ASTRONOMY and ASTROPHYSICS an Atlas of Mid-Infrared Dust
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A&A manuscript no. ASTRONOMY (will be inserted by hand later) AND Your thesaurus codes are: ASTROPHYSICS missing; you have not inserted them February 27, 2001 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 µmand as characterized by e.g.)]Desert, tracing different physical 15 µm/7 µm intensity ratios of selected regions in 43 spiral conditions. A stellar contribution can also exist at 7 µm galaxies observed with ISOCAM. This atlas is a comple- (Boselli et al. 1998), but is negligible except in a few early- ment to studies based on these observations, dealing with type galaxies. star formation in centers of barred galaxies and in spiral The general description of the sample and detailed disks. It is accompanied by a detailed description of data photometric results are given in Paper I, where the spectra reduction and an inventory of generic morphological prop- are also published. Here, we provide the details of obser- erties in groups defined according to bar strength and HI vations and data reduction, total fluxes at 7 and 15 µm, gas content. a morphological description of galaxies grouped into ap- propriate categories and maps at 7 µm, along with optical Key words: atlas – galaxies: spiral – galaxies: ISM – images. The 15 µm maps have very similar morphology infrared: ISM: continuum – infrared: ISM: lines and bands and are therefore not shown. These images demonstrate the peculiar character of circumnuclear regions seen in the mid-infrared. Information on the F15=F7 colors of bright 1. Introduction complexes and other selected regions is added. All of the maps are published for the first time, ex- The mid-infrared maps presented here are part of a larger cept those of M 51 (Sauvage et al. 1996) and NGC 6946 sample of nearby spiral galaxies analyzed in Roussel et (Malhotra et al. 1996), which are both re-analyzed here. al. (2001a and b: Papers I and II). Paper I primarily investigates the dynamical effects of bars on circumnu- clear star formation activity, studied through dust emis- 2. Observations sion at 7 and 15 µm, and proposes an interpretation of All galaxies were observed with two broadband filters, these mid-infrared data in conjunction with information LW3 centered at 15 µm (12–18 µm) and LW2 centered at on molecular gas and stellar populations. Paper II deals 7 µm (5–8.5 µm), that we shall hereafter designate by their with the use of mid-infrared fluxes as a star formation central wavelength. Maps covering the whole infrared- indicator in spiral disks. Observations belong to guaran- emitting disk were built in raster mode, with sufficient teed time programs of ISOCAM, a camera operating be- overlap between adjacent pointings (half the detector field tween 5 and 18 µm onboard the satellite ISO (?, described of view in most cases) to avoid border artifacts and to pro- by)]Cesarsky. The sample consists of a first group of large vide redundancy for spoiled exposures. In all cases, the and regular spirals, mainly barred, and of a second group field of view is large enough to obtain a reliable determi- of spiral galaxies belonging to the Virgo cluster. The an- nation of the background level, except for NGC 4736 and gular resolution of 1000 (HPBW), combined with spec- 6744. NGC 5457, the largest spiral in the sample with an ≈ troscopic information from the same instrument, enables optical size of nearly 300, was imaged with a combination a detailed view of the variations of two dust phases (?, of two overlapping observations made during different rev- unidentified infrared band carriers and very small grains, olutions of the satellite. The pixel size is either 300 or 600, Send offprint requests to: H. Roussel (e-mail: [email protected]) depending on the galaxy size, but for Virgo galaxies it ? Based on observations with ISO, an ESA project with in- was always 600, in order to increase the signal to noise ra- struments funded by ESA Member States (especially the PI tio. The half-power/half-maximum diameters of the point countries: France, Germany, the Netherlands and the United spread function are respectively 6:800/ 3:100 at 7 µmwith ' Kingdom) and with the participation of ISAS and NASA. a300 pixel size, 9:500/5:700 at 7 µmwitha600 pixel size, 2 H. Roussel et al.: An atlas of mid-infrared dust emission in spiral galaxies 9:600/3:500 at 15 µmwitha300 pixel size and 14:200/6:100 at 3 – The removal of cosmic ray impacts is complicated by 15 µmwitha600 pixel size. Table 1 summarizes the rele- the long-duration memory effects they can produce. The vant information regarding the observations. arrival of an energetic cosmic ray may be followed by ei- To estimate