tion is mainly due to the dust associated with the molecular gas because they have high molecular to atomic gas column density ratios. On the other hand, for galaxies whose gas column den- sity is dominated by the atomic gas, the extinction seems to be mainly caused by the dust associated with atomic gas.

The molecular gas content of spiral galaxies in the coma/a1367 supercluster

Abstract: We present 12 CO(J=1{0) line observations of 73 spi- ralgalaxiesmostlyintheComa/A1367supercluster.Fromthese data, combined with data available in the literature, we extract the rst complete, optically selected sample (mpg < 15.2) of 37 isolated and of 27 cluster galaxies. Adopting a standard conversion factor X=N(H2)/I(CO), we estimate that the molecular hydrogen content of isolated spiral galaxies is, on average, 20% of the atomic hydrogen reservoir, signicantly lower than previous estimates based on samples selected by FIR criteria, thus biased towards CO rich objects. We show that the frequency distributions of the CO de- ciencyparameter,denedasthedifferencebetweentheexpected and the observed molecular gas content of a galaxy of given lu- minosity (or linear diameter), computed separately for cluster and isolated galaxies, are not signicantly different, indicating that the environment does not affect the molecular gas content of spiral discs. A well dened relationship exists between Mi(H2) and the star formation activity in bright galaxies, while it is weaker at lower luminosities. We interpret this nding as indicating that CO emission traces relatively well the H2 mass only in high- mass galaxies, such as the Milky Way. On the other hand, in low-mass spirals the higher far{UV radiation eld produced by young O{B stars and the lower metallicity cause the photodis- sociation of the diffuse molecular gas, weakening the expected relationship between star formation and the CO emission. The conversion factor between the CO line intensity and the amount of molecular hydrogen being ill-determined and variable with the UV flux and abundances, it is difcult to assess the relation-