Melanie A. Gendre

The relation between morphology, accretion modes and environmental factors in local radio AGN

The goal of this work is to determine the nature of the relation between morphology and accretion mode in radio galaxies, including environmental parameters. The CoNFIG extended catalogue (improved by new KS-band identifications and estimated redshifts from UKIDSS, and spectral index measurements from new GMRT observations) is used to select a sub-sample of 206 radio galaxies with z60.3 over a wide range of radio luminosity, which are morphology-classified using the Fanaroff-Riley (FR) classification of extended radio sources. For each galaxy, spectroscopic data are retrieved to determine the high/low excitation status of the source, related to its accretion mode. Environmental factors, such as the host galaxy luminosity and a richness factor are also computed, generally using SDSS data. We find the following results: (1) At a given radio luminosity, the FR morphological split of sources is consistent with being the same for both accretion modes. This remains true if analysis is restricted to only rich or only poor environments. If confirmed with a larger sample, this would imply that extended radio morphology is independent of the accretion mode of the black hole, depending only on the power of the resultant jet, and its interactions with the larger-scale environment. (2) Excitation modes seem to be linked to the source environment, with high-excitation galaxies found almost exclusively in low-density environments while low-excitation galaxies occupy a wider range of densities; this result is independent of FR morphology, and is consistent with the different fuelling mechanisms expected for these excitation modes. (3) Independent of excitation mode, FRI sources are found to lie in higher density environments, on average, than FRII sources, consistent with FRI sources having their jets disrupted by a denser surrounding medium. However, there is a significant overlap in environment between the two classes, and no clear driving factor between the FRI and FRII sources is found even when combining radio luminosity, accretion mode, large-scale environment and host galaxy luminosity.

New insights on the z–α correlation from complete radio samples

The existence of a correlation between observed radio spectral index and redshift has long been used as a method for selecting high-redshift radio galaxy candidates. We use nine highly spectroscopically complete radio samples, selected at different frequencies and flux limits, to determine the efficiency of this method and compare consistently observed correlations between spectral index (α), luminosity (P), linear size (D) and redshift (z) in our samples. We observe a weak correlation between z and α which remains even when Malmquist bias is removed. The strength of the z–α correlation is dependent on both the k-correction and sample selection frequency, in addition to the frequency at which α is measured, and consistent results for both high- and low-frequency-selected samples are only seen if analysis is restricted to just extended radio galaxies. This fits with the popular interpretation that the spectra steepen with z because the radio lobes work against a denser intergalactic medium environment as z increases, out to z ∼ 2–3. However, we also note that the majority of sources known at z > 4 are very compact and often display a negatively curved or peaked spectrum, indicative of youth or merger activity, and therefore the low-frequency radio spectrum as a whole should be determined; this is something for which the new LOw Frequency ARray will be crucial. We quantify both the efficiency and the completeness of various techniques used to select high-z radio candidates. A steep-spectrum cut applied to low-frequency-selected samples can more than double the fraction of high-z sources, but at a cost of excluding over half of the high-z sources present in the original sample. An angular size cut is an almost as equally effective radio-based method as a steep-spectrum cut for maximizing the high-z content of large radio samples, and works for both high- and low-frequency-selected samples. In multiwavelength data, selection first of infrared-faint radio sources remains by far the most efficient method of selecting high-z sources. We present a simple method for selecting high-z radio sources, based purely on combining their observed radio properties of α and angular size, with the addition of the K-band magnitude if available.

The Combined NVSS–FIRST Galaxies (CoNFIG) sample – II. Comparison of space densities in the Fanaroff–Riley dichotomy

This paper focuses on a comparison of the space densities of FRI and FRII sources at different epochs, with a particular focus on FRI sources. First, we present the concluding steps in constructing the Combined NVSS-FIRST Galaxy catalogue (CoNFIG), including new VLA observations, optical identifications and redshift estimates. The final catalogue consists of 859 sources over 4 samples (CoNFIG-1, 2, 3 and 4 with flux density limits of S1.4GHz =1.3, 0.8, 0.2 and 0.05 Jy respectively). It is 95.7% complete in radio morphology classification and 74.3% of the sources have redshift data. Combining CoNFIG with complementary samples, the distribution and evolution of FRI and FRII sources are investigated. We find that FRI sources undergo mild evolution and that, at the same radio luminosity, FRI and FRII sources show similar space density enhancements in various redshift ranges, possibly implying a common evolution.

The Combined NVSS–FIRST Galaxies (CoNFIG) sample – I. Sample definition, classification and evolution

The CoNFIG (Combined NVSS-FIRST Galaxies) sample is a new sample of 274 bright radio sources at 1.4 GHz. It was defined by selecting all sources with S 1.4GHz ≥ 1.3 Jy from the NRAO Very Large Array (VLA) Sky Survey (NVSS) in the north field of the Faint Images of the Radio Sky at Twenty centimetres (FIRST) survey. New radio observations obtained with the VLA for 31 of the sources are presented. The sample has complete Fanaroff-Riley (FRI)/FRII morphology identification; optical identifications and redshifts are available for 80 and 89 per cent of the sample, respectively, yielding a mean redshift of ∼ ∼0.71. One of the goals of this survey is to get better definitions of luminosity distributions and source counts of FRI/FRII sources separately, in order to determine the evolution of the luminosity function for each type of source. We present a preliminary analysis, showing that these data are an important step towards examining various evolutionary schemes for these objects and to confirm or correct the dual population unified scheme for radio active galactic nuclei (AGN). Improving our understanding of radio galaxy evolution will give better insight into the role of AGN feedback in galaxy formation.

