Gemini Preprint #67

A Large-Scale Jet and FR I Radio Source in a Spiral Galaxy: The Host Properties and External Environment

Michael J. Ledlow
Gemini Observatory Southern Operations Center, AURA, Casilla 603, La Serena, Chile

Frazer N. Owen, Min S. Yun
National Radio Astronomy Observatory, Socorro, NM 87801

John M. Hill
Steward Observatory, University of Arizona, Tucson, Arizona, 85721


We have identified a large (approximately 200h75-1 kpc), powerful double radio source whose host galaxy is clearly a disk and most likely a spiral. This FR I-like radio galaxy is located very near the center of the Richness Class 0 cluster Abell 428. The existence of such an object violates a fundamental paradigm for radio loud AGN's. In paper I (Ledlow, Owen & Keel, 1998, ApJ, 495, 227), we showed that this object was most consistent with a spiral host classification with optical emission-line ratios and colors suggestive of an active nucleus. However, we were not able to confirm actual radio jet emission based on the maps available at that time. In this paper, we present new, higher resolution radio imaging, a radio/mm continuum spectrum for the nucleus, a detection of HI absorption against the bright radio core, an upper-limit to CO emission and the gas mass, and 70 (68 new) optical redshifts measured in the direction of Abell 428. We confirm the existence of a radio jet at 20cm, extending 42h75-1 into the southern lobe. At 3.6cm, we also detect a nuclear jet similar in length to that in M87, although 10 times weaker. We believe that this is the first detection of a radio jet on these scales in a disk/spiral host galaxy. The nuclear radio spectrum is similar to many blazar or QSO like objects, suggesting that the galaxy harbors an imbedded and obscured AGN. We model a turnover in the spectrum at low frequencies as a result of Free-Free absorption. We detect very strong and narrow HI absorption, with nearly the entire 20cm continuum flux of the core being absorbed, implying an unusually large optical depth (tau approximately 1). The most consistent model is that we are viewing the nucleus through a disk-like distribution of gas in the ISM, possibly through a spiral arm or a warp to account for the above average column density. From the radial velocity distribution, we find that Abell 428 is in fact made up of at least 2 clumps of galaxies separated by ~3300 km sec-1, which themselves appear to be imbedded in a nearly continuous distribution of galaxies over 13000 km sec-1 in velocity space. Thus, the environment around this unusual radio source is more like that of a poor galaxy group imbedded in a filament-like structure viewed end-on.

To appear in the April 2001 Astrophysical Journal.

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Ruth A. Kneale / / February 20, 2001