arXiv subjects
Publications and source records attributed to D. A. Green.
We present further observations of the Lockman Hole field, made with the Giant Metrewave Radio Telescope at 610 MHz with a resolution of 6 x 5 arcsec^2. These complement our earlier observations of the central approx 5 deg^2 by covering a further approx 8 deg^2, with an r.m.s. noise down to ~80 microJy beam^-1. A catalogue of 4934 radio sources is presented.
A revised catalogue of 274 Galactic supernova remnants (SNRs) is presented, along with some simple statistics of their parameters. It is shown that the remnants that have recently been identified are generally faint, as is expected from the selection effects that apply to the identification of remnants.
G35.6-0.4 is an extended radio source in the Galactic plane which has previously been identified as either a supernova remnant or an HII region. Observations from the VLA Galactic Plane Survey at 1.4 GHz with a resolution of 1 arcmin allow the extent of G35.6-0.4 to be defined for the first time. Comparison with other radio survey observations show that this source has a non-thermal spectral index, with alpha -0.47 +/- 0.07. G35.6-0.4 does not have obvious associated infra-red emission, so it is identified as a Galactic supernova remnant, not an HII region. It is approximately 15 x 11 arcmin**2 in extent, showing partial limb brightening.
We present observations between 1.4 and 18 GHz confirming that G64.5+0.9 is new Galactic shell supernova remnant, using the Very Large Array and the Arcminute Microkelvin Imager. The remnant is a shell ~8 arcmin in diameter with a spectral index of alpha = 0.47 +/- 0.03 (with alpha defined such that flux density S varies with frequency nu as S proportional to nu to the power of -alpha). There is also emission near the centre of the shell, ~1 arcmin in extent, with a spectral index of alpha = 0.81 +/- 0.02. We do not find any evidence for spectral breaks for either source within our frequency range. The nature of the central object is unclear and requires further investigation, but we argue that is most unlikely to be extragalactic. It is difficult to avoid the conclusion that it is associated with the shell, although its spectrum is very unlike that of known pulsar wind nebulae.
We describe multi-frequency data designed to probe the emission mechanism that dominates in faint radio sources. Our analysis is based on observations of the Lockman Hole using the GMRT - the deepest 610-MHz imaging yet reported - together with 1.4-GHz imaging from the VLA, well matched in resolution and sensitivity to the GMRT data. The GMRT and VLA data are cross-matched to obtain the radio spectral indices for the faint radio emitters. Statistical analyses show no clear evolution for the median spectral index, alpha, as a function of flux density. alpha is found to be -0.6 to -0.7, based on an almost unbiased 10-sigma criterion, down to a flux level of S(1.4GHz) >~ 100uJy. The fraction of inverted spectrum sources (alpha > 0) is less than 10%. The results suggest that the most prevalent emission mechanism in the sub-mJy regime is optically-thin synchrotron, ruling out a dominant flat spectrum or ultra-steep spectrum radio population. The spectral index distribution has a significant scatter, 0.4-0.5, which suggests a mixture of different populations at all flux levels. Spectroscopic classification of radio sources with X-ray emission has allowed us to estimate that the fraction of radio-quiet AGN at 30 <~ S(1.4GHz) < 300uJy is roughly 25 +/- 10%, suggesting that star-forming galaxies dominate the sub-mJy regime.
The discovery of the youngest Galactic supernova remnant (SNR) G1.9+0.3 has allowed a look at a stage of SNR evolution never before observed. We analyze the 50 ks Chandra observation with particular regard to spectral variations. The very high column density ($N_H \sim 6 \times 10^{22}$ cm$^{-2}$) implies that dust scattering is important, and we use a simple scattering model in our spectral analysis. The integrated X-ray spectrum of G1.9+0.3 is well described by synchrotron emission from a power-law electron distribution with an exponential cutoff. Using our measured radio flux and including scattering effects, we find a rolloff frequency of $5.4 (3.0, 10.2) \times 10^{17}$ Hz ($h ν_{\rm roll} = 2.2$ keV). Including scattering in a two-region model gives lower values of ν_roll by over a factor of 2. Dividing G1.9+0.3 into six regions, we find a systematic pattern in which spectra are hardest (highest ν_roll) in the bright SE and NW limbs of the shell. They steepen as one moves around the shell or into the interior. The extensions beyond the bright parts of the shell have the hardest spectra of all. We interpret the results in terms of dependence of shock acceleration properties on the obliquity angle $θ_{\rm Bn}$ between the shock velocity and a fairly uniform upstream magnetic field. This interpretation probably requires a Type Ia event. If electron acceleration is limited by synchrotron losses, the spectral variations require obliquity-dependence of the acceleration rate independent of the magnetic-field strength.
