Catherine Cress - Academia.edu (original) (raw)
Papers by Catherine Cress
In the local Universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous ... more In the local Universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (H i ), radio surveys to probe the cosmic evolution of H i in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep H i survey, we report the first untargeted detection of an OHM at z > 0 . 5, LADUMA J033046.20 − 275518.1 (nicknamed “Nkalakatha”). The host system, WISEA J033046.26 − 275518.3, is an infrared-luminous radio galaxy whose optical redshift z ≈ 0 . 52 confirms the MeerKAT emission line detection as OH at a redshift z OH = 0 . 5225 ± 0 . 0001 rather than H i at lower redshift. The detected spectral line has 18.4 σ ...
and statistics of weak lensing from large-scale mass inhomogeneities
Galaxies Galaxy clusters Star forming galaxies LCBGs SEDs model Photometry Spectros... more Galaxies Galaxy clusters Star forming galaxies LCBGs SEDs model Photometry Spectroscopy Stellar mass Dynamical mass Dark matter ii Abstract We determine the stellar mass of star forming galaxies in the X-ray luminous cluster MS 0451.6-0305 at z ∼ 0.54. The stellar masses are estimated from fitting model spectral energy distributions (SEDs) to deep, optical UBRIz observations obtained from WIYN 3.5m telescope and public NIR K-band image from Palomar Observatory telescope. The model SEDs are based on the stellar population synthesis (SPS) model of Bruzual & Charlot (2003) and Conroy et al. (2009) that span a wide range of age, star formation history, Initial Mass Function (IMF), metallicity and dust content. We measure stellar masses for galaxies down to M * ∼ 2×10 8 M. We find a tight correlation between stellar masses derived from the two SPSs. We compare the derived stellar masses to the dynamical masses for a set of 25 star forming galaxies. The dynamical masse...
We study a maximum probability approach to reconstructing spatial maps of the Newtonian gravitati... more We study a maximum probability approach to reconstructing spatial maps of the Newtonian gravitational potential Ψ from peculiar velocities of galaxies at redshifts beyond z ∼ 0.1, where peculiar velocities have been measured from distance indicators (DI) such as the Tully-Fisher relation. With the large statistical uncertainties associated with DIs (of the order ∼ 20 % in distance), our reconstruction method aims to recover the underlying true peculiar velocity field with sufficient precision to be used as a cosmological probe of gravity, by reducing these statistical errors with the use of two physically motivated filtering prior terms. The first constructs an estimate of the velocity field derived from the galaxy over-density δg and the second makes use of the matter linear density power spectrum Pk. Through the use of N-body simulations we demonstrate that, with mea-surements with a suitably high signal-to-noise, we can successfully reconstruct the velocity and gravitational pote...
A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg... more A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, for the Degree of Master of Science
Proceedings of MeerKAT Science: On the Pathway to the SKA — PoS(MeerKAT2016), 2018
The cosmic evolution of galaxies' neutral atomic gas content is a major science driver for the Sq... more The cosmic evolution of galaxies' neutral atomic gas content is a major science driver for the Square Kilometre Array (SKA), as well as for its South African (MeerKAT) and Australian (ASKAP) precursors. Among the H I large survey programs (LSPs) planned for ASKAP and MeerKAT, the deepest and narrowest tier of the "wedding cake" will be defined by the combined L-band+UHF-band Looking At the Distant Universe with the MeerKAT Array (LADUMA) survey, which will probe H I in emission within a single "cosmic vuvuzela" that extends to z = 1.4, when the universe was only a third of its present age. Through a combination of individual and stacked detections (the latter relying on extensive multi-wavelength studies of the survey's target field), LADUMA will study the redshift evolution of the baryonic Tully-Fisher relation and the cosmic H I density, the variation of the H I mass function with redshift and environment, and the connection between H I content and galaxies' stellar properties (mass, age, etc.). The survey will also build a sample of OH megamaser detections that can be used to trace the cosmic merger history. This proceedings contribution provides a brief introduction to the survey, its scientific aims, and its technical implementation, deferring a more complete discussion for a future article after the implications of a recent review of MeerKAT LSP project plans are fully worked out.
