Monsters and Babies from the FIRST/IRAS Survey

Radio continuum emission at cm wavelengths is relatively little affected by extinction. When combined with far-infrared (FIR) surveys this provides for a convenient and unbiased method to select (radio-loud) AGN and starbursts deeply embedded in gas and dust--rich galaxies. Such radio-selected FIR samples are useful for detailed investigations of the complex relationships between (radio) galaxy and starburst activity, and to determine whether ULIRGs are powered by hidden quasars (monsters) or young stars (babies). We present the results of a large program to obtain identifications and spectra of radio-selected, optically faint IRAS/FSC objects using the FIRST/VLA 20 cm survey. These objects are all radio-`quiet' in the sense that their radio power / FIR luminosities follow the well-known radio/FIR relationship for star forming galaxies. We compare these results to a previous study by our group of a sample of radio-`loud' IRAS/FSC ULIRGs selected from the Texas 365 MHz survey. Many of these objects also show evidence for dominant, A-type stellar populations, as well as high ionization lines usually associated with AGN. These radio-loud ULIRGs have properties intermediate between those of starbursts and quasars, suggesting a possible evolutionary connection. Deep Keck spectroscopic observations of three ULIRGs from these samples are presented, including high signal-to-noise spectropolarimetry. The polarimetry observations failed to show evidence of a hidden quasar in polarized (scattered) light in the two systems in which the stellar light was dominated by A-type stars. Although observations of a larger sample would be needed to allow a general conclusion, our current data suggest that a large fraction of ULIRGs may be powered by luminous starbursts, not by hidden, luminous AGN (quasars). [abridged]Comment: 6 Pages, including 2 PostScript figures. To appear in proceedings of "Ultraluminous Galaxies: Monsters and Babies", held at Ringberg Castle, Germany, 20-26 September 199

A Sample of 669 Ultra Steep Spectrum Radio Sources to Find High Redshift Radio Galaxies

Since radio sources with Ultra Steep Spectra (USS; alpha <~ -1.30; S ~ nu^alpha) are efficient tracers of high redshift radio galaxies (HzRGs), we have defined three samples of such USS sources using the recently completed WENSS, TEXAS, MRC, NVSS and PMN radio-surveys. Our combined sample contains 669 sources with S_1400 > 10 mJy and covers virtually the entire sky outside the Galactic plane (|b|>15). For our 2 largest samples, covering declination > -35, we selected only sources with angular sizes Theta < 1'. For 410 sources, we present radio-maps with 0.3" to ~5" resolution from VLA and ATCA observations or from the FIRST survey, which allows the optical identification of these radio sources. We find that the spectral index distribution of 143,000 sources from the WENSS and NVSS consists of a steep spectrum galaxy and a flat spectrum quasar population, with the relative contribution of flat spectrum sources doubling from S_1400 >0.1 Jy to S_1400 >2.5 Jy. The identification fraction of our USS sources on the POSS (R <~ 20) is as low as 15%, independent of spectral index alpha < -1.30. We further show that 85% of the USS sources that can be identified with an X-ray source are probably contained in galaxy clusters, and that alpha < -1.6 sources are excellent Galactic pulsar candidates, because the percentage of these sources is four times higher in the Galactic plane. Our sample has been constructed to start an intensive campaign to obtain a large sample of high redshift objects (z>3) that is selected in a way that does not suffer from dust extinction or any other optical bias [abridged].Comment: 28 Pages, 12 PostScript figures. Accepted for publication in Astronomy & Astrophysics Supplements. Appendices B, C & D available on http://www.strw.LeidenUniv.nl/~debreuck/paper

A correlation between the radio and optical morphologies of distant 3Cr radio galaxies

We report the discovery of a strong correlation between the radio and optical morphologies of distant (z ≥ 0.6), powerful radio galaxies. The isophotal axes of highly elongated distant 3CR galaxies, measured both in the light of stellar continua and extranuclear emission lines, tend to align with the radio source axes. This correlation suggests that there is a profound relation between these extremely powerful radio sources and their host galaxies. We propose that the most natural explanation of the effect is that the radio jets and/or backflows from the radio lobes interact with the interstellar media of the gas-rich galaxies associated with 3CR sources and stimulate large-scale star formation. This proposed mechanism can provide a physical explanation for the high star formation and evolution rates of 3CR galaxies at large look-back times found in earlier photometric studies. Thus, the formation and evolution of powerful radio galaxies and radio-quiet giant ellipticals may differ substantially