CURRENT research



Silicate Dust Properties in Quasar Absorption Systems

Probing Dust Properties Using Overlapping Galaxy Pairs in the Local Universe

In-Collaboration With: Varsha P. Kulkarni, James L. Higdon, & Sarah J.U. Higdon

Star-Formation in Polar Ring Galaxies: Evidence for Stochastic Stimulation Mechanisms?

In-Collaboration With: Varsha P. Kulkarni, James L. Higdon, & Sarah J.U. Higdon

Extragalactic star-formation processes


Understanding the nature of interstellar dust grains in distant galaxies is crucial for investigating the chemical evolution of galaxies and for correcting observations of high−redshift objects. Quasar absorption systems, selected by gas cross-section rather than by galaxy brightness, provide a relatively unbiased tool to probe the chemical composition of dust in high-redshift galaxies. Based on measured depletions of refractory elements and associations of background quasar reddening with absorption line strengths, dust is prevalent in at least some of these systems.

Interstellar Silicate Dust in the z=0.89 Absorber Towards PKS 1830-211: Crystalline Silicates at High Redshift? (2011, ApJ submitted, current draft)
 Aller, M.C., Kulkarni, V.P., York, D.G., Vladilo, G., Welty, D.E., & Som, D.

We present evidence of a >10-σ detection of the 10 μm silicate dust absorption feature in the spectrum of the gravitationally lensed quasar PKS 1830-211, produced by a foreground absorption system at redshift 0.886. We have examined more than 100 optical depth templates, derived from both observations of Galactic and extragalactic sources and laboratory measurements, in order to constrain the chemical structure of the silicate dust. We find that the best fit to the observed absorption profile is produced by laboratory crystalline olivine. The fit is slightly improved upon by including small contributions from additional materials such as silica and serpentine, which suggests that the dust composition may consist of a blend of crystalline silicates. Combining amorphous and crystalline templates, we find that the fraction of crystalline silicates needs to be at least 95%. Given the rarity of extragalactic sources with such a high degree of silicate crystallinity, we also explore the possibility that the observed spectral features are produced by amorphous silicates in combination with other molecular or atomic transitions, or by foreground source contamination. While we cannot rule out these latter possibilities, the profile fits are much poorer than for the crystalline olivine template. If the presence of crystalline interstellar silicates in this distant galaxy is real, it would be highly unusual, given that the Milky Way interstellar matter contains essentially only amorphous silicates. It is possible that the z=0.886 absorber towards PKS 1830-211, well known for its high molecular content, has a unique star-forming environment that enables crystalline silicates to form and prevail.Research_files/ms.pdfshapeimage_1_link_0


In the MW, most of the core mass of interstellar dust grains is found in silicates. At higher redshifts, the existence of carbonaceous dust has been well-established, in at least some systems, through the presence of the 2175 Å bump and the shape of the rest-frame UV extinction curve. More recently, our group has firmly identified the presence of silicate dust in quasar absorbers through the detection of the 10 μm Si-O absorption feature (Kulkarni et al. 2007, 2011; Aller et al. 2011). Comparing observed silicate profiles with reference templates derived from the MW and laboratory experiments, we have ascertained that the dust in these quasar absorbers is most closely matched by either amorphous laboratory silicates of an olivine composition, or by diffuse Galactic interstellar clouds, although small residuals suggest that a crystalline silicate component may also be present.


Quasar-continuum-normalized spectrum of PKS 1830-211 in the rest-frame of the z=0.886 absorber, with the best fits produced by crystalline olivine in combination with either serpentine or silica material overplotted.


We are engaged in an on-going project combining Spitzer IRAC imaging with HST/ground-based optical imaging to characterize the properties of dust in low-redshift normal galaxies beyond the local group. The extragalactic dust is directly probed by utilizing partially superposed galaxy pairs, in which the foreground galaxy dust is backlit by the background galaxy. While this technique has previously been applied to some galaxy pairs, it has primarily used optical-wavelength data, which is subject to substantial amounts of extinction. We improve upon these studies by additionally utilizing IRAC 3.6 and 4.5 micron images which provide essentially un-extinguished reference images, and thus allow us to determine dust extinction more accurately across different parts of the foreground galaxies. Furthermore, we have studied a sample of local galaxy pairs spanning a broader range of morphologies. The very local nature of our sample allows a detailed look at dust properties at different positions within the galaxies, and facilitates an examination of what galaxy properties drive the variation in dust properties. We plan to specifically (1) address the opacity of sample spiral disks as a function of position within the galaxy disk; (2) examine whether the average dust grain size decreases in the outer parts of disks; (3) characterize the large-scale dust structure in a sample of elliptical galaxies; and (4) assess whether dust exhibits fractal structure.


The occulting galaxy method (White & Keel 1992) exploits assumed galaxy symmetry in a partially-overlapping, superposed galaxy pair, in order to isolate the foreground galaxy “backlit” dust absorption in the overlapping region. The surface brightness in this overlap-region is taken to be F+Be, where F and B are the surface brightnesses of the foreground and background galaxies, respectively. These components are removed by assuming that the surface brightness distributions in the non-overlapping portions of the galaxies are identical to those in the obscured, overlapping region. The foreground and background light can be estimated either by assuming rotational or reflection symmetry, or by modeling the predicted surface brightness distribution from the non-overlapping galaxy isophotes.

Schematic illustration of the occulting galaxy method, adapted from White & Keel 1992. The resultant e transmission map constrains the foreground galaxy opacity, as a function of position.


In order to place realistic constraints on galaxy evolutionary models, it is crucial to study a broad range of SF mechanisms and environments. Local polar ring galaxies (PRGs) provide an important glimpse into a potentially unique star-forming environment. PRGs consist of a polar ring (PR) comprised of gas, dust, and stars orbiting in a plane perpendicular to that of the host galaxy, and are largely believed to have formed through merger or accretion processes. The blue colors apparent in optical imaging of PRs indicates significant quantities of SF activity in the rings, which is necessarily stimulated by a mechanism other than spiral density waves. This raises the question of whether the SF in PRs is dominated by stochastic processes, such as supernova shocks propagating around the ring.


We are currently pursuing a project studying the star-formation mechanisms and stellar content in PRGs using recent Spitzer IRAC 3.6 and 4.5 μm images, in combination with imaging data at UV, optical, and radio wavelengths, as well as some spectroscopic data. We are using these multi-wavelength data to address a range of topics including: (1) the stellar masses of the rings; (2) the ages of the stellar material in the polar rings; (3) the star-formation triggering mechanisms in the polar rings; (4) the viability of proposed PRG formation models for our sample systems; (5) the opacity of the material in the portion of the PR which crosses in front of the illuminating host galaxy bulge; and (6) the IR Tully-Fisher relation for PRGs and constraints on the dark matter halo shapes.

Example multi-wavelength data for two of our sample polar ring galaxies.