We present observations between 14.2 and 17.9 GHz of 12 reported supernova remnants (SNRs) made with the Arcminute Microkelvin Imager Small Array (AMI SA). In conjunction with data from the literature at lower radio frequencies, we determine spectra of these objects. For well-studied SNRs (Cas A, Tycho's SNR, 3C58 and the Crab Nebula), the results are in good agreement with spectra based on previous results. For the less well-studied remnants the AMI SA observations provide higher-frequency radio observations than previously available, and better constrain their radio spectra. The AMI SA results confirm a spectral turnover at ~11 GHz for the filled-centre remnant G74.9+1.2. We also see a possible steepening of the spectrum of the filled-centre remnant G54.1+0.3 within the AMI SA frequency band compared with lower frequencies. We confirm that G84.9+0.5, which had previously been identified as a SNR, is rather an HII region and has a flat radio spectrum.
The GMRT has been used to make deep, wide-field surveys of several fields at 610 MHz, with a resolution of about 5 arcsec. These include the Spitzer Extragalactic First Look Survey field, where 4 square degrees were observed with a r.m.s. sensitivity of about 30 microJy/beam, and several SWIRE fields (namely the Lockman Hole, ELAIS-N1 and N2 fields) covering more than 20 square degrees with a sensitivity of about 80 microJy beam or better. The analysis of these observations, and some of the science results are described.
We report on the identification of the X/soft gamma-ray source AX J183039-1002 detected with ASCA and INTEGRAL/IBIS. The source, which has an observed 20-100 keV flux of about 8.6 x 10^-11 erg/cm^2/s, is inside a diffuse radio supernova remnant (SNR) and is spatially coincident with a compact radio source. We analyzed archival Chandra and XMM-Newton observations in order to identify the ASCA/INTEGRAL source. A point-like Chandra X-ray object was found to be positionally coincident with the compact radio source and within the error circle of the ASCA and INTEGRAL sources. Although the association of a compact radio/X-ray source with a radio supernova remnant could be indicative of a pulsar wind nebula (PWN), the XMM-Newton X-ray spectrum is compatible with an absorbed, Seyfert-2 like AGN, since it provides evidence for an iron emission line of about 1 keV equivalent width; furthermore the X-ray source spectrum is similar to that of other Compton thick AGN where the <2 keV data are associated to a warm reflector and the >10 keV one to a cold reflector.
We present observations of the Lynds' dark nebula LDN 1111 made at microwave frequencies between 14.6 and 17.2 GHz with the Arcminute Microkelvin Imager (AMI). We find emission in this frequency band in excess of a thermal free--free spectrum extrapolated from data at 1.4 GHz with matched uv-coverage. This excess is > 15 sigma above the predicted emission. We fit the measured spectrum using the spinning dust model of Drain & Lazarian (1998a) and find the best fitting model parameters agree well with those derived from Scuba data for this object by Visser et al. (2001).
Our 50 ks Chandra observation of the small radio supernova remnant (SNR) G1.9+0.3 shows a complete shell structure with strong bilateral symmetry, about $100''$ in diameter. The radio morphology is also shell-like, but only about $84''$ in diameter, based on observations made in 1985. We attribute the size difference to expansion between 1985 and our Chandra observations of 2007. Expansion is confirmed in comparing radio images from 1985 and 2008. We deduce that G1.9+0.3 is of order 100 years old -- the youngest supernova remnant in the Galaxy. Based on a very high absorbing column density of $5.5 \times 10^{22}$ cm$^{-2}$, we place G1.9+0.3 near the Galactic Center, at a distance of about 8.5 kpc, where the mean remnant radius would be about 2 pc, and the required expansion speed about $14,000$ km s$^{-1}$. The X-ray spectrum is featureless and well-described by the exponentially cut off synchrotron model {\tt srcut}. With the radio flux at 1 GHz fixed at 0.9 Jy, we find a spectral index of 0.65 and a rolloff frequency of $1.4 \times 10^{18}$ Hz. The implied characteristic rolloff electron energy of about $94 (B/10 μ{\rm G})^{-1/2}$ TeV is the highest ever reported for a shell supernova remnant. It can easily be reached by standard diffusive shock acceleration, given the very high shock velocities; it can be well described by either age-limited or synchrotron-loss-limited acceleration. Not only is G1.9+0.3 the youngest known Galactic remnant, it is also only the fourth Galactic X-ray synchrotron-dominated shell supernova remnant.
Recent radio observations of the small Galactic supernova remnant G1.9+0.3 made at 4.86 GHz with the VLA are presented, and compared with earlier observations at 1.49 GHz which have a comparable resolution (10 x 4 arcsec^2). These show that the radio emission from this remnant has expanded significantly, by about 15 per cent over 23 years, with a current outer diameter of approximately 92 arcsec. This expansion confirms that G1.9+0.3 is the youngest Galactic remnant yet identified, only about 150 years old at most. Recent, lower resolution, 1.43-GHz observations are also discussed, and the integrated flux densities from these and the 4.86-GHz observations are compared with earlier results. This shows that the integrated flux density of G1.9+0.3 has been increasing recently.
