The Opacity of Spiral Galaxy Disks
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A&A 444, 109–117 (2005) Astronomy DOI: 10.1051/0004-6361:20053013 & c ESO 2005 Astrophysics The opacity of spiral galaxy disks VI. Extinction, stellar light and color B. W. Holwerda1,2, R. A. González3,P.C.vanderKruit1, and R. J. Allen2 1 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands 2 Space Telescope Science Institute, Baltimore, MD 21218 USA e-mail: [email protected] 3 Centro de Radiastronomía y Astrofísica, Universidad Nacional Autónoma de México, 58190 Morelia, Michoacán, Mexico Received 7 March 2005 / Accepted 7 September 2005 ABSTRACT In this paper we explore the relation between dust extinction and stellar light distribution in disks of spiral galaxies. Extinction influences our dynamical and photometric perception of disks, since it can distort our measurement of the contribution of the stellar component. To characterize the total extinction by a foreground disk, González et al. (1998, ApJ, 506, 152) proposed the “Synthetic Field Method” (SFM), which uses the calibrated number of distant galaxies seen through the foreground disk as a direct indication of extinction. The method is described in González et al. (1998, ApJ, 506, 152) and Holwerda et al. (2005a, AJ, 129, 1381). To obtain good statistics, the method was applied to a set of HST/WFPC2 fields (Holwerda et al. 2005b, AJ, 129, 1396) and radial extinction profiles were derived, based on these counts. In the present paper, we explore the relation of opacity with surface brightness or color from 2MASS images, as well as the relation between the scalelengths for extinction and light in the I band. We find that there is indeed a relation between the opacity (AI) and the surface brightness, particularly at the higher surface brightnesses. No strong relation between near infrared (H − J, H − K) color and opacity is found. The scalelengths of the extinction are uncertain for individual galaxies but seem to indicate that the dust distribution is much more extended than the stellar light. The results from the distant galaxy counts are also compared to the reddening derived from the Cepheids light-curves (Freedman et al. 2001, ApJ, 553, 47). The extinction values are consistent, provided the selection effect against Cepheids with higher values of AI is taken into account. The implications from these relations for disk photometry, M/L conversion and galaxy dynamical modeling are briefly discussed. Key words. radiative transfer – methods: statistical – ISM: dust, extinction – galaxies: ISM – galaxies: spiral – galaxies: photometry 1. Introduction (White & Keel 1992; Andredakis & van der Kruit 1992) and the use of calibrated counts of distant objects (the “Synthetic Dust extinction has influenced our perception of spiral disks Field Method” (SFM), by González et al. 1998). Thus far, the since the first observations of them. The measurements of following picture of the influence of dust on disk photometry disk characteristics, such as the central surface brightness (µ ), 0 has emerged from earlier studies, most of which are based on the typical exponential scale (r ) and the mass-to-light ra- typ the inclination effect on photometry of a large sample of spiral tio (M/L), are all affected by the dust extinction in the photo- disks. Tully et al. (1998) and Masters et al. (2003) reported that metric band of observation. The original assertion by Holmberg disks are more opaque in the blue. Disks are practically trans- (1958) that spiral disks are optically thin to their stellar light parent in the near infrared (Peletier & Willner 1992; Graham came under scrutiny after the paper by Disney (1990) and the 2001), making these bands the best mass-to-luminosity esti- observational result of Valentijn (1990) revealed that they were, mator (de Jong 1996). Disks are practically transparent in the in fact, practically opaque. The debate quickly culminated in outer parts but show significant absorption in the inner regions a conference (Davies & Burstein 1995), during which many (Valentijn 1994; Giovanelli et al. 1994). The radial extent of the methods to measure the opacity of spiral disks were put for- dust has been explored using the sub-mm emission (Alton et al. ward. Notably, two methods do not use the disk’s own stel- 1998b; Davies et al. 1999; Trewhella et al. 2000; Radovich lar light for the measurement: the occulting galaxy technique et al. 2001) and edge-on models (Xilouris et al. 1999). These Research support by NASA through grant number HST-AR- results indicate that the scalelength of the dust is 40% larger 08360 from the Space Telescope Science Institute (STScI), the STScI than that of the light. The disk’s average extinction corre- Discretionary Fund (grant numbers 82206 and 82304 to R. J. Allen) lates with the total galaxy luminosity (Giovanelli et al. 1995; and the Kapteyn Institute of Groningen University. Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20053013 110 B. W. Holwerda et al.: The opacity of spiral galaxy disks. VI. Tully et al. 1998; Masters et al. 2003). And spiral arms are individual fields. For a complete discussion of the uncertain- more opaque than the disk (Beckman et al. 1996; White et al. ties of the SFM, see Holwerda et al. (2005a). To combat poor 2000).This may be attributed to a more clumpy medium in the statistics, the numbers of distant galaxies in several images are arms in addition to a disk (González et al. 1998). Stevens et al. combined, based on common characteristics of the foreground (2005) found evidence based on the infrared and sub-mm emis- disks. Holwerda et al. (2005b) combined numbers based on sion from dust for two thermal components of the dust, a warm radius and Hubble types. In this paper we compare the num- component associated with star formation and a colder compo- bers of distant galaxies for image sections of common surface nent in a more extended disk. brightness and color. In this paper we explore the relation between the light from González et al. (2003) and Holwerda et al. (2005c) con- a spiral galaxy’s disk and the opacity measured using the SFM. cluded that the optimal distance for the SFM is that of Virgo This paper is organized as follows: Sect. 2 summarizes the cluster for the HST instruments. Hence our sample of fields is “Synthetic Field Method”, Sect. 3 discusses possible system- taken for disks at this range of distances. atic effects in the method and sample, Sect. 4 describes the rela- tion between surface brightness and average extinction, Sect. 5 3. Discussion of systematic effects explores this relation for arm and disk regions, and Sect. 6 the relation between extinction and near-infrared color. In Sect. 7, There are two possible sources of systematics in the follow- the scalelengths for light and extinction are compared. A brief ing results: the selection of the sample of foreground galaxies comparison between Cepheid reddening and opacity is made and possible systematics in the method itself. The systematics in Sect. 8. The implications for measurements involving spiral and uncertainties of the method are also discussed in detail in disks are discussed in Sect. 9, and we list our conclusions in Holwerda et al. (2005a) but we briefly list possible systemat- Sect. 9. ics here. The selection of the sample is discussed in Holwerda et al. (2005b) but the effects of the smaller sample on the new segmentation of distant galaxy counts are discussed below. 2. The “Synthetic Field Method” The number of distant galaxies seen through a foreground spi- 3.1. Systematics of the Synthetic Field Method ral disk is a function of dust extinction as well as crowding and confusion in the foreground disk. Distant galaxy numbers were A systematic can creep into the SFM if there is a difference in used by several authors to measure extinction in the Magellanic the object identification in the science and the synthetic fields. Clouds and other galaxies1. The “Synthetic Field Method” was This was one of our main drivers to automate the identifica- developed by González et al. (1998) to calibrate an extinc- tion process to the highest possible degree. However, a visual tion measurement based on the number of distant galaxies in check of the candidate objects is still necessary. Therefore an a Hubble Space Telescope (HST) image. It quantifies the ef- observer bias can not be completely excluded. See for an in- fects of crowding and confusion by the foreground spiral disk. depth discussion regarding the identification process Holwerda The SFM consists of the following steps. First, the number of et al. (2005a). distant galaxies in the science field is identified. Secondly, syn- There are, however, several reasons why we consider the thetic fields are constructed. These are the original science field counts calibrated sufficiently for any systematics. The ra- with a suitable deep field added, which is dimmed to mimic the dial opacity profile does seem to end at zero extinction at effects of dust. The added distant galaxies in the resulting syn- higher radii (Holwerda et al. 2005b). The radial opacity profile thetic field are identified, based on object appearance and color. agrees with the values from occulting galaxies (Holwerda et al. The number of these synthetic galaxies identified in the syn- 2005b). And there is good agreement by different observers on thetic fields as a function of dimming can then be characterized: the counts in NGC 4536 (Holwerda et al. 2005a). The Synthetic Field Method was conceived to calibrate any observer bias. If N A = −2.5 C log · (1) our identification process has resulted in the removal of blue N0 distant galaxies together with the HII regions, this has been done in the same way for both synthetic and science fields and A is the dimming in magnitudes, N the number of synthetic therefore does not affect the derived opacity value.