153 research outputs found
Probing dust-obscured star formation in the most massive gamma-ray burst host galaxies
Context. As a result of their relation to massive stars, long-duration gamma-ray bursts (GRBs) allow the pinpointing of star formation in galaxies independent of redshift, dust obscuration, or galaxy mass/size, thus providing a unique tool to investigate star formation history over cosmic time. Aims. About half of the optical afterglows of long-duration GRBs are missed owing to dust extinction and are primarily located in the most massive GRB hosts. It is important to investigate the amount of obscured star formation in these GRB host galaxies to understand this bias. Methods. Radio emission of galaxies correlates with star formation, but does not suffer extinction as do the optical star formation estimators. We selected 11 GRB host galaxies with either large stellar mass or large UV-based and optical-based star formation rates (SFRs) and obtained radio observations of these with the Australia Telescope Compact Array and the Karl Jansky Very Large Array. Results. Despite intentionally selecting GRB hosts with expected high SFRs, we do not find any radio emission related to star formation in any of our targets. Our upper limit for GRB 100621A implies that the earlier reported radio detection was due to afterglow emission. We detect radio emission from the position of GRB 020819B, but argue that it is in large part, if not completely, due to afterglow contamination. Conclusions. Half of our sample has radio-derived SFR limits, which are only a factor 2-3 above the optically measured SFRs. This supports other recent studies that the majority of star formation in GRB hosts is not obscured by dust.R.H.I. acknowledges support from the Spanish MINECO through grants AYA2012-38491-C02-02 and AYA2015-63939-C2-1-P, partially funded by FEDER funds. P.S., J.F.G., and T.K. acknowledge support through the Sofja Kovalevskaja award to P. Schady from the Alexander von Humboldt Foundation Germany. S.K. acknowledges support by DFG grant K1 766/16-1.Peer Reviewe
Radio continuum and X-ray emission from the most extreme FIR-excess galaxy NGC 1377: An extremely obscured AGN revealed
Galaxies which strongly deviate from the radio-far IR correlation are of
great importance for studies of galaxy evolution as they may be tracing early,
short-lived stages of starbursts and active galactic nuclei (AGNs). The most
extreme FIR-excess galaxy NGC1377 has long been interpreted as a young dusty
starburst, but millimeter observations of CO lines revealed a powerful
collimated molecular outflow which cannot be explained by star formation alone.
We present new radio observations at 1.5 and 10 GHz obtained with the Jansky
Very Large Array (JVLA) and Chandra X-ray observations towards NGC1377. The
observations are compared to synthetic starburst models to constrain the
properties of the central energy source. We obtained the first detection of the
cm radio continuum and X-ray emission in NGC1377. We find that the radio
emission is distributed in two components, one on the nucleus and another
offset by 4.5 to the South-West. We confirm the extreme FIR-excess of the
galaxy, with a 4.2, which deviates by more than
7- from the radio-FIR correlation. Soft X-ray emission is detected on
the off-nucleus component. From the radio emission we estimate for a young
( Myr) starburst a star formation rate SFR0.1 M yr. Such
a SFR is not sufficient to power the observed IR luminosity and to drive the CO
outflow. We find that a young starburst cannot reproduce all the observed
properties of the nucleus of NGC1377. We suggest that the galaxy may be
harboring a radio-quiet, obscured AGN of 10M, accreting at
near-Eddington rates. We speculate that the off-nucleus component may be
tracing an hot-spot in the AGN jet.Comment: 14 pages, accepted for publication on Astronomy and Astrophysics on
08/07/201
No AGN evidence in NGC 1614 from deep radio VLBI observations
We present deep dual-band 5.0- and 8.4-GHz European VLBI Network (EVN) observations of NGC 1614, a local luminous infrared galaxy with a powerful circumnuclear starburst ring, and whose nuclear engine origin is still controversial. We aim at detecting and characterizing compact radio structures both in the nuclear region and in the circumnuclear ring. We do not find any compact source in the central 200 pc region, setting a very tight 5s upper limit of 3.7 × 1036 and 5.8 × 1036 erg s-1, at 5.0 and 8.4 GHz, respectively. However, we report a clear detection at both frequencies of a compact structure in the circumnuclear ring, 190 pc to the north of the nucleus, whose luminosity and spectral index are compatible with a core-collapse supernova, giving support to the high star formation rate in the ring. Our result favours the pure starburst scenario, even for the nucleus of NGC 1614, and shows the importance of radio VLBI (very long baseline interferometry) observations when dealing with the obscured environments of dusty galaxies
Star formation and AGN activity in a sample of local Luminous Infrared Galaxies through multi-wavelength characterization
Nuclear starbursts and AGN activity are the main heating processes in
luminous infrared galaxies (LIRGs) and their relationship is fundamental to
understand galaxy evolution. In this paper, we study the star-formation and AGN
activity of a sample of 11 local LIRGs imaged with subarcsecond angular
resolution at radio (8.4GHz) and near-infrared (m) wavelengths. This
allows us to characterize the central kpc of these galaxies with a spatial
resolution of pc. In general, we find a good spatial correlation
between the radio and the near-IR emission, although radio emission tends to be
more concentrated in the nuclear regions. Additionally, we use an MCMC code to
model their multi-wavelength spectral energy distribution (SED) using template
libraries of starburst, AGN and spheroidal/cirrus models, determining the
luminosity contribution of each component, and finding that all sources in our
sample are starburst-dominated, except for NGC6926 with an AGN contribution of
\%. Our sources show high star formation rates ( to
), supernova rates (0.4 to
), and similar starburst ages (13 to
), except for the young starburst (9Myr) in NGC6926. A
comparison of our derived star-forming parameters with estimates obtained from
different IR and radio tracers shows an overall consistency among the different
star formation tracers. AGN tracers based on mid-IR, high-ionization line
ratios also show an overall agreement with our SED model fit estimates for the
AGN. Finally, we use our wide-band VLA observations to determine pixel-by-pixel
radio spectral indices for all galaxies in our sample, finding a typical median
value () for synchrotron-powered LIRGs.Comment: Accepted for publication in MNRAS. 20 pages, 12 figure
A multi-wavelength view of the central kiloparsec region in the Luminous Infrared Galaxy NGC1614
The Luminous Infrared Galaxy NGC1614 hosts a prominent circumnuclear ring of
star formation. However, the nature of the dominant emitting mechanism in its
central ~100 pc is still under debate. We present sub-arcsecond angular
resolution radio, mid-infrared, Pa-alpha, optical, and X-ray observations of
NGC1614, aimed at studying in detail both the circumnuclear ring and the
nuclear region. The 8.4 GHz continuum emission traced by the Very Large Array
(VLA) and the Gemini/T-ReCS 8.7 micron emission, as well as the Pa-alpha line
emission, show remarkable morphological similarities within the star-forming
ring, suggesting that the underlying emission mechanisms are tightly related.
