The detection of the 63 microns line of [O I] is reported for three positions in the H II region complex Sgr A at the galactic center. Velocity resolution of the line indicates that the emitting material has both rotational and radial motion of magnitude similar to that of the ionized gas in the core and that a substantial amount of the emitting material lies within the central few parsecs of the Galaxy. A model in which [O I] is collisionally excited by neutral hydrogen, either from the warm region ahead of an ionization front or behind a shock, is proposed and gives a total mass of hot, neutral gas within the central 3 pc of the Galaxy of between 10 and 10(exp 3) solar mass. A limit on the flux of this line has been set for Sgr B2

Lester, D. F.

Storey, J. W. V.

Townes, C. H.

Watson, Dan M.

Werner, M. W.

English

NASA Technical Reports Server

NASAJCR- 207513
THE ASTROPHYSICAL JOURNAL, 248: L 109- L 112, 1981 September 15
1981. The American Astronomical Society. All rights reserved. Printed in U.S.A.
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DETECTION OF [O 1] 63 MICRON EMISSION FROM THE GALACTIC CENTER
D. F. LESTER 1 AND M. W. WERNER
Ames Research Center, NASA, Moffett Field, California
AND
J. W. V. STOREY, DAN M. WATSON, AND C. H. TOWNES
University of California, Berkeley
Received 1981 April 23; accepted 1981 June 11
ABSTRACT
The detection of the 63 _m fine of [O l] is reported for three positions in the H Il region complex
Sgr A at the galactic center. Velocity resolution of the line indicates that the emitting material has
both rotational and radial motion of magnitude similar to that of the ionized gas in the core and that
a substantial amount of the emitting material lies within the central few parsecs of the Galaxy. A
model in which [O l] is colllsionally excited by neutral hydrogen, either from the warm region ahead
of an ionization front or behind a shock, is proposed and gives a total mass of hot, neutral gas within
the central 3 pc of the Galaxy of between l0 and 103 Mo. A limit on the flux of this line has been set
for Sgr B2.
Subject headings: galaxies: Milky Way -- galaxies: nuclei -- nebulae: H lI regions
I. INTRODUCTION
Recent studies of Sgr A, the H n region at the center
of our Galaxy, have shown that it is distinctive in a
number of ways. Measurements of the spatial distribu-
tion of [Nen] 12.8/tm emission (Lacy et aL 1979, 1980)
suggest that the central parsec of Sgr A contains at least
10 dense, ionized clouds that have large relative veloci-
ties, ranging over more than 400 km s- 1, and superther-
mal internal velocity dispersions. While the large relative
velocities are thought to be caused, in part, by Keplerian
rotation of the ensemble of clouds in the gravitational
field of a massive cluster of stars, the velocity field
shows evidence for chaotic motions as well. The inferred
expansion time scale for these clouds is comparable to
their collision time scale, suggesting that cloud-cloud
collisions may play an important role in their destruc-
tion. In this violent re#on, the formation of shocks in
the ionized clouds and associated neutral gas might be
expected. The estimated lifetime of these clouds is short
enough that their formation must be frequent. If they
are originally formed from neutral gas, whether stripped
off stars or from pieces of a disintegrating molecular
cloud, ionization fronts moving into the originally neu-
tral matter could be a common phenomenon. The inter-
cloud medium is traversed by ionizing radiation which,
in proceeding outward, also encounters neutral material
at some distance from the galactic center.
1National Academy of Science/National Research Council
Associate.
The 63 ttm 3P, _3p: [O I] line is expected to be an
important coolant both in shock-heated neutral clouds
and in ionization fronts moving into these clouds
(Dalgarno and McCray 1972; Hill and Hollenbach 1978).
This line has already been observed in several H n
regions in which the emission is thought to originate in
the neutral gas just ahead of the ionization front as it
expands into an adjacent molecular cloud (Melnick,
Gull, and Harwit 1979; Storey, Watson, and Townes
1979). We have looked for this line in Sgr A in an
attempt to assess the importance of neutral gas in that
region. We find that the [O I] fine is spectrally resolved
by our instrument, allowing a comparison of the velocity
structure of the neutral gas with that of the ionized gas,
and that the velocity of peak emission appears to vary
systematically with position across the source, along the
galactic plane. In addition, we have set an upper limit to
the [O I] strength for the dense H n region Sgr B2.
II. OBSERVATIONS
Observations were made with the 91.4 cm telescope of
the NASA Kuiper Airborne Observatory on 1980 July 8
during a flight to Honolulu, Hawaii. At the observing
altitude of 12.5 km, the line-of-sight column density of
