Jet-disturbed molecular gas near the Seyfert 2 nucleus in M51
Astronomy & Astrophysics manuscript no. m51pdbi-main c© ESO 2018
October 28, 2018
Letter to the Editor
Jet-disturbed molecular gas near the Seyfert 2 nucleus in M51
S. Matsushita, S. Muller, and J. Lim
Academia Sinica, Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 106, Taiwan, R.O.C.
Preprint online version: October 28, 2018
ABSTRACT
Context. 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.
Aims. 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.
Methods. We carried out CO(2-1) and CO(1-0) line observations of the nuclear region of M51 with the new A configu-
ration of the IRAM Plateau de Bure Interferometer, yielding a spatial resolution lower than 15 pc.
Results. 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.
Key words. galaxies: individual (M51, NGC 5194) – galaxies: ISM – galaxies: Seyfert
1. Introduction
Active Galactic Nuclei (AGNs) are believed to be powered
by gas accretion. This gas is supplied from interstellar mat-
ter in host galaxies, and the gas may form rotationally-
supported structures around the central supermassive black
hole. If they are viewed close to edge-on, they may ob-
scure the central activity from direct view. AGNs can be
categorized as type 1 if seen face-on, and type 2 if seen
edge-on; this explanation is known as a unified model (e.g.
Antonucci & Miller 1985). Indeed, a few hundred pc res-
olution molecular gas imaging toward the central regions
of the Seyfert 2 galaxies NGC 1068 (Planesas et al. 1991;
Jackson et al. 1993) and M51 (Kohno et al. 1996) show
strong peaks at the nuclei with velocity gradients perpen-
dicular to radio jets, which suggest the existence of edge-
on circumnuclear rotating disks. Recent ∼ 50 pc resolu-
tion imaging studies toward NGC 1068 and the Seyfert
1 galaxy NGC 3227 support this view, showing more de-
tailed structures, namely warped disks (Schinnerer et al.
2000a,b). However, observations toward a few low activ-
ity AGN galaxies with < 100 pc resolution show lopsided,
weak, or no molecular gas emission toward the nuclei (e.g.
Garćıa-Burillo et al. 2003, 2005).
M51 (NGC 5194) has also been observed in detail
with molecular lines in the past, since it is one of the
nearest (7.1 Mpc; Takáts & Vinkó 2006) Seyfert galax-
Send offprint requests to: S. Matsushita, e-mail:
satoki@asiaa.sinica.edu.tw
ies. A pair of radio jets emanates from the nucleus and
narrow line regions (NLRs) are associated with the jet
(e.g., Crane & van der Hulst 1992; Grillmair et al. 1997;
Bradley et al. 2004). Interferometric images in molecular
gas show blueshifted emission on the eastern side of the
Seyfert 2 nucleus, and redshifted gas on the western side
(Kohno et al. 1996; Scoville et al. 1998). This shift is al-
most perpendicular to the jet axis, and the estimated col-
umn density is consistent with that estimated from X-ray
absorption toward the nucleus, suggesting that the molecu-
lar gas can be a rotating disk and play an important role in
obscuring the AGN. Interferometric CO(3-2) observations
suggest a velocity gradient along the jet in addition to that
perpendicular to the jet (Matsushita et al. 2004). These re-
sults imply more complicated features than a simple disk
structure. We therefore performed sub-arcsecond resolution
CO(2-1) and CO(1-0) imaging observations of the center of
M51 to study the distribution and kinematics of the molec-
ular gas around the AGN in more detail.
2. Observation and data reduction
We observed CO(2-1) and CO(1-0) simultaneously toward
the nuclear region of M51 using the IRAM Plateau de
Bure Interferometer. The array was in the new A configu-
ration, whose maximum baseline length extends to 760 m.
