Issue |
A&A
Volume 517, July 2010
|
|
---|---|---|
Article Number | A82 | |
Number of page(s) | 7 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200913926 | |
Published online | 12 August 2010 |
Detection of the H92
recombination line from NGC 4945
A. L. Roy1,2,3,4
- T. Oosterloo5 - W. M. Goss2
- K. R. Anantharamaiah3,
1 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
2 - NRAO, PO Box O, Socorro, NM 87801, USA
3 - Raman Research Institute, CV Raman Ave, Sadashivanagar, Bangalore
560080, India
4 - Australia Telescope National Facility, PO Box 76, Epping 1710, NSW,
Australia
5 - ASTRON, PO Box 2, 7990 AA, Dwingeloo, The Netherlands
Received 21 December 2009 / Accepted 19 May 2010
Abstract
Context. Hydrogen ionized by young, high-mass stars
in starburst galaxies radiates radio recombination lines (RRLs), whose
strength can be used as a diagnostic of the ionization rate, conditions
and gas dynamics in the starburst region, without problems of dust
obscuration. However, the lines are weak and only few extragalactic
starburst systems have been detected.
Aims. We aimed to increase the number of known
starburst systems with detectable RRLs for detailed studies, and we
used the line properties to study the gas properties and dynamics.
Methods. We searched for the RRLs H91
and H92
with rest frequencies of 8.6 GHz and 8.3 GHz in the
nearby southern Seyfert galaxy NGC 4945 using the
Australia Telescope Compact Array with resolution of 3''. This
yielded a detection from which we derived conditions in the starburst
regions.
Results. We detected RRLs from the nucleus of
NGC 4945 with a peak line strength integrated over the source
of 17.8 mJy, making it the strongest extragalactic RRL emitter
known at this frequency. The line and continuum emission from
NGC 4945 can be matched by a model consisting of a collection
of 10 to 300 H II regions with
temperatures of 5000 K, densities of 103 cm-3
to 104 cm-3 and
a total effective diameter of 2 pc to 100 pc. The
Lyman continuum production rate required to maintain the ionization is
6
to 3
,
which requires 2000 to 10 000 O5 stars to be
produced in the starburst, inferring a star formation rate of
2
yr-1
to 8
yr-1.
We resolved the rotation curve within the central 70 pc region
and this is well described by a set of rotating rings that were
coplanar and edge on. We found no reason to depart from a
simple flat rotation curve. The rotation speed of
120 km s-1 within the
central 1'' (19 pc) radius infers an enclosed mass of
3
,
and an average surface density with the central 19 pc of
25 000 pc-2, which exceeds the
threshold gas surface density for star formation.
Conclusions. We discovered RRLs from
NGC 4945. It is the strongest known extragalactic RRL emitter
and is suited to high-quality spectroscopic study. We resolved the
dynamics of the ionized gas in the central 70 pc and derived
conditions and star formation rates in the ionized gas.
Key words: galaxies: individual: NGC 4945 - galaxies: nuclei - radio lines: galaxies
1 Introduction
NGC 4945 is a nearby almost edge-on spiral galaxy at a
distance of (3.8 0.3) Mpc
(Karachentsev et al. 2007;
implying a scale of 19 pc per arcsec). It has
prominent dust lanes obscuring the nucleus, and is one of the brightest
extragalactic sources seen by IRAS. The nuclear optical spectrum shows
no sign of the Seyfert nucleus, and shows a purely starburst emission
(Krabbe et al. 2001,
and references therein). The presence of an active galactic
nucleus (AGN) was finally settled when X-ray emission from a variable
compact nuclear source was detected in the range 2 keV to
10 keV (Iwasawa et al. 1993) with heavy
absorption (
1024 cm-2)
typical of Seyfert 2 galaxies. The absorption was predicted to
be much less at higher X-ray energies, and indeed at 100 keV
Done et al. (1996)
found it to be the second-brightest known Seyfert galaxy in the sky.
