Publications by Garret Cotter

Deep spectroscopy of a young radio source at z = 0.521

Astronomische Nachrichten 327 (2006) 171-174

KJ Inskip, D Lee, G Cotter, ACS Readhead, TJ Pearson

9C J1503+4528 is a very young CSS radio galaxy with an age of order 10 4 years. This source is an ideal laboratory for the study of the intrinsic host galaxy/IGM properties, radio source interactions, evidence for young stellar populations and the radio source triggering mechanism. Here we present the results of a spectroscopic analysis of this source, considering each of these aspects of radio source physics. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

5-GHz MERLIN and VLBA observations of compact 9C sources

Monthly Notices of the Royal Astronomical Society 367 (2006) 323-330

RC Bolton, CJ Chandler, G Cotter, TJ Pearson, GG Pooley, ACS Readhead, JM Riley, EM Waldram

ABSTRACT In this paper, we present subarcsecond resolution observations of 36 compact sources from the 15 h region of the 15-GHz 9th Cambridge survey. These sources all have previously measured simultaneous continuum radio spectra spanning 1.4-43 GHz and we classify each source by fitting a quadratic function to its spectrum. Using the Multi-Element Radio-Linked Interferometer Network and the Very Long Baseline Array, both at 5 GHz, we resolve all six steep-spectrum objects and four of the 13 flat-spectrum objects. However, none of the 16 objects with convex spectra peaking above 2.5 GHz is resolved even at <3-mas resolution. These results, in combination with the findings of a 15-GHz variability study, suggest that emission from the high-frequency peaking objects is affected by relativistic beaming, and that these objects are not necessarily as young as the synchrotron self-absorption interpretation of their peak frequencies would imply. © 2006 RAS.

15-GHz variability of 9C sources

Monthly Notices of the Royal Astronomical Society 370 (2006) 1556-1564

RC Bolton, CJ Chandler, G Cotter, TJ Pearson, GG Pooley, ACS Readhead, JM Riley, EM Waldram

We present results from a 3-yr study of the 15-GHz variability of 51 9C sources. 48 of these sources make up a subsample of a larger one complete to 25 mJy in 9C, and as the sources are selected pseudo-randomly the results should be representative of the complete sample. 29 per cent of this subsample are found to be variable above the flux calibration uncertainties of ∼6 per cent. 50 per cent of the flat-spectrum objects are variable whilst none of the steep-spectrum objects or the objects with convex spectra peaking below 5 GHz are variable. Nine of the objects studied have convex spectra and peak frequencies above 5 GHz; eight of these were found to vary at 15 GHz, suggesting that the high-frequency peaking class in this sample is largely populated by objects with jets aligned close to the line of sight whose emission is dominated by beamed components. © 2006 RAS.

MRC B1221-423: A compact steep-spectrum radio source in a merging galaxy

Monthly Notices of the Royal Astronomical Society 356 (2005) 515-523

HM Johnston, RW Hunstead, G Cotter, EM Sadler

We present BVRIK images and spectroscopic observations of the z = 0.17 host galaxy of the compact steep-spectrum (CSS) radio source MRC B1221 -423. This is a young (∼10 5 yr) radio source with double lobes lying well within the visible galaxy. The host galaxy is undergoing tidal interaction with a nearby companion, with shells, tidal tails and knotty star-forming regions all visible. We analyse the images of the galaxy and its companion pixel-by-pixel, first using colour-magnitude diagrams and then fitting stellar population models to the spectral energy distributions (SEDs) of each pixel. We also present medium-resolution spectroscopy of the system. The pixels separate cleanly in colour-magnitude diagrams, with pixels of different colours occupying distinct regions of the host galaxy and its companion. Fitting stellar population models to these colours, we have estimated the age of each population. We find three distinct groups of ages: an old population (τ ∼ 15 Gyr) in the outskirts of the host galaxy; an intermediate-age population (τ ∼ 300 Myr) around the nucleus and tidal tail, and a young population (τ 10 Myr) in the nucleus and blue knots. The spectrum of the nucleus shows numerous strong emission lines, including [O I] λ6300, [O II] λ3727, [S II] λλ6716, 6731, Hα and [N II] λλ6548, 6583, characteristic of a low-ionization nuclear emission-line region (LINER) spectrum. The companion galaxy shows much narrower emission lines with very different line ratios, characteristic of a starburst galaxy. We have evidence for three distinct episodes of star formation in B1221-423. The correlation of age with position suggests the two most recent episodes were triggered by tidal interactions with the companion galaxy. The evidence points to the active galactic nucleus (AGN) in the centre of B1221-423 having been caught in the act of ignition. However, none of the components we have identified is as young as the radio source, implying that the delay between the interaction and the triggering of the AGN is at least 3 × 10 8 yr.

