36 research outputs found
Assessing the galaxy population out to z ~ 2 using the Hubble Deep Field South
In this work we use the Hubble Deep Field South (HDF-S) version 2 images to
assess the galaxy population out to z ~ 2. We have used two methods of
templates fitting of the spectral energy distributions to obtain photometric
redshifts and classify the objects. The Bayesian photometric redshifts gave
better results when compared with 54 spectroscopic redshifts available in the
literature. Analysis of the rest-frame colour distribution shows a bimodality
out to z ~ 1.4. We separated our sample in a blue and a red population at B-V =
0.29. At low redshifts (0.2 0.29
whereas at higher redshifts ~ 60% of the galaxies are bluer than B-V < 0.29.
Although in low numbers, a population of early-type galaxies (or heavily
obscured low redshift galaxies) is seen out to z ~ 2.Comment: 7 pages, 7 figures, online material (the gallery) available at
http://www.oso.chalmers.se/~theresaw/Deep/gallery.htm
CHANG-ES V: Nuclear Radio Outflow in a Virgo Cluster Spiral after a Tidal Disruption Event
We have observed the Virgo Cluster spiral galaxy, NGC~4845, at 1.6 and 6 GHz
using the Karl G. Jansky Very Large Array, as part of the `Continuum Halos in
Nearby Galaxies -- an EVLA Survey' (CHANG-ES). The source consists of a bright
unresolved core with a surrounding weak central disk (1.8 kpc diameter). The
core is variable over the 6 month time scale of the CHANG-ES data and has
increased by a factor of 6 since 1995. The wide bandwidths of
CHANG-ES have allowed us to determine the spectral evolution of this core which
peaks {\it between} 1.6 and 6 GHz (it is a GigaHertz-peaked spectrum source).We
show that the spectral turnover is dominated by synchrotron self-absorption and
that the spectral evolution can be explained by adiabatic expansion (outflow),
likely in the form of a jet or cone. The CHANG-ES observations serendipitously
overlap in time with the hard X-ray light curve obtained by Nikolajuk \& Walter
(2013) which they interpret as due to a tidal disruption event (TDE) of a
super-Jupiter mass object around a black hole. We outline a
standard jet model, provide an explanation for the observed circular
polarization, and quantitatively suggest a link between the peak radio and peak
X-ray emission via inverse Compton upscattering of the photons emitted by the
relativistic electrons. We predict that it should be possible to resolve a
young radio jet via VLBI as a result of this nearby TDE.Comment: 45 pages, 10 figures, accepted July 2, 2015 to the Astrophysical
Journa
CHANG-ES VII: Magnetic outflows from the Virgo cluster galaxy NGC 4388
We investigate the effects of ram pressure on the ordered magnetic field of a
galaxy hosting a radio halo and strong nuclear outflows. New radio images in
total and polarized intensity of the edge-on Virgo galaxy NGC\,4388 were
obtained within the CHANG-ES EVLA project. The unprecedented noise level
reached allows us to detect striking new features of the ordered magnetic
field. The nuclear outflow extends far into the halo to about 5\,kpc from the
center and is spatially correlated with the and X-ray emission.
