The distribution of the atomic and ionized hydrogen along the inner spiral arms of M51 are compared. As is the case in M83, the location of both these phases of the interstellar medium with respect to the major dust lanes suggests that molecular hydrogen is dissociated on kpc scales in active star-forming regions, and that this dissociation process may strongly affect the observed morphology of atomic hydrogen in spiral arms

Allen, R. J.

Tilanus, R. P. J.

English

NASA Technical Reports Server

N87-24311
LARGE SCALE DISSOCIATION OF MOLECULAR GAS IN THE SPIRAL ARMS OF M51
R.P.J. Tilanus *_ and R.J. Allen #
#Department of Astronomy
University of Illinois
1011W. Springfield Ave.
Urbana, I1 61801
U.S.A.
(on leave from)
Kapteyn Astronomical Institute
University of Groningen
Postbus 800, 9700 AV Groningen
The Netherlands
Abstract. In this paper we compare the distribution of the atomic and ionized
hydrogen along the inner spiral arms of H51. As is the case in H83, the location
of both these phases of the interstellar medium with respect to the major dust
lanes suggests that molecular hydrogen is dissociated on kpc scales in active
star-forming regions, and that this dissociation process may strongly affect the
observed morphology of atomic hydrogen in spiral arms.
i. INTRODUCTION
Since the introduction of the notion of spiral density-waves by C.C. Lin
and F.H. Shu in the mid-1960's the gaseous spiral arms observed in galaxies have
been explained as shocked gas arising from the non-linear response of the
gaseous disk to a relatively weak density-wave disturbance in the gravitational
field of the old stellar disk (summarized e.g. in Shu 1982). In this picture the
shock is thought either to stimulate the formation of giant cloud complexes or
to cause the collapse of pre-existing clouds, eventually resulting in the
formation of new stars downstream from the the shock. Young, bright stars and
giant HII complexes are distributed like 'beads on a string' along the outer
edges of spiral arms. The position of the shock front is outlined by the dust
lanes, which thus represent the locus of highest (total) gas density. However,
in several cases, e.g. M81 (Visser 1980) and M83 (Allen et al. 1986), the
highest density of the atomic gas is observed to be shifted downstream with
respect to dust lanes. In the case of M83 the shift puts the atomic gas along
the same locus as the HII complexes. Visser has shown that such a shift could
arise from beam-smoothing, but we have ruled out this possibility in the case of
M83 (Allen et al. 1986). We suggest that, since no atomic and ionized gas is
observed at the position of the dust lane, the interstellar gas is apparently
mostly molecular there and remains molecular for some time downstream of the
shock. After the formation of sufficiently hot stars the molecular gas is
dissociated giving rise to the observed arm of atomic gas, or ionized yielding
giant HII complexes. In this scenario the correspondence between the ridge of
highest HI density and the 'string' of HII regions is thus explained quite
naturally.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 3O9
_ _"_'_'_- PAGE BLA_4K NOT FILMED
https://ntrs.nasa.gov/search.jsp?R=19870014878 2020-03-20T10:56:34+00:00Z
R. P. J. TILANUS AND R. J. ALLEN
We have emphasized however that the region studied in M83 is exceptional in
that the displacements described are very clear. Our interpretation of this fact
is that, for this region, the density-wave streaming is well-ordered enough to
allow the time sequence of star formation to be stretched out in space. Since
such large-scale ordered motions have been identified previously in M51
(Segalovitz 1976), we have obtained high resolution HI and Ha observations of
this galaxy.
2. OBSERVATIONS
High resolution HI observations of M51 were carried out using the
Westerbork Synthesis Radio Telescope in a 2 x 12 hrs. observation. The data have
been Hanning smoothed resulting in a resolution of 16.5 km s-I , the 63 channels
spanning a total rang_ in velocity of 520 km s-I centered on the systemic
velocity of 440 km s . The spatial resolution is 12" x 18" (_ x 6). The data
showed some moderate phase errors resulting in extra grating rings at I0', and
we are still in the process of removing these errors. The inner parts presented
appear to be unaffected by these problems, except possibly for a region close to
the nucleus. The continuum has been formed by averaging the channels free from
line emission. Subsequently the continuum-subtracted line channels have been
added using a cutoff of 3_. The resulting total HI map is shown in Figure I,
with the contours representing -4, 4, 8, 14 and 20 mJy/beam (4 mJy/beam = 3_;