Vigorous star formation in a bulge-dominated extremely red object at z = 1.34

Monthly Notices of the Royal Astronomical Society 360 (2005) 685-692

G Cotter, C Simpson, RC Bolton

We present near-infrared (near-IR) spectroscopy of three extremely red objects (EROs) using the OHS/CISCO spectrograph at the Subaru Telescope. One target exhibits a strong emission line, which we identify as Ha at z = 1.34. Using new and existing ground-based optical and near-IR imaging, and archival Hubble Space Telescope imaging, we argue that this target is essentially an elliptical galaxy, with an old stellar population of around 4 × 10 11 M ⊙, but having a dust-enshrouded star-forming component with a star formation rate (SFR) of some 50-100 M ⊙ yr -1. There is no evidence that the galaxy contains an active galactic nucleus. Analysis of a further two targets, which do not exhibit any features in our near-IR spectra, suggests that one is a quiescent galaxy in the redshift range 1.2 < z < 1.6, but that the other cannot be conclusively categorized as either star-forming or quiescent. Even though our first target has many of the properties of an old elliptical, the ongoing star formation means that it cannot have formed all of its stellar population at high redshift. While we cannot infer any robust values for the SFR in ellipticals at z > 1 from this one object, we argue that the presence of an object with such a high SFR in such a small sample suggests that a non-negligible fraction of the elliptical galaxy population may have formed a component of their stellar population at redshifts z ∼ 1-2. We suggest that this is evidence for ongoing star formation in the history of elliptical galaxies. © 2005 RAS.

The radio source population at high frequency: Follow-up of the 15-GHz 9C survey

Monthly Notices of the Royal Astronomical Society 354 (2004) 485-521

RC Bolton, G Cotter, GG Pooley, JM Riley, EM Waldram, CJ Chandler, BS Mason, TJ Pearson, ACS Readhead

We have carried out extensive radio and optical follow-up of 176 sources from the 15-GHz 9th Cambridge survey. Optical identifications have been found for 155 of the radio sources; optical images are given with radio maps overlaid. The continuum radio spectrum of each source spanning the frequency range 1. 4-43 GHz is also given. Two flux-limited samples are defined, one containing 124 sources complete to 25 mJy and one of 70 sources complete to 60 mJy. Between one-fifth and one-quarter of sources from these flux-limited samples display convex radio spectra, rising between 1.4 and 4.8 GHz. These rising-spectrum sources make up a much larger fraction of the radio source population at this high selection frequency than in lower frequency surveys. We find that by using non-simultaneous survey flux density measurements at 1.4 and 15 GHz to remove steep-spectrum objects, the efficiency of selecting objects with spectra rising between 1.4 and 4.8 GHz (as seen in simultaneous measurements) can be raised to 49 per cent without compromising the completeness of the rising-spectrum sample.

Searching for clusters with SUMSS

Revista Mexicana de Astronomia y Astrofisica: Serie de Conferencias 17 (2003) 263-

HJ Buttery, G Cotter, RW Hunstead, EM Sadler

Searches for statistical overdensities of radio sources in the NRAO VLA Sky Survey (NVSS) survey have led to the detection of high redshift clusters of galaxies (Cotter et al. 2002; Croft et al. 2001). We have carried out a similar search for overdensities using the Sydney University Molonglo Sky Survey (SUMSS). Radio observations at 20/13 cm have been made of these candidates at the Australia Telescope Compact Array (ATCA) revealing sources of diffuse cluster emission as well as groups of FR1 sources showing structures associated with clusters, indicating a cluster "hit-rate" of about 70%.

