4 June 2005 Island Universes Benne W. Holwerda Opacity of Spiral Galaxy Disks Extinction measurements from counts of distant galaxies. B.W. Holwerda (Kapteyn/ STScI) R.J. Allen (STScI) R.A. González (UNAM, Morelia) P.C. van der Kruit (Kapteyn)

4 June 2005 Island Universes Benne W. Holwerda Finding dust in spirals To characterize dust in spirals: –Emission: infrared/sub-mm –Extinction: SED models, background objects. Different strengths & weaknesses: –Emission depends on dust temperature and hence illumination and dust emissivity is still uncertain. –Extinction needs a known background source. => We use the number of distant galaxies as the background.

4 June 2005 Island Universes Benne W. Holwerda Counting Galaxies Calibration issues: –Crowding: are the distant galaxies hidden by stars or dust? –Confusion: what is a foreground object and what is a distant galaxy? –Clustering: are the distant galaxies blocked by dust or simply not there?

4 June 2005 Island Universes Benne W. Holwerda The “Synthetic Field Method” 1. Identify distant galaxies in science field. 2. Combine image with dimmed HDF-N/S. 3. Identify background galaxies in simulations. 4. Fit relation between simulated background galaxies and dimming: A = -2.5 Log(N/N 0 ) 5. Estimate opacity from the intersection of fit and number from the science field. González et al. (1998) Holwerda et al. (2005a)

4 June 2005 Island Universes Benne W. Holwerda Beating statistics The HDF has ~100 easily recognizable field galaxies, in a crowded field, ~30 remain. Beating statistics with more data; deeper impractical, analyze more fields with automated method Holwerda et al. (2005a). Combine 32 HST/WFPC2 fields based on common characteristics e.g.: –Radius, distance from foreground galaxy center –Arm, inter-arm and outside disk regions –Disk surface brightness –HI column density

4 June 2005 Island Universes Benne W. Holwerda Radial extinction plots Scale all radii with R 25 (de Vaucouleur et al. 1991) Find background galaxies, synthetic and real, in radial annuli. Combine counts and derive opacity (A I ) Holwerda et al. (2005b)

4 June 2005 Island Universes Benne W. Holwerda Arm and disk regions Divide fields into arm/inter-arm and outside disk regions. Compare to similar results from occulting galaxy technique Points from White et al. (2000) and Domingue et al. (2000). Holwerda et al. (2005b)

4 June 2005 Island Universes Benne W. Holwerda Galaxy colors: patchy dust Average color of background galaxies independent of the inferred opacity. Grey extinction known effect of patchy dust. Extinction measured from missing number, color from remaining galaxies. Distant galaxies are extended objects Holwerda et al. (2005b)

4 June 2005 Island Universes Benne W. Holwerda Dust and light Flag each background galaxy with the 2MASS pixel value Sort the galaxy counts per surface brightness SB-A relation in arms, not in the disk. Holwerda et al. (2005d) submitted

4 June 2005 Island Universes Benne W. Holwerda Compare to SCUBA sub-mm Radial Profiles of sub- mm emission and opacity values match. Individual measurements of opacity are uncertain. Little overlap in current samples. Much future work for SCUBA and ACS. Meijerink et al Holwerda et al. (2005c) A&A accepted

4 June 2005 Island Universes Benne W. Holwerda Comparison to HI Radial surface density profiles  HI (r) from the literature. Compare to individual opacity profiles. No apparent relation between HI and opacity. Holwerda et al. (2005c) A&A accepted

4 June 2005 Island Universes Benne W. Holwerda Average Dust-to-HI ratio Average opacity profile Average HI profile Compare ratio. Higher than earlier estimates. Similar to sub-mm estimates. Mayya et al. (1997) Issa et al. (1990) Boissier et al. (2004) Thomas et al. (2005) Cuillandre et al. (2001) Holwerda et al. (2005c) A&A accepted

4 June 2005 Island Universes Benne W. Holwerda Conclusions (1) SFM calibrates biases and errors in field galaxy numbers. Spiral disk show A I < 1 magnitude of extinctions within the R 25 Later type spirals slightly more opaque than earlier types. Arms are more opaque than the disk. Dust in the disk is patchy.

