A&A 462, L27-L30 (2007)
DOI: 10.1051/0004-6361:20066711
LETTER TO THE EDITOR
E. Schinnerer1 - T. Böker2 - E. Emsellem3 - D. Downes4
1 - Max-Planck-Institut für Astronomie, Königstuhl 17,
69117 Heidelberg, Germany
2 -
European Space Agency, Dept. RSSD, Keplerlaan 1, 2200 AG
Noordwijk, The Netherlands
3 -
CRAL-Observatoire, 9 avenue Charles André, 69231 Saint
Genis Laval, France
4 -
Institut de RadioAstronomie Millimétrique, 300 rue de la
Piscine, Domaine Universitaire, 38406 Saint Martin d'Hères,
France
Received 7 November 2006 / Accepted 5 December 2006
Abstract
We have used the new extended A configuration of the
IRAM Plateau de Bure interferometer to study the dense molecular
gas in the nucleus of the nearby spiral galaxy NGC 6946 at
unprecedented spatial resolution in the HCN(1-0) and 12CO(2-1) lines. The gas distribution in the central 50 pc has been
resolved and is consistent with a gas ring or spiral driven by
the inner 400 pc long stellar bar. For the first time, it is
possible to directly compare the location of (dense) giant
molecular clouds with that of (optically) visible HII regions in
space-based images. We use the 3 mm continuum and the HCN emission to estimate in the central
50 pc the star formation
rates in young clusters that are still embedded in their parent
clouds and hence are missed in optical and near-IR surveys of
star formation. The amount of embedded star formation is about
1.6 times as high as that measured from HII regions alone, and
appears roughly evenly split between ongoing dust-obscured star
formation and very young giant molecular cloud cores that are
just beginning to form stars. The build-up of central mass seems
to have continued over the past 10 Myr, to have occurred
in an extended (albeit small) volume around the nucleus, and to
be closely related to the presence of an inner bar.
Key words: galaxies: nuclei - galaxies: ISM - galaxies: kinematics and dynamics - galaxies: individual: NGC 6946
The nearby (D = 5.5 ,
= 27
)
Scd spiral galaxy NGC 6946
is an ideal laboratory to study the causes (and consequences) of
intense star formation in the vicinity of a galaxy nucleus, although
its low galactic latitude hampers studies at optical and shorter
wavelengths. Within its central
,
NGC 6946 is currently
undergoing an intense burst of star formation, as indicated by strong
far-infrared emission (Devereux & Young 1993) and the presence of numerous
hydrogen recombination lines in its nuclear spectrum (Engelbracht et al. 1996).
The proximity of NGC 6946, combined with recent technological
advances in mm-interferometers which now routinely reach sub-arcsec
resolution, enables a detailed study of the interplay between
infalling dense molecular gas, active star formation, and the
energetic feedback from young massive stars on scales of individual
giant molecular clouds (GMCs). We have recently undertaken such a study which has shown that NGC 6946 appears to be a "textbook case''
for molecular gas responding to the gravitational potential of a small-scale stellar bar (Schinnerer et al. 2006, hereafter Paper I). The gas
flows inward along an S-shaped spiral structure and accumulates in a massive nuclear clump with a size of about
(
). This
clump contains about 1.6
of molecular gas, and
appears to completely obscure the very center of NGC 6946.
In this letter, we describe results from recent mm-observations
obtained with the new, expanded baselines of the IRAM Plateau de Bure
interferometer (PdBI) which yield a spatial resolution of about
,
and thus allow a direct comparison with the best available
(space-based) optical and/or near-infrared maps. The new PdBI observations
were designed to address a number of open issues related to the
connection between active star formation and the gas properties.
Of particular interest is the question how the sites of active star
formation compare to the molecular gas flow, and whether optical or
even near-infrared recombination lines yield a complete picture of the
current star formation rate in the nucleus of NGC 6946. We briefly
describe the observations and data reduction procedures in
Sect. 2, and present the resulting maps of the 12CO(2-1) and
HCN emission in Sect. 3. In Sect. 4, we
analyze the gas dynamics and various star formation tracers, and
briefly discuss the implications of our results.