Searching for Clusters with SUMSS

New Astronomy Reviews 47 (2003) 329-332

HJ Buttery, G Cotter, RW Hunstead, EM Sadler

Statistical overdensities of radio sources in the Sydney University Molonglo Sky Survey (SUMSS) are used as signposts to identify high-redshift clusters of galaxies. These potential clusters have been observed at 20 and 13 cm at the Australia Telescope Compact Array (ATCA) to obtain better positional accuracy for the sources. A subsample have been imaged in V, R and I at the 2.3-m telescope at Siding Spring and in J and K at the Anglo Australian Telescope (AAT) and the New Technology Telescope (NTT) at La Silla, Chile. The colours obtained from these observations will be used to estimate redshifts for the potential cluster members. © 2003 Elsevier B.V. All rights reserved.

Follow-up of the 9C survey: initial results

NEW ASTRONOMY REVIEWS 47 (2003) 367-371

RC Bolton, G Cotter, TJ Pearson, GG Pooley, ACS Readhead, JA Riley, EM Waldram

Ground-based mid infra-red observations of nearby starburst and AGN galaxies


B Nikolic, P Alexander, G Cotter, M Longair, M Clemens

Deep spectroscopy of z ∼ 1 6C radio galaxies - I. The effects of radio power and size on the properties of the emission-line gas

Monthly Notices of the Royal Astronomical Society 337 (2002) 1381-1406

KJ Inskip, PN Best, S Rawlings, MS Longair, G Cotter, HJA Röttgering, S Eales

The results of deep long-slit optical spectroscopy for a sample of eight 6C radio galaxies at redshift z ∼ 1 are presented. Emission-line ratios are derived for many emission lines with rest-frame wavelengths of 1500-4500 Å and the kinematic properties of the emission-line gas are derived from an analysis of the two-dimensional structure of the [O II] 3727-Å emission line at ≈5 Å spectral resolution. In general, the 6C spectra display many characteristics similar to those of more powerful 3CR sources at the same redshifts. The emission-line region gas kinematics are more extreme for the smaller radio sources in the sample, which often display distorted velocity profiles. The ionization state of the emission-line region also varies with radio size: the spectra of large radio sources (> 120 kpc) are consistent with photoionization by an obscured active galactic nucleus (AGN), whilst smaller (< 120 kpc) sources typically exist in a lower ionization state and have spectra that are better explained by additional ionization due to shocks associated with the expanding radio source. The kinematic and ionization properties of the 6C radio galaxies are clearly linked. As for the 3CR sources, smaller radio sources also typically possess more extensive emission-line regions, with enhanced emission-line luminosities. A high-velocity emission-line gas component is observed in 6C 1019+39, similar to that seen in 3C 265. It is clear that the best interpretation of the spectra of radio sources requires a combination of ionization mechanisms. A simple model is developed, combining AGN photoionization with photoionization from the luminous shock associated with the expanding radio source. The relative contributions of ionizing photons from shocks and the central AGN to an emission-line gas cloud vary with radio source size and the position of the cloud. This model provides a good explanation for both the ionization properties of the emission-line regions and the radio size evolution of the emission-line region extents and luminosities.

Searching for clusters of galaxies with SUMSS

NEW ERA IN COSMOLOGY 283 (2002) 245-246

HJ Buttery, G Cotter, RW Hunstead, EM Sadler

The death of FR II radio sources and their connection with radio relics


CR Kaiser, G Cotter

Deep optical imaging of the field of PC 1643+4631A&B - II. Estimating the colours and redshifts of faint galaxies


G Cotter, T Haynes, JC Baker, ME Jones, R Saunders

Observations of the Sunyaev-Zel'dovich effect in the z=0.78 cluster MS 1137.5+6625


G Cotter, HJ Buttery, R Das, ME Jones, K Grainge, GG Pooley, R Saunders

Deep spectroscopy of z ∼ 1 6C radio galaxies - II. Breaking the redshift-radio power degeneracy

