20 research outputs found
Neutral hydrogen gas, past and future star-formation in galaxies in and around the 'Sausage' merging galaxy cluster
CIZA J2242.8+5301 (, nicknamed 'Sausage') is an extremely massive
( ), merging cluster with shock waves
towards its outskirts, which was found to host numerous emission-line galaxies.
We performed extremely deep Westerbork Synthesis Radio Telescope HI
observations of the 'Sausage' cluster to investigate the effect of the merger
and the shocks on the gas reservoirs fuelling present and future star formation
(SF) in cluster members. By using spectral stacking, we find that the
emission-line galaxies in the 'Sausage' cluster have, on average, as much HI
gas as field galaxies (when accounting for the fact cluster galaxies are more
massive than the field galaxies), contrary to previous studies. Since the
cluster galaxies are more massive than the field spirals, they may have been
able to retain their gas during the cluster merger. The large HI reservoirs are
expected to be consumed within Gyr by the vigorous SF and AGN
activity and/or driven out by the out-flows we observe. We find that the
star-formation rate in a large fraction of H emission-line cluster
galaxies correlates well with the radio broad band emission, tracing supernova
remnant emission. This suggests that the cluster galaxies, all located in
post-shock regions, may have been undergoing sustained SFR for at least 100
Myr. This fully supports the interpretation proposed by Stroe et al. (2015) and
Sobral et al. (2015) that gas-rich cluster galaxies have been triggered to form
stars by the passage of the shock.Comment: Accepted to MNRAS, 14 pages, 9 figures, 7 table
Photometric redshifts for the next generation of deep radio continuum surveys - II. Gaussian processes and hybrid estimates
Building on the first paper in this series (Duncan et al. 2018), we present a study investigating the performance of Gaussian process photometric redshift (photo-z) estimates for galaxies and active galactic nuclei detected in deep radio continuum surveys. A Gaussian process redshift code is used to produce photo-z estimates targeting specific subsets of both the AGN population - infrared, X-ray and optically selected AGN - and the general galaxy population. The new estimates for the AGN population are found to perform significantly better at z > 1 than the template-based photo-z estimates presented in our previous study. Our new photo-z estimates are then combined with template estimates through hierarchical Bayesian combination to produce a hybrid consensus estimate that outperforms both of the individual methods across all source types. Photo-z estimates for radio sources that are X-ray sources or optical/IR AGN are significantly improved in comparison to previous template-only estimates - with outlier fractions and robust scatter reduced by up to a factor of âŒ4. The ability of our method to combine the strengths of the two input photo-z techniques and the large improvements we observe illustrate its potential for enabling future exploitation of deep radio continuum surveys for both the study of galaxy and black hole co-evolution and for cosmological studies
Resolving the nucleus of Centaurus A at mid-IR wavelengths
We have observed Centaurus A with the MID-infrared Interferometric instrument
(MIDI) at the Very Large Telescope Interferometer (VLTI) at resolutions of 7 -
15 mas (at 12.5 micron) and filled gaps in the (u,v) coverage in comparison to
earlier measurements. We are now able to describe the nuclear emission in terms
of geometric components and derive their parameters by fitting models to the
interferometric data. With simple geometrical models, the best fit is achieved
for an elongated disk with flat intensity profile with diameter 76 +/- 9 mas x
35 +/- 2 mas (1.41 +/- 0.17 pc x 0.65 +/- 0.03 pc) whose major axis is oriented
at a position angle (PA) of 10.1 +/- 2.2 degrees east of north. A point source
contributes 47 +/- 11 % of the nuclear emission at 12.5 micron. There is also
evidence that neither such a uniform nor a Gaussian disk are good fits to the
data. This indicates that we are resolving more complicated small-scale
structure in AGNs with MIDI, as has been seen in Seyfert galaxies previously
observed with MIDI. The PA and inferred inclination i = 62.6 +2.1/-2.6 degrees
of the dust emission are compared with observations of gas and dust at larger
scales.Comment: Accepted for the PASA special issue on Centaurus
A Simple Prediction on the Surface Density of Galaxies at z ~ 6
Systematic surveys are being proposed to discover a significant number of
galaxies at z ~ 6, which is now suggested as the epoch when the reionization
era of the Universe ends. To plan such surveys, we need a reasonable
expectation of the surface density of high redshift galaxies at different flux
limits. Here we present a simple prediction of the surface density of 5.5 < z <
6.5 galaxies in the optical regime, extrapolating from what is already known
about galaxies at z ~ 3. This prediction is consistent with the results of
