2. The IRAS/FOCA sample
2.1. Construction of the sample of galaxies
The FOCA balloon borne wide-field UV camera (Milliard et al. 1994) has observed a cumulated sky surface of square degrees in a 150 wide band-pass centered near 0.2 µm. The camera (a 40-cm Cassegrain telescope with an image intensifier coupled to a IIaO emulsion film) was operated in two modes, the FOCA 1000 (f/2.6) and FOCA 1500 (f/3.8), which provide 2.3 field of view, 20 " resolution, and 1.5 field of view, 12 " resolution, respectively. The typical limiting depth in one hour observing time is where the magnitude is defined by where the flux f is in (Donas et al. 1991). Here we have considered the 22 calibrated fields ( 70 square degrees), in order to cross correlate them with the observations of the IRAS satellite. Table 1 gives the coordinates of the guide star (near the field center), the total exposure time and the size for each field.
Table 1. Fields at 0.2 µm observed by the FOCA experiment with the coordinates of the field center (guide star), the total exposure time in each field and the size of the field (diameter)
The infrared objects of the IRAS Faint Sources Catalog (FSC) have been associated to sources from other astronomical catalogs. Such cross-correlations are very useful to determine the nature of the sources detected. Unfortunately only a small proportion of UV sources have an identification in an other catalog. So we have chosen to start from the IR detections for which much data are available and to search for their UV counterparts. Therefore our sample will be FIR selected .
For each UV field, we have extracted the FIR sources detected by IRAS at 60 and/or 100 microns and listed in the IRAS FSC. This has been done using the VIZIER facility of the Centre de Données astronomiques de Strasbourg (CDS). 364 IR sources have been selected.
Since we are only interested by the extragalactic targets we have kept only the objects associated to known galaxies from the catalog of associations of the FSC. 102 from the 364 sources at 60 or 100 microns have been securely identified as galaxies. We have only kept galaxies which are not confused with a neighboured source present in cross-correlated catalogs.
Then we have searched for a UV source matching each FIR detection of a galaxy in a circle of 45 arcsec radius centered on the IRAS coordinates. 94 galaxies have been identified both in UV and FIR. 8 FIR sources identified as galaxies have no UV counterpart. Few cases of several UV sources present in the circular area have been judged doubtful and discarded.
To avoid a contamination in the IR detection only galaxies with a cirrus flag lower than or equal to 2 are selected as adviced in the IRAS Faint Source Catalog. We are then left with 80 galaxies with a UV measurement and with 8 galaxies detected by IRAS and not identified in UV.
Endly we exclude nearby ellipticals and S0 galaxies present in our sample since we are only concerned by star forming galaxies. Very extended galaxies like M101 or M51 are excluded from the study since the photometry of these objects needs a special treatment. Our final sample contains 76 galaxies.
Complementary data necessary to the study of this sample like the optical identification, the B magnitude, the distance modulus are taken from the LEDA and NED databases. The fluxes are corrected for Galactic extinction using the Milky Way extinction curve of Pei (1992). Throughout the paper h will be defined as .
2.2. The galaxies not detected at 0.2 µm
8 galaxies detected by IRAS at least at 60 µm do not appear in our UV frames. For one of them (F15451+0132), the non detection is explained by the fact that the galaxy is located on the edge of the UV image. Another source, F13038+2919, lies on the wings of the UV bright guide star (spectral type A3, ). The UV identification of F12041+6519 (identified as MCG 11-15-022 in NED) at 60 arcsecs from the IRAS coordinates is quite uncertain. In Table 2 are gathered the galaxies for which we can estimate an upper limit for their UV flux and F12041+6519 whose identification is uncertain. These galaxies are faint even in FIR: 4 of them are only detected at 60 µm with a very low flux.
Table 2. Galaxies detected in FIR with no or an uncertain UV detection. The B magnitude are taken from Yuan et al. 1996and the CDS database.
F13041+2907 and F11431+2037 have no optical counterpart, F13041+2907 is also detected at 1.4 GHz by FIRST (NED database).
F12259+1141, F12235+0914 and F12242+0919 are three galaxies located in the Virgo cluster area and identified by Yuan et al. (1996): F12259+1141 (VCC1099) is a faint galaxy classified as dE; F12242+0919 (VCC0934) a background blue galaxy classified Sa with a radial velocity equal to 6938 km s-1; F12235+0914 is identified as VCC0864 and classified as Im or dE.
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999