X-ray imaging by Schurch et al. (2002) with X-ray
Multi-Mirror-Newton and Chandra revealed a conical
plume extending 500 pc NW which they interpret as a
mass-loaded superwind driven by the starburst, and they found a
6.4 keV iron K
line
and a Compton reflection component, which are characteristic
of AGNs.
A compact (<30 mas) radio core with high
brightness temperature (107 K) detected
by Sadler et al. (1995)
at 2.3 GHz and 8.4 GHz indicates the presence of
bright synchrotron emission, additional evidence for an AGN, though
could possibly be radio supernovae. Mid-infrared spectroscopy was
carried out with the Infrared Space Observatory by Spoon
et al. (2000)
to penetrate the dust obscuration, and they found no high excitation
lines like [Ne V] that are common in AGNs, implying huge
obscuration of
assuming that a narrow-line region were present like that seen in,
for example, the Circinus galaxy (which is similar in many
ways to NGC 4945).
The nucleus is the source of abundant infrared and molecular
emission, and optical emission line images show gas outflowing from the
nucleus perpendicular to the galaxy plane. This activity originates
from a composite starburst and AGN. An obscured nuclear starburst ring
with diameter 50 pc is seen in Pa
(Marconi et al. 2000).
The molecular content of the NGC 4945 nucleus is rich
and varied, producing the strongest known extragalactic molecular lines
of many species. Over 80 transitions from
19 molecules have been found with single dishes, principally
OH, H2O, CO, HCN, HCO+, CH3OH,
but also many others with the Swedish-ESO-Submillimetre Telescope
(SEST) and Parkes from 1.6 GHz to 354 GHz (Gardner
& Whiteoak 1974;
Whiteoak & Gardner 1974;
Whiteoak et al. 1980;
Whiteoak & Wilson 1990;
Whiteoak & Gardner 1986;
Dos Santos & Lépine 1979;
Batchelor et al. 1982;
Henkel et al. 1990,
1994; Curran
et al. 2001;
Wang et al. 2004).
Many require densities of 105 cm-3
for excitation and yield cloud temperatures of
K.
Wang et al. (2004)
could determine isotope ratios of C, N, O, and S, and see isotope
enrichment by ejecta from massive stars relative to that of fresh gas
inflowing through a bar showing that the starburst is old enough to
have affected the isotopic composition of the surrounding interstellar
medium. Interferometric imaging of the HNC and HCO+
molecular distributions by Cunningham & Whiteoak (2005) with the
Australia Telescope Compact Array (ATCA) showed rotation of the
molecular disk in the central 6'' with rotation speed of
135 km s-1.
Imaging with very long baseline interferometry of the H2O
masers by Greenhill et al. (1997) resolved
the rotation curve over the central 40 milliarcsec
(0.7 pc diameter) with a velocity range of 150 km s-1
inferring a central mass of 1.4
.
The rotation is in the same sense and in the same plane as the galaxy
disk.
CO has been imaged at resolution from 43'' to 15'' with SEST
by Dahlem et al. (1993),
Ott et al. (2001),
and Mauersberger et al. (1996), and
interferometrically at 4'' with the Submillimeter Array (SMA) by Chou
et al. (2007).
All groups
found the CO to be strongly concentrated towards the centre in a disk
of molecular material 16''
11'' (310 pc
210 pc).
Chou et al. (2007)
show that the disk rotates, within the central 5''
(95 pc) radius in the plane of the galaxy disk with simple
rigid-body circular rotation. At larger radius the isovelocity contours
show an ``S'' shaped asymmetry due to a bar potential, and at
the centre there is an unresolved kinematically decoupled component
with a broad (340 km s-1)
velocity range. They find that this last component
is a good candidate for CO emission from the obscuring AGN
torus, and too dense to be part of the starburst-driven molecular
outflow.
The ionized gas phase has been imaged in H +
N II, [O III], and Br
by Moorwood & Oliva (1994),
Moorwood et al. (1996)
and Lípari et al. (1997)
and in Pa
by Marconi et al. (2000).
