195 research outputs found
Observational evidence that positive and negative AGN feedback depends on galaxy mass and jet power
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. ©: 2017 The Author (s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Several studies support the existence of a link between the AGN and star formation activity. Radio jets have been argued to be an ideal mechanism for direct interaction between the AGN and the host galaxy. A drawback of previous surveys of AGN is that they are fundamentally limited by the degeneracy between redshift and luminosity in flux-density limited samples. To overcome this limitation, we present far-infrared Herschel observations of 74 radio-loud quasars (RLQs), 72 radio-quiet quasars (RQQs) and 27 radio galaxies (RGs), selected at 0.9zPeer reviewedFinal Published versio
Forecasting Solar Energetic Particle Fluence with Multi-Spacecraft Observations
Forecasting Solar Energetic Particle (SEP) fluence, as integrated over an SEP event, is an important element when estimating the effect of solar eruptions on humans and technology in space. Current real-time estimates are based on SEP measurements at a single location in space. However, the interplanetary magnetic field corotates with the Sun approximately 13 each day with respect to Earth, thus in 4 days a near-Earth spacecraft will have changed their connection about 60 from the original SEP source. We estimate the effect of the corotation on particle fluence using a simple particle transport model, and show that ignoring corotation can cause up to an order of magnitude error in fluence estimations, depending on the interplanetary particle transport conditions. We compare the model predictions with STEREO observations of SEP event
Star formation in high-redshift quasars: excess [O II] emission in the radio-loud population
We investigate the [O II] emission line properties of 18,508 quasars at z<1.6
drawn from the Sloan Digital Sky Survey (SDSS) quasar sample. The quasar sample
has been separated into 1,692 radio-loud and 16,816 radio-quiet quasars (RLQs
and RQQs hereafter) matched in both redshift and i'-band absolute magnitude.
We use the [O II]\lambda3726+3729 line as an indicator of star formation.
Based on these measurements we find evidence that star-formation activity is
higher in the RLQ population. The mean equivalent widths (EW) for [O II] are
EW([O II])_RL=7.80\pm0.30 \AA, and EW([O II])_RQ=4.77\pm0.06 \AA, for the RLQ
and RQQ samples respectively. The mean [O II] luminosities are \log[L([O
II])_RL/W]=34.31\pm0.01 and \log[L([O II])_RQ/W]=34.192\pm0.004 for the samples
of RLQs and RQQs respectively. Finally, to overcome possible biases in the EW
measurements due to the continuum emission below the [O II] line being
contaminated by young stars in the host galaxy, we use the ratio of the [O II]
luminosity to rest-frame i'-band luminosity, in this case, we find for the RLQs
\log[L([O II])_RL/L_opt]=-3.89\pm0.01 and \log[L([O
II])_RQ/L_opt]=-4.011\pm0.004 for RQQs. However the results depend upon the
optical luminosity of the quasar. RLQs and RQQs with the same high optical
luminosity \log(L_opt/W)>38.6, tend to have the same level of [O II] emission.
On the other hand, at lower optical luminosities \log(L_opt/W)<38.6, there is a
clear [O II] emission excess for the RLQs. As an additional check of our
results we use the [O III] emission line as a tracer of the bolometric
accretion luminosity, instead of the i'-band absolute magnitude, and we obtain
similar results.
