71 research outputs found
Molecular absorption in the cores of AGN : On the unified model
One of the fundamental concepts in the unified scheme of AGN is that both Seyfert 1 and Seyfert 2 galaxies harbour supermassive black holes that act as nuclear engines, but in Seyfert 2 galaxies the nucleus is blocked from direct view by an optically and geometrically thick molecular torus. However, whilst the unified scheme has proven very successful, little is known about the physical properties of the torus itself. Searches for absorption lines of common molecules like CO and OH have mostly yielded non-detections. Before concluding that tori are not molecular, radiative excitation effects, which uppress the opacity in the lowest transitions due to the proximity to the high brightness temperature radio radiation from the nucleus causing excitation into higher states, must be investigated. To explore these effects, we conducted a survey searching for excited OH from a sample of 31 Seyfert 2 galaxies. Here, I present the results of Effelsberg observations at 6.0 GHz yielding five new detections, and at 4.7 GHz yielding no detections. I also present spectral-line VLBI observations carried out at 13.4 GHz, which detected excited OH towards the cores of Cygnus A and NGC 1052. These observations confirm the presence of a molecular torus in these sources. A subsample of the 31 Seyfert 2 galaxies was further searched for methanol at 6.7 GHz, yielding the first ever extragalactic detection of methanol towards NGC 3079. It is, however, unclear if the methanol is associated with the active nucleus or the starburst around the source or possibly connected to the superbubble in this source. A blue-shifted feature in the spectrum also indicates that methanol is present in the large-scale molecular outflows. Finally, I present the exciting new discovery of a water maser at redshift 2.64, which is the most distant source where water has ever been found. Luminous extragalactic water masers are associated with the nuclear activity of their host galaxies. The maser emission originates from within a few parsecs of the central engine, being associated nuclear accretion disks or through interactions between the jet emerging from the nucleus and ambient molecular clouds, or nuclear outflows. The transition observed requires gas temperatures in excess of 300 K and particle densities n(H) > 10e7 cm^(-3). It is the first time that such a dense interstellar gas component has been observed in the early Universe. This discovery has been possible only due to the magnification provided by a foreground galaxy; it acts as a cosmic telescope reducing the integration time required for a detection by a factor of ~ 1000.</p
Detection of 183 GHz water megamaser emission towards NGC 4945
Aim: The aim of this work is to search Seyfert 2 galaxy NGC 4945, a
well-known 22 GHz water megamaser galaxy, for water (mega)maser emission at 183
GHz. Method: We used APEX SEPIA Band 5 to perform the observations. Results: We
detected 183 GHz water maser emission towards NGC 4945 with a peak flux density
of ~3 Jy near the galactic systemic velocity. The emission spans a velocity
range of several hundred km/s. We estimate an isotropic luminosity of > 1000
Lsun, classifying the emission as a megamaser. A comparison of the 183 GHz
spectrum with that observed at 22 GHz suggests that 183 GHz emission also
arises from the active galactic nucleus (AGN) central engine. If the 183 GHz
emission originates from the circumnuclear disk, then we estimate that a
redshifted feature at 1084 km/s in the spectrum should arise from a distance of
0.022 pc from the supermassive black hole (1.6 x 10(5) Schwarzschild radii),
i.e. closer than the water maser emission previously detected at 22 GHz. This
is only the second time 183 GHz maser emission has been detected towards an AGN
central engine (the other galaxy being NGC 3079). It is also the strongest
extragalactic millimetre/submillimetre water maser detected to date.
Conclusions: Strong millimetre 183 GHz water maser emission has now been shown
to occur in an external galaxy. For NGC 4945, we believe that the maser
emission arises, or is dominated by, emission from the AGN central engine.
Emission at higher velocity, i.e. for a Keplerian disk closer to the black
hole, has been detected at 183 GHz compared with that for the 22 GHz megamaser.
This indicates that millimetre/submillimetre water masers can indeed be useful
probes for tracing out more of AGN central engine structures and dynamics than
previously probed. Future observations using ALMA Band 5 should unequivocally
determine the origin of the emission in this and other galaxies.Comment: 4 pages, accepted by A&A Letter
Refining the M_BH-V_c scaling relation with HI rotation curves of water megamaser galaxies
Black hole - galaxy scaling relations provide information about the
coevolution of supermassive black holes and their host galaxies. We compare the
black hole mass - circular velocity (MBH - Vc) relation with the black hole
mass - bulge stellar velocity dispersion (MBH - sigma) relation, to see whether
the scaling relations can passively emerge from a large number of mergers, or
require a physical mechanism, such as feedback from an active nucleus. We
present VLA H I observations of five galaxies, including three water megamaser
galaxies, to measure the circular velocity. Using twenty-two galaxies with
dynamical MBH measurements and Vc measurements extending to large radius, our
best-fit MBH - Vc relation, log MBH = alpha + beta log(Vc /200 km s^-1), yields
alpha = 7.43+/-0.13, beta = 3.68+1.23/-1.20, and intrinsic scatter epsilon_int
= 0.51+0.11/-0.09. The intrinsic scatter may well be higher than 0.51, as we
take great care to ascribe conservatively large observational errors. We find
comparable scatter in the MBH - sigma relations, epsilon_int = 0.48+0.10/-0.08,
while pure merging scenarios would likely result in a tighter scaling with the
dark halo (as traced by Vc) than baryonic (sigma) properties. Instead, feedback
from the active nucleus may act on bulge scales to tighten the MBH - sigma
relation with respect to the MBH - Vc relation, as observed.Comment: 27 pages, 15 figures, ApJ accepte
Long term Arecibo monitoring of the water megamaser in MG J0414+0534
We monitored the 22 GHz maser line in the lensed quasar MG J0414+0534 at
z=2.64 with the 300-m Arecibo telescope for almost two years to detect possible
additional maser components and to measure a potential velocity drift of the
