A program of radio line and continuum studies of star formation in nearby spiral galaxies is reported. The objective is a search for hot gas and peculiar dynamics in spiral nuclei with 10" to 30" angular resolution. Vigorous star formation is found to be a common phenomenon in the inner kpc of spirals. Arcsecond resolution observations of radio continuum emission at 6 and 2 cm were used to separate the thermal and nonthermal radio components. It was found that thermal and nonthermal emission are well mixed even on sizescales of 10 pc. To understand the reason for the increased level of star formation activity in spiral nuclei, HI and CO emission in these galaxies is studied. The CO transition was detected in M51, M82, NGC 253, NGC 6946 and IC 342 with T sub a approx. 0.5 to 2.0 K, at 20" angular resolution. The dynamics and spatial distribution of nuclear gas are being studied using VLA HI maps with 30" synthesized beams. Evidence for noncircular motions in HI was found in the nucleus of IC 342

Ho, Paul T. P.

Martin, Robert N.

Turner, Jean L.

English

NASA Technical Reports Server

N87- 24322
NUCLEAR STAR FORMATION ON 100 PARSEC SCALES:
10 H RESOLUTION RADIO CONTINUUM, HI, AND CO OBSERVATIONS
Jean L. Turner and Paul T. P. Ho
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138
Robert N. Martin
Steward Observatory, University of Arizona, Tucson, AZ 85721
ABSTRACT
We report on a program of radio line and continuum studies of star formation in nearby
(< 10 Mpc) spiral galaxies. The objective is a search for hot gas and peculiar dynamics
in spiral nuclei with 10" to 30" angular resolution. Vigorous star formation is found to
be a common phenomenon in the inner kpc of spirals. Arcsecond-resolution observations
of radio continuum emission at 6 and 2 cm have been used to separate the thermal and
nonthermal radio components. We find that thermal and nonthermal emission are well-
mixed even on sizescales of 10 pc. To understand the reason for the increased level of star
formation activity in spiral nuclei, we are studying HI and CO emission in these galaxies. The
CO (J = 3 _ 2) transition has been detected in M51, M82, NGC 253, NGC 6946 and IC 342
with Ta* ,,-0.5-2.0 K, at 20 H angular resolution. The dynamics and spatial distribution of
nuclear gas are being studied using VLA HI maps with 30" synthesized beams. Evidence for
noncircular motions in HI has been found in the nucleus of IC 342.
INTRODUCTION
It is clear that nuclear activity is common in spiral galaxies, as evidenced by strong
radio continuum (e.g. Hummel 1981), infrared (e.g. Telesco and Harper 1980), and optical
(e.g. Keel 1980) emission. In many cases, the emission regions resemble regions of intense star
formation activity (Rieke et al. 1980). A useful approach toward understanding the "star-
burst" phenomenon is to study nearby galaxies, where the nuclear emission can be studied
with great sensitivity and spatial resolution. We describe a program of radio observations
of the ionized and neutral gas in spiral galaxies closer than 10 Mpc, where we can resolve
regions of 100-200 pc extent. We use radio continuum emission to trace the ionized regions in
these nuclei; dual-frequency observations allow estimates of the thermal emission, and hence
the number of young stars and their spatial distribution. The hot molecular component as-
sociated with these OB stars can be studied using higher transitions of CO. The enormous
numbers of young stars posited for these sources imply high rates of mass consumption. A
central issue becomes: is this activity in the nucleus fed from gas in the disk? This question
can be addressed with high resolution aperture synthesis maps of HI.
RADIO CONTINUUM STUDIES OF NEARBY SPIRAL NUCLEI
In order to accurately measure thermal radio continuum fluxes in spiral galaxies, it is
desirable to spatially resolve out the extended synchrotron emission that dominates single-dish
radio continuum observations (Klein and Emerson 1981). Observations of seventeen nearby
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 383
https://ntrs.nasa.gov/search.jsp?R=19870014889 2020-03-20T10:56:48+00:00Z
J. L. TURNER ET AL.
spiral galaxies were made with the NRAO Very Large Array, at 1" resolution, corresponding
to roughly 10- 20 pc linear extent. Matched beam observations were obtained at 6 and
