NASA Technical Memorandum 88184 Airborne Astronomy Program 

Medium Altitude Mission Branch 

Preprint Serres 038 

NASA-TM-88184 

11 tip tilJtJ776 7 

Star Formation in the Inner 

Galaxy: A Far-Infrared and 

Radio Study of Two H II 

Regions 

D.F. Lester, H.L. Dinerstein, 

M.W. Werner, P.M. Harvey, 

N.J. Evans II, and 

R.L. Brown 

JI\N 1 7 19~o 

UGtl;;r.v, NASA 

HAMPTON, VIRGltllPi 

December 1985 ~ ____ ~ 

, """" "" "" '"'' "'" "'" "'" '''' "" 

NI\S/\ 

National Aeronautics and 

Space Administration 

NF01654 
NASA Technical Memorandum 88184 Airborne Astronomy Program 

Medium Altitude Mission Branch 

Preprlnt Series 038 

Star Formation in the Inner 

Galaxy: A Far-Infrared and Radio 

Study of Two HII Regions 

D. F. Lester, 

H L. Dlnersteln, University of Texas at Austin, Austin, Texas 

M. W. Werner, Ames Research Center, Moffett Field, California 

P. M. Harvey, 

N. J. Evans II, University of Texas at Austin, Austin, Texas 

R. L. Brown, National RadiO Astronomy Observatory, Charlottesville, Virginia 

December 1985 

NI\SI\ 

National Aeronautics and 

Space Administration 

Ames Research Center 

Moffett Field, California 94035 

-#=-' 
This Page Intentionally left Blank 
3 

I. INTRODUCTION 

It now seems well established that the inner spiral ann region of our galaxy, at galactocentric 

radii from 4 to 6 kpc, is a region of greatly enhanced star formation. Surveys of CO emission 

(Scoville and Solomon 1975; Gordon and Burton 1976) indicate that the mean density of 

molecular cloud material may be nearly an order of magnitude higher there than in both the solar 

neighborhood and the region between this 5 kpc ring and the galactic center. Enhancements at 5 

kpc are also seen in the distribution of HIT regions (Lockman 1976), OH/IR stars (Baud, Habing 

and Oort 1979), pulsars (Taylor 1979), and cool giant stars (Ito, Matsumoto and Uyama 1977), all 

of which reflect enhanced star formation in the recent past. The conditions under which stars must 

form in this part of the Galaxy are unusual compared with solar neighborhood conditions. Not 

only is the mean density of radiation and matter much higher than in the solar neighborhood, but 

there is evidence that this region has been enriched in heavy elements relative to regions more 

distant from the galactic center, presumably because of the enhanced star formation rate (cf. 

Mezger et. al. 1979, Lester et. al. 1985). 

In view of the unusual conditions and the lack of detailed studies of star-forming regions in this 

part of the Galaxy, we undertook a study of two particularly bright HIT region complexes, G30.8-

0.0 and G25.4-0.2. We observed these regions photometrically at 40, 50, 100, and 175 Jlm with 

50" resolution and with considerably higher spatial resolution in the 6 cm continuum. Additional 

information about G25.4-0.2 was provided by measurements of the strength and distribution of 

[SIll] 9532A as well as the velocity structure in H86a and CO. These data give us considerable 

insight into the structure and stellar population makeup of these inner galaxy HIT regions which 

are, presumably, excited by very young OB stars. We compare these complexes with those at 

larger galactocentric distances which form the basis for the present general picture of massive star 

formation. Complementary infrared spectral data on these regions are discussed in separate papers 

(Lester et. al. 1983, 1985), in which it is shown that they may have high N/O abundance ratios. 
4 

n. OBSERVATIONS 

The infrared measurements were made using a photometer and fIlter set similar to that 

described by Harvey (1979). The effective wavelengths of 40,50, 100, and 175 Jlffi are those 

applicable to sources with the observed color temperatures. Observations were made from the 

Kuiper Airborne Observatory 91 cm telescope using a bolometer system at the straight-Cassegrain 

focus. A 50" (FWHM) beam was used, and the chopping secondary was operated with a beam 

spacing of 6' at PA=70° and 850 for G30.8-0.0 and G25.4-0.2 respectively. Maps in the 50 and 

100 Jlffi fIlters were made by measuring the two wavelengths simultaneously at each point on the 

sky, which simplifies the derivation of color temperature. Measurements in all four fIlters were 

made at the main peaks in the two regions. The absolute positional accuracy of the maps is 

estimated to be about 10-15". The signal at each position and in each filter was corrected for 

residual water vapor using extinction coefficients of 0.01,0.02 ,0.03, and 0.02 per micron of 

precipitable H20 at 40, 50, 100, and 175 J.l.m respectively. During the observations of each 

source, the water column density varied between 10 and 15 J.l.m. 

The far-infrared maps were made by scanning the telescope in azimuth (along the direction of 

the chop) at a range of elevations in increments of 40" in both axes. The azimuth scans were 

approximately 7' long and covered a range of approximately 9'. Each azimuth scan was 

deconvolved to remove the effects of spatial chopping under the assumption that the average of the 

signal at the two endpoints of the scan defined zero flux. After this procedure was applied, the 

lowest dependable contours were 100 and 300 Jy-beam-l at 50 and 100 J.l.m respectively. The 

50/1 00 Jlffi color temperature was derived by taking the ratio of the 50 and 100 J.l.m fluxes at each 

position and by determining the corresponding color temperature. Since the color temperatures will 

have substantial uncertainties at low flux levels, only the values at positions with substantial flux 

are presented below. 

Flux density calibration was accomplished by observing S140, which was assumed to have 

4600,5700,6000, and 4200 Jy at 40,50, 100 and 175 J.l.m respectively (Harvey, Campbell and 
S 

Hoffman 1977; see also their note added in proof). The accuracy of the flux density is, except for 

the lowest contour in each map, limited by the absolute calibration and water vapor measurements 

to approximately 30%, as the signal-to-noise was high at most positions. The far-infrared spectral 

distributions at the peak positions in G30.8 and G2S.4 were derived, and total flux densities at SO 

and 100 Jlm were determined by integrating the maps. 

