arXiv subjects

Judith A. Irwin

Publications and source records attributed to Judith A. Irwin.

25 records · Page 2Linked to original sources

arXiv2000

Ultra-High-Density Molecular Core and Warped Nuclear Disk in the Deep Potential of Radio-Lobe Galaxy NGC 3079

We have performed high-resolution synthesis observations of the 12CO (J=1-0) line emission from the radio lobe edge-on spiral NGC 3079 using a 7-element mm-wave interferometer at Nobeyama, which consisted of the 45-m telescope and 6-element Array. The molecular nuclear disk (NMD) of 750 pc radius is found to be inclined by 20 degrees from the optical disk, and the NMD has spiral arms. An ultra-high-density molecular core (UHC) was found at the nucleus. The gaseous mass of the UHC within 125 pc radius is as large as about 3 x 10^8 Msun, an order of magnitude more massive than that in the same area of the Galactic Center, and the mean density is as high as approximately 3 x 10^3 H2 cm^-3. A position-velocity diagram along the major axis indicates that the rotation curve starts at a finite velocity exceeding 300 \kms already from the nucleus. The surface mass density in the central region is estimated to be as high as about 10^5 Msun pc^-2, producing a very deep gravitational potential. We argue that the very large differential rotation in such a deep potential will keep the UHC gravitationally stable from current star formation.

astro-ph↗

arXiv2000

Radio Jet-blown Neutral Hydrogen Supershells in Spiral Galaxies?

Taking a clue from the pair of HI supershells found in the Scd galaxy NGC 3556 (M 108), we propose a new mechanism for the origin of HI supershells in gas-rich massive galaxies. In this scenario, the two supershells were inflated out of the neutral hydrogen disk due to the localised flaring of a pair of radio lobes formed by the jets ejected from the nucleus during an active phase about 10^7 years ago, but have faded away by now. It is shown that the salient features of this supershell pair, such as their symmetrical locations about the galactic centre, the anomalously large energy requirements, the large galactocentric distances, as well as the Z-symmetric hemispherical shapes, find a more natural explanation in terms of this scenario, as compared to the standard models which postulate either a massive starburst, or the infall of external gas clouds. Other possible implications of this hypothesis are briefly discussed.

astro-ph↗

arXiv2000

Galactic Worm 123.4-1.5: A Mushroom-shaped HI Cloud

The Dominion Radio Astrophysical Observatory's Synthesis Telescope provides the highest resolution data (1 arcmin and 0.82 km/s) to date of an HI worm candidate. Observed as part of the Canadian Galactic Plane Survey, mushroom-shaped GW 123.4-1.5 extends only a few hundred parsecs, contains ~10^5 solar masses of neutral hydrogen, and appears unrelated to a conventional shell or chimney structure. Our preliminary Zeus-2d models use a single off-plane explosion with a modest (~10^{51} ergs) energy input. These generic simulations generate, interior to an expanding outer blast wave, a buoyant cloud whose structure resembles the morphology of the observed feature. Unlike typical model superbubbles, the stem can be narrow because its width is not governed by the pressure behind the blast wave nor the disk scale height. Using this type of approach it should be possible to more accurately model the thin stem and other details of GW 123.4-1.5 in the future.

astro-ph↗

arXiv1999

An Observational Test of Dark Matter as Cold Fractal Clouds

Using the VLA, we have performed the first observational test of dark matter in the form of cold, primordial fractal clouds, as envisioned by Pfenniger, Combes, & Martinet (1994) and Pfenniger & Combes (1994). We show that, after a Hubble Time, primordial fractal clouds will convert most of their HI to H2, but a small fraction of HI remains which is optically thick. This opens up a new window for detecting dark matter which may exist in this form. The detectability of such gas depends on its filling factor and temperature and therefore should be observable in absorption against a background source, with observations of sufficient sensitivity and resolution. The current VLA observations have made a first step towards this goal by taking advantage of a fortuitous alignment between the extension of the HI disk of the nearby galaxy, NGC 3079, and a background quasar Q 0957+561. Our observations probe 28 independent beams against the quasar and all of velocity space between the extension of a flat rotation curve and a Keplerian decline for the halo region of NGC 3079. We do not detect any absorption features and investigate, in detail, the implication of this result for the hypothesis that dark matter is in the form of fractal clouds. While our analysis does not rule out fractal clouds as dark matter, it does lay out the groundwork for future, more sensitive observations and we consider what form these might take to probe the range of possible cloud properties more deeply. It is interesting that the observations can rule out cold optically thin HI gas, if it exists, to a limit of 0.001% of the dark matter. In contrast, the existence of cold HI in a fractal hierarchy would be an efficient way of hiding dark matter.

