1 Introduction

As the high frequency of bars in galaxies becomes more evident (e.g. Eskridge et al. 2000), and as new techniques emerge to both observationally quantify their strength (Seigar & James 1998; Buta & Block 2001) and numerically simulate them, their effects on their host galaxies are of major interest, and in particular, it is worth checking whether they are indeed very efficient systems to drive nuclear starbursts in spiral galaxies.

Numerous studies have dealt with the respective star formation properties of barred and non-barred spirals, mostly in the infrared, since this is the wavelength regime where starbursts are expected to be most easily detectable. Yet conclusions derived from such studies appear to contradict each other, partly because the different selection criteria result in samples with a more or less pronounced bias toward starburst objects. For instance, in the IR-bright sample analyzed by Hawarden et al. (1986), an important fraction of SB and SAB galaxies (respectively strongly barred and weakly barred spirals in the classification of de Vaucouleurs et al. 1991) shows a 25$\mu$m emission excess (with respect to 12 and 100$\mu$m) absent in the SA subsample (non-barred spirals), which can be accounted for by a highly increased contribution of Galactic-like H II regions to the total emission. On the other hand, Isobe & Feigelson (1992), using a volume-limited sample and performing a survival analysis to take into account the frequent IRAS non-detections, found that the far-IR to blue flux ratio ( $F_{\rm FIR}/F_{\rm B}$) is rather independent of the bar class. The contradiction is marginal since $F_{\rm FIR}/F_{\rm B}$ does not give a direct estimation of the star formation activity, especially when dealing with quiescent normal galaxies: the blue light originates partly from young stars and, as Isobe & Feigelson (1992) emphasize, $F_{\rm FIR}/F_{\rm B}$ depends on the amount and spatial distribution of dust with respect to stars. The relationship between the 25$\mu$m excess, quantified by F₂₅/F₁₂, and $F_{\rm FIR}/F_{\rm B}$in a galaxy sample with good quality data is indeed highly dispersed. Huang et al. (1996) investigated the 25$\mu$m excess as a function of IR brightness and reconciled the two previous analyzes: a significant excess can occur only if $F_{\rm FIR}/F_{\rm B}$ is larger than a threshold value of $\simeq$0.3. Therefore, a statistical effect of bars on star formation can be demonstrated only in suitably selected samples. Huang et al. (1996) also emphasized that the difference between barred and unbarred spirals concerns only early types (S0/a to Sbc).

Studies of the infrared excess in barred galaxies mostly rest on the integrated IRAS measurements, which do not allow the determination of the nature and location of regions responsible for this excess. However, dynamical models and observations at other wavelengths give evidence that the infrared activity should be concentrated in circumnuclear regions (see for instance the study of NGC5383 by Sheth et al. 2000). In addition, high-resolution ground-based observations near 10$\mu$m of galaxy centers (Devereux 1987; Telesco et al. 1993) have shown that the dust emission is more concentrated in barred galaxies.

Theoretically, bars are known to be responsible for large-scale redistribution of gas through galactic disks. In a strong barred perturbation of the gravitational potential, shocks develop along the rotation-leading side of the bar and are associated with strong shear, as shown by Athanassoula (1992) and references therein (also Friedli & Benz 1993). They induce an increase of gas density which is traced by the thin dust lanes widely observed in bars, producing a contrasting absorption of optical light (Prendergast 1962, unpublished; Huntley et al. 1978). Due to these shocks, gas loses angular momentum and flows towards the circumnuclear region. This picture is confirmed by direct observations of inward velocity gradients across bars in ionized gas lines, CO and H I (e.g. Lindblad et al. 1996; Reynaud & Downes 1998; Mundell & Shone 1999). Regan et al. (1997) derive a gas accretion rate of $\approx$ $1\,M_{\odot}\,{\rm yr}^{-1}$ into the circumnuclear ring of NGC1530.

Statistical evidence is also found for higher gas concentrations in the center of barred galaxies (Sakamoto et al. 1999, who however observed only SABs, except NGC1530), and for more frequent circumnuclear starbursts in barred galaxies, as reported by Heckman (1980), Hawarden et al. (1986), Arsenault (1989) (who, more exactly, found more probable starbursts in galaxies with both bar and inner ring, supposed to be a signature of one or two inner Lindblad resonance(s)), Huang et al. (1996), Martinet & Friedli (1997) and Bonatto et al. (1998). Aguerri (1999) has moreover reported that the global star formation intensity of isolated spirals (mostly of late types) is correlated with bar strength as quantified by means of its projected axial ratio, which is surprising in view of the very different timescales of bar evolution ($\approx$1Gyr) and star formation in kpc-scale regions ($\approx$10^7-8yr). Indeed, Martinet & Friedli (1997), using carefully selected late-type galaxies, found no such correlation, the bar strength being quantified either by its deprojected axis ratio or its deprojected length relative to the disk diameter. The fact that only a fraction of strongly barred galaxies exhibit star formation excess (as evidenced by their IRAS colors) is explained by these authors with numerical simulations of bar evolution including gas physics. They show that a strong starburst occurs shortly after bar formation and quickly fades away (in typically less than 1Gyr); meanwhile, the strength and other properties of the bar evolve, but the bar remains strong if it was initially strong. The existence of strongly barred galaxies in a quiescent state is thus to be expected, presumably because the available gas supply has been consumed in previous bursts.

This paper is aimed at characterizing the mid-infrared excess in barred galaxies, with the possibility to carry out a detailed and systematic spatial analysis due to the good angular resolution of ISOCAM (the half-power beam diameter is less than $10\hbox{$^{\prime\prime}$ }$ at 7$\mu$m), and hence to locate unambiguously sites of enhanced infrared activity. Although dust is a more indirect tracer of young stars than far-ultraviolet ionizing radiation or optical recombination lines, the infrared emission suffers relatively minor extinction effects, which are very difficult to correct and hamper shorter wavelength studies. In a companion paper (Roussel et al. 2001a, hereafter Paper II), we have shown that in galactic disks, mid-infrared emission is a reliable star formation indicator. Here, we concentrate on central regions of galaxies where the dust heating regime is markedly different from that in disks.

For this purpose, we have analyzed a sample of 69 nearby spiral galaxies, imaged at 7 and 15$\mu$m with the camera ISOCAM on board ISO (described by Cesarsky et al. 1996c). We have also obtained low-resolution spectroscopic information for a few galaxies, enabling us to identify and separate the various dust components emitting between 5 and 18$\mu$m. 7$\mu$m images and F₁₅/F₇ flux density ratios of selected regions, together with optical images, are presented in Roussel et al. (2001b) (hereafter the Atlas). For a description of data reduction and analysis, and a summary of morphological properties of the sample, the reader is also referred to the Atlas.