Jean L. Turner

CALIBRATING EXTINCTION-FREE STAR FORMATION RATE DIAGNOSTICS WITH 33 GHz FREE-FREE EMISSION IN NGC 6946

Using free-free emission measured in the Ka band (26-40 GHz) for 10 star-forming regions in the nearby galaxy NGC 6946, including its starbursting nucleus, we compare a number of star formation rate (SFR) diagnostics that are typically considered to be unaffected by interstellar extinction. These diagnostics include non-thermal radio (i.e., 1.4 GHz), total infrared (IR; 8-1000 μm), and warm dust (i.e., 24 μm) emission, along with hybrid indicators that attempt to account for obscured and unobscured emission from star-forming regions including Hα + 24 μm and UV + IR measurements. The assumption is made that the 33 GHz free-free emission provides the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24 μm, Hα + 24 μm, and UV + IR SFR calibrations are in good agreement with the 33 GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimates the nuclear SFR by a factor of ~2 relative to the 33 GHz free-free SFR. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the 33 GHz free-free SFRs for the nucleus and underestimate the SFRs from the extranuclear star-forming regions by an average factor of ~2 and ~4-5 before and after subtracting local background emission, respectively. This result likely arises from the cosmic-ray (CR) electrons decaying within the starburst region with negligible escape, whereas the transient nature of star formation in the young extranuclear star-forming complexes allows for CR electrons to diffuse significantly further than dust-heating photons, resulting in an underestimate of the true SFR. Finally, we find that the SFRs estimated using the total 33 GHz flux density appear to agree well with those estimated using free-free emission due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33 GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

Spatially Resolved Chemistry in Nearby Galaxies. I. The Center of IC 342

We have imaged emission from the millimeter lines of eight molecules—C2H, C 34 S, N2H + ,C H3OH, HNCO, HNC, HC3N, and SO—in the central half-kiloparsec of the nearby spiral galaxy IC 342. The 5 00 (� 50 pc) resolution images were made with the Owens Valley Millimeter Array. Using these and previously published CO and HCN images, we obtain a picture of the chemistry within the nuclear region on the size scales of individual giant molecular clouds. Bright emission is detected from all but SO. There are marked differences in morphology for the different molecules. A principal-component analysis is performed to quantify similarities and differences among the images. This analysis reveals that while all molecules are to zeroth order correlated, that is, that they are all found in dense molecular clouds, there are three distinct groups of molecules distinguished by the location of their emission within the nuclear region. N2H + ,C 18 O, HNC, and HCN are widespread and bright, good overall tracers of dense molecular gas. C2 Ha nd C 34 S, tracers of photodissociation region chemistry, originate exclusively from the central 50–100 pc region, where radiation fields are high. The third group of molecules, CH3OH and HNCO, correlates well with the expected locations of bar-induced orbital shocks. The good correlation of HNCO with the established shock tracer molecule CH3OH is evidence that this molecule, whose chemistry has been uncertain, is indeed produced by the processing of grain mantles. HC3N is observed to correlate tightly with 3 mm continuum emission, demonstrating that the young starbursts are the sites of the warmest and densest molecular gas. We compare our HNC images with the HCN images of Downes and coworkers to produce the first highresolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unity across the nucleus, and the correlation of both of these gas tracers with star formation is excellent. The ratio exhibits no obvious correlation with gas temperature or star formation strength. Subject headings: astrochemistry — galaxies: individual (IC 342) — galaxies: ISM — galaxies: starburst — radio lines: galaxies

The First ALMA view of IRAS 16293-2422: Direct detection of infall onto source B and high-resolution kinematics of source A

Aims: In this paper, we focus on the kinematical properties of a proto-binaryto study the infall and rotation of gas towards its two protostellarcomponents. Methods: We present ALMA Science Verification observations withhigh-spectral resolution of IRAS 16293-2422 at 220.2 GHz. The wealth ofmolecular lines in this source and the very high spectral resolution offered byALMA allow us to study the gas kinematics with unprecedented detail. Results:We present the first detection of an inverse P-Cygni profile towards source Bin the three brightest lines. The line profiles are fitted with a simpletwo-layer model to derive an infall rate of 4.5x10^-5 Msun/yr. This infalldetection would rule-out the previously suggested possibility of source B beinga T Tauri star. A position velocity diagram for source A shows evidence forrotation with an axis close to the line-of-sight.

