Scylla (PI: Murray) is a 500-orbit parallel project to ULLYSES, the HST/COS director's project. Scylla will observe alongside ULLYSES with HST/WFC3, to get resolved stellar population information in the Large and Small Magellanic Clouds (LMC & SMC), which will inform our understanding of how dust, gas, and stars relate across a range of metallicities. 

PAHs with JWST | Understanding the Smallest Dust Particles, Across Metallicities

In the mid-infrared part of the spectrum, emission from the ISM is dominated by carbonaceous emission features, produced by small carbonaceous particles that straddle the boundary between "large molecule" and "small dust grain". These particles are variously called carbonaceous nanoparticles, very small grains, or polycyclic aromatic hydrocarbons – PAHs. Although they make up less than one percent of the ISM by mass, PAHs account for up to 25% of the mid-infrared emission of star-forming galaxies, and dominate the photoelectric heating of the diffuse ISM.

But despite their importance, we only have limited understanding of what drives the abundance of PAHs, with respect to gas density, dust shielding, and radiation field intensity, ionized gas – and most especially metallicity. It has long been known that PAHs become much less abundant at metallicities <25% Solar. And until now, there has never been a detection of PAHs in a galaxies with metallicity <10% Solar.

Fortunately, the fantastic infrared capabilities of JWST are allowing members the ISM*@ST to tackle the questions surrounding PAHs. We are leading two JWST proposals targeting PAHs, totalling over 90 hours of observing time. One of these programs is searching for PAHs at the lowest metallicities ever probed, down to 7% Solar (PI Julia Roman-Duval, postdoc Elizabeth Tarantino). The other program is benchmarking how PAH properties vary across an order of magnitude of metallicity in M101 (PI Christopher Clark, postdoc Logan Jones).

Kinetic Tomography | Velocity fields in the Milky Way

Kinetic Tomography (KT) is the process of reconstructing the velocity structure of the interstellar medium. By being able to measure both the distance and radial velocity of a cloud we are able to constrain whether flows are converging or diverging along the line of sight. 

Classically, it has been possible to determine the velocity of a parcel of gas either by its emission (usually in the radio) or by spectral line absorption to background sources. Distances to ISM structures are determined by looking for reddening or absorption to background sources of known distance. With the recent explosion in the fidelity of these 3D dust reddening maps (see Green, Schlafly, & Finkbeiner 2015) it has become possible to try to combine this information. 

Our first results in this exploration have shown that there exist large-scale (many kpc) fast moving (up to 30 km/s) flows of gas in the interstellar medium of our Milky Way beyond the expected flat rotation curve. Our work on diffuse interstellar bands now further confirm these results. We are now exploring how this method can constrain theories of spiral structure and the Milky Way's disk-halo interface and accretion.

This project is funded by the National Science Foundation.

The BEAST | Mapping Dust & Stars in External Galaxies

The Bayesian Extinction and Stellar Tool (BEAST) fits the ultraviolet to near-infrared photometric SEDs of stars to extract stellar and dust extinction parameters. The stellar parameters are age (t), mass (M), and metallicity (Z). The dust extinction parameters are dust column (Av), average grain size (Rv), and mixing between type A and B extinction curves (f_A). 

The MegaBEAST is a currently under development extension that will provide maps based on the ensemble of BEAST results the correctly account for the star/dust geometry and spatially varying survey completeness.

The initial BEAST development was done as part of the Panchromatic Hubble Andromeda Treasury (PHAT) that mapped 1/3 of M31.  The BEAST has grown beyond PHAT and is being used to not only map dust in M31 (PHAT), but also in the LMC (HTTP), the SMC (SMIDGE), and M33 (PHAT Jr?).  

The BEAST technique is given by Gordon et al. (2016, ApJ, 826, 104).   The development of the open source BEAST code can be found at the BEAST github repository.