Measurements of the 9S32A [SIII] line in G2S.4 were made using a sequential scanner with a 

4A bandpass on the Crossley telescope at the Lick Observatory. A Varian VPM-I64 

photomultiplier was used for these observations. Measurements were made with a 30" aperture 

and are referenced to the sky approximately 10' away. The fact that the source is not seen on the 

Palomar Sky Survey red print implies that there is substantial visual extinction. Near the peak of 

the emission in G2S.4-0.2 the line is the bright enough that the calibration procedure is the 

dominant source of error, which is estimated to be about 20%. The instrument is further described 

by Hawley and Grandi (1977). The absolute positions on this [SIll] map are accurate to about 

10". Although only the region around the SE component of G2S.4-0.2 was mapped, no 

emission was detected in a single beam position at the NW peak of that complex. 

The radio continuum maps were made at 4866l\t1Hz with the VLA.S A concatenation of 

SOperated by Associated Universities Inc., under contract with the National Science 

Foundation. 

snapshot (u,v)-data was made from observations taken at separate epochs in the two short 

configurations, the C- and D-arrays. This provides a continuous distribution of projected baselines 

from 40 to 1800 m and resultant synthesized Gaussian beams of 9.2x6.8" at PA= sr and 

7.4x6.5" at PA=- 560 for G30.8 and G25.4 respectively. The flux density calibration was referred 

to 3C286; we adopted 7.41 Jy for this source at our observing frequency. The bandwidth used for 

these observations, 12.S MHz, was sufficiently narrow that we could map the entire primary 

telescope beam and thus obtain a CLEAN map free from effects due to confusing sources of 
6 

galactic emission anywhere in the beam. For 025.4, the total flux in the map is 6.1 Jy (3.5 and 2.6 

Jy from SE and NW respectively). This is somewhat smaller than the 19.1 Jy found by Downes 

et. al. (1980), leading us to believe that our VLA map is not sampling some extended emission in 

the vicinity. Especially short baselines were used in the concatenation for 030.8 and, as a result, 

very little extended emission is likely to be missed for that source. The total flux density in our 

030.8 map is 57.4 Jy, which agrees well with the single dish results. The total flux densities for 

the separate components of 030.8 are thus listed in Table 1. 

In order to obtain the velocity of the ionized gas associated with the two separate components 

025.4SE and 025.4NW, we observed the H89a 9.173 OHz radio recombination line from both 

of these objects. These observations were made with the Cassegrain upconverter-maser receiver 

on the NRAO 140-foot telescope.5 The system temperature on cold sky was 42°K. A single 

1024-channel spectrum covering 20 MHz bandwidth was produced in the autocorrelation 

spectrometer; the velocity resolution was 0.6 km-s-1. The telescope beamwidth at the line 

frequency, 3.' 1 (FWHM) as determined from observations of Virgo A, was sufficiently small to 

allow us to distinguish the line emission from the two peaks of 025.4 seen in the far-infrared and 

VLA maps. We obtained spectra at both VLA positions. In order to verify that we were indeed 

pomted at the proper poSitIOns, unmediately prior to the spectra1line observations we established 

the telescope pointmg using 1730-130 (NRAO 530). Following the spectroscopy at each 

position, we again observed 1730-130 and confIrmed that the pointing was accurate and stable to 

within 5". These single dish measurements, which cleanly separate the two sources, provide the 

best continuum flux densities for them, and these are given in Table 1. 

The 025.4 regIOn was mapped in the 2-1 transitIon of 13CO using the 4.9m antenna of the 

Millimeter Wave Observatory of the University of Texas6. An area of about 

6ne Millimeter Wave Observatory is operated by the Electrical Engineering Research 

Laboratory of the University of Texas at Austin., with partial support from the National 

Science Foundation and McDonald Observatory. 
7 

6xl0' centered on the far-infrared and radio continuum sources was surveyed, using a beam 

spacing of 1 " which is smaller than the 1.3' FWHM beam. The velocity resolution was 0.33 lans-l, and data were taken in 256 velocity channels. Thus the main components at 65 and 95 lan-s-1 

were mapped simultaneously. The system noise temperature was typically 1400°K. The absolute 

position reference was checked by scans of Jupiter and Saturn, and is estimated to be accurate to 

about 15". 
S 

m. MORPHOLOGY AND INFRARED LlTh1lNOSITY 

a) G30.S-0.0 (W43) 

The galactocentric distance of 5.4 kpc adopted for this source is based on the +90 km-s- 1 

component of HIlOa. Recombination line spectra taken around the region covered by our 6x6' 

maps show that this velocity component is by far the dominant one over this restricted area 

(Gardner and Thomasson 1975). The distance ambiguity is resolved by lack ofH2CO absorption 

at velocities larger than this (Downes et. al. 19S0), and a heliocentric distance of7 kpc is 

indicated. 

Pipher, Grasdalen and Soifer (1974) mapped the region at 12 Jllll and found the hot dust to be 

distributed similarly to the 5 GHz emission which they synthesis-mapped with very limited 

baseline coverage. The single-dish radio map that best matches the far-infrared beam IS the 23 

GHz map at SO" resolution by Rodrfguez and Chaisson (197S). At 100 Jlm, five peaks are 

distinguishable in the infrared maps. The far-infrared positions and fluxes for these sources are 

given in Table 1. The brightest two sources, which we will designate G30.SN and G30.SS, 

correspond to the main radio and 12Jlm peaks. 

Our VLA map allows the comparison of radIo structure to infrared structure to be made in 

detail. Comparing the infrared maps ofG30.8 (Figures la, b, c) with the VLA map (Figure 2), it 

is seen that four of the five far-IR sources are also radio continuum sources. The westernmost 

source in the VLA map, corresponding to infrared source #4, resembles a thin rim seen edge-on. 

This morphology is shared by source #3 to the northeast, and, to some extent, by G30.8N (source 

#1) at top center. Infrared source #5 is associated with extended low-surface-brightness 

nebulosity. The 0.2 Jy compact radio source seen near that position (at ISh 45m 05.s7 -02 0

03' 

43") is unresolved, and may not be associated with the W43 complex. The general agreement 

between the far-infrared and radio recombination emission indicates that absorption of photons 
9 

from the hot stars which ionize the H II region is responsible for heating the dust radiating in the 

far-infrared. 

Comparison of the 50 anc. 100 Jlm maps reveals an overall similarity, but noticable differences 

in detail. For example, the position of peak flux in G30.0S is significantly different in the two 

maps; the 50 J.1lIl peak of this source is displaced to the north relative to its 100 Jlm position. 