The optical lines show a conical wind-blown cavity, and the infrared
lines (Br
and Pa
)
show a starburst
disk at the base of the cavity extending over some 8'' and so
are embedded within the molecular disk which extends to 16''.
Despite the cone resembling Seyfert ionization cones, it lacks
[O III] emission and so that origin has been
excluded. Spectra in H
+
N II show motions of
km s-1
in the cone and optical line ratios typical of low-ionization nuclear
emission-line region galaxies. Infrared spectra covering Br
,
Pf
and shocked molecular hydrogen by Koornneef (1993), Moorwood
& Oliva (1994),
and Spoon et al. (2003)
resolve the nuclear rotating disk over the central
pc,
showing a velocity range of 500 km s-1.
The neutral hydrogen optical emission region is within the molecular
hydrogen region, with shock excitation from a nuclear wind from the
central cluster.
The starburst seen in Br
is powerful enough to supply the 2
radiated in the infrared.
The ionized gas component can also be imaged using radio
recombination lines (RRLs), which are not affected by extinction unlike
at optical and near-infrared wavelengths. In this paper we describe RRL
imaging of the nucleus of NGC 4945 in which we detect H92
emission and resolve the
rotation curve. RRLs have been used to derive mass, density and
ionizing photon fluxes for the ionized gas in other galaxies, from
which star formation rates have been derived. The diagnostic methods
are described by Anantharamaiah et al. (2000, and
references therein), and in detail by Mohan (2002). The mere
detection of RRL emission requires the presence of thousands
of H II regions or of stimulated emission since a
single H II region like the Orion nebula placed at
that distance would be undetectably weak.
There are now 15 known extragalactic RRL detections (including
NGC 4945), all in bright starburst galaxies, which are listed
by Roy et al. (2008).
Most of those detections resulted from improving the search sensitivity
by a factor of ten during the 1990s and by making surveys of promising
bright candidates using the Very Large Array (VLA) and ATCA. Our
observation of NGC 4945 reported here was the third detection
made in our ATCA survey for H92 emission. The other
two detections from this survey (NGC 3256 and the Circinus
galaxy) were reported by Roy et al. (2005, and 2008).
NGC 4945 has proven to be the strongest known extragalactic
RRL emitter on the sky.
We give velocities in the heliocentric frame using the optical velocity definition throughout this paper.
2 Observations
We observed NGC 4945 with the ATCA simultaneously in
the lines H91
and H92
with two orthogonal linear polarizations. The observing parameters and
results are summarized in Table 1.
Calibration and imaging were done using the AIPS software using standard methods. The flux-density scale assumed that PKS B1934-638 had a flux density of 2.99 Jy at 8295 MHz and 2.87 Jy at 8570 MHz, based on the Baars et al. (1977) flux-density scale. A phase calibrator was observed every half hour to correct the instrumental phase response. A bandpass calibrator was observed every few hours for correcting the instrumental frequency response (bandpass). Phase corrections obtained from self calibration of the continuum source were applied to the spectral line data. Continuum emission was subtracted from the line data using the method UVLSF (Cornwell et al. 1992) in which the continuum is determined for each baseline by a linear fit to the spectrum. The final continuum and line images were made using natural weighting of the (u, v) data to achieve maximum possible signal-to-noise ratio.
Uncertainties on the flux densities have an 11% rms random multiplicative component due to flux-density bootstrapping and atmospheric opacity, a 0.21 mJy rms random additive component due to thermal noise in a 1 MHz channel and a systematic multiplicative component of 11% rms due mainly to the uncertainty in the Baars et al. flux-density scale.
Table 1: ATCA Observational Parameters and Results for NGC 4945.
3 Results
The ATCA continuum and line images, and integrated spectrum are shown in Fig. 1, and the velocity field and position-velocity diagram are shown in Fig. 2.
