WATER MASERS IN STAR-FORMING GALAXIES
Yoshiaki Hagiwara
ASTRON, Postbus 2, Dwingeloo, 7990AA, The Netherlands,
National Astronomical Observatory Japan, 2-21-1 Osawa, Mitaka Tokyo Japan. 181-8588
[email protected]
Abstract
1.
The water masers in starburst or star-forming galaxies have not yet been intensively studied, and could be a promising tool for diagnosing star-forming activity
in galaxies. In this contribution, a couple of recent results of Very Large Array
(VLA) observations are presented, and general properties of these masers are
briefly discussed.
Introduction
Milli-arcsec imaging of H O masers using Very Long Baseline Interfeormetry (VLBI) has proved itself that high-luminosity water masers are powerful
tools for investigating innermost regions of Active Galactic Nucleus (AGN).
While the studies of high-luminosity ( 10 ) water masers shed light on
a sub-pc-scale rotating disk around a super massive black hole, only a little
has been known on low-luminosity water masers ( 1 ). It is important to
study those masers at high angular resolution, as they could advance the study
of eccentric extra-galactic star-formation.
2.
Observations
22 GHz spectral-line observations using the NRAO Very large Array (VLA)
were carried out for mapping H O maser emission (6 -5 transition) towards
three galaxies M 82, M 51, and NGC 405 between 2002 and 2003. The velocity
resolutions of these observations were 1.3 or 5.3 km s per spectral channel.
The typical angular resolution of these observations was 0.1 arcsec using the
VLA A-Configuration.
3.
Results and Discussion
M 82 A nearby starburst galaxy. Weak H O maser emission with L 0.01–0.1 L is known in the starburst galaxy M 82 (e.g. Baudry et al. 1994).
Several H O maser components were detected at 1 arcsec resolution in the
2
VLA observations (Baudry & Bruillet 1996). New observations in this paper
were motivated by an idea to investigate the association of the masers with radio sources such as a nucleus, jet, compact HII regions, supernova remnants
(SNR), or whatever. The velocity coverage was V 70 to 150 km s , that
is blue-shifted w.r.t. the systemic velocity of V =203 km s .
In Fig.1, the positions of the four detected H O masers are overlaid on the
22 GHz continuum, all of them remain unresolved at 0.1 arcsec. The detections of S1 and S2 were already reported in Baudry & Bruillet (1996), while
two masers at N1 and N2 are newly detected. S1 coincides with a hyper compact HII region, while no continuum emission has been found near S2, N1 or
N2. Follow-up MERLIN observations barely resolved the S2 maser into two
clusters with an angular separation of 0.02 arcsec, or 0.35 pc.
What is the maser in M 82 ?. The location of the S2 maser approximately
coincides with the inner edge of the CO (1-0) super-bubble (Matsushita et al.
2000). The velocity range of the super-bubble (V = 118–212 km s ) is
similar to that of the maser (V 85–115 km s ). The extent of CO(2-1)
molecular clouds and the velocities of the maser are also consistent (Weiss et
al. 2001). However, the Galactic molecular outflows are observed using VLBI
in H O masers typically at 10 AU scales, which is at least 100 times smaller
than the M 82 masers measured with VLA in this paper: The masers would
not be resolved at this scale by analogy with the case of Young Stellar Object
(YSO) in our Galaxy. Water masers in our Galaxy generally indicate sites
of star-formation and appear in some stages of evolution of stars. I conclude
that these masers (at S2, N1, or N2) without accompanying any compact HII
region appeared at the early stage of star-formation and are most likely to be
associated with an extragalactic YSO, such as a molecular outflow.
M 51
Fig.2 displays the unresolved maser emission towards two different positions in the star-forming galaxy M 51, one of them is about 0.1 arcsec (5 pc; d
= 9.6 Mpc) north of an 8.4 GHz continuum nucleus and the other is 0.6 arcsec (30 pc) north-west of the nucleus. The former, red-shifted by 100 km
s w.r.t. the systemic velocity, shows a weak velocity gradient nearly along
the axis of the radio continuum structure. The luminosity of the blue-shifted
emission is small, 1 L and not associated with any known radio sources in the
galaxy, suggesting that AGN-activity does not give rise to the maser. Whether
or not the red-shifted maser is related to the weak AGN activity in the galaxy
is of great interest. The velocity gradient indicates the kinematics near the
nucleus, that could be a part of a rotating maser disk.