Furthermore, the peripheral peaks #3 and #5 are much stronger at 100 Jlm than at 50 Jlm, as 

compared with G30.8N and G30.8S. This effect shows up well on the color temperature map, in 

which G30.8N and G30.8S have peak color temperatures of ===65°K while sources #3 and #5 

have color temperatures of ===4,soK, which is unusually cool compared with other lllI regions 

(Harvey, Campbell and Hoffman 1977; Thronson and Harper 1979). A recent map ofH2CO 

absorption across G30.8 (Beiging et. al. 1982) offers one explanation for this situation. This map 

was used to construct the map of H2CO equivalent width that is shown in Figure 1d. Although the 

H2CO beam is three times larger than that for the infrared measurements, it is clear that there is 

considerable structure in the molecular gas within the boundaries of our far-infrared map, with the 

largest absorptions toward sources #3 and #5. Using the relation of Federman and Evans (1981), 

AV===12W(H2CO), we expect several hundred magnitudes of visual extinction along the line of 

sight to at least parts of these sources. Assuming a A-I extinction law beyond 10 Jlm, this 

corresponds to several extinction optical depths around 100 Jlm and a differential extmction of 

order unity between 50 and 100 J.1lIl. We note that this molecular gas along the line of sight need 

only be colder than the emitting dust in the lllI region in order that flux be removed from the beam. 

This extinction could account for the faintness of these sources at 50 Jlm. 

The far-infrared sources along the northern border of G30.8 (sources #3, #l=N, and #4) might 

be interpreted as ionization fronts along neighboring molecular clouds, an interpretation supported 

by their ridge like structure at 6 cm. Even the lowest contour of the H2CO absorption map predicts 

a rather large extinction in the near-infrared, so the visibility of the complex at 12Jlm and, indeed, 

at shorter wavelengths (see below) would suggest that the molecular gas is clumped on a scale 

considerably smaller than the 2.'5 H2CO beam. 
10 

It can be seen from the spectra of G30.8N and G30.8S and the integrated spectrum for the 

entire complex (see Figure 3) that the extended emission around G30.8 is cooler than that from the 

core. This is also evident from the color temperature map in Figure 1c. This result is not 

unexpected in view of the extinction at the periphery of the nebula. Using a mean 50/100 Jlm color 

temperature of 51 oK for the entire mapped region, we derive Ftotal= 2.2 x 10-9 W-m-2 for the total 

integrated far-infrared flux of the mapped region, assuming a blackbody spectral distnbution as 

indicated in Figure 3. The contribution of the short wavelength wing of the spectral distribution 

(which is especially evident in the spectra at the peaks) to the total flux is negligible because the 

wing does not share the spatial extent of the 100 JlITI emission. At the assumed distance of7 kpc, 

the integrated flux corresponds to a luminosity of 3.3 x 106 Le. Within one beamsize (a projected 

diameter of 1.7 pc at the distance of G30.8), the luminosities of G30.8N and G30.8S are similar, 

about 2 x 105 LQ for each source. 

The fact that the dust color temperature peaks near the center of the complex indicates that a 

discrete luminosity source is in the vicinity. The peak of the color temperature is, interestingly, not 

coincident with either G30.8N and G30.8S, but lies in between them. Concentrations of 6 cm I 

emission are seen not only at the two main far infrared peaks, but also between them, where the 

infrared contours show a saddle minimum. This regIOn was observed with the NASA IRTF to 

look for a near-IR counterpart to this source. A bright source was detected at this peak, with 

K=8.3 at (1950) 18h 45m OO.sO -010 59' 54" ±2". In a 6" circular aperture, this source 

appeared unresolved. Although a complete survey of the region was not made, and measurements 

at other wavelengths were not obtamed, it seems plausible that this near-infrared source is the 

dominant lUminOSIty source for the core of G30.8. Morphologically, this source is near the center 

of symmetry of the rim sources discussed above. The VLA map shows a local maximum at the 

position of this near infrared source. Assuming that this source is a hot star, K=8.3 corresponds 

to a main sequence star of 5 x loS L9 in the absence of any 2Jlm extinction. In order to supply the 

observed total luminosity of 3.3 x 106 L@, we need to mvoke an extinctIOn of two magnitudes, or 

A V=20m , which is not unreasonable. A more complete understanding of the role of this near-
1 1 

infrared source will have to await a more thorough survey of the region. It will also be necessary to 

consider the possibility that the 2JlII1 emission may be seriously contaminated with dust or gas 

emission. 
12 

b) G25.4-0.2 

Our infrared map for this source (Figure 4) clearly distinguishes two components, which we 

will designate as G25.4SE and G25.4NW. We note that G25.4NW is somewhat more extended 

in the far-infrared than is G25.4SE, and that the blackbody color temperature at the peak of 

G25.4NW (59°K) is substantially lower than that for G25.4SE (800K). These two components are 

also seen in the VLA map of the complex (Figure 5). These two radio components have also been 

detected, though with much more limited uv-plane coverage, by Felli, Tofani, and D'Addario 

(1974) and Krassner et. al. (1983). The 15 GHz single-dish map of Schraml and Mezger (1969) 

shows the source to have a pronounced asymmetry, which we now understand in terms of its 

duplicity. 

There has been considerable uncertainty about the distance to the G25.4-0.2 complex. The 

brightest component of H109a in this direction is at 59 km-s-1, which corresponds to a 

galactocentric distance of 6.2 kpc (Churchwell et. al. 1978'). Absorption near the tangent point 

velocity is seen in H2CO (Wllson 1972; Downes et. al. 1980) and OH (Turner 1979), which 

would ordinarily imply the far kinematic distance of, m tills case, 13.5 kpc. This value can be 

questioned, however. Comparing the measured 9532A [SIll] surface bnghtness of G25.4SE 

(Figure 6) with the radio continuum surface brightness, as estimated by the flux in a siffillar beam 

from our VLA data, and making the reasonable assumption that most of the sulfur m the nebula is 

doubly ionized, we require 4.7 magmtudes of extinction at 9532A, which corresponds to about 11 

magnitudes of visual extinction. If G25.4SE were at a distance of 13.5 kpc, such a small 

extinction, for a source seen directly through the galactic disk, would be very surprising. We note 

parenthetically that, to our knowledge, this is the first map of a heavily obscured HIT region in the 

9532A line. It should be noted that this line will dominate I-band photometry ofHII regions and, 

in the case of intrinsic silicon detectors, which exclude HeI 10830A, will dominate the nebular flux 

at wavelengths longer than Hex. 
13 

There are several possible explanations that might be invoked to overcome this problem. One 

possibility, that of a low extinction "tunnel" through the galactic plane, seems rather far-fetched. 

