Existing observations are consistent with rich clusters of galaxies having no dust on > ∼ Mpc sca... more Existing observations are consistent with rich clusters of galaxies having no dust on > ∼ Mpc scales, while galaxy groups most probably do have dust distributed over Mpc scales. Dust in groups accounts for the observed redshift asymmetries of their galaxy distributions, and about E(B − V ) ∼ 0.1 − 0.2 mag of reddening. Motivated by these results, we develop a new technique for determining the degree of reddening and extinction due to widely-distributed dust in nearby moderately rich and poor galaxy clusters. The method compares the color-magnitude plane distributions of galaxies from cluster and control regions on the sky, where control regions are assumed to be unaffected by dust. The method is statistical in nature; it can distinguish between uniformly, nonuniformly, and clumpily distributed dust, and can determine the amount of reddening and obscuration without a priori assuming an A R /E(B J − R) ratio. We apply the method to nearby, z ≤ 0.08, medium rich and poor APM galaxy clusters. We detect no dust in these on 1.3 Mpc scales (we assume h = 0.75), and derive 99% confidence upper limits on extinction of A R = 0. m 025 and reddening of E(B J − R) = 0. m 025 (which corresponds to E(B − V ) ≈ 0. m 02). We test the method using clusters whose galaxies have been artificially reddened and obscured by various amounts, and conclude that it robustly recovers the input values for reddening, its distribution, and the ratio of total-to-selective extinction. The method can be applied to any set of galaxy clusters or groups constructed out of homogeneous and uniform two-color galaxy catalogs.
Monthly Notices of the Royal Astronomical Society, 2000
We have carried out an investigation of the environments of low redshift HII galaxies by cross-co... more We have carried out an investigation of the environments of low redshift HII galaxies by cross-correlating their positions on the sky with those of faint field galaxies in the Automatic Plate Measuring Machine catalogues. We address the question of whether violent star formation in HII galaxies is induced by low mass companions by statistically estimating the mean space density of galaxies around them. We argue that even if low mass companions were mainly intergalactic HI clouds, their optical counterparts should be detectable at faint limits of the APM scans.
Monthly Notices of the Royal Astronomical Society, 1998
Using complete samples of steep-spectrum quasars, we present evidence for a correlation between r... more Using complete samples of steep-spectrum quasars, we present evidence for a correlation between radio and optical luminosity which is not caused by selection effects, nor caused by an orientation dependence (such as relativistic beaming), nor a byproduct of cosmic evolution. We argue that this rules out models of jet formation in which there are no parameters in common with the production of the optical continuum. This is arguably the most direct evidence to date for a close link between accretion onto a black hole and the fuelling of relativistic jets. The correlation also provides a natural explanation for the presence of aligned optical/radio structures in only the most radio luminous high-redshift galaxies.
We present identifications of Abell/ACO clusters of galaxies with APM clusters using the metric s... more We present identifications of Abell/ACO clusters of galaxies with APM clusters using the metric separation between the APM clusters' coordinates and the nominal Abell cluster positions at both the APM and the (measured or estimated) Abell cluster redshift. Wherever available, we also take into account the positions of the APM galaxies which have redshift measurements. Finally, chance alignments of physically unrelated APM and ACO clusters at discordant redshifts are removed after visual inspection of the corresponding optical UK Schmidt plates. Our compilation contains only truly physical identifications, and so supersedes the identifications listed by Dalton et al. and also their subsequent erratum, which both contain a significant fraction of chance coincidences between clusters at different distances that appear close together in projection on the sky.
We present a correlation function analysis for the catalogue of photometric redshifts obtained fr... more We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto et al., 1998. By dividing the catalogue into redshift bins of width $\Delta z=0.4$ we measured the angular correlation function $w(\theta)$ as a function of redshift up to $z\sim 4.8$. From these measurements we derive the trend of the correlation length $r_0$. We find that $r_0(z)$ is roughly constant with look-back time up to $z \simeq 2$, and then increases to higher values at $z\simgt 2.4$. We estimate the values of $r_0$, assuming $\xi(r,z)=(r/r_0(z))^{-\gamma}$, $\gamma=1.8$ and different geometries. For $\Omega_0=1$ we find $r_0(z=3)\simeq 7.00\pm 4.87 h^{-1}$ Mpc, in good agreement with the values obtained from analysis of the Lyman Break Galaxies.
