988 research outputs found
Molecular line probes of activity in galaxies
The use of specific tracers of the dense molecular gas phase can help to
explore the feedback of activity on the interstellar medium (ISM) in galaxies.
This information is a key to any quantitative assessment of the efficiency of
the star formation process in galaxies. We present the results of a survey
devoted to probe the feedback of activity through the study of the excitation
and chemistry of the dense molecular gas in a sample of local universe
starbursts and active galactic nuclei (AGNs). Our sample includes also 17
luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the
analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007)
we find the first clear observational evidence that the star formation
efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is
significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting
evidence of overabundant HCN in active environments would even reinforce the
reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt
law around L_FIR=10^11 L_sun. This result has major implications for the use of
HCN as a tracer of the dense gas in local and high-redshift luminous infrared
galaxies.Comment: 4 pages, 2 figures, contributed paper to Far-Infrared Workshop 07
(FIR 2007
The feeding of activity in galaxies: a molecular line perspective
What are the main drivers of activity in the local universe? Observations
have been instrumental in identifying the mechanisms responsible for fueling
activity in galaxy nuclei. In this context we summarize the main results of the
NUclei of GAlaxies (NUGA) survey. The aim of NUGA is to map, at high resolution
and high sensitivity, the distribution and dynamics of the molecular gas in the
central kiloparsec region of 25 galaxies, and to study the different mechanisms
responsible for gas fueling of low-luminosity AGNs (LLAGN). Gas flows in NUGA
maps reveal a wide range of instabilities. The derived gravity torque maps show
that only about 1/3 of NUGA galaxies show evidence of ongoing fueling. Secular
evolution and dynamical decoupling are seen to be key ingredients to understand
the AGN fueling cycle. We discuss the future prospects for this research field
with the advent of instruments like the Atacama Large Millimeter Array (ALMA).Comment: 6 pages, 3 figures, Proceedings of "The Central Kiloparsec in
Galactic Nuclei-Astronomy at High Angular Resolution 2011", to appear in
Journal of Physics, Conf Series, IOP Publishin
Molecular gas chemistry in AGN. II. High-resolution imaging of SiO emission in NGC1068: shocks or XDR?
This paper is part of a multi-species survey of line emission from the
molecular gas in the circum-nuclear disk (CND) of the Seyfert 2 galaxy NGC1068.
Single-dish observations have provided evidence that the abundance of silicon
monoxide(SiO) in the CND of NGC1068 is enhanced by 3-4 orders of magnitude with
respect to the values typically measured in quiescent molecular gas in the
Galaxy. We aim at unveiling the mechanism(s) underlying the SiO enhancement. We
have imaged with the IRAM Plateau de Bure interferometer the emission of the
SiO(2-1) and CN(2--1) lines in NGC1068 at 150pc and 60pc spatial resolution,
respectively. We have also obtained complementary IRAM 30m observations of HNCO
and methanol (CH3OH) lines. SiO is detected in a disk of 400pc size around the
AGN. SiO abundances in the CND of (1-5)xE-09 are about 1-2 orders of magnitude
above those measured in the starburst ring. The overall abundance of CN in the
CND is high: (0.2-1)xE-07. The abundances of SiO and CN are enhanced at the
extreme velocities of gas associated with non-circular motions close to the AGN
(r<70pc). Abundances measured for CN and SiO, and the correlation of CN/CO and
SiO/CO ratios with hard X-ray irradiation, suggest that the CND of NGC1068 has
become a giant X-ray dominated region (XDR). The extreme properties of
molecular gas in the circum-nuclear molecular disk of NGC1068 result from the
interplay between different processes directly linked to nuclear activity.
Whereas XDR chemistry offers a simple explanation for CN and SiO in NGC1068,
the relevance of shocks deserves further scrutiny. The inclusion of dust grain
chemistry would help solve the controversy regarding the abundances of other
molecular species, like HCN, which are under-predicted by XDR models.Comment: 18 pages, 13 figures, 2 tables; accepted for publication in A&
Nonlinear quantum optics in the (ultra)strong light-matter coupling
The propagation of photons in one dimensional waveguides coupled to
qubits is discussed, both in the strong and ultrastrong qubit-waveguide
coupling. Special emphasis is placed on the characterisation of the nonlinear
response and its linear limit for the scattered photons as a function of ,
, qubit inter distance and light-matter coupling. The quantum evolution is
numerically solved via the Matrix Product States technique. Both the time
evolution for the field and qubits is computed. The nonlinear character (as a
function of ) depends on the computed observable. While perfect reflection
is obtained for , photon-photon correlations are still resolved
for ratios . Inter-qubit distance enhances the nonlinear response.