We used the dual-receiver capability of the PdBI to obtain
simultaneous observations of the HCN(1-0) and 12CO(2-1) lines at
and
,
respectively. The 9 h long observations were performed
on January 15th, 2006, with all six PdBI antennas in their new,
extended A configuration providing baselines between
and
.
For calibration and mapping, we used the standard IRAM
GILDAS software packages CLIC and MAPPING (Guilloteau & Lucas 2000). The phase
center of the observations was at 20
34
52.36
+ 60
09
15.96
(J2000.0). All velocities are reported relative to
.
During the observations, the typical
system temperature in the sideband containing the line of interest at 89 (230) GHz was about 110 (300) K.
The new 12CO(2-1) observations were combined
with our previous dataset (presented in Paper I) that was obtained with
the same spectral setup in the old AB configuration. The data reduction
and imaging closely followed the procedures described in Paper I.
In order to obtain emission line-only maps, we subtracted (in the plane)
continuum maps obtained from line-free channels over the
following spectral ranges: 88.335-88.579 GHz, 88.654-88.704 GHz, and
88.767-88.886 GHz for the
band, and 230.211-230.367 GHz and
230.605-230.761 GHz for the
band.
Using uniform weighting, the resulting CLEAN beams are
(PA
)
and
(PA
)
for the HCN(1-0) and 12CO(2-1) line, respectively. For both
lines, we defined a CLEAN region based on the 0th moment map. The rms
in the
wide channels is 2.3 mJy beam-1 for HCN(1-0) and
4.8 mJy beam-1 for CO(2-1). The rms in the continuum maps is
0.2 (0.21) mJy beam-1 and 0.7 mJy beam-1 for natural
(uniform) weighting at
and
,
respectively. Moment maps
were calculated using the GIPSY task "MOMENTS'' requiring that emission
is present above a 3(2)
clipping level in at least two adjacent channels of the CO (HCN) data cube.
![]() |
Figure 1:
Intensity map of the 12CO(2-1) line emission at
0.4'' ![]() ![]() |
Open with DEXTER |
![]() |
Figure 2:
Comparison of the intensity map of the HCN(1-0) line emission
(color) and the neighboring continuum emission (contours) at
1.0'' ![]() ![]() ![]() |
Open with DEXTER |
As evident from Fig. 1, the new 12CO(2-1) observations clearly resolve the
structure of the inner CO spiral described in Paper I. The south-eastern
gas lane splits into two components while the
north-western lane still appears as a single structure. More
importantly, the structure of the nuclear clump is resolved into two peaks north and south of our derived dynamical center with
extensions to the east and west which connect to the gas lanes. Overall, the molecular gas is in clumps that are mostly
unresolved at our resolution of (11
8) pc2, suggesting that the
GMCs are rather compact, dense, and gravitationally bound.
The emission lines in the central
have complex profiles we
fit with 2- or 3-component Gaussians. They often have two peaks, at
and
relative to the systemic
velocity, with a third peak at
in the inner 0.4'', near
the dynamical center (Fig. 3). The velocity
dispersions are 20 to 30 km s-1 in the two main peaks, and <
in the third, central component. Most of the negative velocity
component is confined to one side of the dynamical minor axis, a result which is consistent with the gas kinematics being dominated by
circular motions. This is also true for the positive velocity
component besides an extra peak about
NE of the dynamical
center, close to the northern CO peak in the integrated map. This is
also illustrated by the double-peaked line profile of the northern
peak (Fig. 3) when compared with the simpler line
profile of the southern peak.
![]() |
Figure 3:
Gas kinematics in the central 3
![]() ![]() |
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The HCN(1-0) emission associated with the nuclear and the southern clump
is shown in Fig. 2. The map contains a total flux of about
4.5 Jy
above a 2
threshold, that is about 8% of the
flux of 11.4 K
measured for the central position by
Gao & Solomon (2004) using the IRAM 30 m telescope (assuming a temperature-to-flux conversion of 4.8 Jy
). 60% of the
recovered flux originates in the nuclear clump. The average line
intensity ratio of HCN(1-0)/12CO(2-1) is 0.02-0.03. Thus, the
non-detection of the northwestern part of the spiral structure is
probably due to our S/N limit combined with the lack of short spacings
in the HCN data. While there is a good correspondence between the HCN
and 12CO(2-1) emission in the southeastern spiral structure, the
integrated HCN emission has an additional peak in the
nuclear disk about
1
west of the dynamical center.