![]() |
Figure 1:
Top: ATCA 8.3 GHz + 8.6 GHz continuum image
with uniform weight
of NGC 4945. Middle: ATCA uniformly
weighted zeroth-moment image of H91 |
Open with DEXTER |
The continuum image shows a well resolved structure extended along the
plane of the galaxy in position angle
.
Line emission was detected from the nuclear region with a well resolved
structure that is extended along the plane of the galaxy like the
continuum, though more compact and clumpy than the continuum emission.
The line emitting region has a total deconvolved
extent of 8.1''
1.7'' (150 pc
30 pc). The peak of line emission is coincident with the peak
of continuum emission within 1''. The total area of line
emission is 1.5 times the naturally-weighted beam area
(8.6 times the uniformly-weighted beam area), or
4500 pc2. The H91
+ H92
spectrum integrated over the line emitting region shows a strong
line detection with significance of 52
,
with centroid at 581 km s-1 in
good agreement with the CO systemic velocity of
585 km s-1 (Chou
et al. 2007)
and with the velocity of the HI absorption towards the nucleus
of 585 km s-1, though
significantly higher than the HI systemic velocity inferred
from the whole galaxy of 557 km s-1
(Ott et al. 2001).
The line FWHM is 220 km s-1
after deconvolving the instrumental velocity resolution
of 42 km s-1.
![]() |
Figure 2:
Top: ATCA first-moment image of H91 |
Open with DEXTER |
The position-velocity diagram (Fig. 2) shows a well resolved symmetric structure indicating undisturbed rotation with a flat rotation curve extending in close to the nucleus. The rotation curve is modelled in Sect. 5.
4 Modelling line formation in the ionized gas
Two types of models have been considered for the RRL emission from the nuclei of external galaxies: one based on a uniform slab of ionized gas and the other based on a collection of compact H II regions. Such models have been discussed by Anantharamaiah et al. (2000, and references therein), and are documented in detail by Mohan (2002). These models take as constraints the integrated RRL strength at one or more frequencies, the radio continuum spectrum, and the geometry of the line emitting region.
For modelling the RRL emission from NGC 4945 we used the
integrated line strength (17.8 mJy), line width
(280 km s-1), size of the
line-emitting region (equivalent to a 70 pc diameter sphere),
continuum emission (1424 mJy), and spectral
index (-0.75) to constrain conditions in the ionized gas.
Using the collection of H II regions model, models
with 10 to 300 H II regions, all
with K,
cm-3
to 104 cm-3 and
a
total effective diameter of the line-emitting gas of 2 pc to
100 pc produced good matches to the line and continuum
emission. Parameters derived for typical allowed models are given in
Table 2.
Table 2:
Derived properties for NGC 4945, using model results for
K.
The range of possible filling factors can be estimated by
comparing the total volume of the line-emitting gas (8 pc3
to 1 106 pc3
derived from the total effective diameter) to the volume of the
line-emitting region (5.3
105 pc3,
assuming cylindrical
geometry with diameter of 150 pc and height of
30 pc). The range of possible values is then 2
10-5
to 1.
The inferred mass of ionized gas is 2
to 6
,
depending on the model conditions, which requires a Lyman
continuum flux of 6
1052 s-1 to
3
1053 s-1 to
maintain the ionization. This flux is equivalent to the Lyman continuum
output of 2000 to 10 000 stars of
type O5, which infers a star-formation rate of 2
yr-1
to 8
yr-1
when averaged over the lifetime of OB stars.
This can be compared to star formation rates derived from
other indicators following Hopkins et al. (2003). Taking the
peak 1.4 GHz flux density of 4.0 Jy beam-1
in the 30''
18'' beam of the ATCA at the nucleus of NGC 4945
(Elmouttie et al. 1997)
yields a 1.4 GHz luminosity of 7.0
1021 W m-2
and a corresponding star formation rate of 3.9
yr-1.
This can be attributed to the central 1.8'' (30 pc)
diameter region following the argument of Elmouttie et al. (1997) based on
the high resolution 8.4 GHz image. The IRAS
m and
m flux
densities yield a far-infrared (FIR) star formation rate of
4.1
yr-1.