3
Water Masers in Star-Forming Galaxies
M82, 22 GHz Radio Continuum
IR
Hard−Xray
IR
S2
N2
S1
N1
Compact HII Region
41.95+57.5
180
S1
50
140
30
120
20
100
MilliJY
MilliJY
S2
160
40
10
80
60
0
40
−10
20
−20
0
−30
70
80
90
100
110
Vlsr (km/s)
120
130
140
−20
150
70
80
90
100
110
Vlsr (km/s)
120
130
140
150
90
100
110
Vlsr (km/s)
120
130
140
150
30
50
N1
25
N2
40
20
30
20
10
MilliJY
MilliJY
15
5
10
0
0
−5
−10
−10
−20
70
80
90
100
110
Vlsr (km/s)
120
130
140
150
70
80
The 22 GHz continuum (grey-scaled) superposed on water masers
(crosses) at S1, S2, N1, and N2, infrared (IR) peaks (stars) (Dietz et al. 1986)
and a hard X-ray point (triangle) (Kaaret et al. 2001). The water maser spectra
obtained by the VLA towards four different locations (S1, S2, N1, and N2) are
displayed below.
Figure 1.
4
Plot file version 5 created 04-JAN-2001 16:47:42
M51 IPOL 8460.100 MHZ M51-C01.ICLN.1
8
6
150
4
MilliJY
47 11 44
PLot file version 30 created 07-SEP-2004 16:15:14
CONT: 13299+47 IPOL TEST UN.MOM1.1
2
47 11 42.95
0
43
-2
380
42
400
420
440
460
Kilo VELO-LSR
480
500
42.90
520
42.85
DECLINATION (J2000)
41
50
50
40
40
30
39
555 km/s
560 km/s
42.80
0
565 km/s
20
42.70
10
38
565 km/s
42.75
MilliJY
DECLINATION (J2000)
100
0
500
37
53.0
52.9
52.8
52.7
52.6
RIGHT ASCENSION (J2000)
Grey scale flux range= -48.2 155.1 MicroJY/BEAM
Peak contour flux = 1.5510E-04 JY/BEAM
Levs = 5.000E-06 * (2, 4, 8, 12, 16, 20, 24, 28,
32, 36, 40)
520
540
560
580
Kilo VELO-LSR
600
620
640
42.65
13 29 53.1
!#"%$ :
13 29 52.720
52.715
52.710
52.705
52.700
RIGHT ASCENSION (J2000)
Cont peak flux = 5.9160E+05 M/S
Levs = 1.000E+03 * (545, 550, 555, 560, 565)
3.4-cm VLA continuum map of the nuclear region of M 51
(Bradley et al. 2004) and positions of blue- (upper) and red-shifted (lower)
masers. &('!)+*,$ : Velocity contour map of the red-shifted masers.
Figure 2.
5
Water Masers in Star-Forming Galaxies
45
40
35
30
MilliJY
25
20
15
10
5
0
-5
640
660
680
700
720
Kilo VELO-LSR
740
760
780
Figure 3. VLA spectra of water maser towards a center of NGC 4051 (Vsys =
730 km s )
NGC 4051
NGC 4051 is a narrow-line Seyfert 1 galaxy (NLS1) which exhibits ’broad’
emission line (FWHM 2000 km s ), that is narrower than the one typically
observed at V(FWHM)=1500 - 10000 km/s in a broad-line Seyfert 1 galaxy
(BLS1). The reason for the unusual line-width was understood that a part of the
broad-line region (BLR) producing the broad emission line is obscured from
the direct view in NLS1. Nowadays, it is explained that the narrower emission lines result from smaller black hole masses that are caused by smaller
Keplerian rotation velocities in BLR clouds (e.g. Boller et al. 1996). The
Doppler-shifted maser emission in NGC 4051 appears to straddle symmetrically maser lines lying near the systemic velocity of the galaxy (Hagiwara et
al. 2003). VLA spectrum in Fig.3 covers all known velocity components in the
range V = 645 – 770 km s , the locations of these components are confined to within about 0.1 arcsec (5 pc; d = 9.7 Mpc). The maser luminosity is
1 L , and the maser is possibly a star-forming maser. However, it is possible
that the low-luminosity might be due to the low-gain maser amplification, resulting from a less edge-on disk-like structure around an AGN, and the smaller
Keplerian rotation velocity of this galaxy is due to the smaller black hole mass
in its nucleus.
6
Table1: Low-luminosity extragalactic H O masers
.
Galaxy
d(Mpc)
L
M 33
IC 342
IC 10
NGC 253
M 82
M 51
NGC 4051
NGC 2146
0.72
1.8
2.0
2.5
3.6
9.6
9.7
14.5
0.2
0.01
0.4
1
0.1-1
1-5
2
0.5-5
4.
Association
HIIR
YSO?
HIIR
?
HIIR and YSO?
AGN or YSO?
AGN?
HIIR
Reference
Greenhill et al. (1990)
Tarchi et al. (2002a)
Greenhill et al. (1990)
Henkel et al. (2004)
This paper
This paper
Hagiwara et al. (2003)
Tarchi et al. (2002b)
Conclusion
As shown in Table 1, low-luminosity H O masers are identified in different
physical environments related to star-formation such as compact HII regions,
and YSO. Many of the host galaxies of the masers house star-forming regions
in the galaxy, or exhibit starburst-activity, but do not contain an AGN. These
H O masers may reveal new stellar phenomena relevant to the earliest stages
of extra-galactic star-forming activity.
References
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