Showing papers by "Elaine M. Sadler published in 2013"

Galaxy And Mass Assembly (GAMA): Spectroscopic analysis

Abstract: The Galaxy And Mass Assembly (GAMA) survey is a multiwavelength photometric and spectroscopic survey, using the AAOmega spectrograph on the Anglo-Australian Telescope to obtain spectra for up to ∼300 000 galaxies over 280 deg2, to a limiting magnitude of rpet < 19.8 mag. The target galaxies are distributed over 0 < z ≲ 0.5 with a median redshift of z ≈ 0.2, although the redshift distribution includes a small number of systems, primarily quasars, at higher redshifts, up to and beyond z = 1. The redshift accuracy ranges from σv ≈ 50 km s−1 to σv ≈ 100 km s−1 depending on the signal-to-noise ratio of the spectrum. Here we describe the GAMA spectroscopic reduction and analysis pipeline. We present the steps involved in taking the raw two-dimensional spectroscopic images through to flux-calibrated one-dimensional spectra. The resulting GAMA spectra cover an observed wavelength range of 3750 ≲ λ ≲ 8850 A at a resolution of R ≈ 1300. The final flux calibration is typically accurate to 10–20 per cent, although the reliability is worse at the extreme wavelength ends, and poorer in the blue than the red. We present details of the measurement of emission and absorption features in the GAMA spectra. These measurements are characterized through a variety of quality control analyses detailing the robustness and reliability of the measurements. We illustrate the quality of the measurements with a brief exploration of elementary emission line properties of the galaxies in the GAMA sample. We demonstrate the luminosity dependence of the Balmer decrement, consistent with previously published results, and explore further how Balmer decrement varies with galaxy mass and redshift. We also investigate the mass and redshift dependencies of the [N II]/Hα versus [O III]/Hβ spectral diagnostic diagram, commonly used to discriminate between star forming and nuclear activity in galaxies.

The local radio-galaxy population at 20 GHz

Abstract: We have made the first detailed study of the high-frequency radio-source population in the local universe, using a sample of 202 radio sources from the Australia Telescope 20 GHz (AT20G) survey identified with galaxies from the 6dF Galaxy Survey (6dFGS). The AT20G-6dFGS galaxies have a median redshift of z=0.058 and span a wide range in radio luminosity, allowing us to make the first measurement of the local radio luminosity function at 20 GHz. Our sample includes some classical FR-1 and FR-2 radio galaxies, but most of the AT20G-6dFGS galaxies host compact (FR-0) radio AGN which appear lack extended radio emission even at lower frequencies. Most of these FR-0 sources show no evidence for relativistic beaming, and the FR-0 class appears to be a mixed population which includes young Compact Steep-Spectrum (CSS) and Gigahertz-Peaked Spectrum (GPS) radio galaxies. We see a strong dichotomy in the Wide-field Infrared Survey Explorer (WISE) mid-infrared colours of the host galaxies of FR-1 and FR-2 radio sources, with the FR-1 systems found almost exclusively in WISE `early-type' galaxies and the FR-2 radio sources in WISE `late-type' galaxies. The host galaxies of the flat- and steep-spectrum radio sources have a similar distribution in both K--band luminosity and WISE colours, though galaxies with flat-spectrum sources are more likely to show weak emission lines in their optical spectra. We conclude that these flat-spectrum and steep-spectrum radio sources mainly represent different stages in radio-galaxy evolution, rather than beamed and unbeamed radio-source populations.

Australia Telescope Compact Array observations of Fermi unassociated sources

Abstract: We report results of the first phase of observations with the Australia Telescope Compact Array (ATCA) at 5 and 9 GHz of the fields around 411 gamma-ray sources with declinations < +10 deg detected by Fermi but marked as unassociated in the 2FGL catalogue. We have detected 424 sources with flux densities in a range of 2 mJy to 6 Jy that lie within the 99 per cent localisation uncertainty of 283 gamma-ray sources. Of these, 146 objects were detected in both the 5 and 9 GHz bands. We found 84 sources in our sample with a spectral index flatter than -0.5. The majority of detected sources are weaker than 100 mJy and for this reason were not found in previous surveys. Approximately 1/3 of our sample, 128 objects, have the probability of being associated by more than 10 times than the probability of being a background source found in the vicinity of a gamma-ray object by chance. We present the catalogue of positions of these sources, estimates of their flux densities and spectral indices where available.