Symposium - International Astronomical Union, 2004
The metal enrichment of the intra-cluster medium within a hierarchical structure formation scenar... more The metal enrichment of the intra-cluster medium within a hierarchical structure formation scenario is investigated using a combination of N-body + Hydro simulations and semi-analytic models of galaxy formation. In our simulation, rates and sites of star formation are identified and gas particles in the environment of these ‘galaxies’ are then enriched with metals using various prescriptions. We investigate the paths of enriched particles, demonstrating how gas enriched at higher redshifts is distributed in a cluster after it has formed. We are then able to predict a number of observable quantities such as the radial profiles of metals from both SNII and SNIa and the evolution of metal content with redshift.
Symposium - International Astronomical Union, 1999
We compare the angular correlation function measured for FIRST sources (Becker et al., Cress et a... more We compare the angular correlation function measured for FIRST sources (Becker et al., Cress et al.) with COBE-normalized CDM-model predictions (Cress…
Proceedings of the International Astronomical Union, 2009
We combine 610 MHz GMRT data, 1.4 GHz VLA data, and 1.4 GHz WSRT observations, encompassing a ~ 4... more We combine 610 MHz GMRT data, 1.4 GHz VLA data, and 1.4 GHz WSRT observations, encompassing a ~ 4 square degree field centered on the verification strip of the Spitzer First Look Survey field, to study radio sources down to fluxes of about 0.1 mJy. The spectral index (Figure 1) analysis shows that the majority of multi-component sources are steep-spectrum sources. Nevertheless the spread in the spectral distribution is wide, with a significant number of ultra-steep, flat or inverted sources, possibly indicating a wider range of accretion modes in fainter samples. By cross-correlating 107 multi-component radio sources with the optical catalogues of Marleau et al. (2007) and Papovich et al. (2006), 23 objects were identified.
Proceedings of the International Astronomical Union, 2011
The MeerKAT (64 x 13.5m dish radio interferometer) is South Africa's precursor instrument for... more The MeerKAT (64 x 13.5m dish radio interferometer) is South Africa's precursor instrument for the Square Kilometre Array (SKA), exploring dish design, instrumentation, and the characteristics of a Karoo desert site and is projected to be on sky in 2016. One of two top-priority, Key Projects is a single deep field, integrating for 5000 hours total with the aim to detect neutral atomic hydrogen through its 21 cm line emission out to redshift unity and beyond. This first truly deep HI survey will help constrain fueling models for galaxy assembly and evolution. It will measure the evolution of the cosmic neutral gas density and its distribution over galaxies over cosmic time, explore evolution of the gas in galaxies, measure the Tully-Fisher relation, measure OH maser counts, and address many more topics. Here we present the observing strategy and envisaged science case for this unique deep field, which encompasses the Chandra Deep Field-South (and the footprints of GOODS, GEMS and ...
Proceedings of the International Astronomical Union, 2015
We investigate the clustering of clusters detected by Planck via the Sunyaev-Zeldovich (SZ) effec... more We investigate the clustering of clusters detected by Planck via the Sunyaev-Zeldovich (SZ) effect. The bias (the offset of the clustering relative to what is expected for dark matter) is then used to estimate the average mass of clusters in the sample, using mass-bias relations found in simulations. We also compare the clustering of Planck clusters with those detected in the Sloan Digital Sky Survey (SDSS) using the GMBCG method. Our results indicate that Planck clusters have a higher average mass than that inferred from their SZ signatures.
Monthly Notices of the Royal Astronomical Society, 2016
We present the detection of a giant radio halo (GRH) in the Sunyaev-Zel'dovich (SZ)selected mergi... more We present the detection of a giant radio halo (GRH) in the Sunyaev-Zel'dovich (SZ)selected merging galaxy cluster ACT-CL J0256.5+0006 (z = 0.363), observed with the Giant Metrewave Radio Telescope at 325 and 610 MHz. We find this cluster to host a faint (S 610 = 5.6 ± 1.4 mJy) radio halo with an angular extent of 2.6 arcmin, corresponding to 0.8 Mpc at the cluster redshift, qualifying it as a GRH. J0256 is one of the lowest mass systems, M 500, SZ = (5.0 ± 1.2) × 10 14 M , found to host a GRH. We measure the GRH at lower significance at 325 MHz (S 325 = 10.3 ± 5.3 mJy), obtaining a spectral index measurement of α 610 325 = 1.0 +0.7 −0.9. This result is consistent with the mean spectral index of the population of typical radio haloes, α = 1.2 ± 0.2. Adopting the latter value, we determine a 1.4 GHz radio power of P 1.4 GHz = (1.0 ± 0.3) × 10 24 W Hz −1 , placing this cluster within the scatter of known scaling relations. Various lines of evidence, including the intracluster medium morphology, suggest that ACT-CL J0256.5+0006 is composed of two subclusters. We determine a merger mass ratio of 7:4, and a line-of-sight velocity difference of v ⊥ = 1880 ± 210 km s −1. We construct a simple merger model to infer relevant timescales in the merger. From its location on the P 1.4 GHz-L X scaling relation, we infer that we observe ACT-CL J0256.5+0006 just before first core crossing.