Flux density variations of radio sources in M82 over the last three decades

This paper presents the results of the 2009-2010 monitoring sessions of the starburst galaxy M82, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN) at 5GHz and e-MERLIN at 6GHz. Combining several 5GHz MERLIN epochs to form a map with 33.0 uJy/bm noise level, 52 discrete sources, mostly supernova remnants and HII regions, are identified. These include three objects which were not detected in the 2002 5GHz MERLIN monitoring session: supernova SN2008iz, the transient source 43.78+59.3, and a new supernova remnant shell. Flux density variations, in the long (1981 to 2010), medium (2002 to 2010) and short (2009 to 2010) term, are investigated. We find that flux densities of SNRs in M82 stay constant in most of the sample (~95%). In addition, aside from SN2008iz and the well-known variable source 41.95+57.5, two sources display short and medium term variations over the period 2009-2010. These sources being among the most compact SNR in M82, these flux density variations could be due to changes in the circumstellar and interstellar medium in which the shocks travel.

CO-EVOLUTION OF CENTRAL BLACK HOLES AND GALAXIES

We focus on a comparison of the space densities of FRI and FRII extended radio sources at different epochs, and find that FRI and FRII sources show similar space density enhancements in various redshift ranges, possibly implying a common evolution.

Resolving the masers in M82

Despite first being detected in the 1970s, surprisingly little is known about the OH main line maser population in the nearby starburst galaxy M82. Sometimes referred to as ‘kilomasers’, they have isotropic luminosities intermediate between Galactic masers and those found in more distant megamasers. Several observations have been carried by this group over the last ten years in an attempt to get a better handle on their nature. High velocity resolution VLA observations in 2006 showed that almost all of the maser spots, distributed across the central arcminute of the galaxy, were apparently coincident with background continuum features, and a handful displayed multiple velocity components. The majority of those with velocity structure are located on a blueshifted arc in the pv-plane, spatially located on an arc northward of the peculiar source known as B41.95+57.5. Now, new results from high spatial and spectral resolution observations with the EVN have resolved several of these masers into multiple spatial components for the first time. The maser emission is compared with known continuum sources in the galaxy, and we conclude that at least some of the maser emission is from high-gain maser action.

Flux density variations of radio sources in M82

This paper presents the results of the 2009-2010 monitoring sessions of the starburst galaxy M82, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN) at 5GHz and e-MERLIN at 6GHz. Combining several 5GHz MERLIN epochs to form a map with 33.0 µJy/bm noise level, 52 discrete sources, mostly supernova remnants and Hii regions, are identified. These include three objects which were not detected in the 2002 5GHz MERLIN monitoring session: supernova SN2008iz, the transient source 43.78+59.3, and a new supernova remnant shell. Flux density variations, in the long (1981 to 2010), medium (2002 to 2010) and short (2009 to 2010) term, are investigated. We find that flux densities of SNRs in M82 stay constant in most of the sample (�95%). In addition, aside from SN2008iz and the well-known variable source 41.95+57.5, two sources display short and medium term variations over the period 2009-2010. These sources being among the most compact SNR in M82, these flux density variations could be due to changes in the circumstellar and interstellar medium in which the shocks travel.

FR dichotomy, accretion modes and environmental factors in the local Universe

The goal of this work is to determine the nature of the relatio n between morphology and accretion mode in radio galaxies, including environmental parameter s. The CoNFIG extended catalogue, improved by including additional information such as K S-magnitudes from UKIDSS (and the related K-z estimated redshift) and spectral index derived us ing new GMRT observation, is used to select a morphology-classified (using the Fanaroff-Rile y (FR) classification of extended radio sources) sub-sample of galaxies with z ≤0.3. For each source, spectroscopic data are retrieved to determine the high/low excitation status of the source re lated to its accretion mode. Environmental factors, such as the host galaxy luminosity and a rich ness factor computed using SDSS or 2MASS, are also included. We find that (1) FR morphologies do n ot seem to be distinguished solely based on the radio luminosity and accretion-mode of t he source. This result is however greatly dependent on the errors; (2) although FRIs generall y reside in denser environments that FRIIs, HEG/LEG in a poor/rich environment has roughly equal probabilities of being of morphological type-I or type-II; (3) excitation modes seem to be li nked to the source environment, with HEGs found almost exclusively in low-density environments while LEGs occupy a wider range of densities.

Evolution of AGN Space Densities and the FR Dichotomy

We focus on a comparison of the space densities of FRI and FRII extended radio sources at different epochs, and find that FRI and FRII sources show similar space density enhancements in various redshift ranges, possibly implying a common evolution.