Observations were carried out on February 4th, 2006. The
system temperatures in DSB at 1 mm were in the range
200-700 K, except for Antenna 6, for which a new genera-
http://arxiv.org/abs/0704.0947v1
2 Matsushita et al.: Jet-disturbed molecular gas near the Sy 2 nucleus in M51
379.0 km/s 399.4 km/s 419.7 km/s
440.0 km/s 460.3 km/s 480.6 km/s
501.0 km/s 521.3 km/s 541.6 km/s
582.2 km/s 602.6 km/s561.9 km/s
2 0 −2 2 0 −22 0 −2
R.A. Offset [arcsec]
Fig. 1. Channel maps of the CO(2-1) line. The contour lev-
els are −3, 3, 5, 7, and 9σ, where 1σ corresponds to 5.2 mJy
beam−1 (= 0.96 K). The cross in each map indicates
the position of the 8.4 GHz radio continuum peak posi-
tion of R.A. = 13h29m52.s7101 and Dec. = 47◦11′42.′′696
(Hagiwara et al. 2001; Bradley et al. 2004). The R.A. and
Dec. offsets are the offsets from the phase tracking center of
R.A. = 13h29m52.s71 and Dec. = 47◦11′42.′′6. The synthe-
sized beam is shown at the bottom-left corner of the first
channel map.
tion receiver gave system temperatures of 150-230 K. Those
in SSB at 3 mm were in the range 140-250 K for Antenna
6, and 220-550 K for other antennas. Four of the corre-
lators were configured to cover a 209 MHz (272 km s−1)
bandwidth for the CO(2-1) line, and a 139 MHz (362 km
s−1) bandwidth for the CO(1-0) line. The remaining four
units of the correlator were configured to cover a 550 MHz
bandwidth for continuum observations and calibration. The
strong quasar 0923+392 was used for the bandpass calibra-
tion, and the quasars 1150+497 and 1418+546 were used
for the phase and amplitude calibrations.
The data were calibrated using GILDAS, and were im-
aged using AIPS. The data were CLEANed with natural
weighting, and the synthesized beam sizes are 0.′′40× 0.′′31
(14 pc × 11 pc) with a position angle (P.A.) of 0◦ and
0.′′85× 0.′′55 (29 pc × 19 pc) with a P.A. of 13◦ for CO(2-1)
and CO(1-0) images, respectively. Fig. 1 shows the channel
maps of CO(2-1) emission with a 20.3 km s−1 velocity res-
olution. The channel maps of CO(1-0) emission show simi-
lar features to that of CO(2-1) emission with lower spatial
resolution. Fig. 2 shows integrated intensity and intensity
weighted mean velocity maps of the CO(2-1) and CO(1-
34 pc
3 2 1 −1 −2 −30
1 20 [Jy/beam km/s]
450 550[km/s]500
3 2 1 0 −1 −2 −3
0.0 0.5 1.0 [Jy/beam km/s]
3 2 1 0 −1 −2 −3
03 12 −1 −2 −3
450 500 550 [km/s]
R.A. Offset [arcsec]
440460
(b) CO(1−0)
R.A. Offset [arcsec]
(d) CO(1−0)
(a) CO(2−1)
Moment 0 Map
(c) CO(2−1)
Moment 1 Map
Moment 0 Map
Moment 1 Map
Fig. 2. Integrated intensity (moment 0) and intensity
weighted velocity (moment 1) maps of the CO(2-1) and
CO(1-0) lines. The synthesized beams are shown at the
bottom-left corner of each image. The crosses and the ref-
erence positions of the R.A. and Dec. offsets are the same
as in Fig. 1. (a) The CO(2-1) moment 0 image. The contour
levels are (1, 3, 5, 7, 9, and 11) × 0.334 Jy beam−1 km s−1
(= 62.0 K km s−1). (b) The CO(1-0) moment 0 image. The
contour levels are (1, 3, 5, and 7) × 0.257 Jy beam−1 km
s−1 (= 50.6 K km s−1). (c) The CO(2-1) moment 1 image.
(d) The CO(1-0) moment 1 image.
0) lines. The noise levels for continuum maps are 1.2 mJy
beam−1 at 1.3 mm and 0.54 mJy beam−1 at 2.6 mm, respec-
tively. We did not detect any significant continuum emission
at either frequency.
3. Results
Most of the CO(2-1) emission is detected within ∼ 1′′
(34 pc) of the center, and is located mainly on the eastern
and western sides of the nucleus. There is also weak emis-
sion located∼ 2.′′7 northwest of the nucleus. The overall dis-
tribution and kinematics are consistent with past observa-
tions (Kohno et al. 1996; Scoville et al. 1998), if we degrade
our image to lower angular resolution; a blueshifted feature
with the average velocity of ∼ 460 km s−1 at the eastern
side of the nucleus, and a redshifted feature with an average
velocity of ∼ 500 km s−1 at the western side (Figs. 1, 2; see
also Fig. 3b). We refer to these main structures with the
same labels as in Scoville et al. (1998) (Fig. 2a).