Downes et. al. (1980) noted that the apparent constancy of H2CO line depth with beamsize 

suggested that this source was unusual, and fIrst suggested the possibility of beam contamination 

by other sources at different distances along the line of sight. Our CO and small-beam 

recombination line measurements have verifIed this suggestion, with the surprising twist that it is 

G25.4SE and G25.4NW themselves that lie at very different distances, and that chance 

superposition accounts for their apparent proximity. Figure 7 shows the H86c:x profiles in the 

direction of these two components. Since the two components are separated by 2.'6, we can 

obtain a line measurement from each of them separately, and expect only slight contamination from 

the other in the skirt of the 3. '15 FWHM beam of the 140-foot telescope. It is easily seen that the 

recombination line velocities differ greatly for the two components. G25.4SE, whose spectrum is 

shown at the top of Figure 7, has the stronger line of the two, with VLSR= +59.1 km-s-1. This 

is the component that dominated the earlier, large-beam, emission line velocities. G25.4NW, 

whose spectrum is shown at the bottom of Figure 7, has VLSR= +99.0 km-s-I; its spectrum is 

slightly contaminated on the low velocity side by the stronger line of its apparent neighbor. Both 

lines are also visible in an HI03a spectrum centered on G25.4SE in a 5.1' beam. The +99.0 kms-I component was actually detected in the earlier work on this source by Churchwell et. al., but 

was inconspicuous in their 2.6' beam which was centered on G25.4SE. It seems likely that it is 

G25.4NW against which the tangent-point H2CO absorption is seen in the large-beam studies, an 

interpretation that can be directly checked when H2CO absorption maps are made of this complex 

with a small beam. Barring an extraordinary dynamical situation, a picture thus emerges in which 

G25.4SE is at the near distance of 4.6 kpc, while G25.4NW is just beyond the tangent point, at a 

distance of about 12 kpc, and their apparent proximity is a projection effect. In view of the much 

larger distance to G25.4NW and the fact that its emission measure is similar to that of G25.4SE, 

the difference in 9532A flux between the two sources is understandable as an extinction difference. 
14 

Our CO measurements add further to our understanding of this region. Figure 8 shows the 

MWO maps of 13CO (2-1) in the two strongest components, which correspond approximately to 

the dominant radio recombination line velocities. Shaver et. at. (1982) and Leisawitz and Bash 

(1984) find evidence in several HII regions for velocities which are anomalous with respect to 

their galactic position, though the differences are generally an order of magnitude smaller than the 

velocity difference between G25.4NW and G25.4SE. On the other hand, the difference between 

the ionized gas velocity of +59 km-s-1 and the strong CO component at +65 km-s-l for G25.4SE 

is not unusual. Israel (1978) has shown that such velocity differences between compact IllI 

regIons and their associated molecular clouds are common, and seen in many less obscured IllI 

regions. The less obscured regions are presumably on the near sides of their molecular clouds, 

and the ionized gas is seen boiling off the cloud surface at thermal velocities. 

It is clear from the CO map that the velocity structure of the molecular gas is also complicated 

in thIS region, with the 95 km-s-1 cloud lying farther to the west than the 65 km-s-l cloud. This 

offset is in the same direction as that seen in the H86a. data. Although the compact IllI regions are 

not at the centers of their associated molecular clouds, it can be seen from this map that the G25.4 

region has two distinct, high-column-density, molecular clouds at very different velocities that are 

superimposed. 

The small-scale near infrared structure was investigated with the IRTF using a broadband 10 

JlII1 fIlter, 6" aperture, and 30" chopper throw. We find a 6" diameter (FWHM) source at the 

center of G25.4SE with an unresolved core. The peak position of this source is (1950) 18h 35m 

33.s6 -6 0

50' 31" ±2", which is coincident with the radio, far-infrared and [SIll] peaks. The flux 

of this source is 10 Jy at 10 ~m. This compares with values of 55 Jy in an 18" beam measured by 

Zeihlc (private communication), and 130 Jy from the AFGL survey. Therefore, a large part of the 

10 ~m emission from G25.4SE is in an extended envelope. A southern extension to the peak can 

be fit with a second source with one-third the flux of the main peak at a distance of 6". The near 

infrared structure of G25.4SE is thus very SImilar to that seen at 6 cm in our VLA map. 
15 

The spectral energy distribution for 025.4-0.2 and the two peaks is shown in Figure 9. We 

find that the color temperature peaks at the position of each component, as it must for separate 

luminosity sources. Assumir.g an average 50/100 Jlm color temperature of 65°K for the entire 

mapped region, and using the integrated fluxes at those wavelengths, we derive Ftotal= 1.4 x 10-9 

W-m-2. Slightly more than half of tlus flux lies within a 3' radius of G25.4SE. Like the peaks m 

G30.8-0.0, these sources are optically thin in emission when the flux is averaged over the farinfrared beam, with 't< 0.01 at 100 11m. It can be seen from Figures lc and 4c that 025.4SE is 

hotter than G25.4NW and all of the sources in G30.8-0.0. Since G25.4SE is also the closest of 

all of these sources, the higher color temperature within our fixed-angular-size beam may result 

simply from sampling the hottest dust, if all of the sources have a common structure with 

temperature gradients decreasing outward from the sources of excitation. 

G25.4SE may be ionized and heated by a single star, since the dominant core component is 

unresolved at a projected size of 0.1 pc at 4.6 kpc. At this distance, the source has a luminosity of 

4 x 105 L0 , which is consistent with a single hot, main-sequence star. G25.4NW is seen from the 

VLA map to have a double structure, with the second peak lying 15" farther toward the NW. This 

structure may at least partly account for the somewhat extended appearance of G25.4NW in the 

far-infrared maps. Although it is somewhat fainter than G25.4SE at all wavelengths, the large 

inferred distance (12 kpc) gives it a luminosity of 2.5 x 106 L0 , much higher than that of 

G25.4SE. 
16 

N. COMPARISON OF 1NFRARED wrrn IONIZING LUMINOSITY 

Most of the short-wavelength (A< 20 J.l.m) emission from hot dust in galactic IllI regions can 

be accounted for by the absorption of resonantly trapped Lyc:x photons within the nebulae (see 

Wynn-Williams and Becklin 1974). In an ionization bounded IllI region, each original Lyman 

continuum photon from the ionizing star is degraded by successive ionization and recombination. 

This results in a single Lyc:x photon that is eventually absorbed by a dust grain, plus some number 

of higher order recombination lines that escape the nebula with negligible cooling effect. 