nearly all known searches for objects at z ~ 6, giving confidence that we may
use it to plan optimal combination of survey depth and sky coverage in
searching for such objects. We suggest that the most efficient strategy with
existing ground-based facilities is to do medium-depth (m_{AB}= 24.0 -- 24.5
mag), wide-field (a couple of square degrees) survey using a wide-field camera
at a 4m-class telescope. As the predicted surface density at this brightness
level is very sensitive to the value of L*, the result of such a survey can be
easily used to constrain the luminosity evolution from z ~ 3 to 6.Comment: Accepted for publication in the Astrophysical Journa
The Infrared Counterparts of the Optically Unidentified CDF-S 1Ms Sources
The Chandra Deep Field South (CDF-S) 1Ms exposure produced a catalog of 346
X-ray sources, of which 59 were not visible on the VLT/FORS1 and the
ESO-MPI/WFI deep R-band images to a limit of R_{vega}=26.1--26.7 mag. Using the
first release of the ESO VLT/ISAAC JHK_s data on the CDF-S, we identified six
of the twelve such objects that were within the coverage of these IR
observations. The VLT/FORS1 I-band data further confirms that five of these six
objects are undetected in the optical. The photometric properties of these six
counterparts are compared against those of the optically brighter counterparts
of Chandra sources in the same field. We found that the location of these
optically brighter Chandra sources in the near-IR color space was bifurcated,
with the color of one branch being consistent with that of E/S0 galaxies at 0
<= z <= 1.5 and the other branch being consistent with that of unreddened
AGN/QSOs at 0 <= z <= 3.5. The six counterparts that we identified seemed to
lie on the E/S0 branch and its extension, suggesting that these X-ray source
hosts are mostly luminous E/S0 galaxies (M_V ~ -20 mag in AB system) at 1 <= z
<= 2.5. On the other hand, some of them can also be explained by AGN/QSOs over
a wide redshift range (0 <= z <= 5) if a range of internal extinction (A_V=0--1
mag) is allowed. However, the later interpretation requires fine-tuning
extinction together with redshift for these objects individually. If they are
indeed AGN/QSOs, the most luminous of them is just barely qualified for being a
QSO. Finally, we point out that neither high-redshift (z > 5) star-forming
galaxies nor irregular galaxies at lower redshift can be a viable explanation
to the nature of these six counterparts.Comment: Accepted for publication in Ap
Finding AGN remnant candidates based on radio morphology with machine learning
Remnant radio galaxies represent the dying phase of radio-loud active
galactic nuclei (AGN). Large samples of remnant radio galaxies are important
for quantifying the radio galaxy life cycle. The remnants of radio-loud AGN can
be identified in radio sky surveys based on their spectral index, or,
complementary, through visual inspection based on their radio morphology.
However, this is extremely time-consuming when applied to the new large and
sensitive radio surveys. Here we aim to reduce the amount of visual inspection
required to find AGN remnants based on their morphology, through supervised
machine learning trained on an existing sample of remnant candidates. For a
dataset of 4107 radio sources, with angular sizes larger than 60 arcsec, from
the LOw Frequency ARray (LOFAR) Two-Metre Sky Survey second data release
(LoTSS-DR2), we started with 151 radio sources that were visually classified as
'AGN remnant candidate'. We derived a wide range of morphological features for
all radio sources from their corresponding Stokes-I images: from simple source
catalogue-derived properties, to clustered Haralick-features, and
self-organising map (SOM) derived morphological features. We trained a random
forest classifier to separate the 'AGN remnant candidates' from the not yet
inspected sources. The SOM-derived features and the total to peak flux ratio of
a source are shown to be most salient to the classifier. We estimate that
of sources with positive predictions from our classifier will be
labelled 'AGN remnant candidates' upon visual inspection, while we estimate the
upper bound of the confidence interval for 'AGN remnant candidates' in
the negative predictions at . Visual inspection of just the positive
predictions reduces the number of radio sources requiring visual inspection by
.Comment: 23 pages; accepted for publication in A&
Optical and Near-IR Imaging of Ultra Steep Spectrum Radio Sources - The K-z diagram of radio and optically selected galaxies
We present optical and/or near-IR images of 128 ultra steep spectrum (USS)
radio sources. Roughly half of the objects are identified in the optical images
(R 94% are detected at K<~ 22. The mean
K-magnitude is K=19.26 within a 2" diameter aperture. The distribution of R-K
colors indicates that at least 1/3 of the objects observed have very red colors
(R-K>5). The major axes of the identifications in K-band are preferentially
oriented along the radio axes, with half of them having compact morphologies.