Our higher resolution images, however, show more com-
plicated structures and kinematics than the previous low
angular resolution observations. Molecular gas on the west-
ern side of the nucleus, S1, is elongated in the north-south
direction and separated into two main peaks (S1a and b).
S1a is located 0.′′9 (30 pc) northwest of the nucleus, and S1b
Matsushita et al.: Jet-disturbed molecular gas near the Sy 2 nucleus in M51 3
is 1.′′0 (34 pc) to the southwest. On the eastern side of the
nucleus, the molecular gas has an intensity peak 0.′′6 (20 pc)
to the northeast (labeled S2), which is located closer to the
nucleus in projected distance than that of S1a/b.
The feature S1 shows a clear velocity gradient along
the north-south direction, which is shown in Fig. 2(c) and
also in the position-velocity (PV) diagram (Fig. 3a). This
gradient was previously suggested by the CO(3-2) data
(Matsushita et al. 2004), but the magnitude of the velocity
gradient is different. The computation of the magnitude of
the velocity gradient is similar to that used for the CO(3-2)
data. The fitting result indicates a velocity gradient within
S1 of 2.2 ± 0.3 km s−1 pc−1, which is larger than that re-
ported previously, 0.77± 0.01 km s−1 pc−1 (the value has
been modified by the different distance of the galaxy used).
This difference is partially due to the larger beam size of
the previous result; the CO(3-2) data set has a beam size
of 3.′′9× 1.′′6 with a P.A. of 146◦, and the velocities of S2/C
and S3 contaminate that of S1.
The CO(1-0) maps show very similar molecular gas
distribution and kinematics as those in CO(2-1) maps
(Fig. 2b,d). Only the western emission was detected in
previous observations (Aalto et al. 1999; Sakamoto et al.
1999), but our map clearly shows the emission from both
side of the nucleus.
In addition to the previously known features, our CO(2-
1) image also shows a weak emission near the nucleus with
a structure elongated in the northeast-southwest direction
(feature C in Fig. 2a). This structure could be a part of
S2, since the velocity map (Fig. 2c) and the PV diagram
(Fig. 3b) show a smooth velocity gradient, although most
of the emission in C comes from only one velocity channel
(419.7 km s−1 map in Fig. 1). The velocity gradient between
S2 and C is in an opposite sense to that previously seen
with the lower angular resolution observations mentioned
above. This structure is not detected in the CO(1-0) line,
but a hint of a velocity gradient can be seen in Fig. 2d.
The total CO(2-1) integrated intensity of S1, S2, and C
is 25.01 Jy km s−1, and that of S1 and S2 in Scoville et al.
(1998) is 33.44 Jy km s−1, so that our data detected 75%
of their intensity. Scoville et al. (1998) detected ∼ 50%
and 20% of the single dish CO(2-1) flux in redshifted and
blueshifted emission, respectively, so that our data recov-
ered ∼ 25% of the single dish flux.
4. Discussion
4.1. Jet-entrained molecular gas
Our molecular gas data show a clear north-south velocity
gradient within the feature S1. We suggested from our pre-
vious study that this velocity gradient may be due to molec-
ular gas entrainment by the radio jet (Matsushita et al.
2004). Here we revisit this possibility with higher spa-
tial and velocity resolution data. Fig. 4 shows our CO(2-
1) image overlaid on the 6 cm radio continuum image
(Crane & van der Hulst 1992). The radio continuum image
shows a compact radio core coincident with the nucleus,
and the southern jet emanating from there (note that the
northern jet is located outside our figure). The CO(2-1)
map clearly shows that S1 is aligned almost parallel to the
jet. In addition, Figs. 2 and 3 show that the velocity gradi-
ent in S1 is also almost parallel to the jet.