Molecular Gas and Star Formation in the Nucleus of IC 342: C18O and Millimeter Continuum Imaging

We present high-resolution maps (D2A) maps of the J \ 1¨0 and J \ 2¨1 transitions of C18 Oi n the central D150 pc of the gas-rich nucleus, IC 342, made with the Owens Valley Millimeter Array. From the C18O maps, we are able to obtain the most accurate map of to date for IC 342. Because of their N H2 low opacities, the transitions of C18O give a more reliable estimate of the true molecular gas columndensity distribution than the more common 12CO and 13CO isotopomers. The morphology of the C18O emission in the nucleus is a minispiral similar to that of the main isotopomer, 12CO, except that it is more symmetric and lacks the enhancements to the north. We suggest that the asymmetries present in 12CO images may re—ect the viewing perspective of the starburst region biased by the high optical depths of 12CO rather than true asymmetries in the amount of molecular gas present. The giant molecular clouds seen in C18O appear to be nonspherical, probably because of tidal arm shearing. Column densities determined from C18O observations, 1.3 mm dust continuum, and the virial theorem indicate that the standard Galactic conversion factor, overestimates the amount of molecular gas in the X CO , center of IC 342 by a factor of D2¨3 at the molecular cloud peaks and by more than this in the diUuse gas away from the starburst. Revised molecular masses based on this conversion factor imply that star formation efficiencies in the starburst region are very high. From the distribution of gas and star formation, it appears that the sites of star formation are dynamically determined rather than driven by density peaks. Near the central star-forming region, evidence is seen for chemical enrichment of C18O caused by massive stars.

The Star Formation in Radio Survey: GBT 33?GHz Observations of Nearby Galaxy Nuclei and Extranuclear Star-forming Regions

We present 33 GHz photometry of 103 galaxy nuclei and extranuclear star-forming complexes taken with the Green Bank Telescope as part of the Star Formation in Radio Survey. Among the sources without evidence for an active galactic nucleus, and also having lower frequency radio data, we find a median thermal fraction at 33 GHz of ≈76% with a dispersion of ≈24%. For all sources resolved on scales ≾0.5 kpc, the thermal fraction is even larger, being ≳90%. This suggests that the rest-frame 33 GHz emission provides a sensitive measure of the ionizing photon rate from young star-forming regions, thus making it a robust star formation rate (SFR) indicator. Taking the 33 GHz SFRs as a reference, we investigate other empirical calibrations relying on different combinations of warm 24 μm dust, total infrared (IR; 8-1000 μm), Hα line, and far-UV continuum emission. The recipes derived here generally agree with others found in the literature, albeit with a large dispersion that most likely stems from a combination of effects. Comparing the 33 GHz to total IR flux ratios as a function of the radio spectral index, measured between 1.7 and 33 GHz, we find that the ratio increases as the radio spectral index flattens which does not appear to be a distance effect. Consequently, the ratio of non-thermal to total IR emission appears relatively constant, suggesting only moderate variations in the cosmic-ray electron injection spectrum and ratio of synchrotron to total cooling processes among star-forming complexes. Assuming that this trend solely arises from an increase in the thermal fraction sets a maximum on the scatter of the non-thermal spectral indices among the star-forming regions of σ_α NT ≾0.13.

An extragalactic supernebula confined by gravity.

Little is known about the origins of globular clusters, which contain hundreds of thousands of stars in a volume only a few light years across. Radiation pressure and winds from luminous young stars should disperse the star-forming gas and disrupt the formation of the cluster. Globular clusters in our Galaxy cannot provide answers; they are billions of years old. Here we report the measurement of infrared hydrogen recombination lines from a young, forming super star cluster in the dwarf galaxy NGC5253. The lines arise in gas heated by a cluster of about one million stars, including 4,000–6,000 massive, hot ‘O’ stars. It is so young that it is still enshrouded in gas and dust, hidden from optical view. The gases within the cluster seem bound by gravity, which may explain why the windy and luminous O stars have not yet blown away those gases. Young clusters in ‘starbursting’ galaxies in the local and distant Universe may also be gravitationally confined and cloaked from view.

