3. The galaxy sample
We have selected all the barred galaxies with 1) bar length L and bar axis ratio determined by Martin (1995), and 2) IRAS fluxes from various authors. The basic source is Soifer et al. (1989) which is completed from lists by Rice et al. (1988), Young et al. (1989), or Devereux (1987).
Five early-type (Sa to Sb) and 42 late-type (Sbc to Scd) galaxies in Martin's list have IRAS data. The sample was reduced, using two more selection criteria: 1) Galaxies should not be too inclined so that the deprojected axis ratio does not deviate too much from the projected value (a difference of 1 ellipticity class as defined by Martin was admitted). 2) Galaxies should be isolated or weakly interacting (no companion detected within ; no morphological disturbances) in order to clearly separate effects coming from the bar from those generated by interactions. An exception is NGC 5457 which presents some moderate morphological disturbances. Seyfert galaxies have not been excluded from the sample, but only two are present (NGC 1365 and NGC 4051).
We have 32 late-type and only 4 early-type objects left. Arguments from the literature can incite to only consider the late types: Bars in them could have spontaneously formed in discs contrary to bars in early types, induced by interactions, as suggested by Noguchi (1996). In fact, in Sect. 5, we will deal with spontaneous bars. Moreover, Sauvage & Thuan (1994) have found a decreasing contribution of the cirrus component to toward later types. These issues are still subjects of debate. Moreover, the number of early-types that would otherwise be in our sample is small (4) and their inclusion or exclusion does not affect the results and the conclusions at all (see below), which will be based on a final sample of 32 Sbc-Sc-Scd galaxies.
The selected galaxies cover nearly two decades in FIR luminosity ( to ). NGC 3486 and NGC 7479 are respectively the least and most luminous objects of the sample. The luminosity of NGC 7479 is similar for instance to M82 but nearly two orders of magnitude below Arp 220. Strong starbursts are generally characterized by and result from significant interactions.
Without being a strong starburst, NGC 7479 has photometric properties comparable to average values given for "Starburst Nucleus Galaxies" (SBNG) as defined by Balzano (1983) or Coziol et al. (1994). For instance, very close to the average value -0.75 inferred for SBNGs by Dultzin-Hacyan et al. (1990). By comparison, we have for the three following well-known starbursts:
Moreover, NGC 7479 has , whereas f is respectively 6.6 and 143 for NGC 253 and NGC 1614.
The final sample of galaxies is listed in Table 1. The column entries are as follows:
Col. (1): NGC designation of the galaxy.
Col. (2): Deprojected bar axis ratio.
Col. (3): Deprojected bar length in unit.
Col. (4): Logarithm of the IRAS color index.
Col. (5): Relative star formation rate from FIR data (Eq. 2).
Col. (6): Star formation rate from FIR data (Eq. 1) in [ ].
Col. (7): Star formation rate from H emission in [ ].
Col. (8): Radial O/H abundance gradient in [ ].
Col. (9): Class of the galaxy as defined in Sect. 6.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998