Dusty Research

(written 2002, needs updating)

My research concentrates on understanding the basic properties of dust grains and the interaction of dust with its environment. As such, I am interested in dust in nearby, simple systems (eg., reflection nebulae, circumstellar disks) as well as dust in more distant, complex systems (eg., starburst & spiral galaxies, active galactic nuclei). The study of nearby systems gives me the opportunity to examine dust grain properties in detail. The study of more distant systems increases the range of environments in which dust grains can be investigated. This combination results in a deeper understanding of dust grain properties and their environmental dependence, with this deeper understanding of dust allowing for a more accurate accounting for the effects of dust in observations of stars, nebulae, and galaxies.

Extragalactic: [Nearby Galaxies], [Active Galactic Nuclei], [High-Redshift]
Galactic: [Nebulae], [Extinction Curves]
DIRTY model


Extragalactic

My research in the coming years will focus on understanding nearby galaxies, active galactic nuclei, and what this tells us about objects at high-redshifts. Galaxies give me the opportunity to study dust in systems which have a much wider range of environments than can be found in the Milky Way or Magellanic Clouds.

Nearby Galaxies

As part of the Multiband Imaging Photometer for Spitzer (MIPS) Science Team, I am leading a project to study giant HII regions in M101. M101 is a nearby spiral galaxy with the largest metallicity gradient known. Planned observations of M101 with the Spitzer Space Telescope (SST) are imaging in 7 bands from 3.5 to 160 µm as well as targeted spectroscopy from 5 to 40 µm of 10 giant HII regions spanning a large range of metallicities. These observations will be combined with new and archival ultraviolet (GALEX & UIT) and optical data to probe the behavior of the dust (PAH features, hot dust, cold dust) as a function of metallicity and star formation rates in a single galaxy. Similar, but less extensive observations are also planned for M31 and M33.

I am deputy-PI of a MIPS Science Team program (PI Charles Engelbracht) to study the properties of starburst galaxies as a function of metallicity. This project will provide imaging from 3.5 to 160 µm as well as spectroscopy from 5 to 40 µm of a sample of 44 starbursts chosen to span the full range of metallicities found in starbursts (log(O/H) = 7.1 to 9.2). New ultraviolet (GALEX), optical, near-infrared imaging and spectroscopy will be combined with the SST observations to enable an excellent characterization of the starburst phenomenon. Such observations of a large sample will allow us to determine how the properties of dust vary over this large metallicity range as well as to accurately account for the effects of dust. This accounting will enable us to study the properties of the stars and gas in these systems without systematic biases due to difficulties in removing the effects of dust.

I am a Co-Investigator of the Spitzer Infrared Nearby Galaxies Survey (SINGS, PI Robert Kennicutt, Kennicutt et al. 2003) project which will use the SST to image 75 galaxies from 3.5 to 160 µm and obtain spectra from 5 to 40 µm of radial strips and nuclei of all 75 galaxies as well as targeted HII regions in a handful of the galaxies. The galaxies in this project span the range of known galaxies. In addition to the SST observations, an extensive, ongoing ground-based program will provide optical, near-infrared, millimeter, and radio observations for most, if not all, of the SINGS galaxies. GALEX is also observing all 75 SINGS galaxies in the ultraviolet and many galaxies have been observed (or will be) with Chandra. As a result, these 75 galaxies will be well observed at almost every wavelength possible which will enable a great deal of science. My specific interests in this project are to understand the behavior of dust and star formation rates in this sample of galaxies. The goals for understanding the dust are the same as described above. I am interested in understanding how dust affects different star formation rate (SFR) indicators (ultraviolet, Hα, infrared, and radio). Properly accounting for the effects of dust should greatly improve the agreement between these SFR indicators as well as point us in the direction of using there remaining differences to probe the star formation history of each galaxy. This is possible as the SFR indicators probe different ages (ie., Hα probes to around 10 Myr, ultraviolet probes to around 100 Myr, and infrared probes all ages).

Part of my work on these three projects will be to use the DIRTY radiative transfer model to accurately study and account for the effects of dust in galaxies. For example, the DIRTY models of simple spherical galactic environments (Witt & Gordon 2000) has shown that ultraviolet observations of starburst galaxies can only be explained if the dust in starbursts is like that found in the Small Magellanic Cloud (Gordon, Calzetti, & Witt 1997). In other words, the dust in starburst galaxies does not have a 2175 Å bump which is ubiquitous in the Milky Way (Gordon et al. 2003). More recently, DIRTY models of disk+bulge galaxies have been run and are being analyzed. The combination of these two different model geometries will be important in understanding dust in the samples described above.

Active Galactic Nuclei

Active Galactic Nuclei (AGN) are interesting from a dust point of view as they provide the a very extreme environment while having a fairly simple geometry. AGNs are also seen to a high redshift providing a a probe of the evolution of dust over the age of the universe. I am a Co-Investigator on a MIPS Science Team program (PI Dean Hines) to investigate the ultraviolet through infrared spectral energy distributions (SEDs) of a large sample of AGN. Most of the dust radiative transfer models in use in the AGN community are restricted to either the optically thin or optically thick cases. The DIRTY radiative transfer model does not suffer from such a restriction and allows for the full range of possible cases. Preliminary work on UV and optical spectroscopy has shown the important of a realistic dust radiative transfer model (Hines et al. 2001). My contribution to this project will be to produce a large range of DIRTY models to probe the dust geometry and type of dust grains found in AGN.