For the first time, the southern outflow is detected. Above and below both
spiral arms we find extended blobs of polarized emission with an ordered field
oriented perpendicular to the disk. The synchrotron lifetime of the cosmic ray
electrons (CREs) in these regions yields a mean outflow velocity of
(270\pm70)\kms, in agreement with a galactic wind scenario. The observed
symmetry of the polarized halo features in NGC 4388 excludes a compression of
the halo gas by the ram pressure of the intra-cluster medium (ICM). The
assumption of equilibrium between the halo pressure and the ICM ram pressure
yields an estimate of the ICM density that is consistent with both the ICM
density derived from X-ray observations and the recent \textit{Planck}
Sunyaev-Zel'dovich measurements. The detection of a faint radio halo around
cluster galaxies could thus be used for an estimate of ICM ram pressure
Compact Resolved Ejecta in the Nearest Tidal Disruption Event
Tidal disruption events (TDEs) occur when a star or sub-stellar object passes
close enough to a galaxy's supermassive black hole to be disrupted by tidal
forces. NGC 4845 (d=17 Mpc) was host to a TDE, IGR J12580+0134, detected in
November 2010. Its proximity offers us a unique close-up of the TDE and its
aftermath. We discuss new Very Long Baseline Array (VLBA) and Karl G. Jansky
Very Large Array (JVLA) observations, which show that the radio flux from the
active nucleus created by the TDE has decayed in a manner consistent with
predictions from a jet-circumnuclear medium interaction model. This model
explains the source's broadband spectral evolution, which shows a spectral peak
that has moved from the submm (at the end of 2010) to GHz radio frequencies (in
2011-2013) to <1 GHz in 2015. The milliarcsecond-scale core is circularly
polarized at 1.5 GHz but not at 5 GHz, consistent with the model. The VLBA
images show a complex structure at 1.5 GHz that includes an east west extension
~40 milliarcsec (3 pc) long as well as a resolved component 52 milliarcsec (4.1
pc) northwest of the flat-spectrum core, which is all that can be seen at 5
GHz. If ejected in 2010, the NW component must have had v=0.96 c over five
years. However, this is unlikely, as our model suggests strong deceleration to
speeds < 0.5c within months and a much smaller, sub-parsec size. In this
interpretation, the northwest component could have either a non-nuclear origin
or be from an earlier event.Comment: 12 pages, 8 figures, ApJ, in press; v2 includes error corrections and
slight additions to the analysi
CHANG-ES XXVII: A Radio/X-ray Catalogue of Compact Sources in and around Edge-on Galaxies
We present catalogues of discrete, compact radio sources in and around the
discs of 35 edge-on galaxies in the Continuum Halos in Nearby Galaxies -- an
EVLA Survey (CHANG-ES). The sources were extracted using the PyBDSF program at
both 1.6 GHz (L-band) and 6.0 GHz (C-band) from matching resolution (
3 arcsec) data. We also present catalogues of X-ray sources from Chandra data
sets for 27 of the galaxies. The sources at the two radio frequency bands were
positionally cross-correlated with each other, and the result cross-correlated
with the X-ray sources. All catalogues are included for download with this
paper. We detect a total of 2507 sources at L-band and 1413 sources at C-band.
Seventy-five sources have been successfully cross-correlated in both radio
bands plus X-ray. Three new nuclear sources are candidates for Low Luminosity
Active Galactic Nuclei in NGC~3877, NGC~4192, and NGC~5792; the one in NGC~3877
also appears to be variable. We also find new nuclear sources in two companion
galaxies: NGC~4435 (companion to NGC~4438) and NGC~4298 (companion to
NGC~4302). We have also discovered what appears to be a foreground double-star;
each star has X-ray emission and there is radio emission at both L-band and
C-band in between them. This could be a colliding wind binary system.
Suggestions for follow-up studies are offered.Comment: Accepted to MNRAS, 57 pages of which pages 30 to 57 are figures. 5
ancillary files containing tabular information, as explained in the
appendice
CHANG-ES VI: Probing Supernova Energy Deposition in Spiral Galaxies Through Multi-Wavelength Relationships
How a galaxy regulates its SNe energy into different
interstellar/circumgalactic medium components strongly affects galaxy
evolution. Based on the JVLA D-configuration C- (6 GHz) and L-band (1.6 GHz)
continuum observations, we perform statistical analysis comparing
multi-wavelength properties of the CHANG-ES galaxies. The high-quality JVLA
data and edge-on orientation enable us for the first time to include the halo
into the energy budget for a complete radio-flux-limited sample. We find tight
correlations of with the mid-IR-based SFR. The normalization of
our relation is 2-3 times of
those obtained for face-on galaxies, probably a result of enhanced IR
extinction at high inclination. We also find tight correlations between and the SNe energy injection rate , indicating
the energy loss via synchrotron radio continuum accounts for of
, comparable to the energy contained in CR electrons. The
integrated C-to-L-band spectral index is for non-AGN
galaxies, indicating a dominance by the diffuse synchrotron component. The
low-scatter /
relationships have super-linear logarithmic slopes at in L-band
(/) while consistent with linear in C-band
(/). The super-linearity could be naturally
reproduced with non-calorimeter models for galaxy disks. Using Chandra halo
X-ray measurements, we find sub-linear relations.