METAL | Depletions, extinction curves, and  Extinction Mapping in the LMC with Hubble

METAL is a large HST program to study the dust properties and abundance in the Large Magellanic Cloud (LMC). Interstellar dust is a key component of galaxy evolution owing to its crucial role in the chemistry and radiative transfer in galaxies. Our interpretation of extragalactic SEDs and our understanding of galaxy evolution thus critically depend on an accurate characterization of how the dust content and properties in a galaxy vary with a range of environmental parameters (metallicity, density, dynamics). Recent observations and modeling suggest that dust grains must grow in the ISM to explain dust masses over cosmic times, leading to changes in the abundance, composition, size, and optical properties of dust grains with environment. Until recently, we were however lacking a comprehensive set of depletion and extinction curve measurements required to characterize dust evolution. The METAL program obtained 33 COS and STIS low and medium resolution spectra in the LMC, which has 1/2 solar metallicity. The goal is to obtain a large set of depletions for the main consistuents of dust over a range of column densities, along with UV extinction curves. By combining the METAL measurements with a set of 21 sightlines in the SMC, which has 1/5 solar metallicity, we are able for the first time to measure the dust-to-gas ratio (D/G), dust-to-metal ratio (D/M), the dust composition, size, and optical properties as a function of surface density and dynamics, characterize dust growth and destruction timescales as a function of metallicity, and test the predicted relation between surface density, D/G, and D/M in models of galaxy evolution. In parallel, METAL obtained WFC3 NUV-NIR imaging to map dust extinction parameters (AV, RV) in the vicinity of our targets and calibrate the FIR emissivity of dust. 

DUSTiNGS | Dust Production in Nearby Galaxies

DUSTiNGS began as a Spitzer program that surveyed 50 nearby dwarf galaxies at 3.6 and 4.5 microns to search for dust-producing evolved stars. These stars have similar IR colors as young evolved stars and unresolved background galaxies (AGN/quasars), so we leveraged stellar pulsation using two epochs of data to distinguish them from other dusty objects. We discovered >500 candidate Asymptotic Giant Branch (AGB) stars, each producing as much dust as the dustiest 'extreme' AGB stars in the Magellanic Clouds that dominate the dust budget (e.g., Srinivasan et al. 2016, MNRAS, 457, 2814). Furthermore, we found no evidence for a decrease in the dust production with metallicity, indicating that AGB stars can produce a substantial amount of dust even in the metal-poor early Universe. [Boyer et al. 2015, ApJS, 216, 10; Boyer et al. 2015, ApJ, 800, 51; McQuinn et al. 2017, 834, 78]

We have since followed up on DUSTiNGS stars with additional Spitzer and HST WFC3/IR data. The HST data use medium-band filters (F127M, F139M, F153M) to separate carbon stars from oxygen-rich stars. Initial results from this program show that, while most DUSTiNGS stars are carbon-rich, there is a small population of oxygen-rich (or M type), dust-producing stars. Theoretical models have no mechanism for dust production in metal-poor M type stars, but we see evidence for dust excess around M-type stars even in the most metal-poor galaxies in our sample (12 + log(O/H) = 7.26–7.50). The low metallicities and inferred high stellar masses (up to ∼10 M_sun) suggest that AGB stars can produce dust very early in the evolution of galaxies (∼30 Myr after they form), and may contribute significantly to the dust reservoirs seen in high-redshift galaxies. [Boyer et al. 2017, ApJ, 851, 152]

We have also obtained time series Spitzer followup of the DUSTiNGS galaxies to measure the stellar light curves and investigate how pulsation and dust production are related. [Goldman et al., 2019]

The next step is to investigate the dust properties (mass and mineralogy) of the DUSTiNGS stars, which are the most metal-poor dusty stars known, and compare that to the ISM dust properties in their host galaxies.  Here, JWST is key.

Swimming with Sharks | Ultraviolet Reflection Nebulae

The Sharks project is a collaboration between PIs Peek, Murray, and Gordon, Drs. Hannah Bish and Catherine Zucker, and Professors Erika Hamden (Arizona) and Susan Clark (Stanford) to unravel some of the dusty mysteries of the Galactic high latitude ISM using reflection nebulae from GALEX. The project leverages three data sets. The first is the deep GALEX imaging in the NUV and the FUV. The second is precision distances to pretty much every FUV and NUV emitting source in the solar neighborhood. The last is PI Murray's new neural-network based maps of the ISM which decompose it into its constituent pieces, phase by phase. By combining these things we can find the distances to these nebulae and how the dust they hold is scattering UV light, and therefore how that dust is evolving.