We find that radio galaxies are strongly clustered and highly biased tracers of the underlying ma... more We find that radio galaxies are strongly clustered and highly biased tracers of the underlying mass distribution. Models for the angular correlation function $w(\theta)$ show good agreement with the observations if we assume a bias factor $b\simeq 2$ at $z\simeq 0.3$.
Monthly Notices of the Royal Astronomical Society, 2000
We present spectra for a sample of radio sources from the FIRST survey, and use them to define th... more We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R ∼ 18.6, corresponding to z ∼ 0.2. Galaxies were assigned spectral types based on emission line strengths. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts 0.15 < ∼ z < ∼ 0.5 ; late-type galaxies make up 15 per cent of the sample and have redshifts 0.05 < ∼ z < ∼ 0.2; starbursting galaxies are a small fraction (∼ 6 per cent), and are very nearby (z < ∼ 0.05). Some 9 per cent of the population have Seyfert1/quasartype spectra, all at z > ∼ 0.8, and there are 4 per cent are Seyfert2 type galaxies at intermediate redshifts (z ∼ 0.2).
Monthly Notices of the Royal Astronomical Society, 2000
We estimate the distribution of intrinsic shapes of APM galaxy clusters from the distribution of ... more We estimate the distribution of intrinsic shapes of APM galaxy clusters from the distribution of their apparent shapes. We measure the projected cluster ellipticities using two alternative methods. The first method is based on moments of the discrete galaxy distribution while the second is based on moments of the smoothed galaxy distribution. We study the performance of both methods using Monte Carlo cluster simulations covering the range of APM cluster distances and including a random distribution of background galaxies. We find that the first method suffers from severe systematic biases, whereas the second is more reliable. After excluding clusters dominated by substructure and quantifying the systematic biases in our estimated shape parameters, we recover a corrected distribution of projected ellipticities. We use the non-parametric kernel method to estimate the smooth apparent ellipticity distribution, and numerically invert a set of integral equations to recover the corresponding distribution of intrinsic ellipticities under the assumption that the clusters are either oblate or prolate spheroids. The prolate spheroidal model fits the APM cluster data best.
Monthly Notices of the Royal Astronomical Society, 1999
We present a correlation function analysis for the catalogue of photometric redshifts obtained fr... more We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto et al., 1998. By dividing the catalogue into redshift bins of width ∆z = 0.4 we measured the angular correlation function w(θ) as a function of redshift up to z ∼ 4.8. From these measurements we derive the trend of the correlation length r 0 . We find that r 0 (z) is roughly constant with look-back time up to z ≃ 2, and then increases to higher values at z ∼ > 2.4. We estimate the values of r 0 , assuming ξ(r, z) = (r/r 0 (z)) −γ , γ = 1.8 and different geometries. For Ω 0 = 1 we find r 0 (z = 3) ≃ 7.00 ± 4.87 h −1 Mpc, in good agreement with the values obtained from analysis of the Lyman Break Galaxies.