We present observations between 14.2 and 17.9 GHz of sixteen Galactic HII regions made with the Arcminute Microkelvin Imager (AMI). In conjunction with data from the literature at lower radio frequencies we investigate the possibility of a spinning dust component in the spectra of these objects. We conclude that there is no significant evidence for spinning dust towards these sources and measure an average spectral index of 0.15+/-0.07 between 1.4 and 17.9 GHz for the sample.
It has recently been suggested that the radio spectrum of the Galactic supernova remnant HB 3 shows flattening at higher frequencies (above about 1 GHz). Here I review the radio spectrum of HB 3, noting the difficulties in deriving accurate flux densities for this remnant, particularly at high frequencies, due to the proximity of bright, thermal emission from W3 and its surroundings. A flux density for HB 3 at 2695 MHz is derived from Effelsberg survey data. The spectrum of HB 3 is well represented by a simple power-law spectrum from 22 to 2695 MHz, with a spectral index of 0.56 +/- 0.03. It is concluded that contamination with thermal emission from adjacent regions is the cause for the reported spectral flattening of HB 3.
We present data probing the spatial and kinematical distribution of both the atomic (HI) and molecular (CO) gas in NGC 5218, the late-type barred spiral galaxy in the spiral-elliptical interacting pair, Arp 104. We consider these data in conjunction with far-infrared and radio continuum data, and N-body simulations, to study the galaxies interactions, and the star formation properties of NGC 5218. We use these data to assess the importance of the bar and tidal interaction on the evolution of NGC 5218, and the extent to which the tidal interaction may have been important in triggering the bar. The molecular gas distribution of NGC 5218 appears to have been strongly affected by the bar; the distribution is centrally condensed with a very large surface density in the central region. The N-body simulations indicate a timescale since perigalacticon of approximately 3 x 10**8 yr, which is consistent with the interaction having triggered or enhanced the bar potential in NGC 5218, leading to inflow and the large central molecular gas density observed. Whilst NGC 5218 appears to be undergoing active star formation, its star formation efficiency is comparable to a `normal' SBb galaxy. We propose that this system may be on the brink of a more active phase of star formation.
We present Giant Metrewave Radio Telescope 21-cm HI observations towards a sample of compact radio sources behind galaxy groups, to search for cool HI. The results -- from high dynamic range spectra for 8 lines-of-sight through 7 galaxy groups -- do not show any evidence for absorption by cool HI. At a resolution of 20 km/s, the optical depth upper limits obtained were between 0.0075 and 0.035 (3sigma); these correspond to upper limits of a few times 10**23 m**-2 for the column density of any cool HI along these lines of sight (assuming a spin temperature of 100 K).
We investigate the atomic and molecular interstellar medium and star formation of NGC 275, the late-type spiral galaxy in Arp 140, which is interacting with NGC 274, an early-type system. The atomic gas (HI) observations reveal a tidal tail from NGC 275 which extends many optical radii beyond the interacting pair. The HI morphology implies a prograde encounter between the galaxy pair approximately 1.5 x 10**8 years ago. The Halpha emission from NGC 275 indicates clumpy irregular star-formation, clumpiness which is mirrored by the underlying mass distribution as traced by the Ks-band emission. The molecular gas distribution is striking in its anti-correlation with the {HII regions. Despite the evolved nature of NGC 275's interaction and its barred potential, neither the molecular gas nor the star formation are centrally concentrated. We suggest that this structure results from stochastic star formation leading to preferential consumption of the gas in certain regions of the galaxy. In contrast to the often assumed picture of interacting galaxies, NGC 275, which appears to be close to merger, does not display enhanced or centrally concentrated star formation. If the eventual merger is to lead to a significant burst of star formation it must be preceded by a significant conversion of atomic to molecular gas as at the current rate of star formation all the molecular gas will be exhausted by the time the merger is complete.
We report the discovery of a young pulsar associated with the supernova remnant G21.5-0.9, using the Giant Metrewave Radio Telescope (GMRT) located near Pune, India. Discovered at a frequency of 610 MHz, J1833-1034 has a period of 61.86 ms and a period derivative of $2.0 \times 10^{-13}$, making it similar to other known young pulsars. The characteristic age of the pulsar is $\approx 4900$ yr, somewhat higher than estimates for the age of the remnant, but not incompatible with it. The pulsar has a spin-down luminosity of $3.3 \times 10^{37}$ erg s$^{-1}$, which is the second highest amongst all the known Galactic pulsars.