When the far-mfrared emission (A>20Jlm) is included in the total luminosity of a typical giant 

HIT region, Lyc:x heating is found to be insufficient to account for the thermal radiation from the 

dust. It seems likely that the additional heating is due to the absorption of stellar photons with 

A>912A by dust in the adjacent cloud material just outside the Stromgren sphere. This 

interpretatIOn is supported by detailed studies of regions like M17 (Gatley et. at. 1979) and 

G333.6-0.2 (Hyland et. at. 1980), which are likely to represent typical HIT region - molecular 

cloud interfaces. The relationship between ionizing flux and infrared emission can be probed 

directly by using the thermal brehmsstrahlung emission at radio wavelengths as a gauge of 

ionizing photon flux. Except for unusually dense and dusty compact HIT regions in which a 

signficant fraction of the Lyman continuum photons are absorbed directly by the dust before they 

are able to ionize hydrogen, It is usually assumed that the ratio of optically thin radio flux to 

integrated infrared flux is a measure of the ratIo of ionizing to total flux from the exciting stars, and 

thus of the spectral character of those stars (Mezger 1978; Mezger and Smith 1975). Following 

Emerson and Jennings (1978), we define an infrared excess for an IllI region as IRE= LIRlNc' 

hva , where LIR is the total luminosity of the IllI region, Nc' is the number of Lyman contmuum 

photons produced by recombination in the gas, and hv (l is the energy of a Lyc:x photon. Rubin 

(1968) gives the number of stellar Lyman continuum photons absorbed by the gas as a function of 

free-free radio luminosity for an ionization bounded nebula. Since one Lyc:x photon is produced 

for a single Lyman continuum photon, we thus denve 
17 

IRE = 1.538x109 (FIR' Sy) v-O.l Te0.45 (1) 

where FIR is the total flux from the region in W-m-2, Sy is the optically thin radio flux in Jy, 

V is the radio frequency in GHz, and T e is the electron temperature of the gas in oK. 

The radio flux with which the infrared flux is to be compared must be measured over the same 

region as the infrared map. Furthermore, since the infrared intensity is assumed to be zero below 

the lowest contour on the maps in Figures 1 and 4, the radio brightness temperatures must be 

treated analogously. The single-dish maps are probably the most appropriate to use to derive total 

radio fluxes. The 23 GHz map of G30.8-0.0 by Rodriguez and Chaisson (1978) is already well 

matched in spatial resolution to our far-infrared map, and gives a total flux of 46±7 Jy. Our 

assumption that the emission at this frequency is optically thin is justified by the similarity of this 

value to the 5 GHz integrated flux from the VLA map (57 Jy). We thus denve for G30.8-0.0 an 

IRE of 2.4 to 3.3, corresponding to an assumed electron temperature in the range 5000 to 10000K. 

This is consistent with the value derived by Emerson and Jennings (1978). 

For G25.4, the Schrarnl and Mezger (1969) map IS the best match to the infrared map. In this 

case, since there is less extended emission than in G30.8, the boundaries of the far-infrared map 

and that of the 15 GHz map are similar. From their total 15 GHz flux of 19.7 Jy, we find that 18.7 

Jy is within our mapped area. For assumed electron temperatures of 5000 and 10000K, we derive 

for G25.4 an IRE of 4.1 and 5.5 respectively. 

The derived IRE values for G25.4 and G30.8 are very similar to the infrared excesses of outerarm HIT regions that have been measured in the same way (Harvey.Campbel1and Hoffman 1977; J 

Thronson and Harper 1979). This result constrasts sharply with a recent study by Boisse et al. 

(1981). These authors have compared the mfrared surface bnghtness along the galactic plane 

measured with a wide-beam balloon-borne experiment with the radio continuum bnghtness 

temperature taken from the maps of Altenhoff et. al. (1970). Their companson IS made for 

measurements integrated into one degree bins of galactic longitude. They find a strong correlatIOn 
18 

of IRE with galactic longitude (and therefore, presumably, with galactocentric distance) such that a 

representative IRE for an HIT region complex in the 5 kpc ring is almost an order of magnitude 

higher than for complexes near the solar circle. Boisse et. al. fmd that in the region 1=30 to 31 0

, a 

region dominated by W43, the IRE is 26 compared with our value of about 3. 

The Boisse et. al. measurement assumed a constant ratio of integrated luminosity to 

luminosity within their long-wavelength passband. This requires that the color temperature not 

vary. As a result, it is worthwhile to check this claim of a high IRE with the more completely 

spectrally sampled data of Low et. al. (1977) and NIshImura, Low and Kurtz (1980), though 

their integrated beamsize of 30' is considerably smaller. Taking F5 GHz = 90 Jy for this beam 

and their derived 60-300 Ilm luminosity of 6xlO-9 W-m-2 , one derives an IRE in the range 4- 6 

which is only slightly larger than we denve m a 6' area. We can mvestigate this dIfference further 

by comparing infrared fluxes for W 43 taken in different apertures. Figure 10 shows the measured 

100 Ilm flux as a function of beam area A. The point correspondmg to the smallest aperture is 

from our study, in which the 6' chopper throw defines the spatial limit of sensitivity. The results 

of Hoffman, Frederick and Emery (1971) ,Low et. al. (1977) and Olthof (1974) with beamsizes 

of 12, 15, and 30' respectively, are also shown. The integrated flux in a 60x22' area IS from 

Boisse et. al. The resulting curve shows that, from eqUIvalent beam dIameters of 6' to 30', the 

100 Ilm flux from W43 increases as A0.35 ,implying a surface brightness varymg as r-1.35. For 

diameters larger than 30', however, the Boisse et. al. point suggests that the 100 Ilm flux varies 

even more rapidly than A. This would indicate a dominant contribution of emission on a scale of 

order 10 that does not correspond to ionized gas. The fact that thIS large-beam result is actually 

larger than an extrapolation assuming constant surface-bnghtness is difficult to understand, 

however, and may indicate an error in the Boisse et. al. analysis, perhaps in their conversion to 

integrated flux. 

