arXiv subjects

Paul J. Francis

Publications and source records attributed to Paul J. Francis.

At least 19 recordsLinked to original sources

arXiv2020

Evidence for Infalling Gas in a Lyman-$α$ Blob

Lyman-$α$ blobs (LABs) are spatially extended nebulae of emission in the Ly$α$ line of hydrogen, seen at high redshifts$^{1,2}$, and most commonly found in the dense environment of star-forming galaxies$^{3,4}$. The origin of Ly$α$ emission in the LABs is still unclear and under debate$^{5}$. Proposed powering sources generally fall into two categories: (1) photoionization, galactic super-winds/outflows, resonant scattering of Ly$α$ photons from starbursts or active galactic nuclei (AGNs)$^{6,7,8,9,10}$ and (2) cooling radiation from cold streams of gas accreting onto galaxies$^{12}$. Here we analyze the gas kinematics within a LAB providing rare observational evidence for infalling gas. This is consistent with the release of gravitational accretion energy as cold streams radiate Ly$α$ photons. It also provides direct evidence for possible cold streams feeding the central galaxies. The infalling gas is not important by mass but hints at more than one mechanism to explain the origin of the extended Ly$α$ emission around young galaxies. It is also possible that the infalling gas may represent material falling back to the galaxy from where it originated, forming a galactic fountain.

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arXiv2007

Using a complete spectroscopic survey to find red quasars and test the KX method

We present an investigation of quasar colour-redshift parameter space in order to search for radio-quiet red quasars and to test the ability of a variant of the KX quasar selection method to detect quasars over a full range of colour without bias. This is achieved by combining IRIS2 imaging with the complete Fornax Cluster Spectroscopic Survey to probe parameter space unavailable to other surveys. We construct a new sample of 69 quasars with measured bJ - K colours. We show that the colour distribution of these quasars is significantly different from that of the Large Bright Quasar Survey's quasars at a 99.9% confidence level. We find 11 of our sample of 69 quasars have signifcantly red colours (bJ - K >= 3.5) and from this, we estimate the red quasar fraction of the K <= 18.4 quasar population to be 31%, and robustly constrain it to be at least 22%. We show that the KX method variant used here is more effective than the UVX selection method, and has less colour bias than optical colour-colour selection methods.

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arXiv2006

Fluorescent Lyman-alpha Emission from Gas Near a QSO at Redshift 4.28

We use integral field spectroscopy with the Gemini North Telescope to detect probable fluorescent Ly-alpha emission from gas lying close to the luminous QSO PSS 2155+1358 at redshift 4.28. The emission is most likely coming not from primordial gas, but from a multi-phase, chemically enriched cloud of gas lying about 50 kpc from the QSO. It appears to be associated with a highly ionised associated absorber seen in the QSO spectrum. With the exception of this gas cloud, the environment of the QSO is remarkably free of neutral hydrogen. We also marginally detect Ly-alpha emission from a foreground sub-Damped-Ly-alpha absorption-line system.

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arXiv2006

Multi-object spectroscopy of the field surrounding PKS 2126-158: Discovery of a z=0.66 galaxy group

The high-redshift radio-loud quasar PKS 2126-158 is found to have a large number of red galaxies in close apparent proximity. We use the Gemini Multi-Object Spectrograph (GMOS) on Gemini South to obtain optical spectra for a large fraction of these sources. We show that there is a group of galaxies at $z\sim0.66$, coincident with a metal-line absorption system seen in the quasar's optical spectrum. The multiplexing capabilities of GMOS also allow us to measure redshifts of many foreground galaxies in the field surrounding the quasar. The galaxy group has five confirmed members, and a further four fainter galaxies are possibly associated. All confirmed members exhibit early-type galaxy spectra, a rare situation for a Mg II absorbing system. We discuss the relationship of this group to the absorbing gas, and the possibility of gravitational lensing of the quasar due to the intervening galaxies.

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arXiv2004

The Distribution of Ly-alpha-Emitting Galaxies at z=2.38: Paper 2, Spectroscopy

In Paper 1 of this series we identified an 80 co-moving Mpc filament of candidate Lyman-alpha emitting galaxies at redshift 2.38. In this paper we present spectroscopy of the 37 galaxy candidates. Our spectroscopy reached a surface brightness limit of 5.0E-17 erg/cm^2/s/arcsec^2. Of the 14 candidates down to this limit, 12 were confirmed to be Lyman-alpha emitting galaxies at the filament redshift. We also obtained spectral confirmation for six of the lower surface brightness candidates, all of which also lay at the filament redshift. In addition, we identify a foreground cluster of QSOs at z=1.65.