Monthly Notices of the Royal Astronomical Society, 2002
We have derived star formation rates (SFRs), gas-phase oxygen abundances and effective dust absor... more We have derived star formation rates (SFRs), gas-phase oxygen abundances and effective dust absorption optical depths for a sample of galaxies drawn from the Stromlo-APM redshift survey using the new Charlot & Longhetti (2001; CL01) models, which provide a physically consistent description of the effects of stars, gas and dust on the integrated spectra of galaxies. Our sample consists of 705 galaxies with measurements of the fluxes and equivalent widths of Hα, [O ii], and one or both of [N ii] and [S ii]. For a subset of the galaxies, 60 and 100 µm IRAS fluxes are available. We compare the star formation rates derived using the models with those derived using standard estimators based on the Hα, the [O ii] and the far-infrared luminosities of the galaxies. The CL01 SFR estimates agree well with those derived from the IRAS fluxes, but are typically a factor of ∼ 3 higher than those derived from the Hα or the [O ii] fluxes, even after the usual mean attenuation correction of A Hα = 1 mag is applied to the data. We show that the reason for this discrepancy is that the standard Hα estimator neglects the absorption of ionizing photons by dust in H ii regions and the contamination of Hα emission by stellar absorption. We also use our sample to study variations in star formation and metallicity as a function of galaxy absolute b J magnitude. For this sample, the star formation rate per unit b J luminosity is independent of magnitude. The gas-phase oxygen abundance does increase with b J luminosity, although the scatter in metallicity at fixed magnitude is large.
Monthly Notices of the Royal Astronomical Society, 2002
We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy. ... more We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy. Optical identifications have been obtained by matching together objects from the APM and FIRST surveys over the region 148.13
Monthly Notices of the Royal Astronomical Society, 2006
We present deep, large area B and r ′ imaging for a sample of 49 brightest cluster galaxies (BCGs... more We present deep, large area B and r ′ imaging for a sample of 49 brightest cluster galaxies (BCGs). The clusters were selected by their x-ray luminosity and redshift to form two volume limited samples, one with z ∼ 0.07 and one at z ∼ 0.17. For each cluster the data cover 41 ′ × 41 ′ . We discuss our data reduction techniques in detail, and show that we can reliably measure the surface brightness at the levels of µ B ∼ 29 and µ r ′ ∼ 28. For each galaxy we present the B and r ′ images together with the surface brightness profile, B − r ′ colour, eccentricity and position angle as a function of radius.
In the course of a weak gravitational lensing survey of 39 clusters of galaxies, covering a total... more In the course of a weak gravitational lensing survey of 39 clusters of galaxies, covering a total sky area of ∼ 1 square degree, we have serendipitously discovered mass concentrations in the fields of A1705 and A1722 which are most probably not associated with the main cluster target. By combining weak lensing information with two-color galaxy photometry in fields centered on our sample clusters, we identify a new cluster candidate at z ∼ 0.5 in the field of A1705. This cluster candidate also displays strong lensing in the form of a giant luminous arc. The new mass concentration in the field of A1722 also seems to be associated with an optically luminous cluster of galaxies at z ∼ 0.5, but in this case there is some evidence for additional structures along the line of sight that may contribute to the lensing signal. A third cluster, A959, has a dark sub-clump which shows interesting morphological evidence in the mass map for being associated with the main cluster. This is the first case where there is any significant evidence for a physical association between a dark sub-clump (discovered from weak lensing) and a normal cluster. Analysis of archival X-ray data shows that the three new mass concentrations are not firmly detected in X-rays and that they are X-ray underluminous.
We constrain f_nu = Omega_nu / Omega_m, the fractional contribution of neutrinos to the total mas... more We constrain f_nu = Omega_nu / Omega_m, the fractional contribution of neutrinos to the total mass density in the Universe, by comparing the power spectrum of fluctuations derived from the 2dF Galaxy Redshift Survey with power spectra for models with four components: baryons, cold dark matter, massive neutrinos and a cosmological constant. Adding constraints from independent cosmological probes we find f_nu < 0.13 (at 95% confidence) for a prior of 0.1< Omega_m <0.5, and assuming the scalar spectral index n=1. This translates to an upper limit on the total neutrino mass and m_nu,tot < 1.8 eV for "concordance" values of Omega_m and the Hubble constant. Very similar results are obtained with a prior on Omega_m from Type Ia supernovae surveys, and with marginalization over n.
The APM Northern Sky Catalogue, a digitised version of the Palomar sky survey, is accessible on-l... more The APM Northern Sky Catalogue, a digitised version of the Palomar sky survey, is accessible on-line.