Herschel-ATLAS/GAMA: a difference between star formation rates in strong-line and weak-line radio galaxies

Abstract: We have constructed a sample of radio-loud objects with optical spectroscopy from the Galaxy and Mass Assembly (GAMA) project over the Herschel Astrophysical Terahertz Large Area Survey (Herschel-ATLAS) Phase 1 fields. Classifying the radio sources in terms of their optical spectra, we find that strong-emission-line sources (‘high-excitation radio galaxies’) have, on average, a factor of ∼4 higher 250-μm Herschel luminosity than weak-line (‘low-excitation’) radio galaxies and are also more luminous than magnitude-matched radio-quiet galaxies at the same redshift. Using all five H-ATLAS bands, we show that this difference in luminosity between the emission-line classes arises mostly from a difference in the average dust temperature; strong-emission-line sources tend to have comparable dust masses to, but higher dust temperatures than, radio galaxies with weak emission lines. We interpret this as showing that radio galaxies with strong nuclear emission lines are much more likely to be associated with star formation in their host galaxy, although there is certainly not a one-to-one relationship between star formation and strong-line active galactic nuclei (AGN) activity. The strong-line sources are estimated to have star formation rates at least a factor of 3–4 higher than those in the weak-line objects. Our conclusion is consistent with earlier work, generally carried out using much smaller samples, and reinforces the general picture of high-excitation radio galaxies as being located in lower-mass, less evolved host galaxies than their low-excitation counterparts.

CO(1-0) detection of molecular gas in the massive Spiderweb Galaxy (z = 2)

Abstract: The high-redshift radio galaxy MRC 1138−262 (‘Spiderweb Galaxy’; z = 2.16) is one of the most massive systems in the early Universe and surrounded by a dense ‘web’ of proto-cluster galaxies. Using the Australia Telescope Compact Array, we detected CO(1–0) emission from cold molecular gas – the raw ingredient for star formation – across the Spiderweb Galaxy. We infer a molecular gas mass of MH2 = 6 × 1010 M⊙ (for MH2/L′CO = 0.8). While the bulk of the molecular gas coincides with the central radio galaxy, there are indications that a substantial fraction of this gas is associated with satellite galaxies or spread across the intergalactic medium on scales of tens of kpc. In addition, we tentatively detect CO(1–0) in the star-forming proto-cluster galaxy HAE 229, 250 kpc to the West. Our observations are consistent with the fact that the Spiderweb Galaxy is building up its stellar mass through a massive burst of widespread star formation. At maximum star formation efficiency, the molecular gas will be able to sustain the current star formation rate (SFR ≈ 1400 M⊙ yr−1, as traced by Seymour et al.) for about 40 Myr. This is similar to the estimated typical lifetime of a major starburst event in infrared luminous merger systems.

On the H i column density–radio source size anticorrelation in compact radio sources

Abstract: Existing studies of the atomic hydrogen gas content in distant galaxies, through the absorption of the 21-cm line, often infer that the total column density, NHI, is anti-correlated with the linear extent of the background radio source, dem. We investigate this interpretation, by dissecting the various parameters from which NHI is derived, and find that the relationship is driven primarily by the observed optical depth, τobs, which, for a given absorber size, is anti-correlated with dem. Therefore, the inferred NHI dem anti-correlation is merely the consequence of geometry, in conjunction with the assumption of a common spin temperature/covering factor ratio for each member of the sample, an assumption for which there is scant observational justification. While geometry can expl ain the observed correlation, many radio sources comprise two radio lobes and so we model the projected area of a two component emitter intercepted by a foreground absorber. From this, the observed τobs dem relationship is best reproduced through models which approximate either of the two Fanaroff & Riley classifications, although the observed scatter in th e sample cannot be duplicated using a single deprojected radio source size. Furthermore, th e trend is best reproduced using an absorber of diameter � 100 1000 pc, which is also the range of values of dem at which the 21-cm detection rate peaks. This may indicate that this is th e characteristic linear size of the absorbing gas structure.