precipitable water was about 15 #m. The instrument
used was the tandem Fabry-Perot spectrometer de-
scribed by Storey, Watson, and Townes 0980), with a
Ge:Ga photoconductive detector. A l' beam (FWHM)
was used with a chopper throw of approximately 3:5
and a chopping frequency of 24 Hz. Wavelength calibra-
tion was provided by the strong H20 line at 63.323/_m
LI09
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LIIO LESTER,WERNER,STOREY,WATSON,ANDTOWNES Vol.248
(McClatcheyet al. 1973) seen through a sample cell
against a spectrometer blackbody source. The same line
is visible in the source spectra from the intervening
terrestrial water, although the column density was large
enough that the line is completely saturated and cannot
itself be used for wavelength calibration. Flux calibra-
tion for the [O l] line is derived directly from the Sgr A
continuum, which has been mapped with 30" resolution
by Becklin, Gatley, and Werner (1981). The spectrome-
ter resolution was )_/AX _ 1600 (Av _ 190 km s-l).
The observations represent 24 minutes of integration
time during which the system noise equivalent power
(NEP) was approximately i × 10 -13 W Hz-i/2
iii. RESULTS
Spectra of Sgr A taken at the nominal position of the
10 p.m peak and 45" north and south along the galactic
plane are shown in Figures i b, l a, and l c, respectively.
Although the relative location of the three measured
positions are well determined, the absolute pointing
position is no better than _ 30". The spectrum of Sgr
B2 was taken at the 60 #m continuum peak and is
shown in Figure ld. The [O q line is clearly detected at
all three points in Sgr A, but is not detected in Sgr B2.
The spectra are smoothed to 1/12 of a resolution ele-
ment and have been fitted with a Lorentzian plus a
linear continuum. They have also been divided by scans
of an internal blackbody calibration source in the spec-
trometer, which removes the transmission function of
the instrument. The continuum slope is largely from the
strong terrestrial water lines that obscure the source on
the long wavelength side of the spectrum. The effect of
these water lines has been estimated from the line-of-
sight precipitable water vapor measurement and is shown
in Figure l e without instrumental broadening. This
spectrum shows that the observed [O I] is substantially
but not severely influenced by terrestrial water. Al-
though observations with less fine-of-sight water are
clearly to be preferred, the effect of the water lines is
cancelled to first order by scaling the [O I] intensity to
the interpolated continuum beneath the lines. The ob-
served line intensities in Sgr A are 7, 1 I, and 8 × l0-13
W m -2 at the northern, central, and southern positions,
respectively, and less than 2 × 10-13 W m -2 from Sgr
B2. The observed line intensities correspond to a
luminosity of approximately 3.5 × l03 Lo in the line at
each position on Sgr A, with a projected beam diameter
of approximately 3 pc.
The laboratory wavelength of the [O I] line is accu-
rately known from the laser magnetic resonance study of
Saykally and Evenson (1979) to be 63.170 ttm. This
allows us to conclude that the line-of-sight velocity of
the emitting region at the center position is +15 -4-40
km s- _with respect to the local standard of rest (LSR),
which is consistent with that derived from the [Ne n]
21 i i
/ A
-1 SGR A 10um PEAK
SGR B2
1
ATM
1000 500 0 -500 -1000
V r (kin s-1)
FIG. l .-- Spectra of Sgr A in a I' beam at (a) 45" N, (b) l0
t_m peak, and (c) 45" S along the galactic plane. The resolution of
the spectrometer is well represented by the [O l] profile at the
northern position (a). The slope in the continuum is caused by
residual telluric water absorption. The velocity scale is defined by
the scan length of the Fabry-Perot, and the zero position is
determined by laboratory wavelengths of [O q and of H20 in a
reference sample cell. The shift in line center velocity across the
source is evident as is a broadening of the [O q line at the center
and south position. (d) Spectrum of Sgr B2 at the 60/_m peak. No
[O I] was detected in this source. (e) Synthetic atmospheric trans-
mission spectrum for 15_m of precipitable water. This spectrum is
shown without instrumental broadening and line overlap. This
spectrum was derived with a single slab atmosphere model based
on the line parameters given in the McClatchey et al. (1973)
compilation.
spectra. There is some evidence for a gradient in line
center velocities, which range from +140 -+ 40 km s -_
at the northern position to -I0-+ 50 km s -t at the
southern position. This effect is similar in both direction
and magnitude to the [Ne n] velocity field. There is no
evidence for such a +140 km s -I cloud in the H2CO
absorption data of Bieging et al. (1980) at the north
position, although a cloud seen in the 21 cm H I map of
Sinha (1979) may correspond to the source of the [O q