Continuum emission has been detected at
and, tentatively, also
at
.
The integrated
flux is
2.8 mJy, with a peak
flux of 1.9 (1.7) mJy beam-1 corresponding to a 9(8)
detection with natural (uniform) weighting. The position of the peak
is at 20:34:52.307 +60:09:14.37 (J2000) and offset by
0.4
from the dynamical center as derived in Paper I. At the low levels the
continuum shows an extension to the west (Fig. 2)
and is slightly resolved in the uniformly weighted map with a Gauss-fit of size (FWHM)
(PA
). Continuum emission at
is present at the 4
level with a peak flux of 3.0 mJy beam-1 in the naturally
weighted data. A recent 0.3
resolution 6 cm continuum map by
Tsai et al. (2006) shows that the nuclear radio continuum breaks up into
two bright peaks with extensions to the north and west. The
continuum peak is off-set by
to the north of the
bright 6 cm peak (Fig. 4) strongly indicating that the
3 mm and 6 cm continuum emission have a different origin.
![]() |
Figure 4:
Top: HCN(1-0) intensity map (contours)
overlaid on the 12CO(2-1) one (color). The HCN contours start at 50% of
the maximum in steps of 10%. Both beams are shown in the bottom right
corner with the HCN one being the larger one.
Bottom: HST
![]() |
Open with DEXTER |
In Paper I, we showed that the CO spiral can be explained by the
response of the gas flow to the gravitational potential of the inner
NIR bar in the central 300 pc. Our qualitative model requires the
existence of an Inner Lindblad resonance (ILR) at a radius of
due to a
400 pc long secondary bar. In this context, we
expect gas to accumulate at or close to the ILR and, thus, star
formation to occur. Such a mechanism is evoked to explain the
formation of the nuclear star formation rings of kiloparsec diameter
in barred galaxies (e.g. Wada & Habe 1992; Allard et al. 2006). While the CO data of
Paper I showed streaming motions associated with the bar, the exact
distribution of the gas in the nuclear clump could not be resolved.
The CO(2-1) intensity distribution is very reminiscent of the
so-called "twin peaks'' structure seen in lower resolution CO(1-0)
images of barred galaxies (Kenney et al. 1992) and associated with gas
accumulation at the ILR. The two peaks of the negative and positive
velocity component are located at a radius of
(Fig. 3), i.e. within the ILR of our model. Dense gas as
traced by the HCN(1-0) line roughly coincides with those CO(2-1) peaks
(Fig. 4). One way to interpret these gas peaks is as the
contact points of two spiral arms connecting in the central 100 pc. Our
derived line intensity ratios (using the integrated flux and
correcting for different beam sizes) correspond to temperature ratios
of
,
similar to values
found in kpc-sized starburst rings (e.g. Kohno et al. 1999). As the
presence and distribution of dense gas is analogous to what is
observed in kpc-sized starburst rings of large-scale bars, our new
data together with the kinematic analysis (presented in detail in
Paper I) are consistent with the interpretation that the inner bar
drives the CO spiral even within the nuclear clump,
The nature of the additional eastern peak in the positive velocity component of the CO(2-1) emission is not clear. However, the young massive stars present there (Fig. 4) will clearly alter the properties of the (dense) molecular gas. In addition, we start to probe size scales where the size and stability of the GMCs themselves is becoming important. Observations of other molecular gas tracers might therefore be needed to clarify the situation.
The high extinction present in the central 100 pc makes it difficult
to estimate the true star formation rate (SFR). In what follows, we
compare different SFR indicators in the central 3
3
to
develop a sense for how much star formation is still deeply embedded
in dense gas clouds and hence does not reveal itself in observable
emission from hydrogen recombination lines.
