After a long-lasting controversy on whether or not galactic disks are optically thick or thin (e.g. Disney et al. 1989; Burstein et al. 1991), a consensus has emerged during the last decade that disks in the local universe (i) behave like optically thick systems as far as integrated photometric properties in the ultraviolet (UV) and visual are concerned (e.g. Shao et al. 2007; Driver et al. 2007; Maller et al. 2009), while at the same time (ii) peripheral and inter-arm regions in spiral galaxies are more transparent than spiral arms or the core of the disk (e.g. White et al. 2000; Holwerda et al. 2005, and references therein).
Beyond the nearby universe, observational constraints on the extinction in disk galaxies have been hard to obtain. A study of the opacity of distant disk galaxies that relies on structural information (e.g., surface brightness) requires both high-resolution imaging and a consistent sampling of the same rest-frame wavelengths. Avoiding so-called `morphological K-corrections' is especially important in this context in view of, e.g., size gradients with color (e.g., de Jong 1996), and also because unrecognized bulge components can induce spurious signals in attenuation-inclination relations (cf. Tuffs et al. 2004).
The limitation of COSMOS HST imaging to a single band (the F814W filter) prevents the construction of disk galaxy samples over a continuous redshift range. We hence resort to a comparative study that takes advantage of the fact that the central wavelength of the SDSS g-band matches the rest-frame wavelength of objects observed in the F814W filter at redshift z ~ 0.7. By artificially redshifting local galaxies to z ~ 0.7 (cf. Kampczyk et al. 2007) it then becomes possible to directly compare the attenuation properties of distant disks to those in the nearby universe.
We have investigated the average opacity of distant (0.6 < z < 0.8) COSMOS spiral galaxies by direct comparison with local disks artificially redshifted to z ~ 0.7. The outcome of inclination-based attenuation measurements is susceptible to selection effects (e.g. Davies et al. 1993; Jones et al. 1996). By processing our two samples identically and by applying a both morphologically and photometrically consistent sample selection, we have taken the necessary precautions to reduce such systematic biases. We recover the expected surface brightness-inclination relation for nearby disks even after the simulated shift to z ~ 0.7 (in the parameterization of the inclination i dependence of surface brightness according to
this implies an opacity constant C(z ~ 0) = 0.47-0.28+0.16). For high-z COSMOS disks a significantly lower value of C(z ~ 0.7) = 0.07±0.06 is found, implying, on average, a nearly constant relation between surface brightness and inclination as is expected for optically thick spiral galaxies.
Previous studies suggest that the extinction laws in star-forming galaxies at similar redshifts as our COSMOS sample do not differ strongly from those in local systems (e.g., Conroy 2009 or Calzetti 2001 and references therein). It thus seems unlikely that the increased opacity of our z ~ 0.7 COSMOS disks is due to a different chemical composition of the dust. Given that our low- and high-z disk samples have similar luminosities we can also rule out that the evolution is due to the locally observed scaling of dust opacity with the blue luminosity of galaxies (Wang & Heckman 1996).
Other possible explanations for the flat surface brightness-inclination relation at z ~ 0.7 are the presence of more attenuating material, or a different spatial arrangement thereof. Evidence that both factors might contribute exists: Genzel et al. 2008 report stronger turbulent motion in disk-like systems at high redshift that could increase the scale height of the dust, and recent measurements of molecular line emission in typical late type galaxies at z ~ 1.5 have revealed large gas fractions in excess of 50% of the baryonic mass (Daddi et al. 2009). The measurements presented here are not sufficient to infer the relative importance of these (at least) two potential contributions. Additional constraints from complementary measurements or different wavelength regions are thus indispensable to determine the causes for the opacity evolution we observe between z ~ 0 and 0.7.
Additional information on my ongoing efforts to measure the opacity of distant disks with COSMOS can be found here:
- 'The Opacity of Galactic Disks at z ~ 0.7'
Sargent, M. T., Carollo, C. M., Kampkczyk, P., et al. 2010, ApJL, 714, 113
- Soon to come:
'The Evolution of Dust Attenuation in Galactic Disks at z < 1.1: Comparing Observations & Models''
Sargent, M. T., et al. 2010, in prep.
Related Links & Literature:
- Collaborators: Paweł Kampczyk, Richard Tuffs, Marcella Carollo, Thomas Bschorr, and the COSMOS Team
- 'Are galaxy discs optically thick?'
Disney, M., Davies, J., & Phillipps, S. 1989, MNRAS, 239, 939 - 'The Dust Opacity of Star-forming Galaxies'
Calzetti, D. 2004, PASP, 113, 1449 - 'Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies'
Tuffs, R., et al. 2004, A&A, 419, 821 - 'The Millennium Galaxy Catalogue: the B-band attenuation of bulge and disc light and the implied cosmic dust and stellar mass densities'
Driver, S., et al. 2007, MNRAS, 379, 1022