A polarization survey of bright extragalactic AT20G sources

Abstract: We present polarization data for 180 extragalactic sources extracted from the Australia Telescope 20 GHz (AT20G) survey catalogue and observed with the Australia Telescope Compact Array during a dedicated, high sensitivity run (σ P ∼ 1 mJy). For the sake of completeness, we extracted the polarization information for seven extended sources from the 9 yr Wilkinson Microwave Anisotropy Probe co-added maps at 23 GHz. The full sample of 187 sources constitutes a �99 per cent complete sample of extragalactic sources brighter than S20GHz = 500 mJy at the selection epoch with declination δ< −30 ◦ . The sample has a 91.4 per cent detection rate in polarization at ∼20 GHz (94 per cent if considering the subsample of point-like sources). We have measurements also at 4.8 and 8.6 GHz within ∼1 month of the 20 GHz observations for 172 sources to reconstruct the spectral properties of the sample in total intensity and in polarization: 143 of them have a polarization detection at all three frequencies. We find that there is no statistically significant evidence of a relationship either between the fraction of polarization and frequency or between the fraction of polarization and the total intensity flux density. This indicates that Faraday depolarization is not very important above 4.8 GHz and that the magnetic field is not substantially more ordered in the regions dominating the emission at higher frequencies (up to 20 GHz). We estimate the distribution of the polarization fraction and the polarized flux density source counts at ∼20 GHz.

A survey for the missing hydrogen in high-redshift radio sources

Abstract: Unlike at lower redshift, where there is a 40% detection rate, surveys for 21-cm absorption arising within the hosts of z > 1 radio galaxies and quasars have been remarkably unsuccessful. Curran et al.(2008) suggest that this is due to the high redshift selection biasing towards the most optically bright objects (those most luminous in the ultra-violetin the rest-frame), where the gas is ionised by the active galactic nucleus. They therefore argue that there must be a population of fainter objects in which the hydrogen is not ionised and which exhibit a similar detection rate as at lower redshifts. In order to find this "missing" gas at high redshift, we have therefore undertaken a survey of z > 2 radio sources, selected by optical faintness. Despite having optical magnitudes which indicate that the targets have ultra-violet luminosities below the threshold where all of the gas is ionised, there were no detections in any of the eight sources for which useable data were obtained. Upon an analysis of the spectral energy distributions, ionising photon rates can only be determined for three of these, all of which suggest that the objects are above the highest luminosity of a current 21-cm detection. The possibility that the other five could be located at lower photon rates cannot be ruled out, although zero detections out of five is not statistically significant. Another possible cause of the non-detections is that our selection biases the sample towards sources which are very steep in the radio band, with a mean spectral index of = -1.0, cf. -0.3 for both the 21-cm detections and UV luminous non-detections. This adds the further possibility that the sources have very extended emission, which would have the effect of reducing the coverage by the putative absorbing gas, thus decreasing the sensitivity of the observation.

Stellar population gradients and spatially resolved kinematics in luminous post-starburst galaxies

Abstract: We have used deep integral field spectroscopy obtained with the GMOS instrument on Gemini-North to determine the spatial distribution of the post-starburst stellar population in four luminous E+A galaxies at z<0.04. We find all four galaxies have centrally-concentrated gradients in the young stellar population contained within the central ~1 kpc. This is in agreement with the Balmer line gradients found in local low luminosity E+A galaxies. The results from higher redshift (z~0.1) samples of luminous E+A galaxies have been varied, but in general have found the post-starburst signature to be extended or a galaxy-wide phenomenon or have otherwise failed to detect gradients in the stellar populations. The ubiquity of the detection of a centrally concentrated young stellar population in local samples, and the presence of significant radial gradients in the stellar populations when the E+A galaxy core is well resolved raises the possibility that spatial resolution issues may be important in interpreting the higher redshift results. The two early type E+A galaxies in our sample that can be robustly kinematically classified, using the LambdaR parameter, are fast-rotators. Combined with previous measurements, this brings the total number of E+A galaxies with measurements of LambdaR to twenty-six, with only four being classified as slow-rotators. This fraction is similar to the fraction of the early-type population as a whole and argues against the need for major mergers in the production of E+A galaxies, since major mergers should result in an increased fraction of slow rotators.

A survey for H i in the distant Universe: the detection of associated 21-cm absorption at z = 1.28

Abstract: We have undertaken a survey for H I 21-cm absorption within the host galaxies of z ∼ 1.2−1.5 radio sources, in the search of the cool neutral gas currently ‘missing’ at z 1. This deficit is believed to be due to the optical selection of high-redshift objects biasing surveys towards sources of sufficient ultraviolet luminosity to ionize all of the gas in the surrounding galaxy. In order to avoid this bias, we have selected objects above blue magnitudes of B ∼ 20, indicating ultraviolet luminosities below the critical value ofLUV ∼ 10 23 WH z −1 , above which 21-cm has never been detected. As a secondary requirement to the radio flux and faint optical magnitude, we shortlist targets with radio spectra suggestive of compact sources, in order to maximize the coverage of background emission. From this, we obtain one detection out of 10 sources searched, which atz = 1.278 is the third highest redshift detection of associated 21-cm absorption to date. Accounting for the spectra compromised by radio frequency interference, as well as various other possible pitfalls (reliable optical redshifts and turnover frequencies indicative of compact emission), we estimate a detection rate of ≈30 per cent, close to that expected for LUV 10 23 WH z −1 sources.

