Showing papers by "Roger Blandford published in 1998"

On the Fate of Gas Accreting at a Low Rate onto a Black Hole

Abstract: Gas supplied conservatively to a black hole at rates well below the Eddington rate may not be able to radiate effectively and the net energy flux, including the energy transported by the viscous torque, is likely to be close to zero at all radii. This has the consequence that the gas accretes with positive energy so that it may escape. Accordingly, we propose that only a small fraction of the gas supplied actually falls onto the black hole and that the binding energy it releases is transported radially outward by the torque so as to drive away the remainder in the form of a wind. This is a generalization of and an alternative to an "ADAF" solution. Some observational implications and possible ways to distinguish these two types of flow are briefly discussed.

The oxygen-II luminosity density of the Universe

Abstract: Equivalent widths of [OII] 3727 A lines are measured in 375 faint galaxy spectra taken as part of the Caltech Faint Galaxy Redshift Survey centered on the Hubble Deep Field. The sensitivity of the survey spectra to the [OII] line is computed as a function of magnitude, color and redshift. The luminosity function of galaxies in the [OII] line and the integrated luminosity density of the Universe in the [OII] line are computed as a function of redshift. It is found that the luminosity density in the [OII] line was a factor of ~10 higher at redshifts z~1 than it is at the present day. The simplest interpretation is that the star formation rate density of the Universe has declined dramatically since z~1.

The O II Luminosity Density of the Universe

Abstract: Equivalent widths of [O II] 3727 A lines are measured in 375 faint galaxy spectra taken as part of the Caltech Faint Galaxy Redshift Survey centered on the Hubble Deep Field. The sensitivity of the survey spectra to the [O II] line is computed as a function of magnitude, color, and redshift. The luminosity function of galaxies in the [O II] line, and the integrated luminosity density of the universe in the [O II] line, are computed as a function of redshift. It is found that the luminosity density in the [O II] line was a factor of ~10 higher at redshifts z ~ 1 than it is at the present day. The simplest interpretation is that the star formation rate density of the universe has declined dramatically since z ~ 1.

A statistical analysis of the galaxy populations of distant luminous X-ray clusters

Abstract: We present a deep, multicolour (UBI) CCD survey using the Palomar 5-m telescope of a sample of high X-ray luminosity, distant clusters selected from the ROSAT All-Sky Survey. The 10 clusters lie in the redshift range z= 0.22–0.28, an era in which evolutionary effects have been reported in the properties of cluster galaxy populations.Our clusters thus provide a well-defined sample of the most massive systems at these redshifts with which to quantify the extent and variability of these evolutionary effects. The relatively low redshifts of these clusters also means that simple connections can be made between the galaxy populations of these clusters and their immediate descendents, local rich clusters. Moreover, by concentrating on a narrow redshift range, we can take advantage of the homogeneity of our cluster sample to combine the galaxy catalogues from all the clusters to analyse statistically the bulk properties of their populations. We present an analysis of the cluster galaxy populations using our multicolour data to probe the distribution, luminosities and star formation histories of galaxies in these regions. Our aim is to chart the characteristics of the galaxy populations of massive intermediate redshift clusters and to combine these into a wider scheme for galaxy evolution in high-density environments. The core regions of clusters in our sample contain only a small proportion of star-forming galaxies, and they therefore do not exhibit a classical “Butcher–Oemler” effect. Focusing on the redder cluster galaxies, we find that their integrated luminosity is well correlated with the X-ray temperatures of the clusters, and hence with cluster mass. Furthermore, the typical rest frame UV–optical colours of the luminous elliptical sequences in the clusters exhibit a remarkably small cluster-to-cluster scatter, ≲2 per cent, indicating that these galaxies are highly homogeneous between cluster environments. However, at fainter magnitudes we observe a marked increase in the range of mid-UV colours of galaxies possessing strong 4000-A breaks, as determined from photometry in ∼7.5 h-1 kpc diameter apertures. In the light of the apparent decline in the population of S0 galaxies seen in distant z≳0.4, clusters, and in view of the luminosities and colours of this population, we propose that they may be the progenitors of the dominant S0 population of rich local clusters, caught in the final stage before they become completely quiescent. Further studies of this population will prove a necessary link with which to connect the evolution observed in cluster populations at high redshift with the nature of the final remnants locally. Observations in the rest frame UV will be important in these studies owing to the relative ease of detecting the signature of previous activity in this spectral region.

