Microwave Continuum Emission and Dense Gas Tracers in NGC 3627: Combining Jansky VLA and ALMA Observations

We present Karl G. Jansky Very Large Array (VLA) Ka band (33 GHz) and Atacama Large Millimeter Array (ALMA) Band 3 (94.5 GHz) continuum images covering the nucleus and two extranuclear star-forming regions within the nearby galaxy NGC 3627 (M 66), observed as part of the Star Formation in Radio Survey (SFRS). Both images achieve an angular resolution of $\lesssim$2\arcsec, allowing us to map the radio spectral indices and estimate thermal radio fractions at a linear resolution of $\lesssim$90 pc at the distance of NGC 3627. The thermal fraction at 33 GHz reaches unity at and around the peaks of each HII region; we additionally observed the spectral index between 33 and 94.5 GHz to become both increasingly negative and positive away from the peaks of the HII regions, indicating an increase of non-thermal extended emission from diffusing cosmic-ray electrons and the possible presence of cold dust, respectively. While the ALMA observations were optimized for collecting continuum data, they also detected line emission from the $J=1\rightarrow0$ transitions of HCN and HCO$^{+}$. The peaks of dense molecular gas traced by these two spectral lines are spatially offset from the peaks of the 33 and 94.5 GHz continuum emission for the case of the extranuclear star-forming regions, indicating that our data reach an angular resolution at which one can spatially distinguish sites of recent star formation from the sites of future star formation. Finally, we find trends of decreasing dense gas fraction and velocity dispersion with increasing star formation efficiency among the three regions observed, indicating that the dynamical state of the dense gas, rather than its abundance, plays a more significant role in the star formation process.Comment: 9 pages, 5 figures, accepted for publication in Ap

Complex Radio Spectral Energy Distributions in Luminous and Ultraluminous Infrared Galaxies

We use the Expanded Very Large Array to image radio continuum emission from local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) in 1 GHz windows centered at 4.7, 7.2, 29, and 36 GHz. This allows us to probe the integrated radio spectral energy distribution (SED) of the most energetic galaxies in the local universe. The 4-8 GHz flux densities agree well with previous measurements. They yield spectral indices \alpha \approx -0.67 (where F_\nu \propto \nu^\alpha) with \pm 0.15 (1\sigma) scatter, typical of nonthermal (synchrotron) emission from star-forming galaxies. The contrast of our 4-8 GHz data with literature 1.5 and 8.4 GHz flux densities gives further evidence for curvature of the radio SED of U/LIRGs. The SED appears flatter near \sim 1 GHz than near \sim 6 GHz, suggesting significant optical depth effects at the lower frequencies. The high frequency (28-37 GHz) flux densities are low compared to extrapolations from the 4-8 GHz data. We confirm and extend to higher frequency a previously observed deficit of high frequency radio emission for luminous starburst galaxies.Comment: 7 pages, 3 figures, 1 table, accepted for publication in the EVLA Special Issue of ApJ Letter

Key Science Goals for the Next Generation Very Large Array (ngVLA): Report from the ngVLA Science Advisory Council

This document describes some of the fundamental astrophysical problems that require observing capabilities at millimeter- and centimeter wavelengths well beyond those of existing, or already planned, telescopes. The results summarized in this report follow a solicitation from the National Radio Astronomy Observatory to develop key science cases for a future U. S.-led radio telescope, the "next generation Very Large Array" (ngVLA). The ngVLA will have roughly 10 times the collecting area of the Jansky VLA, operate at frequencies from 1 GHz to 116 GHz with up to 20 GHz of bandwidth, possess a compact core for high surface-brightness sensitivity, and extended baselines of at least hundreds of kilometers and ultimately across the continent to provide high-resolution imaging. The ngVLA builds on the scientific and technical legacy of the Jansky VLA and ALMA, and will be designed to provide the next leap forward in our understanding of planets, galaxies, and black holes.Comment: ngVLA memo 1

The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wavebands

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S3MC), which imaged the star-forming body of the Small Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the F_8/F_24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 um brightness and F_24/F_70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S3MC images to compile a photometric catalog of ~400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: 1) bright 5.8 um sources with very faint optical counterparts and very red mid-infrared colors ([5.8]-[8.0]>1.2), which we identify as YSOs. 2) Bright mid-infrared sources with mildly red colors (0.16<[5.8]-[8.0]<0.6), identified as carbon stars. 3) Bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars. And, 4) unreddened early B stars (B3 to O9) with a large 24 um excess. This excess is reminiscent of debris disks, and is detected in only a small fraction of these stars (<5%). The majority of the brightest infrared point sources in the SMC fall into groups one to three. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (~7%).Comment: Accepted for publication in The Astrophysical Journal. Given the draconian figure file-size limits implemented in astro-ph, readers are encouraged to download the manuscript with full quality images from http://celestial.berkeley.edu/spitzer/publications/s3mcsurvey.pd