Existing observations are consistent with rich clusters of galaxies having no dust on > ∼ Mpc sca... more Existing observations are consistent with rich clusters of galaxies having no dust on > ∼ Mpc scales, while galaxy groups most probably do have dust distributed over Mpc scales. Dust in groups accounts for the observed redshift asymmetries of their galaxy distributions, and about E(B − V ) ∼ 0.1 − 0.2 mag of reddening. Motivated by these results, we develop a new technique for determining the degree of reddening and extinction due to widely-distributed dust in nearby moderately rich and poor galaxy clusters. The method compares the color-magnitude plane distributions of galaxies from cluster and control regions on the sky, where control regions are assumed to be unaffected by dust. The method is statistical in nature; it can distinguish between uniformly, nonuniformly, and clumpily distributed dust, and can determine the amount of reddening and obscuration without a priori assuming an A R /E(B J − R) ratio. We apply the method to nearby, z ≤ 0.08, medium rich and poor APM galaxy clusters. We detect no dust in these on 1.3 Mpc scales (we assume h = 0.75), and derive 99% confidence upper limits on extinction of A R = 0. m 025 and reddening of E(B J − R) = 0. m 025 (which corresponds to E(B − V ) ≈ 0. m 02). We test the method using clusters whose galaxies have been artificially reddened and obscured by various amounts, and conclude that it robustly recovers the input values for reddening, its distribution, and the ratio of total-to-selective extinction. The method can be applied to any set of galaxy clusters or groups constructed out of homogeneous and uniform two-color galaxy catalogs.
Monthly Notices of the Royal Astronomical Society, 2000
We have carried out an investigation of the environments of low redshift HII galaxies by cross-co... more We have carried out an investigation of the environments of low redshift HII galaxies by cross-correlating their positions on the sky with those of faint field galaxies in the Automatic Plate Measuring Machine catalogues. We address the question of whether violent star formation in HII galaxies is induced by low mass companions by statistically estimating the mean space density of galaxies around them. We argue that even if low mass companions were mainly intergalactic HI clouds, their optical counterparts should be detectable at faint limits of the APM scans.
Monthly Notices of the Royal Astronomical Society, 1998
Using complete samples of steep-spectrum quasars, we present evidence for a correlation between r... more Using complete samples of steep-spectrum quasars, we present evidence for a correlation between radio and optical luminosity which is not caused by selection effects, nor caused by an orientation dependence (such as relativistic beaming), nor a byproduct of cosmic evolution. We argue that this rules out models of jet formation in which there are no parameters in common with the production of the optical continuum. This is arguably the most direct evidence to date for a close link between accretion onto a black hole and the fuelling of relativistic jets. The correlation also provides a natural explanation for the presence of aligned optical/radio structures in only the most radio luminous high-redshift galaxies.
We present identifications of Abell/ACO clusters of galaxies with APM clusters using the metric s... more We present identifications of Abell/ACO clusters of galaxies with APM clusters using the metric separation between the APM clusters' coordinates and the nominal Abell cluster positions at both the APM and the (measured or estimated) Abell cluster redshift. Wherever available, we also take into account the positions of the APM galaxies which have redshift measurements. Finally, chance alignments of physically unrelated APM and ACO clusters at discordant redshifts are removed after visual inspection of the corresponding optical UK Schmidt plates. Our compilation contains only truly physical identifications, and so supersedes the identifications listed by Dalton et al. and also their subsequent erratum, which both contain a significant fraction of chance coincidences between clusters at different distances that appear close together in projection on the sky.
We present a correlation function analysis for the catalogue of photometric redshifts obtained fr... more We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto et al., 1998. By dividing the catalogue into redshift bins of width $\Delta z=0.4$ we measured the angular correlation function $w(\theta)$ as a function of redshift up to $z\sim 4.8$. From these measurements we derive the trend of the correlation length $r_0$. We find that $r_0(z)$ is roughly constant with look-back time up to $z \simeq 2$, and then increases to higher values at $z\simgt 2.4$. We estimate the values of $r_0$, assuming $\xi(r,z)=(r/r_0(z))^{-\gamma}$, $\gamma=1.8$ and different geometries. For $\Omega_0=1$ we find $r_0(z=3)\simeq 7.00\pm 4.87 h^{-1}$ Mpc, in good agreement with the values obtained from analysis of the Lyman Break Galaxies.