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arXiv2004

The Mysterious Absence of Neutral Hydrogen within One Mpc of a Luminous Quasar at Redshift 2.168

The intense UV radiation from a highly luminous QSO should excite fluorescent Ly-alpha emission from any nearby neutral hydrogen clouds. We present a very deep narrow-band search for such emission near the z=2.168 quasar PKS 0424-131, obtained with the Taurus Tunable Filter on the Anglo-Australian Telescope. By working in the UV, at high spectral resolution and by using charge shuffling, we have been able to reach surface brightness limits as faint as 4.7E-19 erg/cm^2/s/arcsec^2. No fluorescent Ly-alpha emission is seen, whereas QSO absorption-line statistics suggest that we should have seen ~6 clouds, unless the clouds are larger than ~ 100 kpc in size. Furthermore, we do not even see the normal population of Ly-alpha emitting galaxies found by other surveys at this redshift. This is very different fromobservations of high redshift radio galaxies, which seem to be surrounded by clusters of Ly-alpha emitters. We tentatively conclude that there is a deficit of neutral hydrogen close to this quasar, perhaps due to the photo-evaporation of nearby dwarf galaxies.

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arXiv2004

Low-Metallicity Gas Clouds in a Galaxy Proto-Cluster at Redshift 2.38

We present high resolution spectroscopy of a QSO whose sight-line passes through the halo of a pair of elliptical galaxies at redshift 2.38. This pair of galaxies probably lies at the center of a galaxy proto-cluster, and is embedded in a luminous extended Ly-alpha nebula. The QSO sight-line intersects two small gas clouds within this halo. These clouds have properties similar to those of high velocity clouds (HVCs) seen in the halo of the Milky Way. The gas is in a cool (< 2 x 10^4 K) and at least 20% neutral phase, with metallicities in the range -3.0 < [Fe/H] < -1.1 and neutral hydrogen column densities of ~10^19.5 /cm^2. The origin of these clouds is unclear. The presence of low metallicity gas within this possible proto-cluster implies either that the intra-cluster medium has not been enriched with metals at this redshift, or the clouds are embedded within a hot, ionized, metal-rich gas phase.

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arXiv2003

The Distribution of Lya-Emitting Galaxies at z=2.3

We present the detection of 34 Ly-alpha emission-line galaxy candidates in a 80x80x60 co-moving Mpc region surrounding the known z=2.38 galaxy cluster J2143-4423. The space density of Ly-alpha emitters is comparable to that found by Steidel et al. when targeting a cluster at redshift 3.09, but is a factor of 5.8 +/- 2.5 greater than that found by field samples at similar redshifts. The distribution of these galaxy candidates contains several 5-10 Mpc scale voids. We compare our observations with mock catalogs derived from the VIRGO consortium Lambda-CDM n-body simulations. Fewer than 1% of the mock catalogues contain voids as large as we observe. Our observations thus tentatively suggest that the galaxy distribution at redshift 2.38 contains larger voids than predicted by current models. Three of the candidate galaxies and one previously discovered galaxy have the large luminosities and extended morphologies of "Ly-alpha blobs".

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arXiv2001

The Spectra of Red Quasars

We measure the spectral properties of a representative sub-sample of 187 quasars, drawn from the Parkes Half-Jansky, Flat-radio-spectrum Sample (PHFS). Quasars with a wide range of rest-frame optical/UV continuum slopes are included in the analysis: their colours range from 2 < B-K < 7. The median H-beta and [O III] emission-line equivalent widths of the red quasar sub-sample are a factor of ten weaker than those of the blue quasar sub-sample. Both the colours and the emission-line equivalent widths of the red quasars can be explained by the addition of a featureless red synchrotron continuum component to an otherwise normal blue quasar spectrum. The relative strengths of the blue and red components span two orders of magnitude at rest-frame 500nm. The blue component is weaker relative to the red component in low optical luminosity sources. This suggests that the fraction of accretion energy going into optical emission from the jet is greater in low luminosity quasars. This synchrotron model does not, however, fit around 10% of the quasars, which have both red colours and high equivalent width emission-lines. We hypothesise that these red, strong-lined quasars have intrinsically weak Big Blue Bumps. There is no discontinuity in spectral properties between the BL Lac objects in our sample and the other quasars. The synchrotron emission component only dominates the spectrum at longer wavelengths, so existing BL Lac surveys will be biassed against high redshift objects.