emission. The line at the north position has a width
consistent with the instrumental response, while at the
central and south positions the line is dearly resolved
with an intrinsic width of ~ 200 km s- I.
No.3,1981
IV.ANALYSISOFLINESTRENGTHS
The[Ol] fluxesfromSgrA canbecomparedwith
those of other H lI regions by scaling to the optically
thin radio flux. The ratio of [O I] flux to radio flux in
Sgr A is comparable to that seen in DR 21, M17, and
the Trapezium region of M42 (Melnick, Gull, and Harwit
1979; Storey, Watson, and Townes 1979). While the
nature of Sgr A is very different from that of these other
H II regions, the relative similarity in [O I] power does
suggest that similar excitation mechanisms may be con-
sidered. Collisional excitation of [O I] has been proposed
for galactic H IXregions, and the applicability of this to
Sgr A is considered in detail in this section.
[O I] EMISSION FROM GALACTIC CENTER
a) Ion Impact Excitation
Excitation by electron and proton impact may be
important in regions where neutral oxygen coexists with
a substantial amount of ionized gas. The similarity of
ionization potentials for neutral oxygen and hydrogen
suggests that this region will be restricted to the edge of
the H II region where the gas is only partially ionized. In
fact, the ionization potentials are so close that charge
exchange of neutral oxygen with protons is nearly a
resonance process (Field and Steigman 1971), and the
product of neutral oxygen density and ion density is
constrained to peak in a narrow region at the boundary
of the H II region. Using the electron impact collision
strength from Saraph (1973) and the proton impact
collision strength derived from the approximate for-
mulae in Bahcall and Wolf (1968), the total neutral mass
can be derived. Assuming an abundance fraction of
neutral oxygen of 10-3f by number, where f is the
fraction of oxygen and hydrogen remaining neutral, we
find a total mass in the region of oxygen excitation of
M_ 104/./(1 - f )n n M o
n e _(1-f)n n <n¢_it _ 103cm -3,
for
and M _ lOll Mo for ne >.ncrit, where nH is in em -3
and a temperature of 104 K has been used. This can be
compared with the mass 7 × lOS/he 31o of ionized gas
in the same beam in Sgr A derived from the 5 GHz flux
of Ekers et al. (1975). In the low-density limit, the
minimum inferred mass occurs at f= 0.5. This mini-
mum mass, assuming ion impact excitation, is compara-
ble to the mass of ionized gas (_ 100 M o for ne _ 104
cm -3) seen in the [Ne n] clouds. In the higher density
regime, 104 < n, < l0 s cm -3, which is considered to
best represent the [Ne Ill clouds, a dominant part of Sgr
A would be required to be only partially ionized. The
substantial ionization of neon throughout the region
argues against a large amount of partially ionized gas,
however, and suggests that ion impact excitation of [O I]
Llli
is not important in the [Ne IX]clouds. It may, however,
apply at the edges of neutral clouds surrounding the
ionized re#on.
b) Neutral Hydrogen Impact Excitation
It is also necessary to consider the excitation of [O I]
by neutral impact. In the galactic center this could occur
either in the warm neutral gas just outside an ionization
front (Hill and Hollenbach 1978) or in gas which is
shock heated. At velocities and densities similar to those
found in the ionized gas at the galactic center, the
destruction in shocked gas of any molecules in which
oxygen and hydrogen might be bound up (CO, OH, 02,
H2) is expected to be complete (Hollenbach and McKee
1979). This should be true either in a shock that moves
ahead of an advancing ionization front or in the more
violent shocks that might be expected as a result of
cloud-cloud collisions. Cross sections for atomic oxygen
excitation by neutral hydrogen impact have been pub-
fished by Launay and Roueff (1977). These yield, for
oH < non t _ 3 × l0 s cm -3,
a neutral mass
M- 1.4× 10' (103/r)/fn. Mo.
Assuming a uniform density within the beam and a
temperature of 103 K, we find n H _ 2 × 10 3 cm -3 and
a mass M _ 700 3/o for f = 1. It is very likely that any
neutral gas is, in fact, distributed very nonuniformly, in
view of the manifest evidence for clumpiness in the
ionized gas from the [Ne ll] studies. As the assumed
clumpiness increases, our estimate of the neutral mass
decreases until the limit nH > ncrit is reached. In
this case, the energy levels in the neutral oxygen are
statistically populated, and the estimated mass is the
same as that derived in the same limit for ion impact
excitation--approximately 10 M o in the region of exci-
tation.
V. DISCUSSION AND CONCLUSIONS
Sgr A is unlike giant H It regions in the spiral arms in
that it shows neither optically thick far-infrared emis-
sion nor strong, localized molecular line emission--
characteristics of dense molecular clouds associated with
H It regions. The velocity structure of the [O l] line in
the direction of Sgr A strongly suggests that its source is