We point out that a variation of IRE with beamsize is not unexpected, however. Such an effect 

can be predicted for a smgle OB association in which subgroups of rather dIfferent ages are being 

sampled by different beamsizes. Ho and Haschik (1981) have investigated the IRE that might be 
19 

expected from a typical initial mass function with different upper and lower mass limits, using the 

data in Panagia (1973).7 

7 Avedisova (1979) has redone the calculations ofPanagia (1973) with non-LTE model 

atmospheres. The agreement with the earlier result is excellent, especially for the 0 stars. For the 

purpose of this study, the Miller and Scalo (1979) IMF that was used by Ho and Haschik is not 

significantly different than that more recently derived by Garmany, ContI, and Chiosi (1982) 

Using their results, we find that an IRE of 2.6 WIll be produced by an ensemble of stars with 

normal IMF and maximum mass of :::::50Me, while an IRE of 26 corresponds to a population WIth a 

maximum mass of 22Mo• A 22Me star has a main sequence lifetime of :::::8x106 years (Chiosi, , 

Nasi, and Sreenivasan 1978). If we assume that the original subgroup that dominates the large 

scale emission from 1=30· had a velocity dIspersion of 20 km-s-1, which IS the turbulent velocity 

now seen in the core ofW43 (fumer et. al. 1974), a subgroup with the most massive star still on 

the main sequence having about 22Me would have expanded to approximately 160 pc by this time. 

This is exactly the projected beamsize (1°) of the Boisse et. al. study at the distance (7 kpc) of 

W 43. Such a velocity dispersion for the stars in an association is also justifiable on the basis of 

observations of young OB associations in the field (Blaauw 1964). 
20 

V. SUMMARY 

We have made far-infrared measurements of two inner-galaxy lID region complexes, G30.8-

0.0 and G25.4-0.2. Measurements of the prominent peaks were made at 40, 50, 100, and 175~1.m, 

and the regions were mapped over an area of 15' at 50" resolution, at 50 and lOOllm. These 

observanons have been combined with high resolution radio continuum maps and, for G25.4, 

radio recombination line and CO line measurements to better understand these regions, and to 

compare their properties with those of star-fonning regions nearer to the solar circle. We 

summarize our fmdings as follows: 

1) The far-infrared emission from each region is dominated by two sources. For both G25.4 

and G30.8, the distrIbution of the far-infrared emission is similar to that of the radio emission, 

indicating that OB stars provide most of the heating. 

2) There is evidence that extinction plays an important role in 030.8, even in the far-infrared. 

3) A near-infrared point source has been detected in G30.8 at the position of peak far-infrared 

color temperature. This source, which is in between the two main far-infrared components, may 

be the ionizing star for the core of G30.8. 

4) Our measurement of [SIll] 9532A from G25.4SE indicates that the extinction toward this 

source is very low (A V < 10m ). This low extincllon is dlfficult to reconcile WIth previously 

determined distance measurements to this source. Our new observations show that the two 

components of 025.4 are probably at very different distances, and are seen comcidentally 

superimposed. Previous large-beam distance determinations for this source were confused by this 
21 

coincidence. G25.4SE is probably at the near distance of only 4 3 kpc, which allows the relatively 

small extinction to be more easily understood, while G25.4NW is about 12 kpc away. 

5) Total luminosities of 3xIQ6 La and 4xl05 La are found for G30.8-0.0 and G25.4-0.2 SE 

respectively. Even though it appears slightly fainter than G25.4SE, G25.4NW is actually much 

more luminous because of its much larger distance, and is probably more like G30 8 in its total 

luminosity. Comparisons with radio continuum fluxes give infrared excesses of between 3 and 5. 

These luminosities and infrared excesses are sirmlar to those found for bright,well-studied Iffi 

regions in the solar circle, when the same projected area is considered. Far-infrared photometnc 

measurements of these regions need not require an anomalous IMF at 5 kpc from the Galactic 

center. 

We thank the staff and crew of the Kuiper Airborne Observatory for their expertise in the 

execution of this project, and Bruce Wllking and Marshall Joy for assistance with the observations. 

This work was supported in part by NASA Grant 05-003-511 to the University of Hawaii and 

NSF Grant AST-8312332 to the UniversIty of Texas. The airborne observations were made whIle 

D.F.L. and H.L.D. held NRC Associateships at the NASA Ames Research Center. H.L.D. was 

partially supported by NASA-U. Texas Research Interchange NCA2-0R781-201 and by the 

Robert A. Welch Foundation. 
22 

TABLE 1 

Positions and Fluxes 

Source lOOllmPeak Fpeak 

RA. Dec. (1 OOllm) (Radio) 

(1950) 

(Jy in 50" beam) 

030.8-0,0 (W43N) (5 GHz) 

(1) 030.8N 18h 45ffi OO.Os _1 0 58' 40" 2010 5.9 

(2) G30.8S 18 45 02.9 -2 01 00 2040 6.8 

(3) 18 45 09.1 -1 57 50 900 

(4) 18 44 47.6 -2 00 00 600 

(5) 18 45 00.9 -2 04 20 1200 

TOTAL 

Near-IR 

point-source 18 45 00.2 -1 59 54 [ K = 8.3 

025.4-0,2 

(1) 025.4SE 18 35 32.8 -6 50 35 3230 

(2) 025.4NW 18 35 25.0 -6 48 25 2630 

TOTAL 

fFdQ 

(lOOllm)(RadlO) 

(Jy) 

(5 GHz) 

11000 16.2 

15700 15.6 

3680 13.8 

6600 9.4 

6100 2.4 

43080 57.4 

<2" at 2J.l.m] 

(10 GHz) 

10700 12,2 

6240 5.8 

16940 18.0 
23 

REFERENCES * 

Altenhoff, W. J., Downes, D., Goad, L. Maxwell, A., and Rinehart, R 1970, Astr. Ap 

Supp/ ,1, 319. 

Avedisova, V. S. 1979, Astr. Zh., 19, 965. 

Baud, B., Habing, H.J., and Dort, J. H. 1979, in IAU Symposium 84, The Large-Scale 

Characteristics of the Galaxy, ed. W. B. Burton (Dordrecht: ReIdel), p. 29. 

Bieging, J. H., WIlson, T. L., and Downes, D. 1982, Astr. Ap. Suppl., 49, 607. 

Blaauw, A. 1964, Ann. Rev. Astr. Ap., 2, 213. 

Boisse, P., Gispert, R, Caron, N. WIjnbergen, J. J., Serra, G.,Ryter, C., and Puget, J. L. 

1981,Astr. Ap, 94, 265. 

Chiosi, C., Nasi, E. and Sreenivasan, S. R. 1978, Astr. Ap., 63, 103. 

Churchwell, E., Smith, L.F., Mathis, J., Mezger, P.G., and Huchtmeier, W. 1978, Astr. Ap., 

70, 719. 

Downes, D., Wilson, T. L., Beiging, J., and Wink, J. 1980, Astr. Ap. Suppl, 40, 379. 

Emerson, J. P., and Jennings, R E. 1978, Astr. Ap., 69, 129. 

Federman, S. and Evans, N. J. 1981 Ap. J., 248, 113. 