(C-13)O in IC 342 - Evidence for a spiral density wave in the nucleus

We present high-resolution (5″, ∼40 pc) maps of the 13 CO emission in the nucleus of the nearby spiral galaxy IC 342. 13 CO emission, a tracer of the H 2 density distribution, indicates that the molecular gas in the nucleus takes the form of two spiral arms of total extent 500 pc. Molecular gas constitutes ∼10% of the total dynamical mass within the inner half-kiloparsec, increasing to ∼50% in the inner 100 pc

Submillimeter ALMA Observations of the Dense Gas in the Low-Luminosity Type-1 Active Nucleus of NGC1097

We present the first 100 pc scale view of the dense molecular gas in the central ~ 1.3 kpc region of the type-1 Seyfert NGC 1097 traced by HCN (J=4-3) and HCO+ (J=4-3) lines afforded with ALMA band 7. This galaxy shows significant HCN enhancement with respect to HCO+ and CO in the low-J transitions, which seems to be a common characteristic in AGN environments. Using the ALMA data, we study the characteristics of the dense gas around this AGN and search for the mechanism of HCN enhancement. We find a high HCN (J=4-3) to HCO+ (J=4-3) line ratio in the nucleus. The upper limit of the brightness temperature ratio of HCN (v2=1^{1f}, J=4-3) to HCN (J=4-3) is 0.08, which indicates that IR pumping does not significantly affect the pure rotational population in this nucleus. We also find a higher HCN (J=4-3) to CS (J=7-6) line ratio in NGC 1097 than in starburst galaxies, which is more than 12.7 on the brightness temperature scale. Combined from similar observations from other galaxies, we tentatively suggest that this ratio appears to be higher in AGN-host galaxies than in pure starburst ones similar to the widely used HCN to HCO+ ratio. LTE and non-LTE modeling of the observed HCN and HCO+ lines using J=4-3 and 1-0 data from ALMA, and J=3-2 data from SMA, reveals a high HCN to HCO+ abundance ratio (5 < [HCN]/[HCO+] < 20: non-LTE analysis) in the nucleus, and that the high-J lines (J=4-3 and 3-2) are emitted from dense (10^{4.5} < n_H2 [/cc] < 10^6), hot (70 < Tkin [K] < 550) regions. Finally we propose that the high temperature chemistry is more plausible to explain the observed enhanced HCN emission in NGC 1097 than the pure gas phase PDR/XDR chemistry.

Item response theory and the assumption of unidimensionality for language tests

Considerable controversy has arisen around the assumption of unidimen sionality underlying the application of latent trait models of measurement. The intent of the present paper is to provide a clearer articulation of the unidimensionality assumption and to investigate the robustness and appli cability of a particular unidimensional model, the Rasch Model, for use with language proficiency tests that consist of batteries of subtests in a variety of skill areas and that are applied in the testing of the abilities of students from diverse educational, linguistic and cultural backgrounds. Results of the analysis of response data from the administration of a 150- item, five-subskill ESL proficiency/placement examination to 312 entering university students indicated that unidimensionality constraints were not violated.

MOLECULAR GAS PROPERTIES OF THE STARBURST NUCLEUS OF IC 342: HIGH-RESOLUTION 13CO (2¨1) IMAGING

We present a map of the J = 2-1 transition of 13CO in the starburst nucleus of IC 342 made with the Owens Valley Millimeter Array. The interferometric 13CO map allows us to directly compare with 12CO (1-0), 12CO (2-1), and 13CO (1-0) maps of nearly identical (~45) resolution. While all four transitions show similar basic morphology, there are spatial differences between the 12CO and 13CO transitions that show up in ratio maps. In particular, the 13CO (2-1)/13CO (1-0) ratio has a markedly different distribution across the nuclear region than does 12CO (2-1)/12CO (1-0), indicating that 12CO and 13CO trace different components of the molecular gas. These differences are explained if 12CO traces the warm, photodissociation region skins of the clouds and 13CO traces the cooler, interior portions constituting the bulk of the molecular gas. We derive excitation temperatures for the bulk of the cloud mass of ~10-20 K and densities of ~103.7 cm-2 along the molecular minispiral.