A Blind Test of Photometric Redshift Prediction

Abstract: Results of a blind test of photometric redshift predictions against spectroscopic galaxy redshifts obtained in the Hubble Deep Field with the Keck Telescope are presented. The best photometric redshift schemes predict spectroscopic redshifts with a redshift accuracy of Δz < 0.1 for more than 68% of sources and with Δz < 0.3 for 100%, when single-feature spectroscopic redshifts are removed from consideration. This test shows that photometric redshift schemes work well, at least when the photometric data are of high quality and when the sources are at moderate redshifts.

Are AGN broad emission lines formed by discrete clouds? Analysis of Keck high‐resolution spectroscopy of NGC 4151

Abstract: We search for a direct signature of discrete ‘clouds’ in the broad-line region (BLR) of the Seyfert galaxy NGC 4151. For this purpose we apply cross-correlation (CC) analysis to a high-resolution Keck spectrum of the galaxy. No such signature is found in the data. In order for cloud models to be compatible with this result, there must be at least ∼3×10^7 emitting clouds in the BLR, where the limit is based on simulation of a homogeneous cloud population. More realistic distributions increase the lower limit to above 10^8. These numbers are an order-of-magnitude improvement on our previous limit from Mrk 335, where the improvement comes from higher signal-to-noise ratio (S/N), broader lines and refined simulations. Combined with the predicted upper limit for the number of emitting clouds in NGC 4151 (10^6–10^7), the derived lower limit puts a strong constraint on the cloud scenario in the BLR of this object. Similar constraints can be placed on models where the emission originates in streams and sheets. Thus, this investigation suggests that the broad emission lines (BELs) in NGC 4151, and by extension in all AGNs, are not made of an ensemble of discrete independent emitters.

B0712+472: a new radio four-image gravitational lens

Abstract: A new four-image gravitational lens system, B0712+472, has been discovered during the Cosmic Lens All-Sky Survey. This system consists of four flat-spectrum radio images that are also seen on a Hubble Space Telescope (HST) image, together with the lensing galaxy. We present MERLIN, VLA and VLBA maps and WHT spectra of the system as well as the HST images. The light distribution of the tensing galaxy is highly elongated and so too is the mass distribution deduced from modelling. We suggest a redshift of similar to 1.33 for the lensed object the lens redshift will require further investigation. The discovery of this new system further increases the ratio of four-image to two-image lens systems currently known, exacerbating problems of required ellipticity of matter distributions in lensing galaxies.

A complete infrared Einstein ring in the gravitational lens system B1938 + 666

Abstract: We report the discovery, using NICMOS on the Hubble Space Telescope, of an arcsec-diameter Einstein ring in the gravitational lens system B1938 + 666. The lensing galaxy is also detected, and is most likely an early-type galaxy. Modelling of the ring is presented and compared with the radio structure from MERLIN maps. We show that the Einstein ring is consistent with the gravitational lensing of an extended infrared component, centred between the two radio components.

Caltech Faint Galaxy Redshift Survey VIII: Analysis for the Field J0053+1234

Abstract: The results of a spectroscopic investigation of a complete sample of objects with $K_s 1.5$, and the galaxies in such groups appear to be coeval and to show little sign of ongoing star formation. The galaxies outside the redshift peaks are also clustered, albeit more weakly, are less luminous and more frequently exhibit strong emission lines. These ``isolated'' galaxies therefore appear, on average, to form stars at later epochs than the strongly clustered galaxies. The galaxy SEDs derived from our $UBVRIK$ photometry are also very closely correlated with the galaxy spectral types and luminosities. These results have strong implications for the analysis of redshift surveys at intermediate redshift.