High-Redshift

One of the large projects being undertaken by the MIPS Science Team (as well as other MIPS/GTO Teams) is 3.5 to 160 µm imaging of a number of Deep Fields to study the evolution of galaxies and AGN. These fields have been carefully chosen for many reasons including overlap with archival data. As such, it is expected that many galaxies and AGN will be found at a range of redshifts which will have fairly complete SEDs. My contribution to this large project will be to provide DIRTY models (which have been calibrated in the local universe) showing where galaxies and AGN should exist in various color-color diagrams as well as to study in more detail specific objects which have fairly complete SEDs.


Galactic

Nebulae

The reflection nebulae I study are usually a young, hot, early-type star embedded in and illuminating its natal interstellar cloud. Such systems give me the opportunity to study the extinction properties of the cloud (extinction curve to the embedded star), the dust grain scattering properties (ultraviolet to near-infrared images of the nebulae), and the dust emission properties (optical through submillimeter images of the nebulae).

In the area of dust scattering properties, I am currently involved in a project to determine the far-ultraviolet, ultraviolet, and optical dust grain scattering properties of NGC 2023, NGC 7023, and IC 435 (using a combination of FUSE, UIT, IUE, optical telescopes, and IRAS/ SST). This project uses the DIRTY radiative transfer model to interpret nebular images and spectroscopy resulting in empirical determinations of the albedo and scattering phase function of dust. See Gordon (2004) for a review of our current knowledge of dust scattering properties.

In the area of dust emissions, I concentrate mainly on understanding the properties of Extended Red Emission (ERE). ERE is a red photoluminescence from dust which has been detected in many dusty astrophysical objects (Gordon et al. 1998). I am leading a project in this area to use HST ACS and NICMOS observations to determine the exciting wavelength of ERE by observing narrow filaments in NGC 2023 and NGC 7023 (6 orbits). In the pursuit of ERE, I have been involved in discovering two new emissions from dust. The first is in the near-infrared (around 1.55 µm) and has been attributed to FeSi2 grains (Gordon et al. 2000). The second is in the blue (around 4000 Å) and is attributed to photoluminescence from large Polycyclic Aromatic Hydrocarbons (PAHs). Continued, sensitive observations of reflection nebulae hold the promise of understanding these two emissions as well as possibly finding other, as yet undiscovered emissions from dust.

Extinction Curves

Extinction curves and their interpretation provide an opportunity to study dust grain properties in the general interstellar medium in the Milky Way and nearby galaxies. Although extinction curves give less information about dust grain properties than the combined observations of a reflection nebulae as it only probes the combined absorption and scattering properties of dust, it does allow for the study of a larger range of environments than reflection nebulae and extends our knowledge of dust to nearby galaxies.

My primary project in this area concentrates on characterizing and understanding the detailed structure of extinction curves from the ultraviolet through mid-infrared. Ultraviolet extinction curves have been well characterized at better than 10 Å resolution for a range of environments (eg., the R(V) dependent CCM relationship). Optical, near-infrared, and mid-infrared extinction curves are mainly studied at broad and sometimes narrow band resolutions. To remedy this disparity, I have started a program to determine the optical through mid-infrared extinction curves for a sample of around 60 sightlines at 10 Å resolution. I have completed the observations of the blue regions (3300 to 6000 Å) using the Steward Observatory 2.3m, have plans to observe the red region (5500 to 8500 Å) with the same telescope, have proposed to obtain spectra for the near-infrared using IRTF/ SpeX (0.8 to 5.5 µm), and plan to submit at proposal to SST/ IRS to obtain 5 to 40 µm spectra for the more reddened portion of this sample. The result of such work will shed light on dust grain properties and enable more accurate corrections for dust at spectroscopic instead of broad-band resolutions.

In addition to this large and fairly new project, I am working on a handful of other projects in the area of extinction curves. I am leading a team which is using HST/ STIS to determine the abundances of two sightlines in the Small Magellanic Cloud (44 orbits). One sightline does not show a 2175 Å bump and has a strong far-ultraviolet rise while the other sightline shows a 2175 Å bump and a Milky Way-like far-ultraviolet rise. Thus, understanding the abundances of these sightlines should directly probe the materials responsible for these prominent dust features for the first time. I also lead a team to obtain spectra of a large sample of hot, reddened stars in the Small Magellanic Cloud (SMC) to determine precise 2D MK spectral types. This work is aimed at producing good candidates for an HST proposal to extend the known extinction curves from 5 to at least 15 in this object. The SMC is the only place where dust like that found in starburst galaxies in the local and high redshift universe is seen. Thus, studying the dust in the SMC is crucial to understanding the dust in starbursts. Finally, I am part of a large project to determine the far-ultraviolet extinction for dust (900 to 1150 Å) in the Milky Way and Magellanic Clouds, this work provides a sensitive probe of small dust grains in the interstellar medium.


DIRTY Model

One of the main tools which I use in my research and continue to improve is the DIRTY (DustI Radiative Transfer, Yeah!) model. This models computes the radiative transfer of photons through dust for arbitrary distributions of both photon emitters and dust using Monte Carlo techniques. Photon emitters can either be stars, gas, accretion disks, etc.\ or the dust itself. The emission from dust is handled self-consistently (self absorption is allowed) in the DIRTY model including PAH emission, small particle, non-equilibrium emission, and large particle, equilibrium emission. The use of Monte Carlo techniques is required by the non-isotropy of dust scattering and the need to model non-symmetrical geometries. The DIRTY model is the result of work by myself (Monte Carlo radiative transfer) and that of Karl Misselt (dust emission) and is described in detail in Gordon et al. (2001) and Misselt et al. (2001).

Currently, we are working on adding the ability to use an active/adaptive mesh to the DIRTY model to allow for a large range of size scales to be simultaneously models. For example, this will make it possible to accurately compute the radiative transfer and dust emission for AGN including both the relative small accretion disk and much larger scattering region. Further improvements to the DIRTY model include releasing it to the community and producing a parallelized version to allow for the computation of even more complex models.


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