Radio jets appear to be the main reason for the [O II] emission excess in the
case of RLQs. In contrast, we suggest AGN feedback ensures that the two
populations acquire the same [O II] emission at higher optical luminosities.Comment: 12 pages, 9 figures, accepted for publication in MNRA
A 610-MHz survey of the ELAIS-N1 field with the Giant Metrewave Radio Telescope - Observations, data analysis and source catalogue
Observations of the ELAIS-N1 field taken at 610 MHz with the Giant Metrewave
Radio Telescope are presented. Nineteen pointings were observed, covering a
total area of 9 square degrees with a resolution of 6" x 5", PA +45 deg. Four
of the pointings were deep observations with an rms of 40 microJy before
primary beam correction, with the remaining fifteen pointings having an rms of
70 microJy. The techniques used for data reduction and production of a
mosaicked image of the region are described, and the final mosaic is presented,
along with a catalogue of 2500 sources detected above 6 sigma. This work
complements the large amount of optical and infrared data already available on
the region. We calculate 610-MHz source counts down to 270 microJy, and find
further evidence for the turnover in differential number counts below 1 mJy,
previously seen at both 610 MHz and 1.4 GHz.Comment: 12 pages, 18 figures, two tables. Table 1 can be found in full via
http://www.mrao.cam.ac.uk/surveys/ . Accepted for publication in MNRA
The Chandra Multi-Wavelength Project: Optical Spectroscopy and the Broadband Spectral Energy Distributions of X-ray Selected AGN
From optical spectroscopy of X-ray sources observed as part of ChaMP, we
present redshifts and classifications for a total of 1569 Chandra sources from
our targeted spectroscopic follow up using the FLWO, SAAO, WIYN, CTIO, KPNO,
Magellan, MMT and Gemini telescopes, and from archival SDSS spectroscopy. We
classify the optical counterparts as 50% BLAGN, 16% NELG, 14% ALG, and 20%
stars. We detect QSOs out to z~5.5 and galaxies out to z~3. We have compiled
extensive photometry from X-ray to radio bands. Together with our spectroscopic
information, this enables us to derive detailed SEDs for our extragalactic
sources. We fit a variety of templates to determine bolometric luminosities,
and to constrain AGN and starburst components where both are present. While
~58% of X-ray Seyferts require a starburst event to fit observed photometry
only 26% of the X-ray QSO population appear to have some kind of star formation
contribution. This is significantly lower than for the Seyferts, especially if
we take into account torus contamination at z>1 where the majority of our X-ray
QSOs lie. In addition, we observe a rapid drop of the percentage of starburst
contribution as X-ray luminosity increases. This is consistent with the
quenching of star formation by powerful QSOs, as predicted by the merger model,
or with a time lag between the peak of star formation and QSO activity. We have
tested the hypothesis that there should be a strong connection between X-ray
obscuration and star-formation but we do not find any association between X-ray
column density and star formation rate both in the general population or the
star-forming X-ray Seyferts. Our large compilation also allows us to report
here the identification of 81 XBONG, 78 z>3 X-ray sources and 8 Type-2 QSO
candidates. Also we have identified the highest redshift (z=5.4135) X-ray
selected QSO with optical spectroscopy.Comment: 17 pages, 16 figures, accepted for publication in ApJS. Full data
table and README file can be found online at
http://hea-www.harvard.edu/~pgreen/Papers.htm
Selection of ULIRGs in Infrared and Submm Surveys
We examine the selection characteristics of infrared and sub-mm surveys with
IRAS, Spitzer, BLAST, Herschel and SCUBA and identify the range of dust
temperatures these surveys are sensitive to, for galaxies in the ULIRG
luminosity range (12<log(LIR)<13), between z=0 and z=4. We find that the extent
of the redshift range over which surveys are unbiased is a function of the
wavelength of selection, flux density limit and ULIRG luminosity. Short
wavelength (<200{\mu}m) surveys with IRAS, Spitzer/MIPS and Herschel/PACS are
sensitive to all SED types in a large temperature interval (17-87K), over a
substantial fraction of their accessible redshift range. On the other hand,
long wavelength (>200{\mu}m) surveys with BLAST, Herschel/ SPIRE and SCUBA are
significantly more sensitive to cold ULIRGs, disfavouring warmer SEDs even at
low redshifts. We evaluate observations in the context of survey selection
effects, finding that the lack of cold ULIRGs in the local (z<0.1) Universe is
not a consequence of selection and that the range of ULIRG temperatures seen
locally is only a subset of a much larger range which exists at high redshift.
We demonstrate that the local luminosity-temperature (L-T) relation, which
indicates that more luminous sources are also hotter, is not applicable in the
distant Universe when extrapolated to the ULIRG regime, because the scatter in
observed temperatures is too large. Finally, we show that the difference
between the ULIRG temperature distributions locally and at high redshift is not
the result of galaxies becoming colder due to an L-T relation which evolves as
a function of redshift. Instead, they are consistent with a picture where the
evolution of the infrared luminosity function is temperature dependent, i.e.
cold galaxies evolve at a faster rate than their warm counterparts.Comment: 11 pages, 6 figures, accepted for publication in MNRA
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