2 cm to allow spectral separation of the thermal and nonthermal emission (Turner and Ho
1983).
We detected nuclear radio continuum emission in sixteen of the seventeen galaxies. The
nuclei show a range of activity, from 1 mJy in M101 at 7.2 Mpc, to 2 Jy in NGC 253 at
2.5 Mpc, a factor of more than 200 in luminosity. Spectral index maps show that the radio
emission is usually dominated by nonthermal emission, even on 10 parsec sizescales. Spectral
indices of-1.2 to -0.8 are common. However, flat and positive spectral indices have also been
observed (M83; IC 342), possibly indicating the presence of large complexes of compact HII
regions in the nuclei. In a number of cases, including Maffei 2, NGC 253, and NGC 4736,
the radio continuum knots are organized in one-dimensional structures. In NGC 253, at 0.1"
resolution, there is a string of compact knots aligned to within a degree on the sky, and 5
degrees in the plane of the galaxy; this 100 pc jet-like structure may be related to ejection
from the central compact (< 0.6 pc) nonthermal (> 105 K) nucleus (Turner and Ho 1985).
Even for our small sample of nearby spirals, the range in activity and the variety of
morphologies are impressive. The overall impression is that nuclei in general are very active.
CO (J = 2 --_ 1) AND CO (J = 3--_ 2) OBSERVATIONS
The goal of the CO observations is to detect molecular gas that has been heated by
star formation in spiral nuclei, and to study the dynamics of this gas. High excitation
CO lines have two advantages Over the CO (J = 1 --_ 0) transition for studying warm
gas in the nucleus: if the gas is optically thin, the warm gas will have significantly higher
line intensities in CO (J = 2 ---* 1) and (3 -* 2); if the gas is optically thick, the smaller
beamsizes afforded by the higher observing frequencies will decrease beam dilution effects.
The CO (J = 2 _ 1) and (3 --* 2) observations were made with the NRAO 12m telescope,
with system temperatures of 2000 K and 12000 K and beamsizes of 35" and 22" at 230 and
345 GHz, respectively.
We have mapped CO (J = 2 ---* 1) in the nuclei of the nearby spiral galaxies IC 342,
M51, NGC 253, and NGC 6946. The beam-diluted brightness temperature, which ranges
from Ta*(2 --* 1) ,_0.2 K in Mbl to 1.4 K in NGC 253, is in each case similar to the
CO (J = 1 _ 0) temperature (e.g. Young and Scoville 1982). Since the (2 _ 1) beam is
four times smaller than the (1 --_ 0) beam, the CO most likely is optically thick, clumpy with
respect to 30", but with a total angular extent greater than the telescope beams. Mapping
shows that the CO (J = 2 _ 1) extent is confined to roughly the inner two kpc. The extent of
the (2 _ 1) emission in the nucleus of IC 342 is -_ 2' x 1', somewhat larger than that mapped
interferometrically by Lo et al. (1984), but in agreement with the (1 ---* 0) Nobeyama map
of Sofue (1986). The (2 _ 1) maps also show that the rotation curve in these galaxies rises
very steeply out of the nucleus: in NGC 253 the velocity centroid shifts by 150 km s -1 over
30" (-_360 pc), implying an enclosed mass of 2 x l0 s M®.
CO (J = 3 --+ 2) was detected in M82, NGC 253, IC 342, M51, and NGC 6946. These
observations represent the first detection of extragalactic CO (J = 3 _ 2). The peak antenna
temperatures Ta* (3 _ 2) range from 0.7 K in NGC 6946, -_1 K in NGC 253, IC 342, and
Mbl, to N2 K in M82. In all cases, Ta*(3 _ 2) is very similar to Wa*(2 --_ 1). This
384
NUCLEARSTARFORMATIONON100 PARSEC SCALES
supports the implications from the (2 _ 1)/ (1 ---* 0) ratio that we are observing well-
distributed but clumpy molecular gas. Even for modest clumping factors, the detected CO
temperatures are higher than the values commonly found in Galactic giant molecular clouds.
For example, if the molecular scale length is 100 pc in M82, we deduce a gas temperature of
N30 K.
With the increasing angular resolution achieved by studying the higher frequency CO
lines, we cart begin to define the morphology and kinematics in the nuclear regions. However,
it appears that arcsecond angular resolution will be needed to define the hotter gas component
that is heated by star formation activity.
VLA HI MAPPING OF NEARBY SPIRAL NUCLEI: IC 342
The objective of HI mapping of nearby spiral nuclei is to study the relation of the disk
gas (HI) and the molecular gas found in the nucleus, as described above. From the large