We find that radio galaxies are strongly clustered and highly biased tracers of the underlying ma... more We find that radio galaxies are strongly clustered and highly biased tracers of the underlying mass distribution. Models for the angular correlation function $w(\theta)$ show good agreement with the observations if we assume a bias factor $b\simeq 2$ at $z\simeq 0.3$.
Monthly Notices of the Royal Astronomical Society, 2000
We present spectra for a sample of radio sources from the FIRST survey, and use them to define th... more We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R ∼ 18.6, corresponding to z ∼ 0.2. Galaxies were assigned spectral types based on emission line strengths. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts 0.15 < ∼ z < ∼ 0.5 ; late-type galaxies make up 15 per cent of the sample and have redshifts 0.05 < ∼ z < ∼ 0.2; starbursting galaxies are a small fraction (∼ 6 per cent), and are very nearby (z < ∼ 0.05). Some 9 per cent of the population have Seyfert1/quasartype spectra, all at z > ∼ 0.8, and there are 4 per cent are Seyfert2 type galaxies at intermediate redshifts (z ∼ 0.2).
Monthly Notices of the Royal Astronomical Society, 2000
We estimate the distribution of intrinsic shapes of APM galaxy clusters from the distribution of ... more We estimate the distribution of intrinsic shapes of APM galaxy clusters from the distribution of their apparent shapes. We measure the projected cluster ellipticities using two alternative methods. The first method is based on moments of the discrete galaxy distribution while the second is based on moments of the smoothed galaxy distribution. We study the performance of both methods using Monte Carlo cluster simulations covering the range of APM cluster distances and including a random distribution of background galaxies. We find that the first method suffers from severe systematic biases, whereas the second is more reliable. After excluding clusters dominated by substructure and quantifying the systematic biases in our estimated shape parameters, we recover a corrected distribution of projected ellipticities. We use the non-parametric kernel method to estimate the smooth apparent ellipticity distribution, and numerically invert a set of integral equations to recover the corresponding distribution of intrinsic ellipticities under the assumption that the clusters are either oblate or prolate spheroids. The prolate spheroidal model fits the APM cluster data best.
Monthly Notices of the Royal Astronomical Society, 1999
We present a correlation function analysis for the catalogue of photometric redshifts obtained fr... more We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto et al., 1998. By dividing the catalogue into redshift bins of width ∆z = 0.4 we measured the angular correlation function w(θ) as a function of redshift up to z ∼ 4.8. From these measurements we derive the trend of the correlation length r 0 . We find that r 0 (z) is roughly constant with look-back time up to z ≃ 2, and then increases to higher values at z ∼ > 2.4. We estimate the values of r 0 , assuming ξ(r, z) = (r/r 0 (z)) −γ , γ = 1.8 and different geometries. For Ω 0 = 1 we find r 0 (z = 3) ≃ 7.00 ± 4.87 h −1 Mpc, in good agreement with the values obtained from analysis of the Lyman Break Galaxies.
