29 research outputs found
Radio continuum properties of luminous infrared galaxies. Identifying the presence of an AGN in the radio
Luminous infrared galaxies are systems enshrouded in dust, which absorbs most
of their optical/UV emission and re-radiates it in the mid- and far-infrared.
Radio observations are largely unaffected by dust obscuration, enabling us to
study the central regions of LIRGs in an unbiased manner. The main goal of this
project is to examine how the radio properties of local LIRGs relate to their
infrared spectral characteristics. Here we present an analysis of the radio
continuum properties of a subset of the Great Observatories All-sky LIRG Survey
(GOALS), which consists of 202 nearby systems (z<0.088). Our radio sample
consists of 35 systems, or 46 individual galaxies, that were observed at both
1.49 and 8.44 GHz with the VLA with a resolution of about 1 arcsec (FWHM). The
aim of the project is to use the radio imagery to probe the central kpc of
these LIRGs in search of active galactic nuclei. We used the archival data at
1.49 and 8.44 GHz to create radio-spectral-index maps using the standard
relation between flux density Sv and frequency v, S~v^-a, where a is the radio
spectral index. By studying the spatial variations in a, we classified the
objects as radio-AGN, radio-SB, and AGN/SB (a mixture). We identified the
presence of an active nucleus using the radio morphology, deviations from the
radio/infrared correlation, and spatially resolved spectral index maps, and
then correlated this to the usual mid-infrared ([NeV]/[NeII] and [OIV]/[NeII]
line ratios and EQW of the 6.2 um PAH feature) and optical (BPT diagram) AGN
diagnostics. We find that 21 out of the 46 objects in our sample are radio-AGN,
9 are classified as starbursts (SB), and 16 are AGN/SB. After comparing to
other AGN diagnostics we find 3 objects out of the 46 that are identified as
AGN based on the radio analysis, but are not classified as such based on the
mid-infrared and optical AGN diagnostics presented in this study.Comment: 33 pages, 7 figures, 5 tables, to appear in A&
The first Swift X-ray Flash: The faint afterglow of XRF 050215B
We present the discovery of XRF 050215B and its afterglow. The burst was
detected by the Swift BAT during the check-out phase and observations with the
X-ray telescope began approximately 30 minutes after the burst. These
observations found a faint, slowly fading X-ray afterglow near the centre of
the error box as reported by the BAT. Infrared data, obtained at UKIRT after 10
hours also revealed a very faint K-band afterglow. The afterglow appear unusual
since it is very faint, especially in the infrared with K>20 only 9 hours post
burst. The X-ray and infrared lightcurves exhibit a slow, monotonic decay with
alpha=0.8 and no evidence for steepening associated with the jet break to 10
days post burst. We discuss possible explanations for the faintness and slow
decay in the context of present models for the production of X-ray Flashes.Comment: 8 pages, 5 figures, accepted for publication in Ap
Probing star formation and ISM properties using galaxy disk inclination: II. Testing typical FUV attenuation corrections out to z ~ 0.7
We evaluate dust-corrected far-ultraviolet (FUV) star formation rates (SFRs) for samples of star-forming galaxies at z ~ 0 and z ~ 0.7 and find significant differences between values obtained through corrections based on UV colour, from a hybrid mid-infrared (MIR) plus FUV relation, and from a radiative transfer based attenuation correction method. The performances of the attenuation correction methods are assessed by their ability to remove the dependency of the corrected SFR on inclination, as well as returning, on average, the expected population mean SFR. We find that combining MIR (rest-frame ~ 13 µm) and FUV luminosities gives the most inclination-independent SFRs and reduces the intrinsic SFR scatter of the methods we tested. However, applying the radiative transfer based method also gives corrections to the FUV SFR that are inclination independent and in agreement with the expected SFRs at both z ~ 0 and z ~ 0.7. SFR corrections based on the UV-slope perform worse than the other two methods we tested. For our local sample, the UV-slope method works on average, but does not remove inclination biases. At z ~ 0.7, we find that the UV-slope correction we used locally flattens the inclination dependence compared to the raw FUV measurements, but was not sufficient to correct for the large attenuation observed at z ~ 0.7.SL acknowledges funding from Deutsche Forschungsgemeinschaft (DFG) Grant BE 1837/13-1 r. ES and PL acknowledge funding from
the European Research Council (ERC) under the European Union’s Horizon
2020 research and innovation programme (grant agreement no. 694343). MTS
acknowledges support from a Royal Society Leverhulme Trust Senior Research
Fellowship (LT150041). BG acknowledges the support of the Australian Research Council as the recipient of a Future Fellowship (FT140101202)
The environments of z~1 Active Galactic Nuclei at 3.6um
We present an analysis of a large sample of AGN environments at z~1 using
stacked Spitzer data at 3.6um. The sample contains type-1 and type-2 AGN in the
form of quasars and radio galaxies, and spans a large range in both optical and
radio luminosity. We find, on average, that 2 to 3 massive galaxies containing
a substantial evolved stellar population lie within a 200-300 kpc radius of the
AGN, constituting a >8-sigma excess relative to the field. Secondly, we find
evidence for the environmental source density to increase with the radio
luminosity of AGN, but not with black-hole mass. This is shown first by
dividing the AGN into their classical AGN types, where we see more significant
over-densities in the fields of the radio-loud AGN. If instead we dispense with
the classical AGN definitions, we find that the source over-density as a
function of radio luminosity for all our AGN exhibits a positive correlation.
