2 The galaxy sample

The sample is intended to be representative of normal quiescent spirals, and contains galaxies of moderate infrared luminosity. It covers three guaranteed time programs of ISOCAM. The first one (Cambarre) consists of nearby barred galaxies, the second one (Camspir) of a few large-size spirals of special interest (NGC1365, 4736, 5194, 5236, 5457 and 6744) and another subsample is drawn from the Virgo cluster sample of Boselli et al. (1998) (Virgo program), containing relatively fainter and smaller galaxies, both barred and unbarred. This sample was supplemented by comparable spirals in the ISOCAM public archive, from the programs Sf_glx (Dale et al. 2000) and Irgal (PI T. Onaka). All of the observations were reduced in the same way to form a homogeneous sample. The final set comprises 69 spiral galaxies at distances between 4 and 60Mpc. We have divided them into three main categories according to morphological classes in the RC3 (de Vaucouleurs et al. 1991): SBs (accounting for about half the sample with 37 galaxies), SABs (20 galaxies) and SAs (12 galaxies). The latter two classes are merged to form the control sample to compare with SB galaxies. This sample, although not statistically complete, has been selected according to the following requirements:

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All objects are relatively nearby, which ensures good spatial resolution with a $6\hbox {$^{\prime \prime }$ }$ or $3\hbox {$^{\prime \prime }$ }$ pixel size (the extension of the central concentration is typically 5-10 pixels in diameter). At the distances of the sample, a $3\hbox {$^{\prime \prime }$ }$ pixel corresponds to linear sizes between 60 and 900pc. Virgo galaxies, less extended and all imaged with $6\hbox {$^{\prime \prime }$ }$ pixels in order to increase the signal to noise ratio, are resolved but with less detail.

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The sample was selected to avoid non-stellar activity as well as strong signs of tidal interaction, with a few exceptions in the Cambarre and Virgo subsamples, detailed in the following.

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Galaxies included in the first two programs are moderately inclined on the line of sight ( $i \leq 50\hbox{$^\circ$ }$). This was not a requirement for the other programs, so that one third of the Virgo galaxies and one third of the supplementary galaxies are inclined by more than $60\hbox{$^\circ$ }$.

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The number of SA-SAB galaxies is comparable to that of SBs and both groups span the whole de Vaucouleurs spiral sequence from types S0/a to Sdm.

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Both barred and unbarred galaxies cover a large range of far-infrared luminosities (between 10^8.6 and $10^{11}\:L_{\odot {\rm bol}}$), but none would be classified as an infrared luminous galaxy, except NGC7771 which is at the lower boundary of this class - defined by $10^{11} < L_{\rm FIR} < 10^{12}\:L_{\odot {\rm bol}}$ (Sanders & Mirabel 1996).

Another property which was not a selection criterion is that absolute blue magnitudes are equal to or greater than the typical magnitude of the Schechter luminosity function in the field, $M_{\rm B}^* \simeq -21$ (more exactly, they range between -21.17 and -17.38).

Despite the incompleteness of the sample, we have checked that it is very similar to the magnitude-limited CfA galaxy sample (Thuan & Sauvage 1992), from the point of view of its infrared brightness normalized by blue starlight. For CfA spiral galaxies detected in all 4 IRAS bands and with blue magnitudes in the RC3, log( $F_{\rm FIR}/F_{\rm B}$) falls in the interval [ -0.97 ; +0.98] with a mean value of 0.05. Using the same IRAS references as those in Thuan & Sauvage (1992), i.e. in order of preference Thuan & Sauvage (1992), Rice et al. (1988), Soifer et al. (1989) and Moshir et al. (1989), galaxies in our sample have log( $F_{\rm FIR}/F_{\rm B}$) in the interval [ -1.58 ; +1.67] with a mean value of 0.01. For that set of references, a Wilcoxon-Mann-Whitney (WMW) test indicates that the probability for the two populations to have the same $F_{\rm FIR}/F_{\rm B}$distribution is about 75%. We note that the IRAS 12$\mu$m fluxes often disagree with our 7 and 15$\mu$m fluxes, although the bandpasses overlap. Thus, when we use IRAS data, we take them from the references we consider the most reliable (i.e. which provide the best match between 12$\mu$m and our 7-15$\mu$m flux densities). In that case, log( $F_{\rm FIR}/F_{\rm B}$) falls in the interval [ -0.77 ; +0.95] with a mean value of 0.02, and the WMW test gives a probability of about 40%. Hence, our sample is not different from optically complete samples regarding the fraction of the energy radiated in the infrared.

