47 research outputs found
Near-infrared and Millimeter Constraints on the Nuclear Energy Source of the Infrared Luminous Galaxy NGC 4418
We present near-infrared and millimeter investigations of the nucleus of the
infrared luminous galaxy NGC 4418, which previous observations suggest
possesses a powerful buried AGN. We found the following main results: (1) The
infrared K-band spectrum shows CO absorption features at 2.3-2.4 micron owing
to stars and very strong H2 emission lines. The luminosity ratios of H2
emission lines are suggestive of a thermal origin, and the equivalent width of
the H2 1-0 S(1) line is the second largest observed to date in an external
galaxy, after the well-studied strong H2-emitting galaxy NGC 6240. (2) The
infrared L-band spectrum shows a clear polycyclic aromatic hydrocarbon (PAH)
emission feature at 3.3 micron, which is usually found in star-forming
galaxies. The estimated star-formation luminosity from the observed PAH
emission can account for only a small fraction of the infrared luminosity. (3)
Millimeter interferometric observations of the nucleus reveal a high HCN (1-0)
to HCO+ (1-0) luminosity ratio of 1.8, as has been previously found in pure
AGNs. (4) The measurements of HCN (1-0) luminosity using a single-dish
millimeter telescope show that the HCN (1-0) to infrared luminosity ratio is
slightly larger than the average, but within the scattered range, for other
infrared luminous galaxies. All of these results can be explained by the
scenario in which, in addition to energetically-insignificant, weakly-obscured
star-formation at the surface of the nucleus, a powerful X-ray emitting AGN
deeply buried in dust and high density molecular gas is present.Comment: 31 pages, 9 figures, Accepted for publication in Astronomical Journal
(2004 November issue
Star Formation on Subkiloparsec Scale Triggered by Non-linear Processes in Nearby Spiral Galaxies
We report a super-linear correlation for the star formation law based on new CO(J = 1-0) data from the CARMA and NOBEYAMA Nearby-galaxies (CANON) CO survey. The sample includes 10 nearby spiral galaxies, in which structures at sub-kpc scales are spatially resolved. Combined with the star formation rate surface density traced by Hα and 24 μm images, CO(J = 1-0) data provide a super-linear slope of N = 1.3. The slope becomes even steeper (N = 1.8) when the diffuse stellar and dust background emission is subtracted from the Hα and 24 μm images. In contrast to the recent results with CO(J = 2-1) that found a constant star formation efficiency (SFE) in many spiral galaxies, these results suggest that the SFE is not independent of environment, but increases with molecular gas surface density. We suggest that the excitation of CO(J = 2-1) is likely enhanced in the regions with higher star formation and does not linearly trace the molecular gas mass. In addition, the diffuse emission contaminates the SFE measurement most in regions where the star formation rate is law. These two effects can flatten the power-law correlation and produce the apparent linear slope. The super-linear slope from the CO(J = 1-0) analysis indicates that star formation is enhanced by non-linear processes in regions of high gas density, e.g., gravitational collapse and cloud-cloud collisions
Molecular outflow in the reionization-epoch quasar J2054-0005 revealed by OH 119 m observations
Molecular outflows are expected to play a key role in galaxy evolution at
high redshift. To study the impact of outflows on star formation at the epoch
of reionization, we performed sensitive ALMA observations of OH 119 m
toward J2054-0005, a luminous quasar at . The OH line is detected and
exhibits a P-Cygni profile that can be fitted with a broad blue-shifted
absorption component, providing unambiguous evidence of an outflow, and an
emission component at near-systemic velocity. The mean and terminal outflow
velocities are estimated to be
and , respectively, making the molecular outflow in
this quasar one of the fastest at the epoch of reionization. The OH line is
marginally spatially resolved for the first time in a quasar at ,
revealing that the outflow extends over the central 2 kpc region. The mass
outflow rate is comparable to the star formation rate
(), indicating rapid
() quenching of star formation. The mass outflow rate in
a sample star-forming galaxies and quasars at exhibits a positive
correlation with the total infrared luminosity, although the scatter is large.
Owing to the high outflow velocity, a large fraction (up to ) of the
outflowing molecular gas may be able to escape from the host galaxy into the
intergalactic medium.Comment: Accepted to Ap
Aperture Synthesis Observations of CO, HCN, and 89GHz Continuum Emission toward NGC 604 in M 33: Sequential Star Formation Induced by Supergiant Hii region
We present the results from new Nobeyama Millimeter Array observations of
CO(1-0), HCN(1-0), and 89-GHz continuum emissions toward NGC 604, known as the
supergiant H ii region in a nearby galaxy M 33. Our high spatial resolution
images of CO emission allowed us to uncover ten individual molecular clouds
that have masses of (0.8 -7.4) 10M_{\sun } and sizes of 5 -- 29 pc,
comparable to those of typical Galactic giant molecular clouds (GMCs).
Moreover, we detected for the first time HCN emission in the two most massive
clouds and 89 GHz continuum emission at the rims of the "H shells".
Three out of ten CO clouds are well correlated with the H shells both
in spatial and velocity domains, implying an interaction between molecular gas
and the expanding H ii region. Furthermore, we estimated star formation
efficiencies (SFEs) for each cloud from the 89-GHz and combination of
H and 24-m data, and found that the SFEs decrease with
increasing projected distance measured from the heart of the central OB star
cluster in NGC 604, suggesting the radial changes in evolutionary stages of the
molecular clouds in course of stellar cluster formation. Our results provide
further support to the picture of sequential star formation in NGC604 initially
proposed by Tosaki et al. (2007) with the higher spatially resolved molecular
clouds, in which an isotropic expansion of the H ii region pushes gases outward
and accumulates them to consecutively form dense molecular clouds, and then
induces massive star formations.Comment: 23 pages, 8 figures, accepted for publication in Ap
Detections of [C II] 158 m and [O III] 88 m in a Local Lyman Continuum Emitter, Mrk 54, and its Implications to High-redshift ALMA Studies
We present integral field, far-infrared (FIR) spectroscopy of Mrk 54, a local
Lyman Continuum Emitter (LCE), obtained with FIFI-LS on the Stratospheric
Observatory for Infrared Astronomy. This is only the second time, after Haro
11, that [C II] 158 m and [O III] 88 m spectroscopy of the known LCEs
have been obtained. We find that Mrk 54 has a strong [C II] emission that
accounts for % of the total FIR luminosity, whereas it has only moderate
[O III] emission, resulting in the low [O III]/[C II] luminosity ratio of
. In order to investigate whether [O III]/[C II] is a useful
tracer of (LyC escape fraction), we examine the correlations of
[O III]/[C II] and (i) the optical line ratio of [O III]
5007 \AA/[O II] 3727 \AA, (ii) specific star formation rate, (iii) [O III] 88
m/[O I] 63 m ratio, (iv) gas phase metallicity, and (v) dust
temperature based on a combined sample of Mrk 54 and the literature data from
the Herschel Dwarf Galaxy Survey and the LITTLE THINGS Survey. We find that
galaxies with high [O III]/[C II] luminosity ratios could be the result of high
ionization (traced by ), bursty star formation, high
ionized-to-neutral gas volume filling factors (traced by [O III] 88 m/[O
I] 63 m), and low gas-phase metallicities, which is in agreement with
theoretical predictions. We present an empirical relation between the [O
III]/[C II] ratio and based on the combination of the [O III]/[C
II] and correlation, and the known relation between
and . The relation implies that high-redshift galaxies with high
[O III]/[C II] ratios revealed by ALMA may have ,
significantly contributing to the cosmic reionization.Comment: 14 pages, 5 figures, Accepted for publication in Ap