Felix J. Lockman

A survey of radio H II regions in the northern sky

Nearly 500 radio continuum sources near the Galactic plane at declinations greater than 37 deg were examined for radio recombination line emission to see if they might be H II regions. A total of 462 were detected, about half of these for the first time. The data are presented in an extensive table. Among the newly discovered nebulae are several with recombination lines so narrow that their electron temperature must be less than 4600 K. 35 refs.

Accurate galactic N(H) values towards quasars and AGN

Integrated Galactic 21-cm column densities toward about 174 quasars and AGN are presented. The data are corrected for stray radiation, using the technique of Lockman et al. (1986). Because of the 21-arcmin beam size of the 140-ft telescope used in the study, the uncertainty in N(H) due to angular variations in the H I of the Galaxy at high latitudes is minimized. The column densities are accurate to about 1 X 10 to the 19th atoms/sq cm, or 5 percent, whichever is larger. Errors above N(H) of about 4 X 10 to the 20th atoms/sq cm are dominated by opacity uncertainties. 24 references.

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.

Diffuse Extreme-Ultraviolet Emission from the Coma Cluster: Evidence for Rapidly Cooling Gases at Submegakelvin Temperatures

The central region of the Coma cluster of galaxies was observed in the energy band from 0.065 to 0.245 kiloelectron volts by the Deep Survey telescope aboard the Extreme Ultraviolet Explorer. A diffuse emission halo of angular diameter ∼30 arc minutes was detected. The extreme-ultraviolet (EUV) emission level exceeds that expected from the x-ray temperature gas in Coma. This halo suggests the presence of two more phases in the emitting gas, one at a temperature of ∼2 × 106 kelvin and the other at ∼8 × 105 kelvin. The latter phase cools rapidly and, in steady state, would have produced cold matter with a mass of ∼1014 solar masses within the EUV halo. Although a similar EUV enhancement was discovered in the Virgo cluster, this detection in Coma applies to a noncooling flow system.

A radio counterpart to a neutron star merger

GROWTH observations of GW170817 The gravitational wave event GW170817 was caused by the merger of two neutron stars (see the Introduction by Smith). In three papers, teams associated with the GROWTH (Global Relay of Observatories Watching Transients Happen) project present their observations of the event at wavelengths from x-rays to radio waves. Evans et al. used space telescopes to detect GW170817 in the ultraviolet and place limits on its x-ray flux, showing that the merger generated a hot explosion known as a blue kilonova. Hallinan et al. describe radio emissions generated as the explosion slammed into the surrounding gas within the host galaxy. Kasliwal et al. present additional observations in the optical and infrared and formulate a model for the event involving a cocoon of material expanding at close to the speed of light, matching the data at all observed wavelengths. Science, this issue p. 1565, p. 1579, p. 1559; see also p. 1554 Radio observations constrain the energy and geometry of relativistic material ejected from a binary neutron star merger. Gravitational waves have been detected from a binary neutron star merger event, GW170817. The detection of electromagnetic radiation from the same source has shown that the merger occurred in the outskirts of the galaxy NGC 4993, at a distance of 40 megaparsecs from Earth. We report the detection of a counterpart radio source that appears 16 days after the event, allowing us to diagnose the energetics and environment of the merger. The observed radio emission can be explained by either a collimated ultrarelativistic jet, viewed off-axis, or a cocoon of mildly relativistic ejecta. Within 100 days of the merger, the radio light curves will enable observers to distinguish between these models, and the angular velocity and geometry of the debris will be directly measurable by very long baseline interferometry.

Detection of 130 “Diffuse” Galactic H II Regions

The paper reports the results of a search for diffuse Galactic H II regions using radio recombination lines. Unlike most previous surveys, which have been directed toward fairly bright, compact radio continuum sources, we observed unprepossessing objects that might be optically obscured analogs to the Rosette Nebula. The sources observed here are typically a factor of 2 fainter in the continuum than those that have been detected before. One hundred thirty nebulae have been measured in radio recombination lines for the first time; most require at least one main-sequence O star to provide their ionization. The diffuse H II regions appear to have essentially the same distribution in the Galaxy as compact H II regions. There is some indication that the diffuse nebulae have a low abundance of ionized helium, possibly due to their low excitation. Several newly discovered nebulae are associated with peculiar kinematic features in the Galactic nucleus. The surface density of radio H II regions in the inner Galaxy follows the surface density of CO fairly closely from 4 ≤ R ≤ 8 kpc. In this area, there is ~1 radio H II region for each 106 M☉ of molecular hydrogen. However, H II regions are a factor ~2 more confined to the Galactic plane than the molecular gas from which they form.

The Extended H I Rotation Curve and Mass Distribution of M31

New H I observations of Messier 31 (M31) obtained with the Effelsberg and Green Bank 100 m telescopes make it possible to measure the rotation curve of that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotation curve is nearly flat at a velocity of ~226 km s-1. A model of the mass distribution shows that at the last observed velocity point, the minimum dark-to-luminous mass ratio is ~0.5 for a total mass of 3.4 × 1011 M☉ at R < 35 kpc. This can be compared to the estimated Milky Way mass of 4.9 × 1011 M☉ for R < 50 kpc.

The Smith Cloud: A High-Velocity Cloud Colliding with the Milky Way

New 21 cm H I observations made with the Green Bank Telescope show that the high-velocity cloud known as the Smith Cloud has a striking cometary appearance and many indications of interaction with the Galactic interstellar medium. The velocities of interaction give a kinematic distance of 12.4 ± 1.3 kpc, consistent with the distance derived from other methods. The Cloud is >3 × 1 kpc in size, and its tip at (l, b) ≈ 39°, –13° is 7.6 kpc from the Galactic center and 2.9 kpc below the Galactic plane. It has >106 M☉ in H I. Its leading section has a total space velocity near 300 km s−1, is moving toward the Galactic plane with a velocity of 73 ± 26 km s−1, and is shedding material to the Galaxy. In the absence of drag, the Cloud will cross the plane in about 27 Myr. The Smith Cloud may be an example of the accretion of gas by the Milky Way that is needed to explain certain persistant anomalies in Galactic chemical evolution.

Vertical distribution and support of Galactic H I

The shape of high-latitude 21 cm H I emission profiles suggests that the local neutral interstellar medium is supported in the Galactic gravitational potential primarily by turbulence, that is, by the motion of individual clouds. Composite H I spectra derived from averages over large areas of the high-latitude sky are modeled by several isothermal components whose equilibrium vertical distribution in a plausible Galactic potential successfully reproduces the observed thickness of the H I layer.

NEUTRAL HYDROGEN CLOUDS IN THE M81/M82 GROUP

We have observed a 3 ◦ × 3 ◦ area centered on the M81/M82 group of galaxies using the Robert C. Byrd Green Bank Telescope in a search for analogs to the high-velocity clouds of neutral hydrogen found around our Galaxy. The velocity range from −605 to −85 km s −1 and from 25 to 1970 km s −1 was searched for Hi clouds. Over the inner 2 ◦ × 2 ◦ the 7σ detection threshold was 9.6 × 10 5 M� . We detect five previously unknown Hi clouds associated with the group, as well as numerous associated filamentary Hi structures, all lying in the range −105 Vhelio +280 km s −1 . From the small angular distance of the clouds to group members, and the small velocity difference between group members and clouds, we conclude that the clouds are most likely relics of ongoing interactions between galaxies in the group.