38 research outputs found
Mid Infrared Spectra of Radio Galaxies and Quasars
Spitzer Infrared Spectrograph (IRS) observations of 3C radio galaxies and
quasars shed new light on the nature of the central engines of AGN. Emission
from silicate dust obscuring the central engine can be used to estimate the
bolometric luminosity of an AGN. Emission lines from ions such as O IV and Ne V
give another indication of the presence or lack of a hidden source of far-UV
photons in the nucleus. Radio-loud AGN with relative-to-Eddington luminosity
ratios of L/L_Edd < 3E-3 do not appear to have broad optical emission lines,
though some do have strong silicate emission. Aromatic emission features from
star formation activity are common in low-luminosity radio galaxies. Strong
molecular hydrogen pure-rotational emission lines are also seen in some mid-IR
weak radio galaxies, caused by either merger shocks or jet shocks in the
interstellar medium.Comment: Conference proceedings to appear in "The Central Engine of Active
Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP
The Spitzer View of FR I Radio Galaxies: On the Origin of the Nuclear Mid-Infrared Continuum
We present Spitzer mid-infrared (MIR) spectra of 25 FR I radio galaxies and investigate the nature of their MIR continuum emission. MIR spectra of star-forming galaxies and quiescent elliptical galaxies are used to identify host galaxy contributions while radio/optical core data are used to isolate the nuclear nonthermal emission. Out of the 15 sources with detected optical compact cores, four sources are dominated by emission related to the host galaxy. Another four sources show signs of warm, nuclear dust emission: 3C15, 3C84, 3C270, and NGC 6251. It is likely that these warm dust sources result from hidden active galactic nuclei of optical spectral type 1. The MIR spectra of seven sources are dominated by synchrotron emission, with no significant component of nuclear dust emission. In parabolic spectral energy distribution fits of the nonthermal cores FR Is tend to have lower peak frequencies and stronger curvature than blazars. This is roughly consistent with the common picture in which the core emission in FR Is is less strongly beamed than in blazars
Shocked Molecular Hydrogen in the 3C 326 Radio Galaxy System
The Spitzer spectrum of the giant FR II radio galaxy 3C 326 is dominated by
very strong molecular hydrogen emission lines on a faint IR continuum. The H2
emission originates in the northern component of a double-galaxy system
associated with 3C 326. The integrated luminosity in H2 pure-rotational lines
is 8.0E41 erg/s, which corresponds to 17% of the 8-70 micron luminosity of the
galaxy. A wide range of temperatures (125-1000 K) is measured from the H2 0-0
S(0)-S(7) transitions, leading to a warm H2 mass of 1.1E9 Msun. Low-excitation
ionic forbidden emission lines are consistent with an optical LINER
classification for the active nucleus, which is not luminous enough to power
the observed H2 emission. The H2 could be shock-heated by the radio jets, but
there is no direct indication of this. More likely, the H2 is shock-heated in a
tidal accretion flow induced by interaction with the southern companion galaxy.
The latter scenario is supported by an irregular morphology, tidal bridge, and
possible tidal tail imaged with IRAC at 3-9 micron. Unlike ULIRGs, which in
some cases exhibit H2 line luminosities of comparable strength, 3C 326 shows
little star-formation activity (~0.1 Msun/yr). This may represent an important
stage in galaxy evolution. Starburst activity and efficient accretion onto the
central supermassive black hole may be delayed until the shock-heated H2 can
kinematically settle and coolComment: 27 pages, 7 figures, accepted for publication in the Astrophysical
Journa
The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infra-Red to X-Ray Spectral Energy Distribution
We present extinction curves derived from the broad emission lines and
continua of large samples of both radio-loud and radio-quiet AGNs. The curves
are significantly flatter in the UV than are curves for the local ISM. The
reddening curves for the radio-quiet LBQS quasars are slightly steeper than
those of the radio-loud quasars in the UV, probably because of additional
reddening by dust further out in the host galaxies of the former. The UV
extinction curves for the radio-loud AGNs are very flat. This is explicable
with slight modifications to standard MRN dust models: there is a relative lack
of small grains in the nuclear dust. Our continuum and broad-emission line
reddening curves agree in both shape and amplitude, confirming that the
continuum shape is indeed profoundly affected by reddening for all but the
bluest AGNs. With correction by our generic extinction curve, all of the
radio-loud AGNs have continuous optical-UV spectra consistent with a single
shape. We show that radio-quiet AGNs have very similar intrinsic UV to optical
shape over orders of magnitude in luminosity. We also argue that radio-loud and
radio-quiet AGNs probably share the same underlying continuum shape and that
most of the systematic differences between their observed continuum shapes are
due to higher nuclear reddening in radio-selected AGNs, and additional
reddening from dust further out in the host galaxies in radio-quiet AGNs. Our
conclusions have important implications for the modelling of quasar continua
and the analysis of quasar demographics.Comment: 41 pages, including 6 figures and 3 tables. To appear in ApJ vol.