The "visible'' SFR can be traced by the NIR Pa line emission
arising from HII regions. Using a Pa
flux of 2.9
from the image by Böker et al. (1999) and assuming an extinction of
= 4.6 mag (Quillen & Yukita 2001), we derive a SFR of 0.13
(see Paper I for
details). This value can be directly compared to the SFR rate derived
from the non-thermal radio continuum which is associated with
synchrotron emission from supernovae explosions
(e.g. Condon 1992). Turner & Ho (1983) find that most (
)
emission at 6 cm in the nuclear region of NGC 6946 is
non-thermal in origin, while the detailed work by Tsai et al. (2006) show
that point-like sources (either due to HII regions or supernova
remnants) contribute about 30% to the total 6 cm flux. We measure a flux density of about 15 mJy in the central 100 pc. Using Eq. (21) of Condon (1992) and assuming that 80% is non-thermal in origin,
we derive a SFR of 0.1
(also taking into account stars below
5
)
in agreement with the SFR derived from the Pa
line.
This agreement might be expected, as the spatial distributions of Pa
and 6 cm radio continuum agree very well (Fig. 4), and it supports our assumption that both are tracers
of stellar populations of similar age.
The millimeter continuum emission, on the other hand, traces free-free
emission from star formation sites that are still embedded in their
parent clouds. This picture is supported by two arguments: first, the
distribution of the mm continuum emission differs from that of the
radio continuum. Secondly, the spectral index
between
the 3 mm and 1 mm continuum flux is about 0.1 (within the
uncertainties on the 1 mm flux), which is expected for free-free
emission. Following the equations summarized in Johnson (2004) we find
an ionizing luminosity
of
,
equivalent to a SFR of 0.1
.
This is an additional
contribution to the total SFR, originating in regions which are not
(yet) seen in hydrogen recombination lines.
Finally, Gao & Solomon (2004) showed that there is a linear relation between
the luminosity of the HCN(1-0) line and the far-infrared luminosity.
This relation appears to not only hold true for entire galaxies, but
also for individual star forming cores within our Galaxy
(Wu et al. 2005). Thus, it seems reasonable to use the HCN(1-0) line flux
as a proxy to estimate the rate of stars that are in the process of
forming. Our measured HCN(1-0) line flux of 2.5 Jy km s-1corresponds to a luminosity of L'(HCN) = 3.1
.
Following Eq. (11) of Gao & Solomon (2004), we obtain another contribution of 0.06
to the
total SFR.
In summary, the different diagnostics (Pa/6 cm, 3 mm, HCN) all
trace star formation, but in different evolutionary stages. This is
also apparent from their different spatial distributions. We find
about twice as much embedded star formation (traced by HCN and 3 mm
continuum) as star formation which has already emerged from its dust
cocoon (traced by Pa
). This implies that
there has been significant massive star formation in the central
100 pc of NGC 6946 over the past >10 Myr. The "older'' star
formation has neither disrupted the GMCs seen in HCN nor has it
prevented the nuclear star formation evident in the 3 mm continuum.
The nucleus of most late-type spiral galaxies is marked by a compact
nuclear star cluster (Böker et al. 2002). In NGC 6946, however, such a nuclear cluster cannot be identified. Possibly, a nuclear cluster
exists, but is obscured by the large amounts of molecular gas and dust
in the central 1''. Another possibility is that we currently witness
the birth of a nuclear cluster. According to our model (Paper I), the
bar-driven CO spiral structure requires the presence of an ILR, which
itself implies a mass concentration on scales of about 20-30 pc
(
). Comparison of the different star formation tracers
shows that stars are being formed in the central 60 pc at a rate of
about 0.1
over the past >10 Myr translating into a total of roughly 10
in stellar mass. The spatial
distribution of the SFR tracers shows that star formation occurs in
distinct areas (clusters, GMCs) spread over the central
(
). This suggests that the build-up of central mass takes place
in an extended (albeit small) volume around the nucleus, rather than
at the very center. This is consistent with the recent identification
of blue, extended disks around a number of nuclear clusters
(Seth et al. 2006). NGC 6946 offers the unique opportunity to study the
processes linked to the build-up of a central mass concentration.
Acknowledgements
E.S. would like to thank Philippe Salome for his help with the IRAM data reduction. We also thank Chao-Wei Tsai and Jean Turner for providing their 6 cm VLA data for comparison and the referee Jonathan Braine for helpful comments.