Felli, M., Tofani, G. and D'Addano, L. R. 1974, Astr. Ap., 31, 431. 

Gardner, C. D., and Thomasson, P. 1975, Astr. Ap., 45, 1. 

Garmany, C. D., Conti, P. S., and Chiosi, C. 1982, Ap. J., 263, 777. 

Gatley, r., Becklin, E. E., Sellgren, K., and Werner, M. W. 1979, Ap. J., 233, 575. 

Gordon, M. A., and Burton, W. B. 1976, Ap. J., 208, 346. 

Harvey, P. M. 1979, Pub. A.S P., 91, 143. 

Harvey, P. M., Campbell, M. F. and Hoffman, W. F. 1977, Ap. J., 211, 786. 

Hawley, S. A., and Grandi, S. A. 1977, Ap. J., 217,420. 

Ho, P. T. P., and Haschik, A. D. 1981, Ap. J., 248, 622. 

Hoffman, W. J., Frederick, C. L. and Emery, R J. 1971, Ap. J. (Letters), 170, L89. 

Hyland, A. R, McGregor, P. J., Robinson, G., Thomas, J. A., Becklin, E. E., Gatley, I., and 

Werner, M. W. 1980, Ap. J., 241, 709. 

*Format dIffers from standard NASA publIcatIOn If you have questIons about a partIcular reference, 

please qUestIOn the author(s) or the NASA contact pomt lIsted on the standard bIblIography page 
24 

Israel, F. 1978, Astr. Ap., 70, 769. 

Ito, K., Matsumoto, T., and Uyama, K. 1977, Nature, 265, 515. 

Krassner, J., Pipher, J. L., Savedoff, M. P., and Soifer, B. T. 1983, A.J., 88, 972. 

Leisawitz, D., and Bash, F. N. 1984, Ap. I., in press. 

Lester, D. F., Dinerstein, H. L., Werner, M. W., Watson, D. M., and Genzel, R. 1983, Ap. 1, 

271, 618. 

___ . 1985, in preparation. 

Lockman, F. J. 1976, Ap. I., 209, 429. 

Low, F. J., Kurtz, R. F., Poteet, W. M., and Nishimura, T. 1977, Ap. 1. (Letters), 214, 

Ll15. 

Mezger, P. G. 1978, Astr. Ap., 70, 565. 

Mezger, P. G., Pankonin, B., Schmid-Burkg, J., Thurn, C., and Wink, J. 1979, Astr. Ap., 

80, L3. 

Mezger, P. G. and Smith, L. F. 1975, in [AU Symposium 75, Star Formation, ed. T. de 

Jong, and A. Maeder (Dordrecht: Reidel), p.133. 

Miller, G. E., and Scalo, J . M. , 1979, Ap. 1. Suppl., 41, 533. 

Nishimura, T., Low, F. J. and Kurtz, R. F. 1980, Ap. 1. (Letters), 239, LlO1. 

Olthof, H. 1974, Astr. Ap., 33,471. 

Panagia, N. 1973, A. I., 78, 929. 

Pipher, J. L., Grasdalen, G. L., and Soifer, B. T. 1974, Ap. I., 193, 283. 

Rodriguez, L. F., and Chaisson, E. J. 1978 Ap. I., 221, 816. 

Rubin, R. H. 1968, Ap. I., 154, 391. 

Schraml, J., and Mezger, P. 1969, Ap. I., 156, 269. 

Shaver, P. A., Radhakrishnan, Y., Anantharamaiah, K. R., Retallack, D. S., Wamstecker, W., 

and Danks, A. C. 1982, Astr Ap., 106, 105. 

Scoville, N. Z., and Solomon, P. M. 1975, Ap. I., 201, 352. 

Taylor, J. H. 1979, m [AU Symposium 84, The Large Scale Characteristics of the 

Galaxy, ed. W. B. Burton (Dordrecht: ReIdel), p. 119. 

Thronson, H. A. Jr., and Harper, D. A. 1979, Ap I., 230, 133. 

Turner, B. E. 1979, Astr. Ap. Suppl., 37, 1. 
25 

Turner, B. E., Balick, B., Cudaback, D. D., Heiles, C., and Boyle, R. J. 1974, Ap J., 194, 

279. 

Wilson, T. L. 1972, Astr. Ap., 19, 354. 

Wynn-Williams, C. G., and Becklin, E. E. 1974, Pub. A. S. P., 86,5. 
26 

FIGURE CAPfIONS 

Figure 1--- (a) upper left: 50Jlm and (b) lower left: 100Jlm maps of the G30.8 complex at an 

angular resolution of 50"; the beam is indicated in the upper left-hand corner. The designated 

source numbers are indicated in the l00Jlm map. The solid triangles (~) indicate the positions of 

the three prominent 100Jlm peaks. The asterisk (*) indicates the position of the unresolved 2Jlm 

source that may correspond to the ionizing star. The contour units for the 50Jlm map are 100, 300, 

600, 900, 1200, 1500, and 1800 Jy per beam. The contour units for the 100Jlm map are 300, 600 

900, 1200, 1500, 1800, and 2100 Jy per beam. (c) upper right: the color temperature map derived 

from the 50 and 100Jlm maps; the contours are 45, 55, 65, 75, and 85°K. (d) lower right: the 

integrated H2CO equivalent width derived from the synthesis maps of Beigmg, Wilson and 

Downes (1982). The much larger beam (2.5' diameter) is indIcated in the corner. The units are 

km-s-1 equivalent width. Comparing this map with the 50 and 100Jlm maps it can be seen that the 

H2CO absorption is strongest over those far infrared components with the lowest apparent color 

temperature. As explained in the text, this is understandable in terms of large extinction optical 

depths. 

Figure 2--- G30.8 is shown in a hybrid array VLA map at 6 cm. Contour levels are 2,3,4,5, 

6, 7,8,9, 10, 15,20,25,30 x 100 mJy per beam. The beam size and orientation is described in 

the text, and the symbols are as for Figure 1. ThIS map is strikmgly similar to the l00Jlm map in 

Figure 1b, showing that the stars which ionize the gas are probably responsible for heating the 

dust. The point source at the lower left may be a background source. The penpheral clouds of 

ionized gas mare seen to be arc-like, and are approximately centered on the near-IR source which is 

indicated by a plus sign (+), and which is also the position of peak dust temperature in thIS region. 

This object may be the source of excitation for most of the complex. 
27 

Figure 3 -- The far-infrared spectral distributions for various components of G30 S are shown. 