lines. The main maser line profile is complex and can be resolved into a number
of broad features with line widths of 30-160 km/s. A new maser component was
tentatively detected in October 2008 at a velocity of +470 km/s. After
correcting for the estimated lens magnification, we find that the H2O isotropic
luminosity of the maser in MG J0414+0534 is about 26,000 solar luminosities,
making this source the most luminous ever discovered. Both the main line peak
and continuum flux densities are surprisingly stable throughout the period of
the observations. An upper limit on the velocity drift of the main peak of the
line has been estimated from our observations and is of the order of 2 km/s per
year. We discuss the results of the monitoring in terms of the possible nature
of the maser emission, associated with an accretion disk or a radio jet. This
is the first time that such a study is performed in a water maser source at
high redshift, potentially allowing us to study the parsec-scale environment
around a powerful radio source at cosmological distances.Comment: 5 pages, 2 figures, to appear in the Proceedings of the IAU Symposium
287, 2012, "Cosmic masers: from OH to H0
Mrk 1419 - a new distance determination
Water vapor megamasers from the center of active galaxies provide a powerful
tool to trace accretion disks at sub-parsec resolution and, through an entirely
geometrical method, measure direct distances to galaxies up to 200 Mpc. The
Megamaser Cosmology Project (MCP) is formed by a team of astronomers with the
aim of identifying new maser systems, and mapping their emission at high
angular resolution to determine their distance. Two types of observations are
necessary to measure a distance: single-dish monitoring to measure the
acceleration of gas in the disk, and sensitive VLBI imaging to measure the
angular size of the disk, measure the rotation curve, and model radial
displacement of the maser feature. The ultimate goal of the MCP is to make a
precise measurement of H0 by measuring such distances to at least 10 maser
galaxies in the Hubble flow. We present here the preliminary results from a new
maser system, Mrk 1419. Through a model of the rotation from the systemic
masers assuming a narrow ring, and combining these results with the
acceleration measurement from the Green Bank Telescope, we determine a distance
to Mrk 1419 of 81\pm10 Mpc. Given that the disk shows a significant warp that
may not be entirely traced by our current observations, more sensitive
observations and more sophisticated disk modeling will be essential to improve
our distance estimation to this galaxy.Comment: 5 pages, 3 figures, to appear in the proceedings of IAU Symposium 287
"Cosmic Masers- from OH to Ho", in Stellenbosch, S
Cosmology and the Hubble Constant: On the Megamaser Cosmology Project (MCP)
The Hubble constant Ho describes not only the expansion of local space at
redshift z ~ 0, but is also a fundamental parameter determining the evolution
of the universe. Recent measurements of Ho anchored on Cepheid observations
have reached a precision of several percent. However, this problem is so
important that confirmation from several methods is needed to better constrain
Ho and, with it, dark energy and the curvature of space. A particularly direct
method involves the determination of distances to local galaxies far enough to
be part of the Hubble flow through water vapor (H2O) masers orbiting nuclear
supermassive black holes. The goal of this article is to describe the relevance
of Ho with respect to fundamental cosmological questions and to summarize
recent progress of the the `Megamaser Cosmology Project' (MCP) related to the
Hubble constant.Comment: 10 pages, 7 postscript figures (8 ps files), IAU Symposium 287, uses
iaus.cl
Using Megamaser Disks to Probe Black Hole Accretion
We examine the alignment between H_2O megamaser disks on sub-pc scales with
circumnuclear disks and bars on <500 pc scales observed with HST/WFC3. The HST
imaging reveals young stars, indicating the presence of gas. The megamaser
disks are not well aligned with the circumnuclear bars or disks as traced by
stars in the HST images. We speculate on the implications of the observed
misalignments for fueling supermassive black holes in gas-rich spiral galaxies.
In contrast, we find a strong preference for the rotation axes of the megamaser
disks to align with radio continuum jets observed on >50 pc scales, in those
galaxies for which radio continuum detections are available. Sub-arcsecond
observations of molecular gas with ALMA will enable a more complete
understanding of the interplay between circumnuclear structures.Comment: Error in Figure 4 corrected, references added. 7 pages, 4 figures, to
be published in the Astrophysical Journa
ALMA Observations of the Physical and Chemical Conditions in Centaurus A
Centaurus A, with its gas-rich elliptical host galaxy, NGC 5128, is the
nearest radio galaxy at a distance of 3.8 Mpc. Its proximity allows us to study
the interaction between an active galactic nucleus, radio jets, and molecular
gas in great detail. We present ALMA observations of low J transitions of three
CO isotopologues, HCN, HCO, HNC, CN, and CCH toward the inner projected
500 pc of NGC 5128. Our observations resolve physical sizes down to 40 pc. By
observing multiple chemical probes, we determine the physical and chemical
conditions of the nuclear interstellar medium of NGC 5128. This region contains
molecular arms associated with the dust lanes and a circumnuclear disk (CND)
interior to the molecular arms. The CND is approximately 400 pc by 200 pc and
appears to be chemically distinct from the molecular arms. It is dominated by
dense gas tracers while the molecular arms are dominated by CO and its
rare isotopologues. The CND has a higher temperature, elevated CN/HCN and
HCN/HNC intensity ratios, and much weaker CO and CO emission than
the molecular arms. This suggests an influence from the AGN on the CND
molecular gas. There is also absorption against the AGN with a low velocity
complex near the systemic velocity and a high velocity complex shifted by about
60 km s. We find similar chemical properties between the CND in emission
and both the low and high velocity absorption complexes implying that both
likely originate from the CND. If the HV complex does originate in the CND,
then that gas would correspond to gas falling toward the supermassive black
hole
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