An Atlas of Mid-Infrared Dust Emission in Spiral Galaxies?
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A&A 369, 473–509 (2001) Astronomy DOI: 10.1051/0004-6361:20010154 & c ESO 2001 Astrophysics An atlas of mid-infrared dust emission in spiral galaxies? H. Roussel1, L. Vigroux1, A. Bosma2, M. Sauvage1, C. Bonoli3, P. Gallais1, T. Hawarden4, J. Lequeux5, S. Madden1, and P. Mazzei3 1 DAPNIA/Service d’Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France 2 Observatoire de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 4, France 3 Osservatorio Astronomico di Padova, 5 Vicolo dell’Osservatorio, 35122 Padova, Italy 4 Joint Astronomy Center, 660 N. A’ohoku Place, Hilo, Hawaii 96720, USA 5 Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France Received 3 November 2000 / Accepted 23 January 2001 Abstract. We present maps of dust emission at 7 µmand15µm/7 µm intensity ratios of selected regions in 43 spiral galaxies observed with ISOCAM. This atlas is a complement to studies based on these observations, dealing with star formation in centers of barred galaxies and in spiral disks. It is accompanied by a detailed description of data reduction and an inventory of generic morphological properties in groups defined according to bar strength and HI gas content. Key words. atlas – galaxies: spiral – galaxies: ISM – infrared: ISM: continuum – infrared: ISM: lines and bands 1. Introduction The general description of the sample and detailed photometric results are given in Paper I, where the spectra The mid-infrared maps presented here are part of a larger are also published. Here, we provide the details of obser- sample of nearby spiral galaxies analyzed in Roussel et al. vations and data reduction, total fluxes at 7 and 15 µm, (2001a and b: Papers I and II). Paper I primarily in- a morphological description of galaxies grouped into ap- vestigates the dynamical effects of bars on circumnuclear propriate categories and maps at 7 µm, along with optical star formation activity, studied through dust emission at images. The 15 µm maps have very similar morphology 7and15µm, and proposes an interpretation of these mid- and are therefore not shown. These images demonstrate infrared data in conjunction with information on molec- the peculiar character of circumnuclear regions seen in the ular gas and stellar populations. Paper II deals with the mid-infrared. Information on the F15/F7 colors of bright use of mid-infrared fluxes as a star formation indicator complexes and other selected regions is added. in spiral disks. Observations belong to guaranteed time All of the maps are published for the first time, ex- programs of ISOCAM, a camera operating between 5 and cept those of M 51 (Sauvage et al. 1996) and NGC 6946 18 µm onboard the satellite ISO (described by Cesarsky (Malhotra et al. 1996), which are both re-analyzed here. et al. 1996). The sample consists of a first group of large and regular spirals, mainly barred, and of a second group 2. Observations of spiral galaxies belonging to the Virgo cluster. The an- 00 All galaxies were observed with two broadband filters, gular resolution of ≈10 (HPBW), combined with spec- troscopic information from the same instrument, enables a LW3 centered at 15 µm (12–18 µm) and LW2 centered at 7 µm (5–8.5 µm), that we shall hereafter designate by their detailed view of the variations of two dust phases (uniden- central wavelength. Maps covering the whole infrared- tified infrared band carriers and very small grains, as char- acterized by e.g. D´esert et al. 1990), tracing different phys- emitting disk were built in raster mode, with sufficient overlap between adjacent pointings (half the detector field ical conditions. A stellar contribution can also exist at 7 µm (Boselli et al. 1998), but is negligible except in a few of view in most cases) to avoid border artifacts and to pro- early-type galaxies. vide redundancy for spoiled exposures. In all cases, the field of view is large enough to obtain a reliable determi- nation of the background level, except for NGC 4736 and 6744. NGC 5457, the largest spiral in the sample with an Send offprint requests to: H. Roussel, e-mail: [email protected] 0 ? Based on observations with ISO, an ESA project with in- optical size of nearly 30 , was imaged with a combination struments funded by ESA Member States (especially the PI of two overlapping observations made during different rev- 00 00 countries: France, Germany, The Netherlands and the UK) and olutions of the satellite. The pixel size is either 3 or 6 , with the participation of ISAS and NASA. depending on the galaxy size, but for Virgo galaxies it Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010154 474 H. Roussel et al.: An atlas of MIR dust emission in spirals