The 22 sources with spectroscopic redshifts and K-band magnitudes follow the
K-z relation found from previous radio samples, but with a larger scatter. We
argue that this may be due to a dependence of K-magnitude on the radio power,
with the highest radio power sources inhabiting the most massive host galaxies.
We present a composite K-z diagram of radio-loud and radio-quiet galaxies,
selected from the HDF-North and the Hawaii surveys. Out to z <~ 1, the
radio-loud galaxies trace the bright envelope of the radio quiet galaxies,
while at z >~ 1, the radio-loud galaxies are >~ 2 magnitudes brighter. We argue
that this is not due to a contribution from the AGN or emission lines. This
difference strongly suggests that radio galaxies pinpoint the most massive
systems out to the highest known redshifts, probably due to the mutual
correlation of the mass of the galaxy and the radio power on the mass of the
central black hole.Comment: 39 Pages, including 10 PostScript figures. Accepted for publication
in the January 2002 Astronomical Journal. Figures 2 and 12 are available from
http://www.strw.leidenuniv.nl/~debreuck/paper
The Spiderweb galaxy: a forming massive cluster galaxy at z~2
We present a deep image of the radio galaxy MRC 1138-262 taken with the
Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to
have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized
structure, identified with a protocluster. The morphology shown on the new deep
HST/ACS image is reminiscent of a spider's web. More than 10 individual clumpy
features are observed, apparently star-forming satellite galaxies in the
process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago.
There is an extended emission component, implying that star formation was
occurring over a 50 times 40 kpc region at a rate of more than 100 M_sun/yr. A
striking feature of the newly named ``Spiderweb galaxy'' is the presence of
several faint linear galaxies within the merging structure. The dense
environments and fast galaxy motions at the centres of protoclusters may
stimulate the formation of these structures, which dominate the faint resolved
galaxy populations in the Hubble Ultra Deep Field. The new image provides a
unique testbed for simulations of forming dominant cluster galaxies.Comment: 5 pages, 2 figures (reduced to grayscale); ApJ Letter
LOFAR/H-ATLAS: The low-frequency radio luminosity - star-formation rate relation
Radio emission is a key indicator of star-formation activity in galaxies, but
the radio luminosity-star formation relation has to date been studied almost
exclusively at frequencies of 1.4 GHz or above. At lower radio frequencies the
effects of thermal radio emission are greatly reduced, and so we would expect
the radio emission observed to be completely dominated by synchrotron radiation
from supernova-generated cosmic rays. As part of the LOFAR Surveys Key Science
project, the Herschel-ATLAS NGP field has been surveyed with LOFAR at an
effective frequency of 150 MHz. We select a sample from the MPA-JHU catalogue
of SDSS galaxies in this area: the combination of Herschel, optical and
mid-infrared data enable us to derive star-formation rates (SFRs) for our
sources using spectral energy distribution fitting, allowing a detailed study
of the low-frequency radio luminosity--star-formation relation in the nearby
Universe. For those objects selected as star-forming galaxies (SFGs) using
optical emission line diagnostics, we find a tight relationship between the 150
MHz radio luminosity () and SFR. Interestingly, we find that a single
power-law relationship between and SFR is not a good description of
all SFGs: a broken power law model provides a better fit. This may indicate an
additional mechanism for the generation of radio-emitting cosmic rays. Also, at
given SFR, the radio luminosity depends on the stellar mass of the galaxy.
Objects which were not classified as SFGs have higher 150-MHz radio luminosity
than would be expected given their SFR, implying an important role for
low-level active galactic nucleus activity.Comment: 21 pages, 12 figures, accepted for publication in MNRA
Giant Lya nebulae associated with high redshift radio galaxies
We report deep Keck narrow-band Lya images of the luminous z > 3 radio
galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100-200 kpc
scale emission line nebulae, centered on these galaxies, which exhibit a wealth
of morphological structure, including extended low surface brightness emission
in the outer regions, radially directed filaments, cone-shaped structures and
(indirect) evidence for extended Lya absorption. We discuss these features
within a general scenario where the nebular gas cools gravitationally in large
Cold Dark Matter (CDM) halos, forming stars and multiple stellar systems.
Merging of these ``building'' blocks triggers large scale starbursts, forming
the stellar bulges of massive radio galaxy hosts, and feeds super-massive black
holes which produce the powerful radio jets and lobes. The radio sources,
starburst superwinds and AGN radiation then disrupt the accretion process
limiting galaxy and black hole growth, and imprint the observed filamentary and
cone-shaped structures of the Lya nebulae.Comment: 36 Pages, including 8 Postscript figures. Accepted for publication in
the Astrophysical Journa