4 June 2005 Island Universes Benne W. Holwerda Conclusions (2) Surface brightness and opacity are linked, especially in the spiral arms. Dust responsible for opacity appears cold when compared to sub-mm emission. HI column density and opacity appear not linked. The ratio of the average opacity profile and the average HI profile poinst to more dust per HI.

4 June 2005 Island Universes Benne W. Holwerda Future Work ACS imaging of M101 and M51 Direct comparison to HI column density and IR/sub-mm emission. Fully automated object classification. Image courtesy of K.D. Kuntz

4 June 2005 Island Universes Benne W. Holwerda More Questions? My thesis is done! Results also in holwerda et. al. (2005a,b) (published) (2005c,d,e)(submitted) Or send me an

4 June 2005 Island Universes Benne W. Holwerda Cepheid reddening & opacity Average reddening of the Cepheids in the fields is converted to extinction. Opacity from number of galaxies mostly similar. Detection bias in Cepheids at higher values. Holwerda et al. (2005d) A&A submitted

4 June 2005 Island Universes Benne W. Holwerda Limiting factors González et al. (2001) predicted that galaxies at Virgo distances are optimum for SFM analysis. A = -2.5 C log(N/N 0 ) Surface Brightness and granularity of the disk responsible for limitations. My data supports this conclusion. Holwerda et al. (2005e) A&A submitted

4 June 2005 Island Universes Benne W. Holwerda An improved HI comparison Cuillandre et al (2001) Compare number of galaxies per HI contour. Optimal for single disk.

4 June 2005 Island Universes Benne W. Holwerda Object Classification Single parameter cut not sensitive enough Total chance computed from all parameters. Some hard cuts to remove blue clusters HII regions etc. Total score works but improvements still possible. HDF All Objects

4 June 2005 Island Universes Benne W. Holwerda Comparing observers NGC 4536 re-analyzed with automated SFM Compared to the number of González et al (1998). Difference due to limiting depth. Holwerda et al. (2005a)

4 June 2005 Island Universes Benne W. Holwerda HDF-N/S as a reference field The HDF-N/S fields; how ``average’’ are they? Compare numbers of objects per magnitude range to cosmological searches. No deviation from the mean field up to the detection limit. Holwerda et al. (2005a)

4 June 2005 Island Universes Benne W. Holwerda Computing the Error Relation between number of synthetic galaxies and dimming. Measurement error in synthetic number Measurement and clustering error in actual number.

4 June 2005 Island Universes Benne W. Holwerda Galactic Extinction Galactic extinction varies for each field. Difference with the reference field (HDF- N/S) important. Nearly negligible except for NGC6946

4 June 2005 Island Universes Benne W. Holwerda Synthetic Dust Models Filling factor of dust clouds observed. Many small clouds or a few big ones? The effects on number of distant galaxies very different. Focus for future work.

4 June 2005 Island Universes Benne W. Holwerda Inclination correction Radial opacity profiles independent of inclination. Implies that the filling factor remains the same. Flat distribution of dust. Holwerda et al. (2005b)

4 June 2005 Island Universes Benne W. Holwerda Disk Opacity and Hubble type A I < 1 until R 25 Disks early types more opaque than later types? LSB transparent. Holwerda et al. (2005b)

4 June 2005 Island Universes Benne W. Holwerda Disk surface brightness Relation between SB and A in radial annuli. More extinction at brighter SB. Combine background galaxy numbers based on disk surface brightness. Holwerda et al. (2005b)

4 June 2005 Island Universes Benne W. Holwerda Comparison to IR/sub-mm. Cold dust best characterized in sub-mm Very few galaxies done with SCUBA Substantial overlap with the Spitzer Infrared Nearby Galaxies (SINGS) data. Compare radial profiles with FIR profile.

4 June 2005 Island Universes Benne W. Holwerda Comparison to IR SINGS overlap in sample: 8 galaxies (Feb 2005) MIPS 160 micron maps Compare radial profiles. Crude but quick first look. Dust causing opacity appears cold (T~15-20 K) Dust emissivity still uncertain.

4 June 2005 Island Universes Benne W. Holwerda The Sample 32 WFPC2 fields in V,I Mostly Cepheid Distance Scale Key Project Drizzled, analyzed & combined. Holwerda et al. (2005a,b)