was always 600, in order to increase the signal to noise ra- 2 – If needed, we mask out the borders of the detector tio. The half-power/half-maximum diameters of the point which are not sufficiently illuminated to allow a proper spread function are respectively 6.800/'3.100 at 7 µm with flat-fielding; a300 pixel size, 9.500/5.700 at 7 µm with a 600 pixel size, 3 – The removal of cosmic ray impacts is complicated by 9.600/3.500 at 15 µm with a 300 pixel size and 14.200/6.100 at the long-duration memory effects they can produce. The 15 µm with a 600 pixel size. Table 1 summarizes the rele- arrival of an energetic cosmic ray may be followed by ei- vant information regarding the observations. ther a slowly decreasing tail or a responsivity drop, as To estimate the relative importance of different emit- explained above. The vast majority of glitches are, how- ting species in different galactic regions, spectral imaging ever, of short duration (one to three exposures) and can between 5 and 16 µm of the inner disks of five bright galax- be rejected by median filtering. To also remove small tails ies was also carried out. These observations are essential and plateaus induced by glitch pile-up, the exposures im- to interpret broadband imaging results. They consist of mediately surrounding exposures flagged by the deglitcher one pointing with two circular variable filters, from 16.5 were examined and their flux level compared with that of to 9 µm and from 9.6 to 5 µm, with a spectral resolution adjacent non-flagged exposures, i.e. we iterated the me- λ/∆λ(FWHM) = 35 to 50. The sampling varies between dian filtering once after rejecting the cosmic ray peaks. 0.24 and 0.45 of ∆λ(FWHM). This filtering cannot blindly be applied everywhere. First, at a step between a faint and a bright illumination level and vice-versa, some real signal variation may be masked 3. Data analysis by the deglitcher. Therefore, the signal in the first and last 3.1. Broadband filter maps exposures for each sky position was compared with that in the neighboring exposures for the same sky position, Data reduction was performed using and adapting the and flags cancelled if necessary. Second, during a point- ISOCAM Interactive Analysis package (CIA). The main ing on a bright peaked source such as galactic circumnu- difficulty and source of uncertainty is the slow time re- clear regions, the noise includes, in addition to the readout sponse of the detector, which is typical of cold photo- and photon noises, high amplitude fluctuations which are conductors. Below 20 K, the mobility of carriers becomes approximately proportional to the signal and due to the very slow, which generates long time constants, both in satellite jitter. For the purpose of glitch detection, an ad- the bulk of the photoconductor and at contact electrodes. ditional noise component proportional to the signal was The typical time spent per pointing is ≈40–100 s whereas thus tuned to reproduce the jitter effects, and for very in general several hundred seconds are needed to reach bright sources, defined by a flux threshold set above the stabilization within 10% of the flux step. background, fluctuations of 20% around the median flux Several causes of memory effects can be distinguished, were allowed. The memory effects following some glitches, although they always involve the same physics of the de- for which no model exists, are temporarily set aside and tector. We will use different designations for convenience: examined at step 6; – (short-term) transients: they are due to the slow stabi- 4 – Short-term transients are corrected for by using the lization following a flux step (either upward or downward), latest available technique taking into account the detector after moving from one sky position to another; they are characteristics (Coulais & Abergel 2000). The parameters the origin of remnant images seen after observing a bright of the model have been determined in low and uniform source; illumination cases only, but are likely to vary with signal – long-term transients: drifts and oscillations visible dur- intensity and are inexact in the presence of strong spa- ing the whole observation and influenced by previous his- tial gradients. Indeed, when switching from a moderate tory; contrary to short-term transients, no model exists to flux level to a bright resolved source, the correction hardly correct for them; modifies the data. After a large amplitude flux step, the – glitch tails: slow return to the sky flux level following stabilization is, however, faster, and thus the last expo- the deposition of energy in the detector by intense cosmic sures are likely closer to the real flux than in the case of rays (this is an unpredictable effect); small flux steps.