Monthly Notices of the Royal Astronomical Society, 2002
We have derived star formation rates (SFRs), gas-phase oxygen abundances and effective dust absor... more We have derived star formation rates (SFRs), gas-phase oxygen abundances and effective dust absorption optical depths for a sample of galaxies drawn from the Stromlo-APM redshift survey using the new Charlot & Longhetti (2001; CL01) models, which provide a physically consistent description of the effects of stars, gas and dust on the integrated spectra of galaxies. Our sample consists of 705 galaxies with measurements of the fluxes and equivalent widths of Hα, [O ii], and one or both of [N ii] and [S ii]. For a subset of the galaxies, 60 and 100 µm IRAS fluxes are available. We compare the star formation rates derived using the models with those derived using standard estimators based on the Hα, the [O ii] and the far-infrared luminosities of the galaxies. The CL01 SFR estimates agree well with those derived from the IRAS fluxes, but are typically a factor of ∼ 3 higher than those derived from the Hα or the [O ii] fluxes, even after the usual mean attenuation correction of A Hα = 1 mag is applied to the data. We show that the reason for this discrepancy is that the standard Hα estimator neglects the absorption of ionizing photons by dust in H ii regions and the contamination of Hα emission by stellar absorption. We also use our sample to study variations in star formation and metallicity as a function of galaxy absolute b J magnitude. For this sample, the star formation rate per unit b J luminosity is independent of magnitude. The gas-phase oxygen abundance does increase with b J luminosity, although the scatter in metallicity at fixed magnitude is large.
Monthly Notices of the Royal Astronomical Society, 2002
We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy. ... more We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy. Optical identifications have been obtained by matching together objects from the APM and FIRST surveys over the region 148.13
Monthly Notices of the Royal Astronomical Society, 2006
We present deep, large area B and r ′ imaging for a sample of 49 brightest cluster galaxies (BCGs... more We present deep, large area B and r ′ imaging for a sample of 49 brightest cluster galaxies (BCGs). The clusters were selected by their x-ray luminosity and redshift to form two volume limited samples, one with z ∼ 0.07 and one at z ∼ 0.17. For each cluster the data cover 41 ′ × 41 ′ . We discuss our data reduction techniques in detail, and show that we can reliably measure the surface brightness at the levels of µ B ∼ 29 and µ r ′ ∼ 28. For each galaxy we present the B and r ′ images together with the surface brightness profile, B − r ′ colour, eccentricity and position angle as a function of radius.
In the course of a weak gravitational lensing survey of 39 clusters of galaxies, covering a total... more In the course of a weak gravitational lensing survey of 39 clusters of galaxies, covering a total sky area of ∼ 1 square degree, we have serendipitously discovered mass concentrations in the fields of A1705 and A1722 which are most probably not associated with the main cluster target. By combining weak lensing information with two-color galaxy photometry in fields centered on our sample clusters, we identify a new cluster candidate at z ∼ 0.5 in the field of A1705. This cluster candidate also displays strong lensing in the form of a giant luminous arc. The new mass concentration in the field of A1722 also seems to be associated with an optically luminous cluster of galaxies at z ∼ 0.5, but in this case there is some evidence for additional structures along the line of sight that may contribute to the lensing signal. A third cluster, A959, has a dark sub-clump which shows interesting morphological evidence in the mass map for being associated with the main cluster. This is the first case where there is any significant evidence for a physical association between a dark sub-clump (discovered from weak lensing) and a normal cluster. Analysis of archival X-ray data shows that the three new mass concentrations are not firmly detected in X-rays and that they are X-ray underluminous.
We constrain f_nu = Omega_nu / Omega_m, the fractional contribution of neutrinos to the total mas... more We constrain f_nu = Omega_nu / Omega_m, the fractional contribution of neutrinos to the total mass density in the Universe, by comparing the power spectrum of fluctuations derived from the 2dF Galaxy Redshift Survey with power spectra for models with four components: baryons, cold dark matter, massive neutrinos and a cosmological constant. Adding constraints from independent cosmological probes we find f_nu < 0.13 (at 95% confidence) for a prior of 0.1< Omega_m <0.5, and assuming the scalar spectral index n=1. This translates to an upper limit on the total neutrino mass and m_nu,tot < 1.8 eV for "concordance" values of Omega_m and the Hubble constant. Very similar results are obtained with a prior on Omega_m from Type Ia supernovae surveys, and with marginalization over n.
The APM Northern Sky Catalogue, a digitised version of the Palomar sky survey, is accessible on-l... more The APM Northern Sky Catalogue, a digitised version of the Palomar sky survey, is accessible on-line.
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Papers by Steve Maddox