H. Alyson Ford

GASS: the Parkes Galactic all-sky survey: II. Stray-radiation correction and second data release

Context. The Parkes Galactic all-sky survey (GASS) is a survey of Galactic atomic hydrogen (H i) emission in the southern sky observed with the Parkes 64-m Radio Telescope. The first data release was published by McClure-Griffiths et al. (2009). Aims. We remove instrumental effects that affect the GASS and present the second data release. Methods. We calculate the stray-radiation by convolving the all-sky response of the Parkes antenna with the brightness temperature distribution from the Leiden/Argentine/Bonn (LAB) all sky 21-cm line survey, with major contributions from the 30-m dish of the Instituto Argentino de Radioastronomia (IAR) in the southern sky. Remaining instrumental baselines are corrected using the LAB data for a first guess of emission-free baseline regions. Radio frequency interference is removed by median filtering. Results. After applying these corrections to the GASS we find an excellent agreement with the Leiden/Argentine/Bonn (LAB) survey. The GASS is the highest spatial resolution, most sensitive, and is currently the most accurate H i survey of the Galactic H i emission in the southern sky. We provide a web interface for generation and download of FITS cubes.

MILKY WAY DISK-HALO TRANSITION IN H I: PROPERTIES OF THE CLOUD POPULATION

Using 21 cm H I observations from the Parkes Radio Telescope’s Galactic All-Sky Survey, we measure 255 H I clouds in the lower Galactic halo that are located near the tangent points at 16.9 ◦ ≤ l ≤ 35.3 ◦ and |b| . 20 ◦ . The clouds have a median mass of 700 M⊙ and a median distance from the Galactic plane of 660 pc. This first Galactic quadrant (QI) region is symmetric to a region of the fourth quadrant (QIV) studied previously using the same data set and measurement criteria. The properties of the individual clouds in the two quadrants are quite similar suggesting that they belong to the same population, and both populations have a line of sight cloud-cloud velocity dispersion of �cc ≈ 16 km s −1 . However, there are three times as many disk-halo clouds at the QI tangent points and their scale height, at h = 800 pc, is twice as large as in QIV. Thus the observed line of sight random cloud motions are not connected to the cloud scale height or its variation around the Galaxy. The surface density of clouds is nearly constant over the QI tangent point region but is peaked near R ∼ 4 kpc in QIV. We ascribe all of these differences to the coincidental location of the QI region at the tip of the Milky Way’s bar, where it merges with a major spiral arm. The QIV tangent point region, in contrast, covers only a segment of a minor spiral arm. The disk-halo H I cloud population is thus likely tied to and driven by large-scale star formation processes, possibly through the mechanism of supershells and feedback. Subject headings: galaxies: structure — Galaxy: halo — ISM: clouds — ISM: structure — radio lines: ISM

AN INTERACTION OF A MAGELLANIC LEADING ARM HIGH-VELOCITY CLOUD WITH THE MILKY WAY DISK

The Leading Arm of the Magellanic system is a tidally formed H I feature extending ~60° from the Magellanic Clouds ahead of their direction of motion. Using atomic hydrogen (H I) data from the Galactic All-Sky Survey (GASS), supplemented with data from the Australia Telescope Compact Array, we have found evidence for an interaction between a cloud in the Leading Arm and the Galactic disk where the Leading Arm crosses the Galactic plane. The interaction occurs at velocities permitted by Galactic rotation, which allows us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the Leading Arm crosses the Galactic plane at a Galactic radius of -->R ≈ 17 kpc.

GASKAP-The Galactic ASKAP Survey

A survey of the Milky Way disk and the Magellanic System at the wavelengths of the 21-cm atomic hydrogen (H i) line and three 18-cm lines of the OH molecule will be carried out with the Australian Square Kilometre Array Pathfinder telescope. The survey will study the distribution of H i emission and absorption with unprecedented angular and velocity resolution, as well as molecular line thermal emission, absorption, and maser lines. The area to be covered includes the Galactic plane (|b| < 10°) at all declinations south of δ = +40°, spanning longitudes 167° through 360° to 79° at b = 0°, plus the entire area of the Magellanic Stream and Clouds, a total of 13 020 deg2. The brightness temperature sensitivity will be very good, typically σT≃ 1 K at resolution 30 arcsec and 1 km s−1. The survey has a wide spectrum of scientific goals, from studies of galaxy evolution to star formation, with particular contributions to understanding stellar wind kinematics, the thermal phases of the interstellar medium, the interaction between gas in the disk and halo, and the dynamical and thermal states of gas at various positions along the Magellanic Stream.

Discovery of a gas-rich companion to the extremely metal-poor galaxy DDO 68

We present H I spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only ~3% Z ☉, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-metallicity and luminosity-metallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival H I spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass ( = 2.8 × 107 M ☉), recently star-forming (SFRFUV = 1.4 × 10–3 M ☉ yr–1, SFRHα < 7 × 10–5 M ☉ yr–1) companion has the same systemic velocity as DDO 68 (V sys = 506 km s–1; D = 12.74 ± 0.27 Mpc) and is located at a projected distance of ~42 kpc. New H I maps obtained with the 100 m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness H I gas forms a bridge between these objects.