These estimates agree well with the star formation rate estimated from
the RRL emission of 2
yr-1
to 8
yr-1.
The H
and U-band-based star formation rates were not
estimated due to the extreme absorption in the optical band. The
supernova rate estimated by Forbes & Norris (1998) from the
4.8 GHz flux density is 0.23 yr-1,
and from the [Fe II] flux is much lower at
0.005 yr-1. These can be converted into
star-formation rates following Fukugita & Kawasaki (2003), yielding
rates of 19
yr-1
and 0.4
yr-1.
These bracket the range we estimated from the RRL emission.
Our RRL-based SFR estimate assumed no dust absorption between
the OB stars and the H II regions producing
the RRL emission. The effect of dust can be estimated by
comparing the bolometric luminosity (
)
from the stellar population needed to ionize the RRL-emitting gas to
the observed FIR output (
)
from the region. If ionizing photons are absorbed by dust
before causing ionization, then
will be less than
.
The ratio
/
thus gives the ratio of the
number of photons absorbed by
the gas to the number of photons absorbed by the dust. We derived
by taking our RRL-based
and dividing by the
/
ratio of 0.29 calculated for a
stellar population with Salpeter IMF,
lower-mass cutoff of
and
upper-mass cutoff of
,
and using the mass-luminosity relation for OB stars in
Table 5 of Vacca et al. (1996). This
resulted in
to 5.7
,
compared to
from region of 1.6
(Brock et al. 1988).
The ratio
/
shows that the gas volume in
the nuclear region is illuminated by
ionizing radiation that represents only 7.5% to 35% of the ionizing
flux from the stars needed to power the FIR output; the rest,
we assume, is absorbed by dust within the
H II regions. Converting to opacity using
Fig. 2
of Petrosian (1972)
yields dust opacities of 1.3 neper to 3.4 neper
within the H II regions.
Given so much absorption, it is unexpected that the
star-formation rate inferred from the RRL emission, which assumed zero
absorption, should agree so well with that inferred from
.
An important assumption made while deriving the RRL-based star
formation rate from
the present inferred number of OB stars was that the stars are
being formed at a steady rate that would maintain the present numbers.
5 Dynamical modelling of the ionized gas
We used the rotation curve to constrain the gas kinematics by fitting to the data a simple model consisting of a set of rings, coplanar, edge-on, with an initially flat rotation curve. The brightness of each ring was determined by deprojecting the observed zeroth-moment image to derive the radial distribution of the line intensity. The radial distribution showed a central peak and a ring of emission 2.5'' (50 pc) from the nucleus.
We refined the model iteratively, varying the systemic velocity, rotation velocity, and velocity dispersion until we achieved a reasonably close match to the data, and found no reason to depart from a simple flat rotation curve. The biggest residual between the model and data was at the nucleus and a smaller one at the 50 pc ring. To improve the model, we refined the radial profile of line emission strength at the nucleus and the ring. The data required us to boost the central peak to 25 times the brightness of the more extended emission and to place that gas at the systemic velocity. This could correspond to gas on circular orbits moving transverse to the line of sight in front of the nucleus. The data also required us to put half that strength in the innermost ring (0.4'' = 7.6 pc) with our best-fit rotation velocity. For the 50 pc ring, we tweaked the radius and brightness. The result was a very good match to the observed data, with residuals at 1.5 times the rms noise in the position-velocity diagram.
The final model had a flat rotation curve with
km s-1,
=
120 km s-1, and
=
15 km s-1. We did not need
to invoke a bar or radial motion, though the data do not strongly
exclude either.
The rotation velocity of 120 km s-1
within the central 1'' (19 pc) radius infers an
enclosed mass of 3
.
The water masers (Greenhill et al. 1997) infer an
enclosed mass of 1
within 0.3 pc radius,
and so most of the 3
is extended between
0.3 pc and 19 pc radius of the
nucleus. The average surface density within the central 19 pc
is
pc-2,
which exceeds the threshold gas surface density for star-formation of
(3 to 10)
pc-2
(Kennicutt 1989)
by four orders of magnitude.
