4 June 2005Island 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 2005Island Universes

Benne W. Holwerda

Finding dust in spiralsFinding 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 2005Island Universes

Benne W. Holwerda

Counting GalaxiesCounting 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 2005Island Universes

Benne W. Holwerda

The “Synthetic Field Method”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/N0)

5. Estimate opacity from the intersection of fit and number from the science field.

González et al. (1998)Holwerda et al. (2005a)

4 June 2005Island Universes

Benne W. Holwerda

Beating statisticsBeating 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 2005Island Universes

Benne W. Holwerda

Radial extinction plotsRadial extinction plots• Scale all radii with

R25 (de Vaucouleur et al. 1991)

• Find background galaxies, synthetic and real, in radial annuli.

• Combine counts and derive opacity (AI)

Holwerda et al. (2005b)

4 June 2005Island Universes

Benne W. Holwerda

Arm and disk regionsArm 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 2005Island Universes

Benne W. Holwerda

Galaxy colors: patchy dustGalaxy 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 2005Island Universes

Benne W. Holwerda

Dust and lightDust 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 2005Island Universes

Benne W. Holwerda

Compare to SCUBA sub-mmCompare 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. 2005Holwerda et al. (2005c)A&A accepted

4 June 2005Island Universes

Benne W. Holwerda

Comparison to HIComparison 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 2005Island Universes

Benne W. Holwerda

Average Dust-to-HI ratioAverage 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 2005Island Universes

Benne W. Holwerda

Conclusions (1)Conclusions (1)

• SFM calibrates biases and errors in field galaxy numbers.

• Spiral disk show AI < 1 magnitude of extinctions within the R25

• Later type spirals slightly more opaque than earlier types.

• Arms are more opaque than the disk.

• Dust in the disk is patchy.

4 June 2005Island Universes

Benne W. Holwerda

Conclusions (2)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 2005Island Universes

Benne W. Holwerda

Future WorkFuture 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 2005Island Universes

Benne W. Holwerda

More Questions?More Questions?

• My thesis is done!• Results also in

holwerda et. al. (2005a,b) (published) (2005c,d,e)(submitted)

• Or send me an email: holwerda@stsci.edu

4 June 2005Island Universes

Benne W. Holwerda

Cepheid reddening & opacityCepheid 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 2005Island Universes

Benne W. Holwerda

Limiting factorsLimiting factors

• González et al. (2001) predicted that galaxies at Virgo distances are optimum for SFM analysis.

• A = -2.5 C log(N/N0)• Surface Brightness and

granularity of the disk responsible for limitations.

• My data supports this conclusion.

Holwerda et al. (2005e)A&A submitted

4 June 2005Island Universes

Benne W. Holwerda

An improved HI comparisonAn improved HI comparison

• Cuillandre et al (2001)

• Compare number of galaxies per HI contour.

• Optimal for single disk .

4 June 2005Island Universes

Benne W. Holwerda

Object ClassificationObject 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 2005Island Universes

Benne W. Holwerda

Comparing observersComparing 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 2005Island Universes

Benne W. Holwerda

HDF-N/S as a reference fieldHDF-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 2005Island Universes

Benne W. Holwerda

Computing the ErrorComputing the Error

• Relation between number of synthetic galaxies and dimming.

• Measurement error in synthetic number

• Measurement and clustering error in actual number.

4 June 2005Island Universes

Benne W. Holwerda

Galactic ExtinctionGalactic Extinction

• Galactic extinction varies for each field.

• Difference with the reference field (HDF-N/S) important.

• Nearly negligible except for NGC6946

4 June 2005Island Universes

Benne W. Holwerda

Synthetic Dust ModelsSynthetic 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 2005Island Universes

Benne W. Holwerda

Inclination correctionInclination 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 2005Island Universes

Benne W. Holwerda

Disk Opacity and Hubble typeDisk Opacity and Hubble type

• AI < 1 until R25

• Disks early types more opaque than later types?

• LSB transparent.

Holwerda et al. (2005b)

4 June 2005Island Universes

Benne W. Holwerda

Disk surface brightness 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 2005Island Universes

Benne W. Holwerda

Comparison to IR/sub-mm.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 2005Island Universes

Benne W. Holwerda

Comparison to IRComparison 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 2005Island Universes

Benne W. Holwerda

The SampleThe Sample

• 32 WFPC2 fields in V,I

• Mostly Cepheid Distance Scale Key Project

• Drizzled, analyzed & combined.

Holwerda et al. (2005a,b)