R.A. Offset [arcsec]Dec. Offset [arcsec]
1.51.5 1.01.0 0.50.5 0.00.0 −0.5−0.5 −1.0−1.0 −1.5−1.5
620 580
600 560
580 540
560 520
540 500
520 480
500 460
480 440
460 420
440 400
420 380
(a) (b)
Fig. 3. Position-velocity (PV) diagrams of the CO(2-1)
line. The contour levels are 3, 5, 7, and 9σ, where 1σ cor-
responds to 5.2 mJy beam−1 (= 0.96 K). (a) PV diagram
along the north-south elongated S1 feature (P.A. of the cut
is 103◦). The positions for S1a and S1b are shown with
labels. (b) PV diagram along R.A. with the cut through
the S1a, C, and S2 features (P.A. of the cut is 90◦). The
positions for S1a, S2, and C are shown with labels.
The velocity increases from ∼ 480 km s−1 at S1a to
∼ 540 km s−1 south of S1b. This increment is very simi-
lar to that observed in the NLR clouds along the radio jet;
Bradley et al. (2004) measured the velocities and velocity
dispersions of the clouds using the [O III] λ5007 line, and
showed that the velocity of the southern <≃ 1
′′ clouds from
the nucleus are at VLSR ∼ 440− 590 km s
−1 and the veloc-
ity increases as the clouds move away from the nucleus (see
Table 2 and Fig. 9 of their paper)1. This velocity range and
increment are consistent with our data. Furthermore recent
observations of H2O masers toward the nucleus also show
a velocity gradient along the jet with the same sense as our
results (Hagiwara 2007), in addtion to the good correspon-
dance of the velocity range (Hagiwara et al. 2001; Hagiwara
2007; Matsushita et al. 2004). These results suggest that
the molecular gas in S1 (and the NLR clouds and the H2O
masers) is possibly entrained by the radio jet. These results
also suggest that some of the material in NLRs is supplied
from molecular gas close to AGNs.
Another example of jet-entrained neutral gas is found in
the radio galaxy 3C293 (Emont et al. 2005). The velocity
of H I gas in absorption spectra toward the AGN matches
that of ionized gas along kpc-scale radio jets. The spatial
coincidence is not clear, since the spatial resolution of the
H I data is lower (25.′′3 × 11.′′9) than that of the ionized
gas data. Our result is therefore the first possible case of
entrainment of molecular gas by a jet at the scale of ten pc.
The better resolution of our new CO data allows us to
revisit the values of the molecular gas mass, momentum,
and energy of the entrained gas. We derive 6 × 105 M⊙,
8 × 1045 g cm s−1, and 3 × 1052 ergs for these quantities.
These values are about half of the previous values derived
from the CO(3-2) data, mainly due to the larger beam, but
the conclusion is similar; the energy of the entrained gas
could be similar to that of the radio jet (> 6.9× 1051 ergs;
1 We selected the clouds with a velocity dispersion of less than
100 km s−1; Clouds 3, 4, and 4a in Bradley et al. (2004). If we
include all the clouds, the velocity is ∼ 440 − 690 km s−1 with
a range of velocity dispersion of ∼ 25 − 331 km s−1; Clouds 2,
3, 3a, 4, 4a, and 4b.
4 Matsushita et al.: Jet-disturbed molecular gas near the Sy 2 nucleus in M51
47 11 46
52.9 52.7 52.5 52.313 29 53.1
RIGHT ASCENSION (J2000)
Fig. 4. The CO(2-1) integrated intensity image (contours)
overlaid on the VLA 6cm radio continuum image (greyscale;
Crane & van der Hulst 1992). The contour levels, the syn-
thesized beam, and the cross are the same as in Fig. 1.
Crane & van der Hulst 1992), but the momentum is much
larger than that of the jet (2 × 1041 g cm s−1). One way
to explain this discrepancy is through a continuous input
of momentum from the jet (see Matsushita et al. 2004, for
more detail discussions).
4.2. Obscuring material around the Seyfert 2 nucleus
The feature C is located in front of the Seyfert 2 nu-
cleus, and the CO(2-1) intensity is about 62.0 K km s−1
(Fig. 2a). Hence the column density can be calculated as
6.2 × 1021 cm−2 using a CO-to-H2 conversion factor of
1.0 × 1020 cm−2 (K km s−1)−1 (Matsushita et al. 2004)
and assuming a CO(2-1)/(1-0) ratio of unity. This value is
far lower than that derived from the X-ray absorption of
5.6× 1024 cm−2 (Fukazawa et al. 2001). As is mentioned in
Sect.3, the missing flux of our data is ∼ 75%. However, even
if all of this missing flux contributes to obscuring the nu-
clear emission, this large column density difference cannot
be explained. Changing the conversion factor or the ratio
by an order of magnitude also cannot explain this large
difference. One way to reconcile this disparity is to assume
that C is not spatially resovled, in which case the computed
column density is a lower limit. Alternatively, the obscur-
ing material preferentially traced by higher-J CO lines or
denser molecular gas tracers such as HCN may be involved.