614, October 20 issue. Some slight wording changes. Some additional
references added. Small changes in the model fit in section 6.2, to the
analytical fit in the Appendix, and to the tabulated reddening curve in the
Appendi
Thermal Emission as a Test for Hidden Nuclei in Nearby Radio Galaxies
The clear sign of a hidden quasar inside a radio galaxy is the appearance of
quasar spectral features in its polarized (scattered) light. However that
observational test requires suitably placed scattering material to act as a
mirror, allowing us to see the nuclear light. A rather robust and more general
test for a hidden quasar is to look for the predicted high mid-IR luminosity
from the nuclear obscuring matter. The nuclear waste heat is detected and well
isolated in the nearest narrow line radio galaxy, Cen A. This confirms other
indications that Cen A does contain a modest quasar-like nucleus. However we
show here that M87 does not: at high spatial resolution, the mid-IR nucleus is
seen to be very weak, and consistent with simple synchrotron emission from the
base of the radio jet. This fairly robustly establishes that there are "real"
narrow line radio galaxies, without the putative accretion power, and with
essentially all the luminosity in kinetic form. Next we show the intriguing
mid-IR morphology of Cygnus A, reported previously by us and later discussed in
detail by Radomski et al. (2002). All of this mid-IR emission is consistent
with reprocessing by a hidden quasar, known to exist from spectropolarimetry by
Ogle et al. (1997) and other evidence.Comment: 21 pages, 5 figure
The Core of NGC 6240 from Keck Adaptive Optics and HST NICMOS Observations
We present results of near infrared imaging of the disk-galaxy-merger NGC
6240 using adaptive optics on the Keck II Telescope and reprocessed archival
data from NICMOS on the Hubble Space Telescope. Both the North and South nuclei
of NGC 6240 are clearly elongated, with considerable sub-structure within each
nucleus. In K' band there are at least two point-sources within the North
nucleus; we tentatively identify the south-western point-source within the
North nucleus as the position of one of the two AGNs. Within the South nucleus,
the northern sub-nucleus is more highly reddened. Based upon the nuclear
separation measured at 5 GHz, we suggest that the AGN in the South nucleus is
still enshrouded in dust at K' band, and is located slightly to the north of
the brightest point in K' band. Within the South nucleus there is strong H2 1-0
S(1) line emission from the northern sub-nucleus, contrary to the conclusions
of previous seeing-limited observations. Narrowband H2 emission-line images
show that a streamer or ribbon of excited molecular hydrogen connects the North
and South nuclei. We suggest that this linear feature corresponds to a bridge
of gas connecting the two nuclei, as seen in computer simulations of mergers.
Many point-like regions are seen around the two nuclei. These are most
prominent at 1.1 microns with NICMOS, and in K'-band with Keck adaptive optics.
We suggest that these point-sources represent young star clusters formed in the
course of the merger.Comment: 50 pages, 13 figures. To be published in the Astrophysical Journal,
March 10, 200
On the nature of optical nuclei in FR I radio-galaxies from ACS/HST imaging polarimetry
We obtained optical imaging polarimetry with the ACS/HRC aboard the HST of
the 9 closest radio-galaxies in the 3C catalogue with an FR I morphology. The
nuclear sources seen in direct HST images in these galaxies are found to be
highly polarized with levels in the range ~2-11 % with a median value of 7 %.
We discuss the different mechanisms that produce polarized emission and
conclude that the only viable interpretation is a synchrotron origin for the
optical nuclei. This idea is strengthened by the analogy with the polarization
properties of BL Lac objects, providing also further support to the FRI/BL Lac
unified model. This confirms previous suggestions that the dominant emission
mechanism in low luminosity radio-loud AGN is related to non-thermal radiation
produced by the base of their jets. In addition to the nuclear polarization
(and to the large scale optical jets), polarization is detected co-spatially
with the dusty circumnuclear disks, likely due to dichroic transmission; the
polarization vectors are tangential to the disks as expected when the magnetic
field responsible for the grains alignment is stretched by differential
rotation. We explored the possibility to detect the polarimetric signature of a
misaligned radiation beam in FR I, expected in our sources in the frame of the
FR I/ BL Lac unification. We did not find this effect in any of the galaxies,
but our the results are not conclusive on whether a misaligned beam is indeed
present in FR I.Comment: 16 page, 10 figures, accepted for publication in A&
The 1.6 micron near infrared nuclei of 3C radio galaxies: Jets, thermal emission or scattered light?
Using HST NICMOS 2 observations we have measured 1.6-micron near infrared
nuclear luminosities of 100 3CR radio galaxies with z<0.3, by modeling and
subtracting the extended emission from the host galaxy. We performed a
multi-wavelength statistical analysis (including optical and radio data) of the
properties of the nuclei following classification of the objects into FRI and
FRII, and LIG (low-ionization galaxies), HIG (high-ionization galaxies) and BLO
(broad-lined objects) using the radio morphology and optical spectra,
respectively. The correlations among near infrared, optical, and radio nuclear
luminosity support the idea that the near infrared nuclear emission of FRIs has
a non-thermal origin. Despite the difference in radio morphology, the
multi-wavelength properties of FRII LIG nuclei are statistically
indistinguishable from those of FRIs, an indication of a common structure of
the central engine. All BLOs show an unresolved near infrared nucleus and a
large near infrared excess with respect to FRII LIGs and FRIs of equal radio
core luminosity. This requires the presence of an additional (and dominant)
component other than the non-thermal light. Considering the shape of their
spectral energy distribution, we ascribe the origin of their near infrared
light to hot circumnuclear dust. A near infrared excess is also found in HIGs,
but their nuclei are substantially fainter than those of BLO. This result
indicates that substantial obscuration along the line-of-sight to the nuclei is
still present at 1.6 micron. Nonetheless, HIGs nuclei cannot simply be
explained in terms of dust obscuration: a significant contribution from light
reflected in a circumnuclear scattering region is needed to account for their
multiwavelength properties.Comment: 20 pages, 16 figures. Accepted for publication on Ap