GHOSTS I: A New Faint Very Isolated Dwarf Galaxy at D = 12 ± 2 Mpc

We report the discovery of a new faint dwarf galaxy, GHOSTS?I, using HST/ACS data from one of our GHOSTS?(Galaxy Halos, Outer disks, Substructure, Thick disk, and Star clusters) fields. Its detected individual stars populate an approximately 1?mag range of its luminosity function (LF). Using synthetic color-magnitude diagrams (CMDs) to compare with the galaxys CMD, we find that the colors and magnitudes of GHOSTS?Is individual stars are most consistent with being young helium-burning and asymptotic giant branch stars at a distance of ~12 ? 2?Mpc. Morphologically, GHOSTS?I appears to be actively forming stars, so we tentatively classify it as a dwarf irregular (dIrr) galaxy, although future Hubble Space Telescope (HST) observations deep enough to resolve a larger magnitude range in its LF are required to make a more secure classification. GHOSTS?Is absolute magnitude is , making it one of the least luminous dIrr galaxies known, and its metallicity is lower than [Fe/H] = ?1.5?dex. The half-light radius of GHOSTS?I is 226 ? 38?pc and its ellipticity is 0.47 ? 0.07, similar to Milky Way and M31 dwarf satellites at comparable luminosity. There are no luminous massive galaxies or galaxy clusters within ~4?Mpc from GHOSTS?I that could be considered as its host, making it a very isolated dwarf galaxy in the local universe.

The RadioAstron Green Bank Earth Station

We present the design, commissioning, and initial results of the Green Bank Earth Station (GBES), a RadioAstron data downlink station located at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. The GBES uses the modernized and refurbished NRAO 140ft telescope. Antenna optics were refurbished with new motors and drives fitted to the secondary mirror positioning system, and the deformable subreflector was refurbished with a new digital controller and new actuators. A new monitor and control system was developed for the 140ft and is based on that of the Green Bank Telescope (GBT), allowing satellite tracking via a simple scheduling block. Tools were developed to automate antenna pointing during tracking. Data from the antenna control systems and logs are retained and delivered with the science and telemetry data for processing at the Astro Space Center (ASC) of the Lebedev Physical Institute (LPI) of the Russian Academy of Sciences and the mission control centre, Lavochkin Association.

Satellite communications with NRAO Green Bank antennas

The National Radio Astronomy Observatory (NRAO) has new opportunities to partner with organizations in addition to the NSF using the assets at the Green Bank site. The 100 m Green Bank Telescope (GBT) is the worlds largest and most sensitive fully-steerable telescope, and can track at 20 deg min−1 in elevation and 40 deg min−1 in azimuth, providing the ability to track medium Earth orbit, geostationary, and deep space spacecraft. At X-band, its gain of 77 dB is unsurpassed by any fully steerable antenna. In addition to the GBT, the fully steerable, polar-mounted NRAO 140-ft telescope, with tracking rates of 20 deg min−1, and the 20 m telescope, with tracking rates of 120 deg min−1, provide a significant suite of instruments that can be deployed for spacecraft communications. Scheduled and target of opportunity observations are supported, allowing regular monitoring of and rapid response to targets.

A Green Bank Telescope Search for Highly Extended HI Disks Around Spiral Galaxies

Recent UV absorption line studies suggest that a large fraction of missing baryons are in the warm ionized and neutral phases, with about half of Milky Way-mass galaxies containing absorption systems with HI column densities of 10 cm or greater. This HI gas, which would have been difficult to detect with previous instruments, could be a significant contributor to the missing baryons. The Green Bank Telescope (GBT) presents a unique opportunity to detect this emission. We present results from GBT 21 cm observations of a sample of ten nearby optically luminous spirals, which reveal extended HI gas in half of our sample. The column densities of this extended HI are typically ∼ 1 × 10 cm , as measured at distances of 100 kpc from the center of the galaxies.

Space situational awareness applications for radio astronomy assets

The National Radio Astronomy Observatory (NRAO) builds, operates, and maintains a suite of premier radio antennas, including the 100m aperture Green Bank Telescope, the largest fully-steerable antenna in the world. For more than five decades the NRAO has focused on astrophysics, providing researchers with the most advanced instruments possible: large apertures, extremely low-noise receivers, and signal processors with high frequency and time resolution. These instruments are adaptable to Space Situational Awareness (SSA) tasks such as radar detection of objects in near-Earth and cis-Lunar space, high accuracy orbit determination, object surveillance with passive methods, and uplink and downlink communications. We present the capabilities of antennas and infrastructure at the NRAO Green Bank Observatory in the context of SSA tasks, and discuss what additions and modifications would be necessary to achieve SSA goals while preserving existing radio astronomy performance. We also discuss how the Green Bank Observatory’s surrounding topography and location within the National Radio Quiet Zone will enhance SSA endeavors.