The complex gravitational lens system B1933+503

Abstract: We report the discovery of the most complex arcsec-scale radio gravitational lens system yet known, B1933+503 was found during the course of the CLASS survey and MERLIN and VLA radio maps reveal up to 10 components. Four of these are compact and have flat spectra: the rest are more extended and have steep spectra, The background lensed object appears to consist of a flat-spectrum core (quadruply imaged) and two compact 'lobes' symmetrically disposed relative to the core, One of the lobes is quadruply imaged while the other is doubly imaged. An HST observation of the system with the WFPC2 shows a galaxy with an axial ratio of 0.5, but none of the images of the background object is detected, A redshift of 0.755 has been measured for the lens galaxy.

A possible gravitational lens in the Hubble Deep Field South

Abstract: We model an apparent gravitational lens system HDFS 2232509-603243 in the Hubble Deep Field South. The system consists of a blue V=25 mag arc separated by 0.9 arcsec from a red V=22 mag elliptical galaxy. A mass distribution which follows the observed light distribution with a constant mass-to-light ratio can fit the arc component positions if external shear is added. A good fit is also obtained with simple parameterized models, and all the models predict a forth image fainter than the detection limit. The inferred mass-to-light ratio is roughly 15 in solar units if the lens is at redshift 0.6. The lens models predict a velocity dispersion of about 280 km/s which could be confirmed with spectroscopy.

The JVAS/Class Gravitational Lens Surveys

Abstract: The JVAS/CLASS surveys are designed to find gravitational lens systems in which the flat spectrum cores of radio sources are multiply imaged. The prime motivation for the searches is to find systems from which the Hubble constant can be determined by measuring time delays and to produce statistically clean samples of lenses which can be used for cosmological investigations. Up to now some 8000 flat spectrum sources have been mapped with the VLA with a resolution of 200 mas. When complete we hope to have surveyed 10,000 sources. High resolution MERLIN and VLBA follow-up of the candidates is still in progress. To date 11 lens systems have been discovered. Time delays have been measured for 2 of these systems. HST and other observations almost always show a lensing galaxy so there is no evidence for “dark lenses”. The maximum image separations peak between 1 and 2 arcsec.

Caltech Faint Galaxy Redshift Survey VII: Data Analysis Techniques and Redshifts in the Field J0053+1234

Abstract: We present the techniques used to determine redshifts and to characterize the spectra of objects in the Caltech Faint Galaxy Redshift Survey in terms of spectral classes and redshift quality classes. These are then applied to spectra from an investigation of a complete sample of objects with $K_s<20$ mag in a 2 by 7.3 arcmin^2 field at J005325+1234. Redshifts were successfully obtained for 163 of the 195 objects in the sample; these redshifts lie in the range [0.173, 1.44] and have a median of 0.58 (excluding 24 Galactic stars). The sample includes two broad lined AGNs and one QSO.

The Extremely Red Objects Found Thus Far in the Caltech Faint Galaxy Redshift Survey

Abstract: We discuss the very red objects found in the first field of the Caltech Faint Galaxy Redshift Survey, for which the observations and analysis are now complete. In this field, which is 15 arcmin$^2$ and at J005325+1234 there are 195 objects with $K_s < 20$ mag, of which 84% have redshifts. The sample includes 24 spectroscopically confirmed Galactic stars, 136 galaxies, three AGNs, and 32 objects without redshifts. About 10% of the sample has $(R-K) \ge 5$ mag. Four of these objects have redshifts, with $0.78 \le z \le 1.23$. Three of these are based on absorption features in the mid-UV, while the lowest redshift object shows the standard features near 4000\AA. Many of the objects still without redshifts have been observed spectroscopically, and no emission lines were seen in their spectra. We believe they are galaxies with $z \sim 1 - 1.5$ that are red due to their age and stellar content and not to some large amount of internal reddening from dust. Among the many other results from this survey of interest here is a determination of the median extinction in the mid-UV for objects with strong emission line spectra at $z \sim 1 - 1.3$. The result is extinction by a factor of $\sim$2 at 2400\AA.

