H1517+656: the Birth of a BL Lac Object?

H1517+656 is an unusual source, even for a BL Lac object. It is one of the most luminous BL Lacs known, with extreme emission properties at radio, optical, and X-ray frequencies. Furthermore, in our WFPC2 snapshot survey we discovered a series of peculiar arcs describing a 2.4 arcsec radius ring surrounding the source. This paper describes follow-up observations with additional WFPC2 bands and the STIS longpass filter, which have revealed this structure to be the remnants of a very recent galaxy merger. Population synthesis modelling has shown that regions of the arcs have stellar populations with age < 20 Myrs. Additionally, the circularity of the arcs indicates that the plane of the collision and hence accretion is very close to the plane of the sky. Given that BL Lac jets are thought to be aligned with the line of sight, this observation may provide a direct link between the transfer of angular momentum in an interaction and the generation of a radio source.Comment: 29 pages, 9 figures, accepted for publication in Ap

The Host Galaxies of Radio-Loud AGN: The Black Hole--Galaxy Connection

We have studied the host galaxies of a sample of radio-loud AGN spanning more than four decades in the energy output of the nucleus. The core sample includes 40 low-power sources (BL Lac objects) and 22 high-power sources (radio-loud quasars) spanning the redshift range z~0.15 to z~0.5, all imaged with the high spatial resolution of HST. All of the sources are found to lie in luminous elliptical galaxies, which follow the Kormendy relation for normal ellipticals. A very shallow trend is detected between nuclear brightness (corrected for beaming) and host galaxy luminosity. Black hole masses are estimated for the entire sample, using both the bulge luminosity--black hole mass and the velocity dispersion--black hole mass relations for local galaxies. The latter involves a new method, using the host galaxy morphological parameters, mu_e and r_e, to infer the velocity dispersion, sigma, via the fundamental plane correlation. Both methods indicate that the entire sample of radio-loud AGN are powered by very massive central black holes, with M_{black hole} ~ 10^8 to 10^10 M_{sun}$. Eddington ratios range from L/L_{Eddington} ~ 2 x 10^-4 to ~1, with the high- power sources having higher Eddington ratios than the low-power sources. Overall, radio-loud AGN appear to span a very large range in accretion efficiency, which is all but independent of the mass of the host galaxy.Comment: 21 pages, 5 figures. Accepted for publication in Ap

Host Galaxy Evolution in Radio-Loud AGN

We investigate the luminosity evolution of the host galaxies of radio-loud AGN through Hubble Space Telescope imaging of 72 BL Lac objects, including new STIS imaging of nine z > 0.6 BL Lacs. With their intrinsically low accretion rates and their strongly beamed jets, BL Lacs provide a unique opportunity to probe host galaxy evolution independent of the biases and ambiguities implicit in quasar studies. We find that the host galaxies of BL Lacs evolve strongly, consistent with passive evolution from a period of active star formation in the range 0.5 <~ z <~ 2.5, and inconsistent with either passive evolution from a high formation redshift or a non-evolving population. This evolution is broadly consistent with that observed in the hosts of other radio-loud AGN, and inconsistent with the flatter luminosity evolution of quiescent early types and radio-quiet hosts. This indicates that active star formation, and hence galaxy interactions, are associated with the formation for radio-loud AGN, and that these host galaxies preferentially accrete less material after their formation epoch than galaxies without powerful radio jets. We discuss possible explanations for the link between merger history and the incidence of a radio jet.Comment: 37 pages, 8 figures, accepted for publication in ApJ, for full PDF incl. figures see http://www.ph.unimelb.edu.au/~modowd/papers/odowdurry2005.pd