101_ 2for this resolution, 1 mJy/beam = 4.6 x atoms/cm along the line of sight
through M51).
H_ observations were obtained using the TAURUS imaging Fabry-P_rot
spectrometer (Atherton et al. 1982) attached to the Isaac Newton telescope on La
-i
Palma. The total velocity range observed is 400 km s over 72 channels, the
resulting resolution in velocity is 15 km s-1, sampling slightly less than 3
channels per resolution element. Again the continuum was formed by averaging
line-free channels and a total Ha map was obtained adding the
continuum-subtracted line channels using a cutoff of 3a. The distribution of the
Ha emission in the inner region of M51 is shown in Figure 2, where the contours
are -250, 250, 500, 1000, 2000, 4000, and 6250 in units which scale linearly
with the number of photons observed. This map and a velocity integrated map of
the original cube showing also the continuum sources are in excellent agreement
with broad-band H_ observations of M51 by Kennicutt (1986, private
communication). We have used his observations and positions of reference sources
from Mathewson et al. (1972) to obtain a position calibration of the TAURUS
observation accurate to approximately I". Figure 3 shows the Ha distribution of
Figure 2 smoothed to the resolution of the HI observation (Figure I).
The thick lines in Figures 1,2 and 3 represent the major dust lanes along the
inner spiral arms of M51, obtained from overlaying a large scale optical
photograph with the Ha observations.
We emphasize that a more thorough analysis of both the Ha and HI data is in
progress, using more advanced methods of treating the data.
310
DISSOCIATIONOFMOLECULARGASIN M51
D
E
C
L
I
N
A
T
I
0
N
47
00 <_ _.
2G 30 _) " -
13 27 S.S $0 45 40
@IGHT ASCENSION
Fig. i HI density distribution in
the central region of M51.
contours: -4, 4 (-3a), 8, 14, and
20 mJy/beam.
The beam (12"x18") is shown in the
bottom left corner. The thick contours
indicate the main dust lanes.
3. DISCUSSION
Although the separation is not as pronounced as in M83, comparing Figure 1
with Figures 2 and 3 shows that the ridge of highest HI density is more closely
coincident with the Ha distribution than with the dust lanes, in particular
along the inner northern arm. This suggests that in the inner parts of M519 as
in M83, the interstellar gas is molecular at the position of the shock and
remains molecular downstream of the shock until the newly formed stars
dissociate or ionize the molecular gas on a kpc scale. Furthermore, our
observations indicate that this dissociation process may significantly affect
the HI morphology in the inner regions of galaxies and that the HI clouds need
not always have key role as precursors of molecular clouds.
More so than in M83, the observed distribution of the ionized gas is likely
to be affected by obscuration effects. High resolution radio continuum
observations at several frequencies have been obtained which should be free of
obscuration, allowing a precise indication of the location of both the thermal
(HII regions) and the non-thermal (shock region) radio continuum (van der Hulst
1986, private communication).
311
R. P. J. TILANUS AND R. J. ALLEN
I I I I I ! I i
I
b'_W
_1" _...I. --_ F I I / (_-
CU
[_.
I I I I I I I
/
/
\
/
L j//
C_) t:_ _ _ 0"I {_
03 L_- CD F._
L',-
C_
'q'Z
0
H
03
_Z
f_L_J
,_- L.J
L'h__
U)
p-
03
w-t
o '_ ,,-_ _=
,--4 o 0_ m o
•1_ IO 0 0 0
_l ._0 m o
_ _,4 ..u_ ,N _ "_
0 0 _ g'_
• ,_ .1_ o',._
•,_ • o o ._ ,_l _ ,-c:
312
rDISSOCIATION OF MOLECULAR GAS IN M51
Acknowledgements. We would like to thank F. (K-Y) Lo, J.M. van der Hulst and
R. Sancisi for usefull discussions, R. Kennicutt for the use of his H= plates of
M51 and F. Lo for providing us with an excellent optical photograph of the
central region of M51. P.D. Atherton and K. Taylor generously provided expertise
in obtaining the TAURUS data. The Westerbork Radio Observatory is operated by
the Netherlands Foundation for Radio Astronomy with the financial support of the
Netherlands Organization for the Advancement of Pure Research (Z.W.0.).
R.P.J. Tilanus is financially supported by Z.W.O.
REFERENCES
Allen, R.J., Atherton, P.D., Tilanus, R.P.J. 1986, Nature 319, 296
Atherton, P.D., Taylor, K., Pike, C.D., Harmer, C.F.W., Parker, N.M.,
Hook, R.N. 1982, MNRAS 201, 661-696
Mathewson, D.S., van der Kruit, P.C., Brouw, W.N. 1972, Astr. Ap. 17,
468-486
Segalovitz, A. 1976, Astr. Ap. 52, 167-174
Shu, F.H. 1982, Th___ePhysical Universe (University Science Books,
Mill Valley, California), p. 281-284
Visser, H.C.D. 1980, Astr. Ap. 88, 159-174
313


Large scale dissociation of molecular gas in the sprial arms of M51

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server