The 50 pc ring might be a Lindblad resonance, which would infer a bar and then our assumption of pure rotation would become invalid. The ring and nucleus may be seen also in CO spectra (Whiteoak & Wilson 1990; Dahlem et al. 1993), and in OH absorption (Whiteoak & Wilson 1990).
The RRL emission contains a bright peak on the nucleus at the systemic velocity. The continuum emission is smooth and does not show a similar peak and so we speculate that this might be stimulated emission along the line of sight for which the gas velocity is transverse to the line of sight and so has a long maser gain path.
![]() |
Figure 3:
H91 |
Open with DEXTER |
6 Comparison with literature
H I kinematics were studied by Ott et al. (2001) with the ATCA
at 3.5'' resolution. They saw HI in emission over the disk,
with an extent of 22'
4' (43 kpc
7.8 kpc)
and in absorption towards the nuclear continuum emission. They see an
asymmetry in the large-scale H I rotation field, which they
argue is due to the bar potential. The H I absorption towards
the nucleus shows a uniform velocity gradient across the nucleus
indicating solid-body rotation peaking at
km s-1
either side of the systemic velocity out to
either side of the centre, with a sign of flattening beyond
to
.
For comparison, our H91
+ H92
rotation curve remained flat at
km s-1
either side of systemic all the way into the central 1''. This
difference could be due to the two-times lower spatial resolution of
the HI observation compared to that of the H91
+
H92
observation (3.5'' compared to 1.4''). Were we to
smooth our position-velocity diagram, or had the noise in our
position-velocity diagram been a little higher, the small wings that
indicate a flat rotation curve would be less visible, giving the
impression of solid-body rotation. Also, in edge-on
systems with a flat rotation curve, one finds the density distribution
often gives the appearance of solid-body rotation.
Rotation of the larger-scale disk (
)
derived from CO(1-0) observations with SEST by Dahlem et al. (1993) at
43'' resolution shows evidence for gas inflow that is
consistent with the inflow model proposed by Ables et al. (1987) from
H I. This was seen also in CO(2-1) by Ott et al. (2001) with SEST at
23'' resolution. However, inflow was not seen in the H91
+
H92
dynamics within 4'' of the nucleus.
Isovelocity contours of CO by Chou et al. (2007) show an
S-shaped asymmetry due to a bar potential at distances larger than 5''
from the nucleus, and show simple circular rotation within that radius.
The isovelocity contours of the H91 + H92
radio recombination line from the ionized gas component seen within
4'' radius of the nucleus likewise shows simple circular
rotation with no asymmetry, consistent with the CO on
that scale.
The rotation of the nuclear disk derived from CO(3-2)
observations with SEST by Mauersberger et al. (1996) with
15'' resolution and with SMA by Chou et al. (2007) at resolution
show that within
of the nucleus, the velocity changes from 390 km s-1
to 730 km s-1, which is
broader than the velocity range of the H91
+ H92
emission (460 km s-1 to
700 km s-1) over the same
region.
The CO emission seen by both Mauersberger et al. (1996) and
Chou et al. (2007)
shows two local intensity peaks, with a radius of 6''
to 8'' from the nucleus. The H91 + H92
line emission radial distribution showed a central peak and a ring of
emission with radius 2.5'' (50 pc) from the nucleus.
Thus, the ring in ionized gas lies within the ring in the molecular
gas, consistent with the illustration by Spoon et al. (2003)
Fig. 8.
The dynamical mass within the central 100 pc radius
derived from the CO by Mauersberger et al. (1996) is
8
and the dynamical mass within
0.3 pc radius derived from the
water masers by Greenhill et al. (1997) is
1.0
.
The dynamical mass enclosed within the central 1''
(19 pc) radius derived from the H91
+ H92
line is 3
,
which lies between the masses within the larger and smaller radii,
as expected.