Proceedings of The International Astronomical Union, 1996
Monthly Notices of the Royal Astronomical Society, 2014
In this paper, we use the MareNostrum Universe Simulation, a large-scale, hydrodynamic, nonradiat... more In this paper, we use the MareNostrum Universe Simulation, a large-scale, hydrodynamic, nonradiative simulation in combination with a simple abundance matching approach to determine the ram pressure statistics for bent radio sources (BRSs). The abundance matching approach allows us to determine the locations of all galaxies with stellar masses ≥10 11 h −1 M in the simulation volume. Assuming that ram pressure exceeding a critical value causes bent morphology, we compute the ratio of all galaxies exceeding the ram pressure limit (RPEX galaxies) relative to all galaxies in our sample. According to our model 50 per cent of the RPEX galaxies at z = 0 are found in clusters with masses larger than 10 14.5 h −1 M the other half resides in lower mass clusters. Therefore, the appearance of bent tail morphology alone does not put tight constraints on the host cluster mass. In low-mass clusters, M ≤ 10 14 h −1 M , RPEX galaxies are confined to the central 500 h −1 kpc whereas in clusters of ≥10 15 h −1 M they can be found at distances up to 1.5 h −1 Mpc. Only clusters with masses ≥10 15 h −1 M are likely to host more than one BRS. Both criteria may prove useful in the search for distant, high-mass clusters.
Monthly Notices of the Royal Astronomical Society, 2015
We present Southern African Large Telescope (SALT) follow-up observations of seven massive cluste... more We present Southern African Large Telescope (SALT) follow-up observations of seven massive clusters detected by the Atacama Cosmology Telescope (ACT) on the celestial equator using the Sunyaev-Zel'dovich (SZ) effect. We conducted multi-object spectroscopic observations with the Robert Stobie Spectrograph in order to measure galaxy redshifts in each cluster field, determine the cluster line-of-sight velocity dispersions, and infer the cluster dynamical masses. We find that the clusters, which span the redshift range 0.3 < z < 0.55, range in mass from (5 − 20) × 10 14 M (M 200c). Their masses, given their SZ signals, are similar to those of southern hemisphere ACT clusters previously observed using Gemini and the VLT. We note that the brightest cluster galaxy in one of the systems studied, ACT-CL J0320.4+0032 at z = 0.38, hosts a Type II quasar. Only a handful of such systems are currently known, and therefore ACT-CL J0320.4+0032 may be a rare example of a very massive halo in which quasar-mode feedback is actively taking place.
General Relativity and Gravitation, 2014
We present a maximum probability approach to reconstructing spatial maps of the peculiar velocity... more We present a maximum probability approach to reconstructing spatial maps of the peculiar velocity field at redshifts z ∼ 0.1, where the velocities have been measured from distance indicators (DI) such as D n − σ relations or Tully-Fisher. With the large statistical uncertainties associated with DIs, our reconstruction method aims to recover the underlying true peculiar velocity field by reducing these errors with the use of two physically motivated filtering prior terms. The first constructs an estimate of the velocity field derived from the galaxy over-density δ g and the second makes use of the matter linear density power spectrum P k. Using N-body simulations we find, with an SDSS-like sample (N gal 33 per deg 2), an average correlation coefficient value of r = 0.55 ± 0.02 between our reconstructed velocity field and that of the true velocity field from the simulation. However, with a suitably high number density of galaxies from the next generation surveys (e.g. N gal 140 per deg 2
Proceedings of the International Astronomical Union, 2011
We investigate the possibility of using Luminous Red Galaxies (LRGs) as “Cosmic chronometers” to ... more We investigate the possibility of using Luminous Red Galaxies (LRGs) as “Cosmic chronometers” to measure the expansion rate of the universe to 3% over a redshift range 0.1 < z < 1.0. In this method, H(z) is directly measured by using the ages of passively evolving galaxies to determine dz/dt. We first present results from our study of LRGs in simulations Crawford et al. where we explore the impact of extended star formation histories on the measurements of the Hubble parameter. We then extract a carefully selected sample of LRGs taken from Sloan Digital Sky Survey (SDSS) Data Released Seven (DR7), stack spectra in redshift bins to increase the signal-to-noise, and use the Lick index modelling presented in Thomas et al. to age-date the sample. We discuss the implications for expansion rate measurements and outline a proposal to observe LRGs with the Southern African Large Telescope (SALT).