Nominalization and Alternations in Biomedical Language

Background: This paper presents data on alternations in the argument structure of common domain-specific verbs and their associated verbal nominalizations in the PennBioIE corpus. Alternation is the term in theoretical linguistics for variations in the surface syntactic form of verbs, e.g. the different forms of stimulate in FSH stimulates follicular development and follicular development is stimulated by FSH. The data is used to assess the implications of alternations for biomedical text mining systems and to test the fit of the sublanguage model to biomedical texts. Methodology/Principal Findings: We examined 1,872 tokens of the ten most common domain-specific verbs or their zerorelated nouns in the PennBioIE corpus and labelled them for the presence or absence of three alternations. We then annotated the arguments of 746 tokens of the nominalizations related to these verbs and counted alternations related to the presence or absence of arguments and to the syntactic position of non-absent arguments. We found that alternations are quite common both for verbs and for nominalizations. We also found a previously undescribed alternation involving an adjectival present participle. Conclusions/Significance: We found that even in this semantically restricted domain, alternations are quite common, and alternations involving nominalizations are exceptionally diverse. Nonetheless, the sublanguage model applies to biomedica

Multi-Scale Stellar Associations across the Star Formation Hierarchy in PHANGS-HST Nearby Galaxies: Methodology and Properties

We develop a method to identify and determine the physical properties of stellar associations using Hubble Space Telescope (HST) NUV-U-B-V-I imaging of nearby galaxies from the PHANGS-HST survey. We apply a watershed algorithm to density maps constructed from point source catalogues Gaussian smoothed to multiple physical scales from 8 to 64 pc. We develop our method on two galaxies that span the distance range in the PHANGS-HST sample: NGC 3351 (10 Mpc), NGC 1566 (18 Mpc). We test our algorithm with different parameters such as the choice of detection band for the point source catalogue (NUV or V), source density image filtering methods, and absolute magnitude limits. We characterise the properties of the resulting multi-scale associations, including sizes, number of tracer stars, number of associations, photometry, as well as ages, masses, and reddening from Spectral Energy Distribution fitting. Our method successfully identifies structures that occupy loci in the UBVI colour-colour diagram consistent with previously published catalogues of clusters and associations. The median ages of the associations increases from log(age/yr) = 6.6 to log(age/yr) = 6.9 as the spatial scale increases from 8 pc to 64 pc for both galaxies. We find that the youngest stellar associations, with ages < 3 Myr, indeed closely trace H ii regions in H$\alpha$ imaging, and that older associations are increasingly anti-correlated with the H$\alpha$ emission. Owing to our new method, the PHANGS-HST multi-scale associations provide a far more complete census of recent star formation activity than found with previous cluster and compact association catalogues. The method presented here will be applied to the full sample of 38 PHANGS-HST galaxies.Comment: Submitted to MNRAS. Referee report received with minor comments, and "request to clarify if the smaller associations are always included in the larger ones and how this may affect the photometric fitting of the larger association if the groups have different ages." Revision in progres

PHANGS: Constraining Star Formation Timescales Using the Spatial Correlations of Star Clusters and Giant Molecular Clouds

In the hierarchical view of star formation, giant molecular gas clouds (GMCs) undergo fragmentation to form small-scale structures made up of stars and star clusters. Here we study the connection between young star clusters and cold gas across a range of extragalactic environments by combining the high resolution (1") PHANGS-ALMA catalogue of GMCs with the star cluster catalogues from PHANGS-HST. The star clusters are spatially matched with the GMCs across a sample of 11 nearby star-forming galaxies with a range of galactic environments (centres, bars, spiral arms, etc.). We find that after 4-6 Myr the star clusters are no longer associated with any gas clouds. Additionally, we measure the autocorrelation of the star clusters and GMCs as well as their cross-correlation to quantify the fractal nature of hierarchical star formation. Young ($\leq$ 10 Myr) star clusters are more strongly autocorrelated on kpc and smaller spatial scales than the >10 Myr stellar populations, indicating that the hierarchical structure dissolves over time.Comment: 15 pages, 11 figures, 4 tables. Accepted to MNRAS Sept 6 202

A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud

To investigate the fundamental principles of H2 formation in a giant molecular cloud (GMC), we derive the HI and H2 surface density (Sigma_HI and Sigma_H2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the HI data from the Galactic Arecibo L-band Feed Array HI Survey and an estimate of the local dust-to-gas ratio, we then derive the Sigma_H2 distribution across Perseus. We find a relatively uniform Sigma_HI ~ 6-8 Msun pc^-2 for both dark and star-forming regions, suggesting a minimum HI surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2. The transition between the HI- and H2-dominated regions occurs at N(HI)+2N(H2) ~ (8-14) x 10^20 cm^-2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of "CO-dark" gas, f_DG ~ 0.3. While significant spatial variations of f_DG are found, we do not find a clear correlation with the mean V-band extinction.Comment: updated to match the final version published in April 201