Rotation of the nuclear ionized gas is traced also by Pf
by Spoon et al. (2003).
They find ionized gas within the central
around the nucleus from the same region in which we find H91
+
H92
emission. Comparing their position-velocity diagram of Pf
(their Fig. 7) to our position-velocity diagram of H91
+
H92
(our Fig. 2),
the diagrams look essentially the same, having the same central
velocity gradient, the same rotation rate, the same systemic velocity,
and the same overall extent. Thus, both emission lines are tracing the
same gas, as expected. Spoon et al. (2003) propose a
geometry in which the HI Pf
originates from the central ionized and rotating disk (their
Fig. 8). The H91
+
H92
kinematics supports this picture. Similar rotation curves are seen in
HCN, HNC, and HCO+ by Cunningham &
Whiteoak (2005),
though the molecular material is seen more strongly concentrated into
an edge-on ring with weak emission or absorption towards the nucleus,
unlike the ionized gas which is always in emission.
Chou et al. (2007)
identify a kinematically decoupled component in their CO
position-velocity diagrams along the minor axis, showing broad velocity
width at or near the disk centre, as a strip in velocity at the origin
of the position-velocity diagram spanning 400 km s-1
to 750 km s-1 in their
Fig. 5. They identify this gas as a promising candidate for
the circumnuclear molecular torus invoked by AGN unification models.
For comparison, we show the position-velocity diagram in H91 +
H92
along the minor axis in Fig. 3.
The H91
+
H92
shows a broad velocity range at the position of the nucleus
(panel 252), spanning 430 km s-1
to 750 km s-1, similar to that
seen in CO by Chou et al. (2007).
This broad velocity width in the centre of our position-velocity
diagram along the minor axis underlines that the rotation curve is
flat; at this position close to the centre, one sees the full receding
and the full approaching velocities, which can be the case only if
the rotation curve rises very quickly in the central beam and stays
flat further out, as seen the other panels of Fig. 3. The gas with broad
velocity width at the nucleus was included when we fit the kinematics
and we obtained a good fit with a simple flat rotation curve extending
into the central ring; we did not need to introduce any departure from
the simple flat-rotation-curve
model.
7 Conclusions
We have discovered H91
+ H92
lines in emission in NGC 4945 with flux density of
17.8 mJy using the ATCA, making NGC 4945 the
brightest known extragalactic RRL source. The detected line strength
infers an ionized gas mass of (2 to 6)
and a corresponding star
formation rate of 2
yr-1
to 8
yr-1
depending on the model
conditions. The star formation rate estimated from the RRL detection in
NGC 4945 agrees well with rates estimated from radio and FIR
luminosities using previously-calibrated relations.
The rotation curve was found to be flat into the
central 1'' with
km s-1,
=
120 km s-1. We found no need
to invoke a bar or radial motion and no indication of a kinematically
decoupled component.
Future observations at high frequencies where RRLs are stronger and resolution is higher will provide measurements of multiple transitions to provide better constraints on the gas conditions.
Since RRLs occur over a wide range of wavelengths, a given array can provide higher resolution for studying the dynamics of ionized gas than can be achieved for studies of neutral hydrogen.
Followup with the Atacama Large Millimeter/submillimeter Array of RRLs in NGC 4945 and others should be rewarding, giving good SNR and high spatial resolution to see finer details in the kinematics and of the possible circumnuclear torus.
AcknowledgementsThe Australia Telescope Compact Array is part of the Australia Telescope, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
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Footnotes
All Tables
Table 1: ATCA Observational Parameters and Results for NGC 4945.
Table 2:
Derived properties for NGC 4945, using model results for
K.
All Figures
![]() |
Figure 1:
Top: ATCA 8.3 GHz + 8.6 GHz continuum image
with uniform weight
of NGC 4945. Middle: ATCA uniformly
weighted zeroth-moment image of H91 |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Top: ATCA first-moment image of H91 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
H91 |
Open with DEXTER | |
In the text |
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