Cherenkov-drift emission mechanism

Abstract: Emission of a charged particle propagating in a medium with a curved magnetic field is reconsidered stressing the analogy between this emission mechanism and collective Cherenkov-type plasma emission It is explained how this mechanism differs from conventional Cherenkov, cyclotron or curvature emission and how it includes, to some extent, the features of each of these mechanisms Presence of a medium supporting subluminous waves is essential for the possibility of wave amplification by particles streaming along the curved magnetic field with a finite curvature drift We suggest an analogy between the curvature drift emission and the anomalous cyclotron-Cherenkov emission Treating the emission in cylindrical coordinates in the plane-wave-like approximation allows one to compute the single particle emissivity and growth rate of the Cherenkov-drift instability We compare the growth rates calculated using the single particle emissivity and using the dielectric tensor of one dimensional plasma streaming along the curved field In calculating the single particle emissivity it is essential to know the normal modes of the medium and their polarization which can be found from the dielectric tensor of the medium This emission mechanism may be important for the problem of pulsar radio emission generation

B2045+265: A New Four-Image Gravitational Lens from CLASS

Abstract: We have discovered a new gravitational lens in the Cosmic Lens All-Sky Survey (CLASS). B2045+265 is a four-image system with a maximum separation of 1.9 arcsec. A fifth radio component is detected, but its radio spectrum and its positional coincidence with infrared emission from the lensing galaxy strongly suggests that it is the radio core of the lensing galaxy. This implies that the B2045+265 system consists of a flat-spectrum radio source being lensed by another flat-spectrum radio source. Infrared images detect the lensed images of the background source and the lensing galaxy. The lensed images have relative positions and flux densities that are consistent with those seen at radio wavelengths. Spectra of the system reveal a lens redshift of z_l=0.8673 and a source redshift of z_s=1.28. The image splitting and system redshifts imply that the projected mass inside the Einstein radius of the lensing galaxy is M_E=4.7x10^{11} h^{-1} M_sun. An estimate of the light emitted inside the Einstein radius gives a mass-to-light ratio in the rest frame B band of (M/L_B)_E = 20 h (M/L_B)_sun. Both the mass and mass-to-light ratio are higher than what is seen in nearby Sa galaxies. In fact, the implied rotation velocity for the lensing galaxy is two to three times higher than what is seen in nearby spirals. The large projected mass inside the Einstein ring radius may be the result of a significant amount of dark matter in the system, perhaps from a compact group of galaxies; however, it may also arise from a misidentification of the source redshift. A simple model of the gravitational potential of the lens reproduces the image positions well, but further modeling is required to satisfy the constraints from the image flux density ratios. With further observations and modeling, this lens may yield an estimate of H_0.

Jets and Transients

Abstract: I have been asked to provide some general introduction to the topic of jets and high energy transients. I have chosen to do this by asking seven questions that seem to be timely following recent observational and theoretical developments, the most important of which may be the growing realization that the extragalactic- massive black hole and Galactic-stellar mass black hole manifestations of these phenomena provide two complementary ways of viewing a common physical mechanism. I have also chosen to illustrate these questions using one specific source in each case. The page limits prevent me from giving the answers. They also prohibit my even attempting to give a skeletal bibliography over so large a field. Fortunately almost all of these topics will be covered in greater depth during this meeting.

The Extremely Red Objects Found Thus Far in the Caltech Faint Galaxy Redshift Survey

Abstract: We discuss the very red objects found in the first field of the Caltech Faint Galaxy Redshift Survey, for which the observations and analysis are now complete. In this field, which is 15 arcmin$^2$ and at J005325+1234 there are 195 objects with $K_s < 20$ mag, of which 84% have redshifts. The sample includes 24 spectroscopically confirmed Galactic stars, 136 galaxies, three AGNs, and 32 objects without redshifts. About 10% of the sample has $(R-K) \ge 5$ mag. Four of these objects have redshifts, with $0.78 \le z \le 1.23$. Three of these are based on absorption features in the mid-UV, while the lowest redshift object shows the standard features near 4000\AA. Many of the objects still without redshifts have been observed spectroscopically, and no emission lines were seen in their spectra. We believe they are galaxies with $z \sim 1 - 1.5$ that are red due to their age and stellar content and not to some large amount of internal reddening from dust. Among the many other results from this survey of interest here is a determination of the median extinction in the mid-UV for objects with strong emission line spectra at $z \sim 1 - 1.3$. The result is extinction by a factor of $\sim$2 at 2400\AA.