These results indicate that the observed radio halo of a starburst galaxy is
close to electron calorimeter, and a galaxy with higher SFR tends to distribute
an increased fraction of SNe energy into radio emission (than X-ray).Comment: 16 pages, 6 figures, 1 table, MNRAS in pres
CHANG-ES XI: Circular Polarization in the Cores of Nearby Galaxies
We detect 5 galaxies in the CHANG-ES (Continuum Halos in Nearby Galaxies --
an EVLA Survey) sample that show circular polarization (CP) at L-band in our
high resolution data sets. Two of the galaxies (NGC~4388 and NGC~4845) show
strong Stokes \%, two (NGC~660 and NGC~3628) have
values of \%, and NGC~3079 is a marginal detection at \%. The two strongest galaxies also have the most luminous X-ray
cores and the strongest internal absorption in X-rays. We have expanded on our
previous Faraday conversion interpretation and analysis and provide analytical
expressions for the expected signal for a general case in which the cosmic
ray electron energy spectral index can take on any value. We provide examples
as to how such expressions could be used to estimate magnetic field strengths
and the lower energy cutoff for CR electrons. Four out of our detections are
{\it resolved}, showing unique structures, including a {\it jet} in NGC~4388
and a CP `conversion disk' in NGC~4845. The conversion disk is inclined to the
galactic disk but is perpendicular to a possible outflow direction. Such CP
structures have never before been seen in any galaxy to our knowledge. None of
the galaxy cores show linear polarization at L-band. Thus CP may provide a
unique probe of physical conditions deep into radio AGNs.Comment: 30 pages, 4 figures, accepted to MNRA
CHANG-ES IV: Radio continuum emission of 35 edge-on galaxies observed with the Karl G. Jansky Very Large Array in D-configuration, Data Release 1
We present the first part of the observations made for the Continuum Halos in
Nearby Galaxies, an EVLA Survey (CHANG-ES) project. The aim of the CHANG-ES
project is to study and characterize the nature of radio halos, their
prevalence as well as their magnetic fields, and the cosmic rays illuminating
these fields. This paper reports observations with the compact D configuration
of the Karl G. Jansky Very Large Array (VLA) for the sample of 35 nearby
edge-on galaxies of CHANG-ES. With the new wide bandwidth capabilities of the
VLA, an unprecedented sensitivity was achieved for all polarization products.
The beam resolution is an average of 9.6" and 36" with noise levels reaching
approximately 6 and 30 microJy per beam for C- and L-bands, respectively
(robust weighting). We present intensity maps in these two frequency bands (C
and L), with different weightings, as well as spectral index maps, polarization
maps, and new measurements of star formation rates (SFRs). The data products
described herein are available to the public in the CHANG-ES data release
available at www.queensu.ca/changes. We also present evidence of a trend among
galaxies with larger halos having higher SFR surface density, and we show, for
the first time, a radio continuum image of the median galaxy, taking advantage
of the collective signal-to-noise ratio of 30 of our galaxies. This image shows
clearly that a typical spiral galaxy is surrounded by a halo of magnetic fields
and cosmic rays.Comment: 70 pages, of which 35 pages present the data of each galax
CHANG-ES V: Nuclear Outflow in a Virgo Cluster Spiral after a Tidal Disruption Event
We have observed the Virgo Cluster spiral galaxy, NGC 4845, at 1.6 and 6 GHz using the Karl G. Jansky Very Large Array, as part of the Continuum Halos in Nearby Galaxies—an EVLA Survey (CHANG-ES). The source consists of a bright unresolved core with a surrounding weak central disk (1.8 kpc diameter). The core is variable over the 6 month timescale of the CHANG-ES data and has increased by a factor of ≈6 since 1995. The wide bandwidths of CHANG-ES have allowed us to determine the spectral evolution of this core, which peaks between 1.6 and 6 GHz (it is a Gigahertz-peaked spectrum source). We show that the spectral turnover is dominated by synchrotron self-absorption and that the spectral evolution can be explained by adiabatic expansion (outflow), likely in the form of a jet or cone. The CHANG-ES observations serendipitously overlap in time with the hard X-ray light curve obtained by Nikolajuk & Walter (2013), which they interpret as due to a tidal disruption event (TDE) of a super-Jupiter mass object around a 10^5 M⊙ black hole. We outline a standard jet model, provide an explanation for the observed circular polarization, and quantitatively suggest a link between the peak radio and peak X-ray emission via inverse Compton upscattering of the photons emitted by the relativistic electrons. We predict that it should be possible to resolve a young radio jet via VLBI as a result of this nearby TDE