SSGSS: The Spitzer-SDSS-GALEX Spectroscopic Survey

The Spitzer-SDSS-GALEX Spectroscopic Survey (SSGSS) provides a new sample of 101 star-forming galaxies at z < 0.2 with unprecedented multi-wavelength coverage. New mid- to far-infrared spectroscopy from the Spitzer Space Telescope is added to a rich suite of previous imaging and spectroscopy, including ROSAT, Galaxy Evolution Explorer, Sloan Digital Sky Survey, Two Micron All Sky Survey, and Spitzer/SWIRE. Sample selection ensures an even coverage of the full range of normal galaxy properties, spanning two orders of magnitude in stellar mass, color, and dust attenuation. In this paper we present the SSGSS data set, describe the science drivers, and detail the sample selection, observations, data reduction, and quality assessment. Also in this paper, we compare the shape of the thermal continuum and the degree of silicate absorption of these typical, star-forming galaxies to those of starburst galaxies. We investigate the link between star formation rate, infrared luminosity, and total polycyclic aromatic hydrocarbon luminosity, with a view to calibrating the latter for spectral energy distribution models in photometric samples and at high redshift. Last, we take advantage of the 5-40 micron spectroscopic and far-infrared photometric coverage of this sample to perform detailed fitting of the Draine et al. dust models, and investigate the link between dust mass and star formation history and active galactic nucleus properties.Comment: 60 pages, 20 figure

The HST Survey of BL Lacertae Objects. II. Host Galaxies

We have used the HST WFPC2 camera to survey 132 BL Lac objects comprising seven complete radio-, X-ray-, and optically-selected samples. We obtained useful images for 110 targets spanning the redshift range 0 < z < 1.3. In two thirds of the BL Lac images, host galaxies are detected, including nearly all for z < 0.5 (58 of 63). The highest redshift host galaxy detected is in a BL Lac object at z=0.664. In 58 of the 72 resolved host galaxies, a de Vaucouleurs profile is significantly preferred, at >99% confidence, over a pure exponential disk; the two fits are comparable in the remaining 14 cases. These results limit the number of disk systems to at most 8% of BL Lacs (at 99% confidence), and are consistent with all BL~Lac host galaxies being ellipticals. The detected host galaxies are luminous ellipticals with a median absolute K-corrected magnitude of M_R= -23.7 +- 0.6 mag, at least one magnitude brighter than M* and comparable to brightest cluster galaxies. The galaxy morphologies are generally smooth and undisturbed, with small or negligible ellipticities (<0.2). There is no correlation between host galaxy and observed nuclear magnitude or estimated jet power corrected for beaming. If black hole mass is correlated linearly with bulge mass in general, this implies a large range in Eddington ratio. Present data strongly support the unification picture with FR I galaxies constituting the bulk of the parent population of BL Lac objects.Comment: Accepted for publication on ApJ. 38 pages, 8 figure

The HST Survey of BL~Lacertae Objects. IV. Infrared Imaging of Host Galaxies

The HST NICMOS Camera 2 was used for H-band imaging of 12 BL Lacertae objects taken from the larger sample observed with the WFPC2 in the R band (Urry et al. 2000; Scarpa et al. 2000). Ten of the 12 BL Lacs are clearly resolved, and the detected host galaxies are large, bright ellipticals with average H-band absolute magnitude M=-26.2+-0.45 mag and effective radius 10+-5 kpc. The rest-frame integrated color of the host galaxies is on average R-H=2.3+-0.3, consistent with the value for both radio galaxies and normal, non-active elliptical galaxies, and indicating the dominant stellar population is old. The host galaxies tend to be bluer in their outer regions than in their cores, with average color gradient Delta(R-H)/Delta(log r)=-0.2 mag, again consistent with results for normal non-active elliptical galaxies. The infrared Kormendy relation, derived for the first time for BL Lac host galaxies, is m(e) = 3.8*log(R)+14.8 (where m(e) is the surface brightness at the effective radius R), fully in agreement with the relation for normal ellipticals. The close similarity between BL Lac host galaxies and normal ellipticals suggests the active nucleus has surprisingly little effect on the host galaxy. This supports a picture in which all elliptical galaxies harbor black holes which can be actively accreting for some fraction of their lifetime.Comment: Accepted for publication on ApJ. 25 pages, 7 figure