We demonstrate a new technique for determining the physical conditions of the broad line emitting gas in quasars, using near-infrared hydrogen emission lines. Unlike higher ionisation species, hydrogen is an efficient line emitter for a very wide ran
ge of photoionisation conditions, and the observed line ratios depend strongly on the density and photoionisation state of the gas present. A locally optimally emitting cloud model of the broad emission line region was compared to measured emission lines of four nearby ($zapprox0.2$) quasars that have optical and NIR spectra of sufficient signal-to-noise to measure their Paschen lines. The model provides a good fit to three of the objects, and a fair fit to the fourth object, a ULIRG. We find that low incident ionising fluxes ($phih<10^{18}$cmsqs), and high gas densities ($ h>10^{12}$cmcu) are required to reproduce the observed hydrogen emission line ratios. This analysis demonstrates that the use of composite spectra in photoionisation modelling is inappropriate; models must be fitted to the individual spectra of quasars.
Aims. The small-scale nature of spacetime can be tested with observations of distant quasars. We comment on a recent paper by Tamburini et al. (A&A, 533, 71) which claims that Hubble Space Telescope observations of the most distant quasars place seve
re constraints on models of foamy spacetime. Methods. If space is foamy on the Planck scale, photons emitted from distant objects will accumulate uncertainties in distance and propagation directions thus affecting the expected angular size of a compact object as a function of redshift. We discuss the geometry of foamy spacetime, and the appropriate distance measure for calculating the expected angular broadening. We also address the mechanics of carrying out such a test. We draw upon our previously published work on this subject (Christiansen et al. 2011), which carried out similar tests as Tamburini et al. and also went considerably beyond their work in several respects. Results. When calculating the path taken by photons as they travel from a distant source to Earth, one must use the comoving distance rather than the luminosity distance. This then also becomes the appropriate distance to use when calculating the angular broadening expected in a distant source. The use of the wrong distance measure causes Tamburini et al. to overstate the constraints that can be placed on models of spacetime foam. In addition, we consider the impact of different ways of parametrizing and measuring the effects of spacetime foam. Given the variation of the shape of the point-spread function (PSF) on the chip, as well as observation-specific factors, it is important to select carefully -- and document -- the comparison stars used as well as the methods used to compute the Strehl ratio.
We compare the near-infrared (NIR) H band photometric and morphological properties of low-redshift (z<0.3) 3CR radio galaxies with samples of BL Lac object and quasar host galaxies, merger remnants, quiescent elliptical galaxies, and brightest cluste
r galaxies drawn from the literature. In general the 3CR host galaxies are consistent with luminous (~L*) elliptical galaxies. The vast majority of FR IIs (~80%) occupy the most massive ellipticals and form a homogeneous population that is comparable to the population of radio-loud quasar (RLQ) host galaxies in the literature. However, a significant minority (~20%) of the 3CR FR IIs appears under-luminous with respect to quasar host galaxies. All FR II objects in this faint tail are either unusually red, or appear to be the brightest objects within a group. We discuss the apparent differences between the radio galaxy and RLQ host galaxy populations. RLQs appear to require >1E11 M_sun host galaxies (and ~1E9 M_sun black holes), whereas radio galaxies and RQQs can exist in galaxies down to 3E10 M_sun. This may be due to biases in the measured quasar host galaxy luminosities or populations studied, or due to a genuine difference in host galaxy. If due to a genuine difference, it would support the idea that radio and optical active galactic nucleii are two separate populations with a significant overlap.
We present the second part of an H-band (1.6 microns) atlas of z<0.3 3CR radio galaxies, using the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (HST NICMOS2). We present new imaging for 21 recently acquired sources, and h
ost galaxy modeling for the full sample of 101 (including 11 archival) -- an 87% completion rate. Two different modeling techniques are applied, following those adopted by the galaxy morphology and the quasar host galaxy communities. Results are compared, and found to be in excellent agreement, although the former breaks down in the case of strongly nucleated sources. Companion sources are tabulated, and the presence of mergers, tidal features, dust disks and jets are catalogued. The tables form a catalogue for those interested in the structural and morphological dust-free host galaxy properties of the 3CR sample, and for comparison with morphological studies of quiescent galaxies and quasar host galaxies. Host galaxy masses are estimated, and found to typically lie at around 2*10^11 solar masses. In general, the population is found to be consistent with the local population of quiescent elliptical galaxies, but with a longer tail to low Sersic index, mainly consisting of low-redshift (z<0.1) and low-radio-power (FR I) sources. A few unusually disky FR II host galaxies are picked out for further discussion. Nearby external sources are identified in the majority of our images, many of which we argue are likely to be companion galaxies or merger remnants. The reduced NICMOS data are now publicly available from our website (http://archive.stsci.edu/prepds/3cr/)