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arXiv2001

Red synchrotron jets in Parkes quasars

We present model fits to spectral energy distributions in the optical and NIR of >100 flat-spectrum radio quasars from the Parkes Half-Jansky Flat-spectrum Sample. We find that approximately 40% of the sources have power-law SEDs, while a similar number show evidence for two primary components: a blue power law and optical synchrotron emission. The blue power law is similar to the dominant component observed in the spectra of optically-selected QSOs. There is strong evidence that the synchrotron component has a turnover in the UV-optical rest frame of the spectrum. In the remaining sources it is likely that the synchrotron peaks at longer wavelengths. This mixture of two components is supported by optical polarisation measurements in a subgroup of the sources. The sources with power law SEDs show evidence for an excess number of red power law slopes compared to optically-selected quasars. There are additional spectral components in some of the sources, such as dust and the underlying galaxy, which have not been considered here.

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arXiv2000

On the Gas Surrounding High Redshift Galaxy Clusters

Francis & Hewett (1993) identified two 10-Mpc scale regions of the high redshift universe that were seemingly very overdense in neutral hydrogen. Subsequent observations showed that at least one of these gas-rich regions enveloped a cluster of galaxies at redshift 2.38. We present improved observations of the three background QSOs with sightlines passing within a few Mpc of this cluster of galaxies. All three QSOs show strong neutral hydrogen absorption at the cluster redshift, suggesting that this cluster (and perhaps all high redshift clusters) may be surrounded by a ~5 Mpc scale region containing ~ 10^12 solar masses of neutral gas. If most high redshift clusters are surrounded by such regions, we show that the gas must be in the form of many small (< 1 kpc), dense (> 0.03 cm^-3) clouds, each of mass < 10^6 solar masses. These clouds are themselves probably gathered into > 20 kpc sized clumps, which may be galaxy halos or protogalaxies. If this gas exists, it will be partially photoionised by the UV background. We predict the diffuse Ly-alpha flux from this photoionisation, and place observational limits on its intensity.

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arXiv2000

Binary quasars

Quasar pairs are either physically distinct binary quasars or the result of gravitational lensing. The majority of known pairs are in fact lenses, with a few confirmed as binaries, leaving a population of objects that have not yet been successfully classified. Building on the arguments of Kochanek, Falco & Munoz (1999), it is shown that there are no objective reasons to reject the binary interpretation for most of these. In particular, the similarity of the spectra of the quasar pairs appears to be an artifact of the generic nature of quasar spectra. The two ambiguous pairs discovered as part of the Large Bright Quasar Survey (Q 1429-053 and Q 2153-0256) are analysed using principle components analysis, which shows that their spectral similarities are not greater than expected for a randomly chosen pair of quasars from the survey. The assumption of the binary hypothesis allows the dynamics, time-scales and separation distribution of binary quasars to be investigated and constrained. The most plausible model is that the quasars' activity is triggered by tidal interactions in a galatic merger, but that the (re-)activation of the galactic nuclei occurs quite late in the interaction, when the nuclei are within 80+/-30 kpc of each other. A simple dynamical friction model for the decaying orbits reproduces the observed distribution of projected separations, but the decay time inferred is comparable to a Hubble time. Hence it is predicted that binary quasars are only observable as such in the early stages of galactic collisions, after which the quiescent super-massive black holes orbit in the merger remnant for some time.

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arXiv1999

The Optical/Near-IR Colours of Red Quasars

We present quasi-simultaneous multi-colour optical/near-IR photometry for 157 radio selected quasars, forming an unbiassed sub-sample of the Parkes Flat-Spectrum Sample. Data are also presented for 12 optically selected QSOs, drawn from the Large Bright QSO Survey. The spectral energy distributions of the radio- and optically-selected sources are quite different. The optically selected QSOs are all very similar: they have blue spectral energy distributions curving downwards at shorter wavelengths. Roughly 90% of the radio-selected quasars have roughly power-law spectral energy distributions, with slopes ranging from Fnu=nu^0 to Fnu=nu^-2. The remaining 10% have spectral energy distributions showing sharp peaks: these are radio galaxies and highly reddened quasars. Four radio sources were not detected down to magnitude limits of H ~ 19.6. These are probably high redshift (z >3) galaxies or quasars. We show that the colours of our red quasars lie close to the stellar locus in the optical: they will be hard to identify in surveys such as the Sloan Digital Sky Survey. If near-IR photometry is added, however, the red power-law sources can be clearly separated from the stellar locus: IR surveys such as 2MASS should be capable of finding these sources on the basis of their excess flux in the K-band.