Monthly Notices of the Royal Astronomical Society 337 (2002) 1407-1416

KJ Inskip, PN Best, HJA Röttgering, S Rawlings, G Cotter, MS Longair

The results of a spectroscopic analysis of 3CR and 6C radio galaxies at redshift z ∼ 1 are contrasted with the properties of lower-redshift radio galaxies, chosen to be matched in radio luminosity to the 6C sources studied at z ∼ 1, thus enabling the redshift-radio power degeneracy to be broken. Partial rank correlations and principal component analysis have been used to determine which of redshift and radio power are the critical parameters underlying the observed variation of the ionization state and kinematics of the emission-line gas. [O II]/Hβ is shown to be a useful ionization mechanism diagnostic. Statistical analysis of the data shows that the ionization state of the emission-line gas is strongly correlated with radio power, once the effects of other parameters are removed. No dependence of ionization state on cosmic epoch is observed, implying that the ionization state of the emission-line gas is solely a function of the properties of the active galactic nucleus rather than the host galaxy and/or environment. Statistical analysis of the kinematic properties of the emission-line gas shows that these are strongly correlated independently with both redshift and radio power. The correlation with redshift is the stronger of the two, suggesting that host-galaxy composition or environment may play a role in producing the less extreme gas kinematics observed in the emission-line regions of low-redshift galaxies. For both the ionization and kinematic properties of the galaxies, the independent correlations observed with radio size are stronger than with either radio power or redshift. Radio source age is clearly a determining factor for the kinematics and ionization state of the extended emission-line regions.

Detection of a cosmic microwave background decrement towards a cluster of mJy radio sources


G Cotter, HJ Buttery, S Rawlings, S Croft, GJ Hill, P Gay, R Das, N Drory, K Grainge, WF Grainger, ME Jones, GG Pooley, R Saunders

Deep optical imaging of the field of PC 1643+4631A&B - I. Spatial distributions and the counts of faint galaxies


T Haynes, G Cotter, JC Baker, S Eales, ME Jones, S Rawlings, R Saunders

Surveying the sky with the Arcminute Microkelvin Imager: Expected constraints on galaxy cluster evolution and cosmology

Monthly Notices of the Royal Astronomical Society 328 (2001) 783-794

R Kneissl, ME Jones, R Saunders, VR Eke, AN Lasenby, K Grainge, G Cotter

We discuss prospects for cluster detection via the Sunyaev-Zel'dovich (SZ) effect in a blank field survey with the interferometer array, the Arcminute MicroKelvin Imager (AMI). Clusters of galaxies selected in the SZ effect probe cosmology and structure formation with little observational bias, because the effect measures integrated gas pressure directly, and does so independently of cluster redshift. We use hydrodynamical simulations in combination with the Press-Schechter expression to simulate SZ cluster sky maps. These are used with simulations of the observation process to gauge the expected SZ cluster counts. Even with a very conservative choice of parameters we find that AMI will discover at least several tens of clusters every year with Mtot ≥ 1014M⊙; the numbers depend on factors such as the mean matter density, the density fluctuation power spectrum and cluster gas evolution. The AMI survey itself can distinguish between these to some degree, and parameter degeneracies are largely eliminated given optical and X-ray follow-up of these clusters; this will also permit direct investigation of cluster physics and what drives the evolution.

Stellar populations in the nuclear regions of nearby radio galaxies

Monthly Notices of the Royal Astronomical Society 325 (2001) 636-648

I Aretxaga, E Terlevich, RJ Terlevich, G Cotter, AI Díaz

We present optical spectra of the nuclei of seven luminous (P178MHz ≳ 1025 W Hz-1 Sr-1 ) nearby (z < 0.08) radio galaxies, which mostly correspond to the FR II class. In two cases, Hydra A and 3C 285, the Balmer and λ4000-Å break indices constrain the spectral types and luminosity classes of the stars involved, revealing that the blue spectra are dominated by blue supergiant and/or giant stars. The ages derived for the last burst of star formation in Hydra A are between 7 and 40Myr, and in 3C 285 about 10 Myr. The rest of the narrow-line radio galaxies (four) have a λ4000-Å break and metallic indices consistent with those of elliptical galaxies. The only broad-line radio galaxy in our sample, 3C 382, has a strong featureless blue continuum and broad emission lines that dilute the underlying blue stellar spectra. We are able to detect the Ca II triplet in absorption in the seven objects, with good quality data for only four of them. The strengths of the absorptions are similar to those found in normal elliptical galaxies, but these values are consistent both with stellar populations of roughly similar ages (as derived from the Balmer absorption and break strengths) and with mixed young+old populations.