Overview
My research focuses on understanding the role of the interstellar medium –- gas, dust, and magnetic fields -- in the evolution of galaxies using the most advanced radio telescopes in the world, including the Atacama Large Millimeter/submillimeter Array (ALMA), the Karl G. Jansky Very Large Array (VLA), and the Robert C. Byrd Green Bank Telescope (GBT). It's an exciting time to be a radio astronomer: these telescopes are all either brand-new or have been significantly upgraded in the last five years. Many new discoveries await!
How is the molecular gas in galaxies related to their star formation?
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The fundamental process at the heart of all star formation theories is the conversion of gas into stars. A key test of these theories, therefore, involves quantifying the observed relationship between gas density and star formation under a wide range of physical conditions. Observations in the Milky Way provide excellent spatial resolution for studying this relationship, but only probe a limited range of physical conditions. Using nearby galaxies, we can access a much wider range of physical conditions while retaining moderate spatial resolution (100s of pc). Until recently, however, mapping the dense molecular gas from which stars presumably form in nearby galaxies was difficult. The primary dense gas tracer HCN (and its close cousin HCO+) is 10-30 times fainter than the most commonly observed molecular gas tracer CO, limiting previous studies to single pointings or single galaxies. These early results show that while the ratio of dense gas to star formation is relatively constant within the Milky Way and across entire galaxy disks, it does vary as a function of environment within galaxies.
Today we can map dense molecular gas tracers across large samples of nearby galaxies using the excellent sensitivities of telescopes like ALMA and the GBT. The 4mm system in the GBT, in particular, has the potential to be an incredible dense gas survey instrument because of its large collecting area, excellent surface accuracy, and good resolution (10′′at 90GHz, comparable to HI surveys). I am leading an on-going project to map the distribution of dense gas tracers in nearby galaxies using the GBT. As a first step, we made the deepest HCN and HCO+ maps to date of the nearby starburst galaxy M82 using the current, effectively single- pixel, 4mm receiver on the GBT (Kepley+ 2014). We found that the relationship between star formation and dense molecular gas in M82 changes with distance from the central starburst. For the first time, we have also identified dense molecular gas (as traced by HCO+) associated with the previously identified CO outflow. These two pieces of evidence suggest that the starburst may play a critical role in regulating star formation in M82 by removing or destroying the fuel for star formation. ARGUS – the new 16-pixel, 4 mm receiver receiver for the GBT – will dramatically increase the GBT survey speed, making mapping large numbers of normal galaxies feasible. I have an accepted GBT shared-risk proposal to use ARGUS to make the highest resolution map to date of dense gas across the dish of an entire disk of a normal spiral galaxy and another survey proposal under consideration to map dense gas tracers in a sample of 36 galaxies. The ultimate goal of both of these projects is to quantify how the fraction of dense gas and ratio of star formation to dense gas change as a function of environment within a galaxy to constrain star formation theories. |
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Dwarf starburst galaxies – also often referred to as blue compact dwarf galaxies – are characterized by their low metallicities, low masses, and high star formation rate surface densities). These combined properties produce one of the most extreme environments for star formation in the local universe, one more akin to that found in high redshift galaxies than in local spirals. Under these conditions, there are fewer molecules, less dust to shield CO and to serve as a formation site for molecular hydrogen, and more feedback and dissociation from young massive stars. As a consequence, different physical effects may dominate the conversion of molecular gas into stars. To understand star formation in dwarf starburst galaxies, we need to quantify the relationship between their young massive stars and molecular gas.
As a case study, I have published a pair of papers exploring the young massive stars and molecular gas in the prototypical dwarf starburst galaxy: II Zw 40. Using high resolution VLA continuum images, we find that three clusters within the central star-forming region of II Zw 40 have masses greater than 30 Doradus, the closest example of a super star cluster. ALMA observations of the molecular gas in this system show that the molecular gas clouds in II Zw 40 have sizes and linewidths similar to those found in the Antennae, a late stage major merger. Together, these observations – along with the tidal features seen in neutral hydrogen – suggest a picture where the current burst of star formation within II Zw 40 is driven by its ongoing merger, not its low mass or metallicity. II Zw 40, however, may not be representative of dwarf starburst galaxies as a whole. I am in the process of expanding this analysis to a larger sample of dwarf starburst galaxies as part of the Fireflies survey. VLA observations of the young massive stars within the Fireflies sample are on-going and we have a recently accepted ALMA Cycle 4 proposal to attempt to detect CO -- the primary molecular gas tracer -- in the lowest metallicity galaxies in our sample. |
What role do magnetic fields play in shaping the dust and gas in galaxies?
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Magnetic fields are an important source of pressure in galaxies. They also influence gas dynamics in galaxies by channeling gas flows, preventing or delaying the breakout of bubbles of hot gas, and distributing and accelerating high energy particles. The handful of magnetic field measurements in low mass galaxies find that their magnetic field strengths are comparable to those found in more massive spiral galaxies. Strong fields in low mass galaxies imply that their magnetic fields may be more dynamically important than in spirals and challenge the standard α-ω dynamo mechanism assumed to amplify galactic magnetic fields because it requires larger shears than typically found in these galaxies.
My dissertation explored these issues using observations of the synchrotron emission, which allows us to derive the magnetic field strength and structure, from two irregular galaxies. Despite their low mass, the magnetic field pressures in these galaxies are comparable to their hot gas pressures and their gravitational “pressures.” The magnetic field structures in each galaxy appears to depend on its star formation and merger history and their fields may be amplified by a variety of mechanisms including a supernova-driven dynamo or a turbulent dynamo. Measurements of magnetic fields in nearby galaxies have been limited because they require very sensitive observations. Today the phenomenal sensitivity of the VLA enables more sensitive and detailed magnetic field observations than ever before. The CHANG-ES survey of edge-on galaxies (PI: J. Irwin) takes advantage of these new capabilities to probe the halos of galaxies including their magnetic field structure, cosmic ray population, and the ionized gas content. With Megan Johnson (USNO), I am leading the GBT portion of the survey to obtain information on the large-scale emission in these galaxies. |
What are the properties of young massive stars?
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Observations of molecular gas in galaxies only tell half the story of star formation. A complete picture of star formation needs information on both its raw materials (molecular gas) and its end product (newly formed stars). Typically, stars are studied in the optical, but the dust surrounding young star-forming regions blocks the optical emission from stars. Longer wavelength tracers are required to penetrate the dust and measure the properties of the young massive stars.
In the radio, we have access to two such tracers: free-free emission and radio recombination lines (RRLs). Free-free emission is easier to observe, but it only offers limited information about the ionized gas including estimates of ionizing photon fluxes, cluster masses, and the total star formation rates. In contrast, RRLs are more difficult to observe because they are both faint (a few percent of the continuum) and broad (100 km/s), but can tell us about the filling factor and density of the ionized gas and its kinematics. At present, only ~12 galaxies have RRL detections. Telescopes like the GBT and the VLA as well as ALMA are poised to change this situation with their superior bandwidth and sensitivity. Through the VLA resident shared risk program, I tested using the new VLA Ka-band receiver to observe RRLs in the nuclear starburst galaxy NGC 253 and tested a new correlator mode designed to simultaneously detect multiple RRLs and continuum at lower frequencies. |