Dark Matter in Astro- and Particle Physics, 2002
If dark matter is cold then structure in the universe is built up hierarchically, smaller structu... more If dark matter is cold then structure in the universe is built up hierarchically, smaller structures forming first. The details of how galaxies form within the underlying dark matter structure are not completely understood and it remains to be seen if hierarchical models of galaxy formation can reproduce the wealth of observational data available. Here we explore predictions of hierarchical structure formation for the metal enrichment of the intra-cluster medium and compare these predictions with X-ray observations.
The Astrophysical Journal, 2004
The coherent image distortions induced by weak gravitational lensing can be used to measure the p... more The coherent image distortions induced by weak gravitational lensing can be used to measure the power spectrum of density inhomogeneities in the universe. We present our ongoing effort to detect this effect with the FIRST radio survey, which currently contains about 400,000 sources over 4,200 square degrees, and thus provides a unique resource for this purpose. We discuss the sensitivity of our measurement in the context of various cosmological models. We then discuss the crucial issue of systematic effects, the most serious of which are source fragmentation, image-noise correlation, and VLA-beam anisotropy. After accounting for these effects, we expect our experiment to yield a detection, or at least a tight upper limit, for the weak lensing power spectrum on 0.2-20 degree scales.
In the local Universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous ... more In the local Universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (H i ), radio surveys to probe the cosmic evolution of H i in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep H i survey, we report the first untargeted detection of an OHM at z > 0 . 5, LADUMA J033046.20 − 275518.1 (nicknamed “Nkalakatha”). The host system, WISEA J033046.26 − 275518.3, is an infrared-luminous radio galaxy whose optical redshift z ≈ 0 . 52 confirms the MeerKAT emission line detection as OH at a redshift z OH = 0 . 5225 ± 0 . 0001 rather than H i at lower redshift. The detected spectral line has 18.4 σ ...
and statistics of weak lensing from large-scale mass inhomogeneities
Galaxies Galaxy clusters Star forming galaxies LCBGs SEDs model Photometry Spectros... more Galaxies Galaxy clusters Star forming galaxies LCBGs SEDs model Photometry Spectroscopy Stellar mass Dynamical mass Dark matter ii Abstract We determine the stellar mass of star forming galaxies in the X-ray luminous cluster MS 0451.6-0305 at z ∼ 0.54. The stellar masses are estimated from fitting model spectral energy distributions (SEDs) to deep, optical UBRIz observations obtained from WIYN 3.5m telescope and public NIR K-band image from Palomar Observatory telescope. The model SEDs are based on the stellar population synthesis (SPS) model of Bruzual & Charlot (2003) and Conroy et al. (2009) that span a wide range of age, star formation history, Initial Mass Function (IMF), metallicity and dust content. We measure stellar masses for galaxies down to M * ∼ 2×10 8 M. We find a tight correlation between stellar masses derived from the two SPSs. We compare the derived stellar masses to the dynamical masses for a set of 25 star forming galaxies. The dynamical masse...