One interpretation of this result is that the Mpc-scale environment is in some
way influencing the radio emission that we observe from AGN. This could be
explained by the confinement of radio jets in dense environments leading to
enhanced radio emission or, alternatively, may be linked to more rapid
black-hole spin brought on by galaxy mergers.Comment: 13 pages, 12 figures, accepted by MNRA
EMU/GAMA: Radio detected galaxies are more obscured than optically selected galaxies
We demonstrate the importance of radio selection in probing heavily obscured
galaxy populations. We combine Evolutionary Map of the Universe (EMU) Early
Science data in the Galaxy and Mass Assembly (GAMA) G23 field with the GAMA
data, providing optical photometry and spectral line measurements, together
with Wide-field Infrared Survey Explorer (WISE) infrared (IR) photometry,
providing IR luminosities and colours. We investigate the degree of obscuration
in star forming galaxies, based on the Balmer decrement (BD), and explore how
this trend varies, over a redshift range of 0<z<0.345. We demonstrate that the
radio detected population has on average higher levels of obscuration than the
parent optical sample, arising through missing the lowest BD and lowest mass
galaxies, which are also the lower star formation rate (SFR) and metallicity
systems. We discuss possible explanations for this result, including
speculation around whether it might arise from steeper stellar initial mass
functions in low mass, low SFR galaxies.Comment: Accepted for publication in PASA, 17 pages, 14 figures, 3 table
The non-linear infrared-radio correlation of low-z galaxies: implications for redshift evolution, a new radio SFR recipe, and how to minimize selection bias
The infrared-radio correlation (IRRC) underpins many commonly used radio luminosity-star formation rate (SFR) calibrations. In preparation for the new generation of radio surveys we revisit the IRRC of low-z galaxies by (a) drawing on the best currently available IR and 1.4 GHz radio photometry, plus ancillary data over the widest possible area, and (b) carefully assessing potential systematics. We compile a catalogue of ∼9,500 z < 0.2 galaxies and derive their 1.4 GHz radio (L1.4), total IR, and monochromatic IR luminosities in up to seven bands, allowing us to parameterize the wavelength-dependence of monochromatic IRRCs from 22-500 μm. For the first time for low-z samples, we quantify how poorly matched IR and radio survey depths bias measured median IR/radio ratios, q¯¯TIR , and discuss the level of biasing expected for low-z IRRC studies in ASKAP/MeerKAT fields. For our subset of ∼2,000 high-confidence star-forming galaxies we find a median q¯¯TIR of 2.54 (scatter: 0.17 dex). We show that q¯¯TIR correlates with L1.4, implying a non-linear IRRC with slope 1.11±0.01. Our new L1.4-SFR calibration, which incorporates this non-linearity, reproduces SFRs from panchromatic SED fits substantially better than previous IRRC-based recipes. Finally, we match the evolutionary slope of recently measured q¯¯TIR -redshift trends without having to invoke redshift evolution of the IRRC. In this framework, the redshift evolution of q¯¯TIR reported at GHz frequencies in the literature is the consequence of a partial, redshift-dependent sampling of a non-linear IRRC obeyed by low-z and distant galaxies
The KHOLOD Experiment: A Search for a New Population of Radio Sources
Published data from long-term observations of a strip of sky at declination
+5 degrees carried out at 7.6 cm on the RATAN-600 radio telescope are used to
estimate some statistical properties of radio sources. Limits on the
sensitivity of the survey due to noise imposed by background sources, which
dominates the radiometer sensitivity, are refined. The vast majority of noise
due to background sources is associated with known radio sources (for example,
from the NVSS with a detection threshold of 2.3 mJy) with normal steep spectra
({\alpha} = 0.7-0.8, S \propto {\nu}^{- \alpha}), which have also been detected
in new deep surveys at decimeter wavelengths. When all such objects are removed
from the observational data, this leaves another noise component that is
observed to be roughly identical in independent groups of observations. We