The CO(3-2) intensity in brightness temperature scale is
∼ 2 times stronger than that of CO(1-0) (Matsushita et al.
2004), and the HCN(1-0) intensity is also relatively stronger
(HCN/CO ∼ 0.4; Kohno et al. 1996) than normal galaxies.
4.3. Molecular gas at ten pc scale from the Seyfert nucleus
Previous studies suggest that the blue shifted eastern fea-
ture S2 and the red shifted western feature S1 may be the
outer part of a rotating disk as in the AGN unified model.
However, our images show a more complicated nature, and
no clear evidence of simple disk characteristics.
The simplest interpretation is that S1 and S2/C are in-
dependent structures. Since S1 is affected by the jet but
S2/C is not, S1 is expected to be located closer to the nu-
cleus than S2/C, and the projection effect makes the po-
sition of S2/C closer in our images. Alternatively, S2/C
may be close to the nucleus, but the entrained gas has
been already swept away or ionized by the jet. S2/C has
a velocity gradient, and therefore can be interpreted as a
streaming gas, presumably infalling toward the nucleus, as
is observed in the Galactic Center (Lo & Claussen 1983;
Ho et al. 1991).
S1 and S2/C can also be interpreted as a rotating disk
that is largely disturbed by the jet, and only a part remains.
According to the velocity gradient along S2/C, the bluesh-
fited gas is expected at S1, which is the opposite sense to
the previous suggestion, but the gas shows no signs of it due
to the jet entrainment. This is possible from the timescale
point of view; under this interpretation, S1 should have
a blueshifted rotation velocity of ∼ 380 km s−1 based on
the velocity gradient in S2/C. S1 has a velocity ∼ 150 km
s−1 higher than the expected rotational velocity, and we
assume that this is the entrained velocity. In this case, it
takes 2 × 105 years to be elongated along the jet by ∼ 1′′
or 34 pc. On the other hand, the rotation timescale at this
radius is about 2× 106 years, an order of magnitude longer
timescale. The rotating disk can therefore be locally dis-
turbed by the jet.
However, the above two explanations have difficulty in
explaining optical images of the nucleus; the Hubble Space
Telescope images show “X” shaped dark lanes in front of the
nucleus (Grillmair et al. 1997), suggesting the existence of a
warped disk or two rings with one tilted far from another.
An alternative explanation of the dark lanes is that, as
previously proposed, there is a rotating edge-on ring with
S2 as blueshifted gas and S1a as redshifted gas. In this case,
the feature C can be the counterpart of another dark lane,
which runs northeast-southwest, although C has to be a
counter-rotating or Keplarian rotating disk to explain the
opposite sense of the velocity gradient to that of the S1a/S2
(Sect. 3). This configuration explains the “X” shape, but
has a rather complicated configuration, and it is difficult to
explain why the inner disk C is not disturbed by the jet.
We imaged the nuclear region of the Seyfert 2 galaxy
M51 at ∼ 10 pc resolution, and we see no clear evidence of
a circumnuclear rotating molecular gas disk as previously
suggested. The molecular gas along the radio jet is most
likely entrained by the jet. The explanations for other gas
components are speculative, possibly involving a circumnu-
clear rotating disk or streaming gas.
Acknowledgements. We thank Arancha Castro-Carrizo and the
IRAM staff for the new A configuration observations. We also thank
the anonymous referee for helpful comments. IRAM is supported by
INSU/CNRS (France), MPG (Germany) and IGN (Spain). This work
is supported by the National Science Council (NSC) of Taiwan, NSC
95-2112-M-001-023.
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	Introduction
	Observation and data reduction
	Results
	Discussion
	Jet-entrained molecular gas
	Obscuring material around the Seyfert 2 nucleus
	Molecular gas at ten pc scale from the Seyfert nucleus
