Identifying Variations to the IMF at High-z Through Deep Radio Surveys

In this article I briefly describe how deep radio surveys may provide a means to identify variations in the upper end of the initial mass function (IMF) in star-forming galaxies at high redshifts (i.e., z ≳ 3). At such high redshifts, I argue that deep radio continuum observations at frequencies ≳10 GHz using next generation facilities (e.g., EVLA, MeerKAT, SKA/NAA) will likely provide the most accurate measurements for the ionizing photon rates (star formation rates; SFRs) of normal galaxies since their non-thermal emission should be highly suppressed due to the increased inverse Compton (IC) losses from the cosmic microwave background (CMB), leaving only thermal (free-free) emission detectable. Thus, a careful analysis of such observations in combination with future ALMA and JWST data, measuring the rest-frame far-infrared and UV emission from the same population of galaxies, may yield the best means to search for variability in the stellar IMF at such epochs

The VLA Sky Survey

The VLA All-Sky Survey (VLASS) builds upon the newly upgraded Karl G. Jansky Very Large Array (VLA) to employ its enhanced bandwidth, time resolution, and survey speed to carry out a next-generation sky survey. VLASS will map the sky at δ > −40°, covering a total of 33,885 square degrees, with 2.005 angular resolution using the VLA B-array at 2–4 GHz, with an expected rms of 69 µJy/beam in the final images. The survey will be carried out over three epochs spanning 32 months, providing a synoptic view of the dynamic radio sky similar to those now available through the new generation of synoptic imagers at other wavelengths. Consequently, VLASS will provide unique measurements of the radio sky at key epochs and sensitivity levels between that from FIRST and NVSS and the new upcoming radio surveys. This will be a critical enabler for early identification and filtering for the most interesting transient events. In this proceeding we highlight some of the key science that will be delivered by VLASS that make optimal utilization of the Jansky VLA’s unique capabilities: high resolution imaging and exquisite point-source sensitivity, critical for source identification; wide bandwidth coverage, enabling instantaneous spectral index determination; and full polarimetry with good performance even in lines of sight with high Faraday depth, enabling instantaneous rotation measure and Faraday structure determinations

The Role of Merger Stage on Galaxy Radio Spectra in Local Infrared-Bright Starburst Galaxies

An investigation of the steep, high-frequency (i.e., ~12 GHz) radio spectra among a sample of 31 local infrared-bright starburst galaxies is carried out in light of their HST-based merger classifications. Radio data covering as many as 10 individual bands allows for spectral indices to be measured over three frequency bins between 0.15-32.5 GHz. Sources having the flattest spectral indices measured at ~2 and 4 GHz, arising from large free-free optical depths among the densest starbursts, appear to be in ongoing through post-stage mergers. The spectral indices measured at higher frequencies (i.e., ~12 GHz) are steepest for sources associated with ongoing mergers in which their nuclei are distinct, but either share a common stellar envelope and/or exhibit tidal tails. These results hold after excluding potential AGN based on their low 6.2µm PAH EQWs. Consequently, the low-, mid-, and high-frequency spectral indices each appear to be sensitive to the exact merger stage. It is additionally shown that ongoing mergers, whose progenitors are still separated and share a common envelope and/or exhibit tidal tails, also exhibit excess radio emission relative to what is expected given the far-infrared/radio correlation, suggesting that there may be a significant amount of radio emission that is not associated with ongoing star formation. The combination of these observations, along with high-resolution radio morphologies, leads to a picture in which the steep high-frequency radio spectral indices and excess radio emission arises from radio continuum bridges and tidal tails that are not associated with star formation, similar to what is observed for so-called "taffy" galaxies. This scenario may also explain the seemingly low far-infrared/radio ratios measured for many high-z submillimeter galaxies, a number of which are merger-driven starbursts

The Relative Importance of Thermal Gas, Radiation, and Magnetic Pressures Around Star-Forming Regions in Normal Galaxies and Dusty Starbursts