suggest this represents a new population of radio sources that are not present
in known catalogs at the 0.6 mJy level at 7.6 cm. The studied redshift
dependence of the number of steep-spectrum objects shows that the sensitivity
of our survey is sufficient to detect powerful FRII radio sources at any
redshift, right to the epoch of formation of the first galaxies. The inferred
new population is most likely associated with low-luminosity objects at
redshifts z < 1. In spite of the appearance of new means of carrying out direct
studies of distant galaxies, searches for objects with very high redshifts
among steep and ultra-steep spectrum radio sources remains an effective method
for studying the early Universe.Comment: 13 pages, 10 figure
Radio observations of the merging galaxy cluster system Abell 3391-Abell 3395
The pre-merging system of galaxy clusters Abell 3391-Abell 3395 located at a
mean redshift of 0.053 has been observed at 1 GHz in an ASKAP/EMU Early Science
observation as well as in X-rays with eROSITA. The projected separation of the
X-ray peaks of the two clusters is 50 or 3.1 Mpc. Here we
present an inventory of interesting radio sources in this field around this
cluster merger. While the eROSITA observations provide clear indications of a
bridge of thermal gas between the clusters, neither ASKAP nor MWA observations
show any diffuse radio emission coinciding with the X-ray bridge. We derive an
upper limit on the radio emissivity in the bridge region of . A non-detection of diffuse radio emission in the X-ray bridge between
these two clusters has implications for particle-acceleration mechanisms in
cosmological large-scale structure. We also report extended or otherwise
noteworthy radio sources in the 30 deg field around Abell 3391-Abell 3395.
We identified 20 Giant Radio Galaxies, plus 7 candidates, with linear projected
sizes greater than 1 Mpc. The sky density of field radio galaxies with largest
linear sizes of Mpc is deg, three times higher than
previously reported. We find no evidence for a cosmological evolution of the
population of Giant Radio Galaxies. Moreover, we find seven candidates for
cluster radio relics and radio halos.Comment: Astronomy & Astrophysics, in pres
Automated mining of the ALMA archive in the COSMOS field (A3COSMOS): I. Robust ALMA continuum photometry catalogs and stellar mass and star formation properties for ∼700 galaxies at z = 0.5–6
The rich information on (sub)millimeter dust continuum emission from distant galaxies in the public Atacama Large Millimeter/submillimeter Array (ALMA) archive is contained in thousands of inhomogeneous observations from individual PI-led programs. To increase the usability of these data for studies deepening our understanding of galaxy evolution, we have developed automated mining pipelines for the ALMA archive in the COSMOS field (A3COSMOS) which efficiently exploit the available information for large numbers of galaxies across cosmic time, and keep the data products in sync with the increasing public ALMA archive: (a) a dedicated ALMA continuum imaging pipeline; (b) two complementary photometry pipelines for both blind source extraction and prior source fitting; (c) a counterpart association pipeline utilizing the multi-wavelength data available (including quality assessment based on machine-learning techniques); (d) an assessment of potential (sub-)mm line
contribution to the measured ALMA continuum; and (e) extensive simulations to provide statistical corrections to biases and uncertainties in the ALMA continuum measurements. Application of these tools yields photometry catalogs with ∼ 1000 (sub-)mm detections (spurious fraction ∼ 8 − 12%) from over 1500 individual ALMA continuum images. Combined with ancillary photometric and redshift catalogs and the above quality assessments, we provide robust information on redshift, stellar mass and star formation rate for ∼700 galaxies at redshifts 0.5-6 in the COSMOS field (with undetermined selection function). The ALMA photometric measurements and galaxy properties are released publicly within our blind-extraction, prior-fitting and galaxy property catalogs, plus the images. These products will be updated on a regular basis in the future