The Opacity of Nearby Galaxies from Colors and Counts of Background Galaxies. I. The Synthetic Field Method and Its Application to NGC 4536 and NGC 3664

Abstract

We describe a new, direct method for determining the opacity of foreground galaxies which does not require any a priori assumptions about the spatial distribution or the reddening law of the obscuring material. The method is to measure the colors and counts of background galaxies which can be identified through the foreground system. The method is calibrated, and the effects of confusion and obscuration are decoupled by adding various versions of a suitable deep reference frame containing only field galaxies with known properties into the image of the foreground galaxy, and analyzing these "synthetic field" images in the same way as the real images.We test the method on HST WFPC2 archived images of two galaxies which are quite different: NGC 4536 is a large Sc spiral, and NGC 3664 is a small Magellanic irregular. The reference frames are taken from the Hubble Deep Field.From the background galaxy counts, NGC 4536 shows an extinction A I ∼ 1 mag in the northwestern arm region, and lower than 0.5 mag in the corresponding interarm region (no correction for inclination has been attempted). However, from the galaxy colors, the same reddening of E(V − I) ∼ 0.2 is observed in both the arm and the interarm regions. In the interarm region, the combination of extinction and reddening can be explained by a diffuse component with a Galactic reddening law (R V ≈ 3). In the spiral arm, however, the same diffuse, low opacity component seems to coexist with regions of much higher opacity. Since the exposures are shorter the results for NGC 3664 are less clear, but also appear to be consistent with a two component distribution.

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“…The model with Salpeter IMF produces more metals and therefore results in a higher metallicity but also consumes more gas for the same SFR and gives a lower extinction after a Hubble time. The model with t * = 8 Gyr (e) shows the highest present day extinction of about 0.3 to 0.4 mag which is also observed in Gonzalez et al (1998). The chemically consistent model with constant SF also shows an evolutionary effect because of the increasing abundances and decreasing gas content not only in the time evolution of its SED but also due to its increasing extinction.…”

The occulting galaxy pair UGC 3995

“…Their technique was then applied to all known pairs using ground-based optical images (Andredakis & van der Kruit 1992;Berlind et al 1997;Domingue et al 1999;White et al 2000), spectroscopy (Domingue et al 2000), and later space-based Hubble Space Telescope (HST) images (Keel & White 2001a,b;Elmegreen et al 2001;Holwerda et al 2009, H09 hereafter). The general results of this technique have been confirmed with the number of distant galaxies seen through spiral discs with HST (González et al 1998(González et al , 2003Holwerda 2005b,a;Holwerda et al 2005aHolwerda et al ,b,c,e,d, 2007aHolwerda et al ,c, 2012b. More recently, more pairs were found in the SDSS spectroscopic catalog (86 pairs in Holwerda et al 2007b), and in the Galaxy Zoo project (1993 pairs in Keel et al 2013).…”

What drives the Balmer extinction sequence in spiral galaxies?

“…Sauvage & Thuan 1994;Dale & Helou 2002). A different approach is to study the transparency of galaxies with respect to background light (see Keel & White 2001 and references therein for methods using one nearby background galaxy, and González et al 1998 for methods using distant galaxies as background sources). Unfortunately, this method suffers from extremely low statistics.…”