Tamara Toby Helfer

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.

Dense Gas in the Milky Way

We present a study of dense gas emission in the Milky Way to serve as a basis for comparison with extragalactic results. This study combines new observations of HCN, CS, and CO in individual giant molecular clouds (GMCs) and in the Milky Way plane with published studies of emission from these molecules in the inner 500 pc of the Milky Way. We find a strong trend in the fraction of emission from dense gas tracers as a function of location in the Milky Way: in the bulge, IHCN/ICO = 0.081 ± 0.004, in the plane, IHCN/ICO = 0.026 ± 0.008 on average, and over the full extent of nearby GMCs, IHCN/ICO = 0.014 ± 0.020. Similar trends are seen in ICS/ICO: in the bulge, ICS/ICO = 0.027 ± 0.006, in the plane, ICS/ICO = 0.018 ± 0.008 on average, and over the full extent of nearby GMCs, ICS/ICO = 0.013 ± 0.02. The low intensities of the HCN and CS emission in the plane suggest that these lines are produced by gas at moderate densities; thus, they are not like the emission produced by the dense, parsec-scale star-forming cores in nearby GMCs. The contrast between the bulge and the disk ratios in the Milky Way is likely to be caused by a combination of higher kinetic temperatures as well as a higher dense gas fraction in the bulge of the Milky Way. We show that the ratio IHCN/ICO is correlated quantitatively with the total hydrostatic gas pressure in the Milky Way as IHCN/ICO ∝ P0.19±0.04.

Dense Gas in the Bulges of External Galaxies

In order to probe the physical conditions in the high-pressure environments of galactic bulges, we have studied the properties of the dense molecular gas in the bulges of a sample of 19 mostly normal spiral galaxies. We observed the 3 mm emission from the molecules HCN and CS, which trace gas densities of ∼10 5 cm -3 . Our high detection rate (68% in HCN, 50% in CS) suggests that most spiral galaxies, not just starburst galaxies, contain appreciable quantities of dense gas in their bulges

The molecular connection to the FIR-radio continuum correlation in galaxies

We have studied the relationships between the radio continuum (RC) and CO emission for a set of galaxies selected from the BIMA Survey of Nearby Galaxies. We find that the global CO-RC correlation is as tight as the global FIR-RC corre- lation for the 24 galaxies studied. Within 9 galaxies with ∼6 �� CO and RC data available, the CO and RC emission is as tightly correlated as its global value; the radially averaged correlation is nearly linear, extends over four order of magnitude and holds down to the smallest linear resolution of the observations, which is ∼100 pc. We define qCO/RC as the log of the ratio of the CO to RC flux as a way to characterize the CO-RC correlation. Combining 6 �� pixel-by-pixel comparisons across all sources yields an average small-scale correlation of qCO/RC = 1.1 ± 0.28; that is, the spatially resolved correlation has a dispersion that is less than a factor of 2. There are however systematic variations in the CO/RC ratio; the strongest organized structures in qCO/RC tend to be found along spiral arms and on size scales much larger than the resolution of the observations. We do not measure any systematic trend in CO/RC ratio as a function of radius in galaxies. The constancy of the CO/RC ratio stands in contrast to the previously measured decrease in the FIR/RC ratio as a function of radius in galaxies. We suggest that the excellent correlation between the CO, RC and FIR emission in galaxies is a consequence of regulation by hydrostatic pressure; this model links all three emissions without invoking an explicit dependence on a star formation scenario.

THE ALLEN TELESCOPE ARRAY Pi GHz SKY SURVEY. I. SURVEY DESCRIPTION AND STATIC CATALOG RESULTS FOR THE BOÖTES FIELD

The Pi GHz Sky Survey (PiGSS) is a key project of the Allen Telescope Array. PiGSS is a 3.1 GHz survey of radio continuum emission in the extragalactic sky with an emphasis on synoptic observations that measure the static and time-variable properties of the sky. During the 2.5 year campaign, PiGSS will twice observe similar to 250,000 radio sources in the 10,000 deg(2) region of the sky with b > 30 degrees to an rms sensitivity of similar to 1 mJy. Additionally, sub-regions of the sky will be observed multiple times to characterize variability on timescales of days to years. We present here observations of a 10 deg(2) region in the Bootes constellation overlapping the NOAO Deep Wide Field Survey field. The PiGSS image was constructed from 75 daily observations distributed over a 4 month period and has an rms flux density between 200 and 250 mu Jy. This represents a deeper image by a factor of 4-8 than we will achieve over the entire 10,000 deg(2). We provide flux densities, source sizes, and spectral indices for the 425 sources detected in the image. We identify similar to 100 new flat-spectrum radio sources; we project that when completed PiGSS will identify 10(4) flat-spectrum sources. We identify one source that is a possible transient radio source. This survey provides new limits on faint radio transients and variables with characteristic durations of months.