The AT20G view of swift/BAT selected AGN: High-frequency radio waves meet hard X-rays

Abstract: We cross-matched the 6-year Swift/Burst Alert Telescope (BAT) survey of active galactic nuclei (AGN) with the AT20G radio survey of the southern sky, which is one of the largest high-frequency radio surveys available. With these data we investigated the possible correlation between the radio and the X-ray emission at the highest radio and X-ray frequencies. We found 37 AGN with a high probability of association (>80 per cent), among which 19 are local Seyfert galaxies (with median redshift z = 0.03) and 18 blazars. We found that ≈20 per cent of the AGN detected in hard X-rays are also bright radio sources at 20 GHz, but the apparent correlation between the radio and hard X-ray luminosity is completely driven by the different median redshifts of the two subgroups of AGN. When we consider only the local Seyfert sample we find no evidence of a correlation between their 20 GHz and 15–55 keV power. Therefore it appears that at high frequencies the radio–X connection, which had been previously observed at lower frequencies, disappears. The disappearance of the radio–X correlation at high radio and X-ray frequencies could be tested through Very Long Baseline Interferometry and the use of the Nuclear Spectroscopic Telescope Array (NuSTAR) satellite.

Broad, weak 21 cm absorption in an early-type galaxy: spectral line finding and parametrization for future surveys

Abstract: We report conclusive verification of the detection of associated H I 21cm absorption inthe early-type host galaxy of the compact radio source PMNJ2054–4242. We estimate aneffective spectral line velocity width of 418 20kms 1 and observed peak optical depthof 2:5 0:2percent, making this one of the broadest and weakest 21cm absorption linesyet detected. For T spin =f > 100K the atomic neutral hydrogen column density is N HI &2 10 21 cm . The observed spectral line profile is redshifted by2 187 46kms 1 , withrespect to the optical spectroscopic measurement, perhaps indicating that the H I gas isinfalling towards the central active galactic nucleus. Our initial tentative detection wouldlikely have been dismissed by visual inspection, and hence its verification here is anexcellent test of our spectral line detection technique, currently under development inanticipation of future next-generation 21cm absorption-line surveys.Key words: methods: data analysis – galaxies: active – galaxies: ISM – radio lines:galaxies

The youngest and most active radio galaxies in the local universe: AT20G sources in the 6dF Galaxy Survey

Abstract: We study a sample of 202 radio sources from the Australia Telescope 20 GHz (AT20G) survey which are identified with nearby galaxies from the 6dF Galaxy Survey (6dFGS). Our sample includes many of the youngest and most active radio galaxies in the local universe, and around 65% of the sample are candidate Compact Steep-Spectrum (CSS) and Gigahertz-Peaked Spectrum (GPS) radio sources. The AT20G-6dFGS galaxies have a median redshift of z=0.058 and span a wide range in radio luminosity. The local radio luminosity function (RLF) of galaxies at 20 GHz roughly matches the local 1.4 GHz RLF for radio-loud active galaxies if we make a simple shift in radio spectral index. While most of the AT20G-6dFGS galaxies are massive ellipticals, at least 30% of the radio sources in our sample are hosted by galaxies with WISE infrared colours characteristic of spiral galaxies with ongoing star formation. We see a strong dichotomy in the WISE colours of the host galaxies of FR-1 and FR-2 radio sources in our sample, with the FR-1 systems found almost exclusively in `WISE early-type' galaxies with [4.6]-[12] 2.0 mag. This strongly suggests that some factor related to the host-galaxy morphology or large-scale environment helps to determine whether a young radio source evolves into an extended FR-1 radio galaxy or an FR-2 system. The host galaxies of flat- and steep-spectrum sources in our sample have a similar distribution in both galaxy stellar mass and WISE colours (though galaxies with flat-spectrum radio sources are more likely to show weak emission lines in their optical spectra). This is consistent with a picture in which these flat-spectrum and steep-spectrum radio sources represent different stages in radio-galaxy evolution, rather than beamed and unbeamed radio-source populations.