We study a maximum probability approach to reconstructing spatial maps of the Newtonian gravitati... more We study a maximum probability approach to reconstructing spatial maps of the Newtonian gravitational potential Ψ from peculiar velocities of galaxies at redshifts beyond z ∼ 0.1, where peculiar velocities have been measured from distance indicators (DI) such as the Tully-Fisher relation. With the large statistical uncertainties associated with DIs (of the order ∼ 20 % in distance), our reconstruction method aims to recover the underlying true peculiar velocity field with sufficient precision to be used as a cosmological probe of gravity, by reducing these statistical errors with the use of two physically motivated filtering prior terms. The first constructs an estimate of the velocity field derived from the galaxy over-density δg and the second makes use of the matter linear density power spectrum Pk. Through the use of N-body simulations we demonstrate that, with mea-surements with a suitably high signal-to-noise, we can successfully reconstruct the velocity and gravitational pote...
A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg... more A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, for the Degree of Master of Science
Proceedings of MeerKAT Science: On the Pathway to the SKA — PoS(MeerKAT2016), 2018
The cosmic evolution of galaxies' neutral atomic gas content is a major science driver for the Sq... more The cosmic evolution of galaxies' neutral atomic gas content is a major science driver for the Square Kilometre Array (SKA), as well as for its South African (MeerKAT) and Australian (ASKAP) precursors. Among the H I large survey programs (LSPs) planned for ASKAP and MeerKAT, the deepest and narrowest tier of the "wedding cake" will be defined by the combined L-band+UHF-band Looking At the Distant Universe with the MeerKAT Array (LADUMA) survey, which will probe H I in emission within a single "cosmic vuvuzela" that extends to z = 1.4, when the universe was only a third of its present age. Through a combination of individual and stacked detections (the latter relying on extensive multi-wavelength studies of the survey's target field), LADUMA will study the redshift evolution of the baryonic Tully-Fisher relation and the cosmic H I density, the variation of the H I mass function with redshift and environment, and the connection between H I content and galaxies' stellar properties (mass, age, etc.). The survey will also build a sample of OH megamaser detections that can be used to trace the cosmic merger history. This proceedings contribution provides a brief introduction to the survey, its scientific aims, and its technical implementation, deferring a more complete discussion for a future article after the implications of a recent review of MeerKAT LSP project plans are fully worked out.
Symposium - International Astronomical Union, 2004
The metal enrichment of the intra-cluster medium within a hierarchical structure formation scenar... more The metal enrichment of the intra-cluster medium within a hierarchical structure formation scenario is investigated using a combination of N-body + Hydro simulations and semi-analytic models of galaxy formation. In our simulation, rates and sites of star formation are identified and gas particles in the environment of these ‘galaxies’ are then enriched with metals using various prescriptions. We investigate the paths of enriched particles, demonstrating how gas enriched at higher redshifts is distributed in a cluster after it has formed. We are then able to predict a number of observable quantities such as the radial profiles of metals from both SNII and SNIa and the evolution of metal content with redshift.
Symposium - International Astronomical Union, 1999
We compare the angular correlation function measured for FIRST sources (Becker et al., Cress et a... more We compare the angular correlation function measured for FIRST sources (Becker et al., Cress et al.) with COBE-normalized CDM-model predictions (Cress…
Proceedings of the International Astronomical Union, 2009
We combine 610 MHz GMRT data, 1.4 GHz VLA data, and 1.4 GHz WSRT observations, encompassing a ~ 4... more We combine 610 MHz GMRT data, 1.4 GHz VLA data, and 1.4 GHz WSRT observations, encompassing a ~ 4 square degree field centered on the verification strip of the Spitzer First Look Survey field, to study radio sources down to fluxes of about 0.1 mJy. The spectral index (Figure 1) analysis shows that the majority of multi-component sources are steep-spectrum sources. Nevertheless the spread in the spectral distribution is wide, with a significant number of ultra-steep, flat or inverted sources, possibly indicating a wider range of accretion modes in fainter samples. By cross-correlating 107 multi-component radio sources with the optical catalogues of Marleau et al. (2007) and Papovich et al. (2006), 23 objects were identified.