close to the ionized core of the Galaxy. For these
reasons, our observations constitute the first definitive
detection of neutral gas within the central few parsecs of
the galactic center.
We have considered two excitation mechanisms for
[O I] in § IV. Excitation by neutral hydrogen impact in
compressed, heated regions can account for the ob-
served emission without requiring unduly large amounts
of neutral gas. Excitation by ion impact in an advancing
Lll2
ionization front, however, requires a large amount of
partly ionized gas, a prediction which conflicts with our
understanding of the ionization equilibrium in the
[Ne n] clouds. We cannot exclude, however, the possi-
bility that the line is produced in a more diffuse, par-
tially ionized region exterior to the core.
Assuming excitation by neutral hydrogen collisions,
the above discussion has shown that I0 M o is a lower
limit to the mass of hot, neutral gas within the central 3
pc. Even if the high-density limit is applicable, this is
likely to underestimate the total neutral mass in Sgr A,
because the [O i] emission will arise only from regions
heated to T-> 228 K and in which the bulk of the
oxygen is in atomic rather than molecular form. By
comparison, the amount of ionized gas in the [Ne u]
emitting clouds is less than 100 Mo, and the total
amount of ionized gas in the central I' is probably less
than 500 Mo.
It is of great interest to understand the i'elationship of
the neutral gas to the ionized clouds. Similar velocity
dispersions suggest that their relation may be intimate.
One possibility is that the clouds are only partially
ionized and that the [O I] is excited outside the ioniza-
tion fronts eating into the clouds (Hill and Hollenbach
i 978). In this picture, we would expect the same velocity
peaks seen in the [Ne 11] spectra to appear in higher
spectral and spatial resolution [O I] spectra, and the
spatial distribution of [O I] and [Ne 11] to be similar.
Although our three beam positions suggest that the [O I]
is more extended than the [Ne u], the limited spatial
coverage in both lines, and the fact that the [O I] beam
positions overlap somewhat, makes this result pre-
liminary. Alternatively, the [O i] emission may arise in
shock fronts produced by colliding neutral clouds. This
would again produce discrete [O I] velocities, although
not necessarily the same as those in the [Ne n] clouds.
Finally, the neutral gas may be not in the form of
individual inclusions but more widely spread. In this
LESTER, WERNER, STOREY, WATSON, AND TOWNES
case, discrete [O 1] velocities would probably not be
seen.
It is also possible that the neutral gas is in organized
rather than chaotic motion. The velocity dispersion at
the center position and the evident velocity gradient
along the galactic plane might then indicate that the
[O I] emission is associated with an expanding or con-
tracting ring of neutral gas which is rotating about the
axis of the Galaxy. The unusual ringlike distribution of
far-infrared emission seen in Sgr A by Becklin, Gatley,
and Werner (1981) makes this a particularly attractive
possibility. More extensive mapping and observations
with higher spectral resolution will be of great value in
testing these hypotheses.
Sgr B2 is a source in which an H n region is seen
embedded in a dense molecular cloud and, for this
reason, strong [O I] emission is expected from the transi-
tion region at the ionization front. The lack of [O 1]
emission in Sgr B2 is perhaps best understood on the
basis of infrared continuum results. The absence of
near-infrared emission from this source and the large
far-infrared optical depth inferred by Gatley et al. (1978)
suggest that the H u region is obscured completely by its
parent molecular cloud, even at 63 #m. Continuum
emission at 63 I_m must then come from well in front of
the H u region, and the ionization front is not exposed
at this wavelength. A similar effect has been noted by
Storey, Watson, and Townes (1981) from the absence in
Sgr B2 of NH 3 lines around 120/_m, a wavelength at
which the cloud should be less opaque than at 63 jam.
We thank the staff and crew of the Kuiper Airborne
Observatory for their support during these observations,
and Dr. R. Genzel for help with the observations. We
thank Dr. D. Hollenbach for many helpful discussions,
and Dr. J. Scargle, Dr. H. Dinerstein, and Dr. W. J.
Forrest for critical readings of the manuscript. This
work is supported by NASA grant NGR 05-003-51 I.
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D. F. LESTER and M. W. WERNER: Astrophysical Experiments Branch, MS245-6, NASA Ames Research Center,
Moffett Field, CA 94035
J. W. V. STOREY, C. H. TOWNES, and DAN M. WATSON: Department of Physics, University of California, Berkeley, CA
94720


Detection of [O I] 63 Micron Emission from the Galactic Center

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