  

Table 1: General properties of sample galaxies. Virgo members are also named from the VCC catalog. Other galaxies belong to the field or loose groups, unless otherwise noted. Distances are from the NGC catalog of Tully (1988), taking into account the Virgo infall and assuming h₁₀₀ = 0.75; morphological types, asymptotic blue magnitudes $m_{\rm BT}$ and major diameters D₂₅ at the isophote $\mu _{\rm B} = 25\,{\rm mag\,arcsec}^{-2}$ are from the RC3 (de Vaucouleurs et al. 1991).

$$\begin{tabular}{\vert l@{\hspace*{0.1cm}}c@{\hspace*{0.1cm}}r... ...4 (GR) & ~ & H{\scriptsize II} (K) & ~ \\ \hline \end{tabular}$$

 

Table 1: continued. The last column indicates the name of the observation project: S for Sf_glx or H I_Q_gal for IC1953 (PI: G. Helou), and I for Irgal (PI: T. Onaka).

$$\begin{tabular}{\vert l@{\hspace*{0.1cm}}c@{\hspace*{0.1cm}}r... ...13.93 & 1.91 & ~ & ~ & ~ & ~ & {\it S} \\ \hline \end{tabular}$$

^a The H I deficiency according to the definition and reference values of Guiderdoni & Rocca (1985). It is normalized by the dispersion in the field sample. Diameters are taken from the RC2 for consistency, and H I fluxes from the indicated references. NGC4567/68 are unresolved in H I. When $D_{\rm HI}/D_{\rm opt} \leq 1.$ and $Def \geq 0.5$ in Cayatte et al. (1994), it corresponds to def > 1.2 here.
^b The given range represents the effect of varying the scale length of the CO distribution from once to twice that of infrared circumnuclear regions (see text). b is the beam HPBW of the observations used. (*) NGC 4535 was observed by A. Bosma, D. Reynaud and H. Roussel at the IRAM 30m telescope.
Signs of tidal interaction. ``doa'': asymmetrical distortion of outer arms. "mbs'': magellanic barred spiral. "am'': amorphous. "pc'': past collision. "note'': On DSS images, the brightness peak is displaced by $\approx$ $10\hbox{$^{\prime\prime}$ }$ ENE from the center of the regular outer isophotes and the NW outer disk seems depressed in stars and gas.
Members of the Fornax cluster of galaxies. Members of the Dorado group of galaxies. § Member of the Grus quartet with NGC7582/90/99. $\natural$ Member of the spiral-rich Ursa Major cluster.
# Classified SA in the RC3, here considered a SB after the morphological arguments of Phillips & Malin (1982) and McLeod & Rieke (1995), and the recent kinematic analysis of Veilleux et al. (1999).
^d The distances of ESO317-G023, NGC6753 and 6156 were assumed to be those of the galaxy groups LGG199, LGG426 and LGG407 (Garcia 1993); those of NGC5786 and 7771 were estimated from the H I redshift, that of NGC3620 from the CO redshift and those of NGC5430, 5937 and 6824 from the optical redshift (with h₁₀₀ = 0.75).

Table 1 lists some general characteristics of the galaxies. The morphological classification adopted is that of the RC3 (de Vaucouleurs et al. 1991). Although it is based on blue images, which may not be as appropriate as near-infrared images for detecting bars, many more galaxies are classified as barred in this catalog than for instance in Sandage & Bedke (1994). We have found only two galaxies classified as SA in the RC3 and possessing a bar (as described in the following). A drawback of using the SB and SAB classes of the RC3 is that they do not constitute a measure of the bar dynamical strength. The bar strength is however difficult to quantify, and reliable measures, such as those of Buta & Block (2001), are scarce. In the following, we will refer to bar lengths, normalized by the disk diameter, because longer bars are able to collect gas from inside a larger area and have low axis ratios, which are among the (unsatisfactory) quantities used to estimate bar strengths; bar lengths are in addition relatively easy to measure.

The two sub-samples of spirals found in the field or loose groups and Virgo galaxies have been separated, because they differ both in their aspect in the infrared (Virgo members are fainter and less extended) and in their environment. Although Virgo is not a very rich cluster, the interaction of central galaxies with the intracluster gas and with their neighbours is likely to cause either a depletion or an enhancement of star formation activity in the outer parts of disks and also to have global dynamical consequences. An extreme case is the galaxy NGC4438 (= VCC1043), whose very perturbed morphological appearence was successfully modelled by Combes et al. (1988) as the result of a collision with NGC4435. Several Virgo members have truncated H I disks due to the interaction with the cluster hot gas (Cayatte et al. 1990); a very clear example is NGC4569 (= VCC1690), which on optical photographs shows the juxtaposition of a bright and patchy inner disk structured by star formation sites and dust lanes, and a low surface brightness and very smooth outer disk with faint spiral arms. Severely H I-stripped galaxies can indeed be recognized in the optical as anemic (defined by van den Bergh 1976 as an intermediate and parallel sequence between lenticulars and spirals), due to the suppression of star formation where the gas density is too low. Table 1 also indicates whether signatures of nuclear activity or tidal interaction exist.

In addition to these, some galaxies deserve special comments (see the Atlas for more details) and should be considered cautiously in the interpretation of the data set:

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NGC337, 1385 and 4027 are strongly asymmetric and fit in the category of magellanic barred spirals.

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NGC4691 and 1022 have an amorphous structure and highly centrally concentrated interstellar tracers. Their morphology is suggestive of merger results. Their star formation activity may therefore not be a consequence of the bar, since the latter was likely produced at the same time by the same cause, i.e. the merger.