Proceedings of the International Astronomical Union, 2011
The MeerKAT (64 x 13.5m dish radio interferometer) is South Africa's precursor instrument for... more The MeerKAT (64 x 13.5m dish radio interferometer) is South Africa's precursor instrument for the Square Kilometre Array (SKA), exploring dish design, instrumentation, and the characteristics of a Karoo desert site and is projected to be on sky in 2016. One of two top-priority, Key Projects is a single deep field, integrating for 5000 hours total with the aim to detect neutral atomic hydrogen through its 21 cm line emission out to redshift unity and beyond. This first truly deep HI survey will help constrain fueling models for galaxy assembly and evolution. It will measure the evolution of the cosmic neutral gas density and its distribution over galaxies over cosmic time, explore evolution of the gas in galaxies, measure the Tully-Fisher relation, measure OH maser counts, and address many more topics. Here we present the observing strategy and envisaged science case for this unique deep field, which encompasses the Chandra Deep Field-South (and the footprints of GOODS, GEMS and ...
Proceedings of the International Astronomical Union, 2015
We investigate the clustering of clusters detected by Planck via the Sunyaev-Zeldovich (SZ) effec... more We investigate the clustering of clusters detected by Planck via the Sunyaev-Zeldovich (SZ) effect. The bias (the offset of the clustering relative to what is expected for dark matter) is then used to estimate the average mass of clusters in the sample, using mass-bias relations found in simulations. We also compare the clustering of Planck clusters with those detected in the Sloan Digital Sky Survey (SDSS) using the GMBCG method. Our results indicate that Planck clusters have a higher average mass than that inferred from their SZ signatures.
Monthly Notices of the Royal Astronomical Society, 2016
We present the detection of a giant radio halo (GRH) in the Sunyaev-Zel'dovich (SZ)selected mergi... more We present the detection of a giant radio halo (GRH) in the Sunyaev-Zel'dovich (SZ)selected merging galaxy cluster ACT-CL J0256.5+0006 (z = 0.363), observed with the Giant Metrewave Radio Telescope at 325 and 610 MHz. We find this cluster to host a faint (S 610 = 5.6 ± 1.4 mJy) radio halo with an angular extent of 2.6 arcmin, corresponding to 0.8 Mpc at the cluster redshift, qualifying it as a GRH. J0256 is one of the lowest mass systems, M 500, SZ = (5.0 ± 1.2) × 10 14 M , found to host a GRH. We measure the GRH at lower significance at 325 MHz (S 325 = 10.3 ± 5.3 mJy), obtaining a spectral index measurement of α 610 325 = 1.0 +0.7 −0.9. This result is consistent with the mean spectral index of the population of typical radio haloes, α = 1.2 ± 0.2. Adopting the latter value, we determine a 1.4 GHz radio power of P 1.4 GHz = (1.0 ± 0.3) × 10 24 W Hz −1 , placing this cluster within the scatter of known scaling relations. Various lines of evidence, including the intracluster medium morphology, suggest that ACT-CL J0256.5+0006 is composed of two subclusters. We determine a merger mass ratio of 7:4, and a line-of-sight velocity difference of v ⊥ = 1880 ± 210 km s −1. We construct a simple merger model to infer relevant timescales in the merger. From its location on the P 1.4 GHz-L X scaling relation, we infer that we observe ACT-CL J0256.5+0006 just before first core crossing.
Proceedings of The International Astronomical Union, 1996
Monthly Notices of the Royal Astronomical Society, 2014
In this paper, we use the MareNostrum Universe Simulation, a large-scale, hydrodynamic, nonradiat... more In this paper, we use the MareNostrum Universe Simulation, a large-scale, hydrodynamic, nonradiative simulation in combination with a simple abundance matching approach to determine the ram pressure statistics for bent radio sources (BRSs). The abundance matching approach allows us to determine the locations of all galaxies with stellar masses ≥10 11 h −1 M in the simulation volume. Assuming that ram pressure exceeding a critical value causes bent morphology, we compute the ratio of all galaxies exceeding the ram pressure limit (RPEX galaxies) relative to all galaxies in our sample. According to our model 50 per cent of the RPEX galaxies at z = 0 are found in clusters with masses larger than 10 14.5 h −1 M the other half resides in lower mass clusters. Therefore, the appearance of bent tail morphology alone does not put tight constraints on the host cluster mass. In low-mass clusters, M ≤ 10 14 h −1 M , RPEX galaxies are confined to the central 500 h −1 kpc whereas in clusters of ≥10 15 h −1 M they can be found at distances up to 1.5 h −1 Mpc. Only clusters with masses ≥10 15 h −1 M are likely to host more than one BRS. Both criteria may prove useful in the search for distant, high-mass clusters.