Moving to the ultrastrong coupling regime, we observe that inelastic processes
are \emph{robust} against the number of qubits and that the qubit-qubit
interaction mediated by the photons is qualitatively modified. The theory
developed in this work modelises experiments in circuit QED, photonic crystals
and dielectric waveguides.Comment: Comments are wellcom
Scattering in the ultrastrong regime: nonlinear optics with one photon
The scattering of a flying photon by a two-level system ultrastrongly coupled
to a one-dimensional photonic waveguide is studied numerically. The photonic
medium is modeled as an array of coupled cavities and the whole system is
analyzed beyond the rotating wave approximation using Matrix Product States. It
is found that the scattering is strongly influenced by the single- and
multi-photon dressed bound states present in the system. In the ultrastrong
coupling regime a new channel for inelastic scattering appears, where an
incident photon deposits energy into the qubit, exciting a photon-bound state,
and escaping with a lower frequency. This single-photon nonlinear frequency
conversion process can reach up to 50\% efficiency. Other remarkable features
in the scattering induced by counter-rotating terms are a blueshift of the
reflection resonance and a Fano resonance due to long-lived excited statesComment: 5+4 page
Single photons by quenching the vacuum
Heisenberg's uncertainty principle implies that the quantum vacuum is not
empty but fluctuates. These fluctuations can be converted into radiation
through nonadiabatic changes in the Hamiltonian. Here, we discuss how to
control this vacuum radiation, engineering a single-photon emitter out of a
two-level system (2LS) ultrastrongly coupled to a finite-band waveguide in a
vacuum state. More precisely, we show the 2LS nonlinearity shapes the vacuum
radiation into a nonGaussian superposition of even and odd cat states. When the
2LS bare frequency lays within the band gaps, this emission can be well
approximated by individual photons. This picture is confirmed by a
characterization of the ground and bound states, and a study of the dynamics
with matrix product states and polaron Hamiltonian methods.Comment: 9 pages, 10 figure
One- and two-photon scattering from generalized V-type atoms
The one- and two-photon scattering matrix S is obtained analytically for a
one-dimensional waveguide and a point-like scatterer with N excited levels
(generalized V -type atom). We argue that the two-photon scattering matrix
contains sufficient information to distinguish between different level
structures which are equivalent for single-photon scattering, such as a V -atom
with N = 2 excited levels and two two-level systems. In particular, we show
that the scattering with the V -type atom exhibits a destructive interference
effect leading to two-photon Coupled-Resonator-Induced Transparency, where the
nonlinear part of the two-photon scattering matrix vanishes when each incident
photon fulfills a single-photon condition for transparency
Full two-photon downconversion of just a single photon
We demonstrate, both numerically and analytically, that it is possible to
generate two photons from one and only one photon. We characterize the output
two photon field and make our calculations close to reality by including
losses. Our proposal relies on real or artificial three-level atoms with a
cyclic transition strongly coupled to a one-dimensional waveguide. We show that
close to perfect downconversion with efficiency over 99% is reachable using
state-of-the-art Waveguide QED architectures such as photonic crystals or
superconducting circuits. In particular, we sketch an implementation in circuit
QED, where the three level atom is a transmon
Jet-disturbed molecular gas near the Seyfert 2 nucleus in M51
Previous molecular gas observations at arcsecond-scale resolution of the
Seyfert 2 galaxy M51 suggest the presence of a dense circumnuclear rotating
disk, which may be the reservoir for fueling the active nucleus and obscures it
from direct view in the optical. However, our recent interferometric CO(3-2)
observations show a hint of a velocity gradient perpendicular to the rotating
disk, which suggests a more complex structure than previously thought. To image
the putative circumnuclear molecular gas disk at sub-arcsecond resolution to
better understand both the spatial distribution and kinematics of the molecular
gas. We carried out CO(2-1) and CO(1-0) line observations of the nuclear region
of M51 with the new A configuration of the IRAM Plateau de Bure Interferometer,
yielding a spatial resolution lower than 15 pc. The high resolution images show
no clear evidence of a disk, aligned nearly east-west and perpendicular to the
radio jet axis, as suggested by previous observations, but show two separate
features located on the eastern and western sides of the nucleus. The western
feature shows an elongated structure along the jet and a good velocity
correspondence with optical emission lines associated with the jet, suggesting
that this feature is a jet-entrained gas. The eastern feature is elongated
nearly east-west ending around the nucleus. A velocity gradient appears in the
same direction with increasingly blueshifted velocities near the nucleus. This
velocity gradient is in the opposite sense of that previously inferred for the
putative circumnuclear disk. Possible explanations for the observed molecular
gas distribution and kinematics are that a rotating gas disk disturbed by the
jet, gas streaming toward the nucleus, or a ring with another smaller counter-
or Keplarian-rotating gas disk inside.Comment: 5 pages, 4 figures, to appear in A&A Letters Special Issue for the
new extended configuration at the IRAM PdB
Emergent Causality and the N-photon Scattering Matrix in Waveguide QED
In this work we discuss the emergence of approximate causality in a general
setup from waveguide QED -i.e. a one-dimensional propagating field interacting
with a scatterer. We prove that this emergent causality translates into a
structure for the N-photon scattering matrix. Our work builds on the derivation
of a Lieb-Robinson-type bound for continuous models and for all coupling
strengths, as well as on several intermediate results, of which we highlight
(i) the asymptotic independence of space-like separated wave packets, (ii) the
proper definition of input and output scattering states, and (iii) the
characterization of the ground state and correlations in the model. We
illustrate our formal results by analyzing the two-photon scattering from a
quantum impurity in the ultrastrong coupling regime, verifying the cluster
decomposition and ground-state nature. Besides, we generalize the cluster
decomposition if inelastic or Raman scattering occurs, finding the structure of
the S-matrix in momentum space for linear dispersion relations. In this case,
we compute the decay of the fluorescence (photon-photon correlations) caused by
this S-matrix
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