We used an HST/NICMOS Pa-alpha map of similar resolution to our radio maps to
disentangle the thermal free-free and non-thermal synchrotron radio emission,
from which we obtained the intrinsic synchrotron power-law for each individual
region within the central kpc of NGC1614. The radio ring surrounds a relatively
faint, steep-spectrum source at the very center of the galaxy, suggesting that
the central source is not powered by an AGN, but rather by a compact (r < 90
pc) starburst. Chandra X-ray data also show that the central kpc region is
dominated by starburst activity, without requiring the existence of an AGN. We
also used publicly available infrared data to model-fit the spectral energy
distribution of both the starburst ring and a putative AGN in NGC1614. In
summary, we conclude that there is no need to invoke an AGN to explain the
observed bolometric properties of the galaxy.Comment: 13 pages, 7 figures, 5 tables. Accepted for publication in Ap
Radio continuum and X-ray emission from the most extreme far-IR-excess galaxy NGC 1377: An extremely obscured AGN revealed
Context. Galaxies which strongly deviate from the radio-far infrared (FIR) correlation are of great importance for studies of galaxy evolution as they may be tracing early, short-lived stages of starbursts and active galactic nuclei (AGNs). The most extreme FIR-excess galaxy NGC 1377 has long been interpreted as a young dusty starburst, but millimeter observations of CO lines revealed a powerful collimated molecular outflow which cannot be explained by star formation alone. Aims. This paper aims to determine the nature of the energy source in the nucleus of NGC 1377 and to study the driving mechanism of the collimated CO outflow. Methods. We present new radio observations of NGC 1377 at 1.5 and 10 GHz obtained with the Jansky Very Large Array (JVLA) and Chandra X-ray observations. The observations are compared to synthetic starburst models to constrain the properties of the central energy source. Results. We obtained the first detection of the cm radio continuum and X-ray emission in NGC 1377. We found that the radio emission is distributed in two components, one on the nucleus and another offset by 4?5 to the south-west. We confirm the extreme FIR-excess of the galaxy, with a qFIR ? 4.2, which deviates by more than 7? from the radio-FIR correlation. Soft X-ray emission is detected on the off-nucleus component. From the radio emission we estimated for a young (<10 Myr) starburst a star formation rate (SFR) of <0.1 M? yr-1. Such a SFR is not sufficient to power the observed IR luminosity and to drive the CO outflow. Conclusions. We found that a young starburst cannot reproduce all the observed properties of the nucleus of NGC 1377. We suggest that the galaxy may be harboring a radio-quiet, obscured AGN of 106M?, accreting at near-Eddington rates. We speculate that the off-nucleus component may be tracing an hot-spot in the AGN jet
Multiple AGN in the crowded field of the compact group SDSSJ0959+1259
We present a multi-wavelength study of a newly discovered compact group (CG),
SDSS J0959+1259, based data from XMM-Newton, SDSS and the Calar Alto optical
imager BUSCA. With a maximum velocity offset of 500 km s, a mean
redshift of 0.035, and a mean spatial extension of 480 kpc, this CG is
exceptional in having the highest concentration of nuclear activity in the
local Universe, established with a sensitivity limit L10
erg s in 2--10 keV band and R-band magnitude . The group is
composed of two type-2 Seyferts, one type-1 Seyfert, two LINERs and three star
forming galaxies. Given the high X-ray luminosity of LINERs which reaches erg s, it is likely that they are also accretion driven,
bringing the number of active nuclei in this group to to 5 out of 8 (AGN
fraction of 60\%). The distorted shape of one member of the CG suggests that
strong interactions are taking place among its galaxies through tidal forces.
Therefore, this system represents a case study for physical mechanisms that
trigger nuclear activity and star formation in CGs.Comment: 8 pages, 4 figures and 4 tables. Accepted for publication in MNRA
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