Monthly Notices of the Royal Astronomical Society, 2015
We present Southern African Large Telescope (SALT) follow-up observations of seven massive cluste... more We present Southern African Large Telescope (SALT) follow-up observations of seven massive clusters detected by the Atacama Cosmology Telescope (ACT) on the celestial equator using the Sunyaev-Zel'dovich (SZ) effect. We conducted multi-object spectroscopic observations with the Robert Stobie Spectrograph in order to measure galaxy redshifts in each cluster field, determine the cluster line-of-sight velocity dispersions, and infer the cluster dynamical masses. We find that the clusters, which span the redshift range 0.3 < z < 0.55, range in mass from (5 − 20) × 10 14 M (M 200c). Their masses, given their SZ signals, are similar to those of southern hemisphere ACT clusters previously observed using Gemini and the VLT. We note that the brightest cluster galaxy in one of the systems studied, ACT-CL J0320.4+0032 at z = 0.38, hosts a Type II quasar. Only a handful of such systems are currently known, and therefore ACT-CL J0320.4+0032 may be a rare example of a very massive halo in which quasar-mode feedback is actively taking place.
General Relativity and Gravitation, 2014
We present a maximum probability approach to reconstructing spatial maps of the peculiar velocity... more We present a maximum probability approach to reconstructing spatial maps of the peculiar velocity field at redshifts z ∼ 0.1, where the velocities have been measured from distance indicators (DI) such as D n − σ relations or Tully-Fisher. With the large statistical uncertainties associated with DIs, our reconstruction method aims to recover the underlying true peculiar velocity field by reducing these errors with the use of two physically motivated filtering prior terms. The first constructs an estimate of the velocity field derived from the galaxy over-density δ g and the second makes use of the matter linear density power spectrum P k. Using N-body simulations we find, with an SDSS-like sample (N gal 33 per deg 2), an average correlation coefficient value of r = 0.55 ± 0.02 between our reconstructed velocity field and that of the true velocity field from the simulation. However, with a suitably high number density of galaxies from the next generation surveys (e.g. N gal 140 per deg 2
Proceedings of the International Astronomical Union, 2011
We investigate the possibility of using Luminous Red Galaxies (LRGs) as “Cosmic chronometers” to ... more We investigate the possibility of using Luminous Red Galaxies (LRGs) as “Cosmic chronometers” to measure the expansion rate of the universe to 3% over a redshift range 0.1 < z < 1.0. In this method, H(z) is directly measured by using the ages of passively evolving galaxies to determine dz/dt. We first present results from our study of LRGs in simulations Crawford et al. where we explore the impact of extended star formation histories on the measurements of the Hubble parameter. We then extract a carefully selected sample of LRGs taken from Sloan Digital Sky Survey (SDSS) Data Released Seven (DR7), stack spectra in redshift bins to increase the signal-to-noise, and use the Lick index modelling presented in Thomas et al. to age-date the sample. We discuss the implications for expansion rate measurements and outline a proposal to observe LRGs with the Southern African Large Telescope (SALT).
Dark Matter in Astro- and Particle Physics, 2002
If dark matter is cold then structure in the universe is built up hierarchically, smaller structu... more If dark matter is cold then structure in the universe is built up hierarchically, smaller structures forming first. The details of how galaxies form within the underlying dark matter structure are not completely understood and it remains to be seen if hierarchical models of galaxy formation can reproduce the wealth of observational data available. Here we explore predictions of hierarchical structure formation for the metal enrichment of the intra-cluster medium and compare these predictions with X-ray observations.
The Astrophysical Journal, 2004
The coherent image distortions induced by weak gravitational lensing can be used to measure the p... more The coherent image distortions induced by weak gravitational lensing can be used to measure the power spectrum of density inhomogeneities in the universe. We present our ongoing effort to detect this effect with the FIRST radio survey, which currently contains about 400,000 sources over 4,200 square degrees, and thus provides a unique resource for this purpose. We discuss the sensitivity of our measurement in the context of various cosmological models. We then discuss the crucial issue of systematic effects, the most serious of which are source fragmentation, image-noise correlation, and VLA-beam anisotropy. After accounting for these effects, we expect our experiment to yield a detection, or at least a tight upper limit, for the weak lensing power spectrum on 0.2-20 degree scales.