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arXiv1999

Introduction to Principal Components Analysis

Understanding the inverse equivalent width - luminosity relationship (Baldwin Effect), the topic of this meeting, requires extracting information on continuum and emission line parameters from samples of AGN. We wish to discover whether, and how, different subsets of measured parameters may correlate with each other. This general problem is the domain of Principal Components Analysis (PCA). We discuss the purpose, principles, and the interpretation of PCA, using some examples from QSO spectroscopy. The hope is that identification of relationships among subsets of correlated variables may lead to new physical insight.

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arXiv1998

The Intermediate Line Region and the Baldwin Effect

Statistical investigations of samples of quasars have established that clusters of properties are correlated. The strongest trends among the ultraviolet emission-line properties are characterized by the object-to-object variation of emission from low-velocity gas, the so-called ``intermediate-line region'' or ILR. The strongest trends among the optical emission-line properties are characterized by the object-to-object variation of the line intensity ratio of [O III] 5007 to optical Fe II. Additionally, the strength of ILR emission correlates with [O III]/Fe II, as well as with radio and X-ray properties. The fundamental physical parameter driving these related correlations is not yet identified. Because the variation in the ILR dominates the variation in the equivalent widths of lines showing the Baldwin effect, it is important to understand whether the physical parameter underlying this variation also drives the Baldwin effect or is a primary source of scatter in the Baldwin effect.

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arXiv1998

Host Galaxy Contribution to the Colours of `Red' Quasars

We describe an algorithm that measures self-consistently the relative galaxy contribution in a sample of radio-quasars from their optical spectra alone. This is based on a spectral fitting method which uses the size of the characteristic 4000Å~ feature of elliptical galaxy SEDs. We apply this method to the Parkes Half-Jansky Flat Spectrum sample of Drinkwater et al. (1997) to determine whether emission from the host galaxy can significantly contribute to the very red optical-to-near-infrared colours observed. We find that at around $2σ$ confidence, most of the reddening in unresolved (mostly quasar-like) sources is unlikely to be due to contamination by a red stellar component.

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arXiv1998

Galaxy Clusters and Large Scale Structure at High Redshifts

We present a detailed study of a rich galaxy cluster at z=2.38. We demonstrate that this cluster contains large overdensities of damped Ly-alpha absorption lines, of Ly-alpha emitting galaxies and of extremely red objects. The overdensity of extremely red objects in this field demonstrates that many are high z galaxies. The huge overdensities we measure for these three classes of object are much larger than the mass overdensities of typical clusters at this redshift, as predicted by CDM and related models. We suggest therefore that the distribution of damped Ly-alpha absorption line systems, of Ly-alpha emitting galaxies and of extremely red objects are all very strongly biassed, and that somehow a small overdensity of mass has increased the fraction of baryons in collapsed objects, in the volume occupied by the cluster, to close to unity (a factor of ~10 increase). We speculate that some unknown physical process, acting on the volume occupied by our cluster, caused the normally diffuse ionised inter-galactic medium to coalesce into small (< 10^8 Solar masses) blobs of neutral hydrogen, which produce the Ly-alpha absorption-lines. Star formation occurred within these blobs at z>5, enriching them with metals and producing stars, which after several mergers and ~ 0.5 Gyr of passive evolution form the extremely red objects. The Ly-alpha emitting galaxies are probably AGN, triggered perhaps by mergers of the small blobs.

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arXiv1997

A Group of Red, Ly-alpha Emitting, High Redshift Galaxies

We have discovered two new high redshift (z=2.38) galaxies, near the previously known z=2.38 galaxy 2139-4434 B1 (Francis et al. 1996). All three galaxies are strong Ly-alpha emitters, and have much redder continuum colors (I-K about 5) than other optically-selected high redshift galaxies. We hypothesize that these three galaxies are QSO IIs; radio-quiet counterparts of high redshift radio galaxies, containing concealed QSO nuclei. The red colors are most easily modelled by an old (> 0.5 Gyr), massive (> 10E11 solar masses) stellar population. If true, this implies that at least one galaxy cluster of mass much greater than 3E11 solar masses had collapsed before redshift five.

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