Hintze Centre for Astrophysical Surveys

Abstract:

We report recent Chandra observations of the ULX in the elliptical galaxy NGC 3379 that clearly detect two flux variability cycles. Comparing these data with the Chandra observation of ∼5 years ago, we measure a flux modulation with a period of ∼12.6 hr. Moreover, we find that the emission undergoes a correlated spectral modulation, becoming softer at low flux. We argue that our results establish this source as a ULX binary in NGC 3379. Given the old stellar population of this galaxy, the ULX is likely to be a soft transient; however, historical X-ray sampling suggests that the current "on" phase has lasted ∼10yr. We discuss our results in terms of ADC and wind-feedback models. If the flux modulation is orbital, we can constrain the donor mass and orbital period at the onset of mass transfer within 1.15-1.4 Ṁ and 12.5-17 hr, respectively. The duration of the mass transfer phase so far is probably ∼ 1 Gyr, and the binary has been a soft X-ray transient throughout this time. These constraints are insensitive to the mass of the accretor. © 2006. The American Astronomical Society. All rights reserved.

Abstract:

We present basic observational data on the 6C** sample. This is a new sample of radio sources drawn from the 151 MHz 6C survey, which was filtered with radio criteria chosen to optimize the chances of finding radio galaxies at z > 4. The filtering criteria are a steep-spectral index and a small angular size. The final sample consists of 68 sources from a region of sky covering 0.421 sr. We present VLA radio maps, and the results of K-band imaging and optical spectroscopy. Near-infrared counterparts are identified for 66 of the 68 sources, down to a 3-sigma limiting magnitude of K ~ 22 mag in a 3-arcsec aperture. Eight of these identifications are spatially compact, implying an unresolved nuclear source. The K-magnitude distribution peaks at a median K=18.7 mag, and is found to be statistically indistinguishable from that of the similarly selected 6C* sample, implying that the redshift distribution could extend to z > 4. Redshifts determined from spectroscopy are available for 22 (32 per cent) of the sources, over the range of 0.2 < z < 3.3 . We measure 15 of these, whereas the other 7 were previously known. Six sources are at z > 2.5. Four sources show broad emission lines in their spectra and are classified as quasars. Three of these show also an unresolved K-band identification. Eleven sources fail to show any distinctive emission and/or absorption features in their spectra. We suggest that these could be (i) in the so-called `redshift desert' region of 1.2 < z < 1.8, or (ii) at a greater redshift, but feature weak emission line spectra.