In this paper, an investigation on the relative importance of the thermal gas, radiation, and (minimum-energy) magnetic pressures around $\approx$200 star-forming regions in a sample of nearby normal and luminous infrared galaxies is presented. Given the range of galaxy distances, pressure estimates are made on spatial scales spanning $\sim$0.1$-3$kpc. The ratio of thermal gas-to-radiation pressures does not appear to significantly depend on star formation rate surface density ($\Sigma_{\rm SFR}$), but exhibits a steady decrease with increasing physical size of the aperture over which the quantities are measured. The ratio of magnetic-to-radiation pressures appears to be relatively flat as a function of $\Sigma_{\rm SFR}$ and similar in value for both nuclear and extranuclear regions, but unlike the ratio of thermal gas-to-radiation pressures, exhibits a steady increase with increasing aperture size. Furthermore, it seems that the magnetic pressure is typically weaker than the radiation pressure on sub-kpc scales, and only starts to play a significant role on few-kpc scales. When the internal pressure terms are summed, their ratio to the ($\Sigma_{\rm SFR}$-inferred) kpc-scale dynamical equilibrium pressure estimates is roughly constant. Consequently, it appears that the physical area of the galaxy disk, and not necessarily environment (e.g., nuclear vs. extranuclear regions) or star formation activity, may play the dominant role in determining which pressure term is most active around star-forming regions. These results are consistent with a scenario in which a combination of processes acting primarily on different physical scales work collectively to regulate the star formation process in galaxy disks.Comment: 21 pages, 9 figures, accepted for publication in Ap

The Role of Radio Observations in Studies of Infrared-Bright Galaxies: Prospects for a next-generation Very Large Array

The bulk of the present-day stellar mass was formed in galaxies when the universe was less than half its current age (i.e., $1 \lesssim z \lesssim 3$). While this likely marks one of the most critical time periods for galaxy evolution, we currently do not have a clear picture on the radial extent and distribution of cold molecular gas and associated star formation within the disks of galaxies during this epoch. Such observations are essential to properly estimate the efficiency at which such galaxies convert their gas into stars, as well as to account for the various energetic processes that govern this efficiency. Long-wavelength (i.e., far-infrared--to--radio) observations are critical to penetrate the high-levels of extinction associated with dusty, infrared-bright galaxies that are driving the stellar mass assembly at such epochs. In this article we discuss how the next-generation Very Large Array will take a transformative step in our understanding of galaxy formation and evolution by delivering the ability to simultaneously study the relative distributions molecular gas and star formation on sub-kpc scales unbiased by dust for large populations of typical galaxies in the early universe detected by future far-infrared space missions.Comment: 20 pages, 5 figures; Invited review for the special issue "Recent Advances in Infrared Galaxies and AGN", edited by Anna Sajina and Asantha R. Cooray, in Univers

A stellar census of the nearby, young 32 Orionis group

The 32 Orionis group was discovered almost a decade ago and despite the fact that it represents the first northern, young (age ~ 25 Myr) stellar aggregate within 100 pc of the Sun ($d \simeq 93$ pc), a comprehensive survey for members and detailed characterisation of the group has yet to be performed. We present the first large-scale spectroscopic survey for new (predominantly M-type) members of the group after combining kinematic and photometric data to select candidates with Galactic space motion and positions in colour-magnitude space consistent with membership. We identify 30 new members, increasing the number of known 32 Ori group members by a factor of three and bringing the total number of identified members to 46, spanning spectral types B5 to L1. We also identify the lithium depletion boundary (LDB) of the group, i.e. the luminosity at which lithium remains unburnt in a coeval population. We estimate the age of the 32 Ori group independently using both isochronal fitting and LDB analyses and find it is essentially coeval with the {\beta} Pictoris moving group, with an age of $24\pm4$ Myr. Finally, we have also searched for circumstellar disc hosts utilising the AllWISE catalogue. Although we find no evidence for warm, dusty discs, we identify several stars with excess emission in the WISE W4-band at 22 {\mu}m. Based on the limited number of W4 detections we estimate a debris disc fraction of $32^{+12}_{-8}$ per cent for the 32 Ori group.Comment: Accepted for publication in MNRAS; 24 pages, 17 figures and 10 table

Gendered Paths to Formal and Informal Resources in Post-Disaster Development in the Ecuadorian Andes

The devastating eruptions of Mount Tungurahua in the Ecuadorian highlands in 1999 and 2006 left many communities struggling to rebuild their homes and others permanently displaced to settlements built by state and nongovernmental organizations. For several years afterward, households diversified their economic strategies to compensate for losses, communities organized to promote local development, and the state and nongovernmental organizations sponsored many economic recovery programs in the affected communities. Our study examined the ways in which gender and gender roles were associated with different levels and paths of access to scarce resources in these communities. Specifically, this article contrasts the experiences of men and women in accessing household necessities and project assistance through formal institutions and informal networks. We found that women and men used different types of informal social support networks, with men receiving significantly more material, emotional, and informational support than women. We also found that men and women experienced different challenges and advantages when pursuing support through local and extralocal institutions and that these institutions often coordinated in ways that reified their biases. We present a methodology that is replicable in a wide variety of disaster, resettlement, and development settings, and we advocate an inductive, evidence-based approach to policy, built upon an understanding of local gender, class, and ethnic dynamics affecting access to formal and informal resources. This evidence should be used to build more robust local institutions that can resist wider social and cultural pressures for male dominance and gendered exclusion