numbers of stars derived for many spiral nuclei (e.g. Beck, Turner, and Ho 1986) it is clear
that not many generations of such star formation activity can proceed before the nucleus
is exhausted of gas. Is the nuclear gas replenished from the disk? Can we see evidence for
nuclear feeding in the HI velocity distribution? To study this problem, we have observed
IC 342 in HI with the C and D configurations of the VLA. The preliminary maps have 30 H
angular resolution and 10 km s -1 velocity resolution.
The integrated intensity distribution in IC 342 shows a flat HI disk distribution and
a ,-,2 _ central HI hole, as shown in Westerbork (Rogstad, Shostak, and Rots 1973) and
Cambridge (Newton 1980) maps. The higher resolution of the VLA map, however, reveals
that the central hole is not completely devoid of HI. There are faint HI features in the
nucleus. One HI filament lines up with the inner portion of a spiral arm, as traced by radio
continuum emission. Another HI filament appears at the center of the nuclear region, slightly
offset from the nuclear radio continuum source and the CO bar of Lo et al. (1984). This
nuclear HI filament has an extent of roughly 2 _ × 1 _, oriented north-south. This feature is
very similar in extent to the CO (J = 2 --* 1) feature mentioned in the previous section
and the Nobevama maps of Sofue (1986). The mass of this HI bar is _l-2x106 M®, or
approximately 1% of the CO mass {Loet el. 1984). The aorth-south orientation _f the HI bar
is -_20 ° different from the more northwesterly orientation of the disk HI gas at the same
velocity.
The HI velocities also indicate that the central HI bar feature is not simply a contin-
uation of the HI disk. Deviations from circular rotation appear in the central 2 _. The HI
bar rotates nearly north-south, with velocities similar in both direction and magnitude with
those of CO (Loet al. 1984). In contrast, the disk HI at these velocities (near the systemic
velocity of IC 342) define a rotating structure with a position angle --,45 ° east of north. Other
deviations from circular rotation appear at the edges of the HI hole near the ends of the bar.
The deviations from circular motion are -_10 to 30 km s -1.
The high angular resolution HI synthesis maps show that the molecular gas in the
nucleus can be connected to the disk atomic gas. The correspondence between the HI filament
and the CO bar, and the presence of noncircular velocities suggest flows into the nucleus and
concurrent conversion of atomic gas to molecular form.
385
J.L. TURNERETAL.
SUMMARY
We report here on a program of radio continuum, CO, and HI studies at high angular
resolution in order to examine the kinematics and spatial distribution of key tracers of young
star formation. These studies have been most fruitful and promising in demonstrating a link
between the nuclear and the disk environments in the case of IC 342.
This research has been supported in part by the Alfred P. Sloan Foundation, NATO
Grant 83/584, and NSF Grant AST85-09907. The National Radio Astronomy Observatory
is operated by Associated Universities, Inc., under contract with the National Science Foun-
dation.
REFERENCES
Beck, S.C., Turner, J.L., and Ho, P.T.P. 1986, this volume.
Hummel, E. 1981, Astr. Ap., 93, 93.
Keel, W. 1984, Ap. J., 282, 75.
Klein, U., and Emerson, D.T. 1981, Astr. Ap., 94, 29.
Lo, K.Y., et al. 1984, Ap. J. (Letters), 282, L59.
Newton, K. 1980, M. N. R. A. S., 191, 169.
Rieke, G.H., Lebofsky, M.J., Thompson, R.I., Low, F.J., and Tokunaga, A.T. 1980, Ap. J.,
238, 24.
Rogstad, D.H., Shostak, G.S., and Rots, A.H. 1973, Astr. Ap., 22, 111.
Sofue, Y. 1986, this volume.
Telesco, C.M. and Harper, D.A. 1980, Ap. J., 235,392.
Turner, J.L., and Ho, P.T.P. 1983, Ap. J. (Letters), 268, L79.
Turner, J.L., and Ho, P.T.P. 1985, Ap. J. (Letters), 299, L77.
Young, J. S., and Scoville, N. 1982, Ap. J., 258,467.
DISCUSSION
APPLETON:
Can you be sure that the weak HI feature you see near the center of IC 342 is not a result of incomplete
continuum subtraction from the line-containing maps? It seems suspicious to me that this weak HI feature
corresponds so closely to the continuum emission.
TURNER:
The feature only shows up in four or five channel maps: it is clearly a spectral and not a continuum
feature.
386


Nuclear star formation on 100 parsec scales: 10" resolution radio continuum, HI and CO observations

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