Thermal and non-thermal components of the interstellar medium at sub-kiloparsec scales in galaxies

Aims. We present new radio continuum observations of ten BIMA SONG galaxies, taken at 1.4 GHz with the Very Large Array. These observations allow us to extend the study of the relationships between the radio continuum (RC) and CO emission to 22 CO luminous galaxies for which single dish CO images have been added to interferometric data. New Spitzer infrared (IR) images of six of these galaxies have been released. The analysis of these high resolution images allowed us to probe the RC-IR-CO correlations down to linear scales of a few hundred pc. Methods. We compare the point-by-point RC, CO and mid-IR intensities across entire galaxy disks, producing radial profiles and spatially resolved images of the RC/CO and RC/mid-IR ratios. Results. For the 22 galaxies analysed, the RC-CO correlation on scales from ~10 kpc down to ~100 pc is nearly linear and has a scatter of a factor of two, i.e. comparable to that of the global correlations. There is no evidence for any severe degradation of the scatter below the kpc scale. This also applies to the six galaxies for which high-resolution mid-IR data are available. In the case of NGC 5194, we find that the non-thermal radio spectral index is correlated with the RC/FIR ratio. Conclusions. The scatter of the point-by-point correlations does not increase significantly with spatial resolution. We thus conclude that we have not yet probed the physical scales at which the correlations break down. However, we observe local deviations from the correlations in regions with a high star formation rate, such as the spiral arms, where we observe a flat radio spectrum and a low RC/FIR ratio. In the intra-arm regions and in the peripheral regions of the disk, the RC/FIR is generally higher and it is characterized by a steepening of the radio spectrum.

Primary Beam and Dish Surface Characterization at the Allen Telescope Array by Radio Holography

The Allen Telescope Array (ATA) is a cm-wave interferometer in California, comprising 42 antenna elements with 6-m diameter dishes. We characterize the antenna optical accuracy using two-antenna interferometry and radio holography. The distortion of each telescope relative to the average is small, with RMS differences of 1% of beam peak value. Holography provides images of dish illumination, characterizing as-built mirror surfaces. Maximal distortions across ~ 2 meter lengths appear to result from mounting stresses or solar radiation. Experimental RMS errors are 0.7 mm at night and 3 mm under worst-case solar illumination. For frequencies 4, 10, and 15 GHz, the nighttime values indicate sensitivity losses of 1, 10 and 20%, respectively. ATAs wide-bandwidth receiver permits observations over a continuous range 0.5-11.2 GHz. We probe the antenna optical gain and beam pattern stability as a function of focus position and observation frequency, concluding that ATA can produce high fidelity images over a decade of simultaneous observation frequencies. We quantify solar heating effects on antenna sensitivity and pointing accuracy. We find that during the day, observations >;5 GHz will suffer some sensitivity loss and it may be necessary to make antenna pointing corrections on a 1-2 hourly basis.

THE DENSE GAS IN THE CENTRAL KILOPARSEC OF NGC 6946

We present observations of the HCN and HCO -->+ -->J = 1–0 transitions in the center of the nearby spiral galaxy NGC 6946 made with the BIMA and CARMA interferometers. Using the BIMA SONG CO map, we investigate the change in the -->IHCN/ICO and -->IHCO+/ICO integrated intensity ratios as a function of radius in the central kiloparsec of the galaxy and find that they are strongly concentrated at the center. We use the 2MASS -->Ks-band image to find the stellar surface density and then construct a map of the hydrostatic midplane pressure. We apply a PDR model to the observed -->IHCN/IHCO+ integrated intensity ratio to calculate the number density of molecular hydrogen in the dense gas tracer emitting region and find that it is roughly constant at -->105 cm -->−3 across our map. We explore two hypotheses for the distribution of the dense gas. If the HCN and HCO -->+ emission comes from self-gravitating density peaks inside of a less dense gas distribution, there is a linear proportionality between the internal velocity dispersion of the dense gas and the size of the density peak. Alternatively, the HCN and HCO -->+ emission could come from dense gas homogeneously distributed throughout the center and bound by ambient pressure, similar to what is observed toward the center of the Milky Way. We find both of these hypotheses to be plausible. We fit the relationships between -->IHCN, IHCO+, and -->ICO. Correlations between the hydrostatic midplane pressure and -->IHCN and -->IHCO+ are demonstrated, and power-law fits are provided. We confirm the validity of a relation found by Blitz & Rosolowsky between pressure and the molecular to atomic gas ratio in the high hydrostatic midplane pressure regime ( -->106- -->108 cm -->−3 K).

The Structure and Kinematics of Cold Molecular Gas in NGC 1068

I summarize fully-sampled observations of the 3 mm emission from CO and HCN in the inner arcminute of NGC 1068. The CO emission is distributed in the form of a molecular bar, coincident with the infrared bar, from which emanate two spiral arms. A relatively weak concentration of CO line emission is found at the nucleus. By contrast, the HCN emission is strongly concentrated at the center, with relatively weak emission in the region of the star-forming arms. The ratio of HCN to CO integrated intensities is about 0.6 over the central r ≉ 175 pc and is the highest ratio measured in the center of any galaxy; the ratio reflects the high thermal pressure (TK ~ 50 K, n[H2] ~ 4 × 106 cm-3) in the few hundred parsecs surrounding the nucleus.The kinematics in the star-forming arms are well described by circular orbits, with ordered noncircular motions of < 30 km s-1 that may be attributed to spiral density wave streaming. Interior to the bar, noncircular motions dominate the gas kinematics. A model of the CO kinematics contrains any Inner Lindblad Resonance to be close to the location of the hundred-parsec scale HCN ‘disk’. At the nucleus, the spatially unresolved CO emission shows a triplet velocity structure characteristic of kinematically independent regions.

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in Northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting panoramic surveys of the astrophysical sky. The fundamental scientific program of this new telescope is varied and exciting; we here discuss some of the first astronomical results.