astro-ph↗

arXiv1998

High Latitude Radio Emission in a Sample of Edge-On Spiral Galaxies

We have mapped 16 edge-on galaxies at 20 cm using the VLA. For 5 galaxies, we could form spectral index, energy and magnetic field maps. We find that all but one galaxy show evidence for non-thermal high latitude radio continuum emission, suggesting that cosmic ray halos are common in star forming galaxies. The high latitude emission is seen over a variety of spatial scales and in discrete and/or smooth features. In general, the discrete features emanate from the disk, but estimates of CR diffusion lengths suggest that diffusion alone is insufficient to transport the particles to the high latitudes seen (> 15 kpc in one case). Thus CRs likely diffuse through low density regions and/or are assisted by other mechanisms (e.g. winds). We searched for correlations between the prevalence of high latitude radio emission and a number of other properties, including the global SFR, supernova input rate per unit star forming, and do not find clear correlations with any of these properties.

astro-ph↗

arXiv1997

Water maser emission and the parsec-scale jet in NGC 3079

We have conducted VLBI observations of water maser and radio continuum emission in the nucleus of NGC 3079. The 22 GHz maser emission arises in compact clumps, distributed along an axis that is aligned with the major axis of the galactic disk. The velocities of the masers are consistent with their lying in the inner parsec of a molecular disk rotating in the same sense as the rest of the galaxy. However, the velocity field has a significant non-rotational component, which may indicate supersonic turbulence. The bright maser emission is not coincident with any detected compact 22 GHz continuum source, suggesting the high apparent luminosity of the maser may not due to beamed amplification of continuum emission. We observed two compact continuum sources that have inverted spectra between 5 and 8 GHz, and steep spectra between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the class of compact symmetric gigahertz-peaked spectrum (GPS) radio sources. We detected a third continuum component that lies along the same axis as the other two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser emission was detected near this axis, which may indicate a second population of masers associated with the jet.

astro-ph↗

arXiv1997

Neutral Hydrogen in the Edge-on Spiral Galaxy NGC 3044 -- Global Properties and Discovery of HI Supershells

The first detailed VLA mapping of the neutral hydrogen distribution in the isolated, edge-on spiral galaxy NGC 3044 is presented. Physical parameters determined for this galaxy are typical for galaxies of its morphological class (SBc). We have modelled the HI spectra in order to derive its global density and velocity distributions. An HI scale height of 420 h^(-1) pc is thus found. This can be compared to the 8 kpc radio continuum halo found previously. The present study reveals an asymmetry in the HI distribution as well as numerous high-latitude HI structures at various galactocentric radii. Twelve high-latitude features were catalogued, of which four exhibit the signature of an expanding shell. There is some correlation of these features with features observed in the radio continuum from independent data. The most massive shell (Feature 10) extends out to 6h^(-1) kpc above the galactic disk. The radii and masses of these shells range from 1.2h^(-1) - 2.0h^(-1) kpc and 1.6 10^7 - 5.5 10^7h^(-2) solar masses, respectively. We have investigated the possibility that the supershells could have been produced by external impacting clouds, but conclude that this scenario is unattractive, given the age of the shells, the isolation of the galaxy, and the lack of any observed features sufficiently massive to form the shells in the vicinity of the galaxy. Therefore, an internal origin is suggested. Since the implied input energies from supernovae are extremely high (e.g. from 1.4 10^(53)h^(-20 - 7.4 10^(55)h^(-2) ergs, corresponding to 400 - 74,000 supernovae), we suggest that some additional energy (e.g. from magnetic fields) may be needed to produce the observed supershells.

astro-ph↗