the relative importance of different emit- ther a slowly decreasing tail or a responsivity drop, as ting species in different galactic regions, spectral imaging explained above. The vast majority of glitches are, how- between 5 and 16 µm of the inner disks of five bright galax- ever, of short duration (one to three exposures) and can ies was also carried out. These observations are essential be rejected by median filtering. To also remove small tails to interpret broadband imaging results. They consist of and plateaus induced by glitch pile-up, the exposures im- one pointing with two circular variable filters, from 16.5 mediately surrounding exposures flagged by the deglitcher to 9 µm and from 9.6 to 5 µm, with a spectral resolution were examined and their flux level compared with that of λ/∆λ(FWHM) = 35 to 50. The sampling varies between adjacent non-flagged exposures, i.e. we iterated the me- 0.24 and 0.45 of ∆λ(FWHM). dian filtering once after rejecting the cosmic ray peaks. This filtering cannot blindly be applied everywhere. First, at a step between a faint and a bright illumination level 3. Data analysis and vice-versa, some real signal variation may be masked 3.1. Broadband filter maps by the deglitcher. Therefore, the signal in the first and last exposures for each sky position was compared with that Data reduction was performed using and adapting the in the neighboring exposures for the same sky position, ISOCAM Interactive Analysis package (CIA). The main and flags cancelled if necessary. Second, during a point- difficulty and source of uncertainty is the slow time re- ing on a bright peaked source such as galactic circumnu- sponse of the detector, which is typical of cold photo- clear regions, the noise includes, in addition to the readout conductors. Below 20 K, the mobility of carriers becomes and photon noises, high amplitude fluctuations which are very slow, which generates long time constants, both in approximately proportional to the signal and due to the the bulk of the photoconductor and at contact electrodes. satellite jitter. For the purpose of glitch detection, an ad- The typical time spent per pointing is 40–100 s whereas ≈ ditional noise component proportional to the signal was in general several hundred seconds are needed to reach thus tuned to reproduce the jitter effects, and for very stabilization within 10% of the flux step. bright sources, defined by a flux threshold set above the Several causes of memory effects can be distinguished, background, fluctuations of 20% around the median flux although they always involve the same physics of the de- were allowed. The memory effects following some glitches, tector. We will use different designations for convenience: for which no model exists, are temporarily set aside and – (short-term) transients: they are due to the slow stabi- examined at step 6. lization following a flux step (either upward or downward), 4 – Short-term transients are corrected for by using the after moving from one sky position to another; they are latest available technique taking into account the detector the origin of remnant images seen after observing a bright characteristics (Coulais & Abergel 2000). The parameters source. of the model have been determined in low and uniform – long-term transients: drifts and oscillations visible dur- illumination cases only, but are likely to vary with signal ing the whole observation and influenced by previous his- intensity and are inexact in the presence of strong spa- tory; contrary to short-term transients, no model exists to tial gradients. Indeed, when switching from a moderate correct for them. flux level to a bright resolved source, the correction hardly – glitch tails: slow return to the sky flux level following modifies the data. After a large amplitude flux step, the the deposition of energy in the detector by intense cosmic stabilization is, however, faster, and thus the last expo- rays (this is an unpredictable effect). sures are likely closer to the real flux than in the case of – responsivity drops: they also follow cosmic ray impacts small flux steps. The readout histories of pixels imaging and produce “holes” which can last more than one hun- galactic nuclei or extranuclear bright complexes were care- dred exposures (this is again an unpredictable effect). fully examined after step 7 to check whether the computed The data reduction proceeded in the following way: flux level was consistent with the level expected from the 1 – The subtraction of the dark current follows a model last valid exposures; if not, it was replaced with the mean which predicts its time evolution for even and odd rows of signal in the last part of the readout history that was ap- the detector.