The lower two curves cOltespond to the spectral flux per beam centered on the peaks G30.SN and 

S (sources 1 and 2 in that ccmplex). Photometry for these sources was done in four bandpasses, 

which are indicated by horizontal lines at the bottom of the figure. A single error bar, which 

typifies the observational uncertainties in all measurements, is shown. Blackbody curves have 

been fitted through the 50 and l00J,lm points of these spectra, and these fits are indicated by the 

solid lines. The dashed lines are the actual spectral distributions, fitted by eye to all four 

wavelength points. The fact that the spectra are wider than blackbodies is indicative of 

nonisothermal dust distributions, but this excess width contributes negliglbly to the total flux from 

each source. In the uppermost curve, a blackbody curve is fitted through the integrated 50 and 

100J,lm fluxes for the entire nebula. This blackbody curve yields an approximate value for the total 

far-infrared flux form the G30.S complex. Note that the two upper curves are normalized by scale 

factors in order to distinguish the curves on the graph. 

Figure 4 --- (a) above: 50J,lm and (b) middle: 100J,lm maps of the G25.4 complex at an angular 

resolution of 50", which is indicated in the upper left-hand corner. The plus slgns (+) indicate the 

positions of the two peaks at 100J,lm. The contours for the 50J,lm map are 100,200,300,500, 

1000, 2000, 2500, 3000, and 3500 Jy per beam. The contours on the 100J,lm map are 300, 500, 

1000, 1500,2000,2500, and 3000 Jy per beam. (c) bottom: color temperature map denved from 

the 50 and 100J,lm maps; contour units are 50, 60, 70, and SOoK. 

Figure 5--- VLA map of G25.4 at 6 cm. Contour levels are 4, 5, 6, 7, 8, 9, 10, 15,20,30, 

40,50,60, and 70 x 100 mJy per beam. The beam size and orientation is described in the text. 

This map was made mainly with a large antenna spacing, so low level extended emission is 

undersampled. G25.4SE and NW are the only sources visible. 
28 

Figure 6---A map of G25.4SE at [SIII] 9532;\ made with a 30" aperture. Contour units are 

10-12 erg-cm-2- s-1 in this aperture. The positions of peak flux at this wavelength is probably 

consistent with that of the radio continuum and far-infrared peak to within the errors of 

measurement 

Figure 7---H86a spectra ofG25.4SE and NW are compared. Note the pronounced velOCIty 

shift (=40 km-s-1 ) between these two compact IllI regions that are only 2.5' apart in the sky. 

These spectra show that the two components, previously thought to be physically associated, are 

just coincidentally superimposed. The corresponding recombination line of helIum is barely visible 

at V",,-80 km-s-1. 

Figure 8--- 13CO (2-1) maps ofG25.4 taken in a l' (FWHM) beam atMWO. Contours in OK 

antenna temperature are indicated separately for the two dominant velocity components. The 

molecular gas reflects the complicated velocity structure that is found from H86a in the ionized gas. 

The far-mfrared centroids of G25.4SE and G25.4NW are indicated for reference. 

Figure 9--- Same as for Figure 3 except for G25.4-0.2. 

Figure 10--- lOO!lm fluxes ofW43 from different investigations are compared. The integrated 

fluxes are plotted as a function of effective beam size. The cited values, in order of decreasmg 

beam size, are from BOlsse et. al (1981), Olthof (1974), Low et. al. (1977), Hoffman, 

Frederick, and Emery (1971), and this paper. 
_1° 551 I-@ 

BEAM 50p.m 

- 2°001 

-

0 

lO 

en 

'>:1 - 2° 051 

~. C 

0 

1-':;: -1 0

55' ~ -I- ~ 1 -I N 

0 \0 

C I 100 p.m 0 \ 

Q) 

a 

~ 3 

- 2°001 

2 

'--~ J j 5 

- 2°051 

10' ISh45mOO' 50' 10' ISh45mOO' 50' 

Right ascension (1950) 
-

o 

d 

&0 

en 

-

-

30 

ISh45mIO' ISh45mO' 

RIGHT ASCENSION (1950.0) 

Fig. 2 
3.6 

3.4 

> :;. 

:I. U. 

Cl 

0 

3.2 

3.0 

G30.8 

TOTAL-8 

G30.8N 

II 

I. 

PEAK X 1.4 / 

/ 

P /. / / / / 

/ 

d 

/ 

/ 

/ 

G30.8S 

PEAK 

I 

I 

31 

T = 511< € = CONST 

h 

I 

I 

Fig. 3 
32 

, i ,--r-, 

I @ -6 4S' ~ 

SO"m 

~ BEAM 

~ L 

4>wt I 

1 I I 

~J, -

-

I 

"1 

I 

i 

-&' 45' I 

I 100 11m 

r 

to L (G) '" i a 

- ~ 

j Z 

0 

~ -6' 50' 

Z 

~ 

'" ~ U 

w 

C 

-S' 55' 

~ 

-&' 45' ~ i 

TC 

~ ~ I 

] 

-6' 50' 

~: J 

~ 

~ ~ 

J 

I 

I 

-6' 55'1- i i I I 

50' 40' 30' 20' 10' 

lSh 35"+ 

RIGHT ASCENSION (1950) 

Fig. 4 
33 

- 6°4910" 0 

-

q 0 0 

It) -b en 

-

z 

0 

ti 

~ 

- 6°50'0" ..J 

u 

"" Q ~ ~ 

• ~ Q 

ISh35m33s ISh35m30s ISh3Sm27s 

RIGHT ASCENSION (1950.0) 

Fig. 5 
o 

an 

en 

-6°49' 

::. -6"50' 

z 

o 

i= 

<t 

Z 

-J 

(,) 

W 

C 

G25.4-0.2SE 

[Sill] 9532A 

34 

36$ 34$ 32$ 30$ 

1Sh 35m + 

RIGHT ASCENSION (1950) 

Fig. 6 
35 

Ha6 a 

0.24 

0.16 G25.4 SE 

G25.4 NW 

·0.04~ ________ ~ ______ ~ ________ ~ ______ ~~ ______ ~ 

-100 0 100 200 

VLSR (KM-S-tl 

Fig. 7 
-c .-E 

u 

~ 

c 

-

U 

LU 

C 

<J 

36 

13CO (2-1) 

G25.4 -0.2 

••••••••• 65 km/s 

95 km/s . , ~ , ~ \ 

, \ ~ : ~ \ 

,., ... ,.... ...------- ~ \ ... ~"" ... ", .... , \ 

.- , .. ~ ~'....... " •• - "e.. -. .....:. \ • , I e. e. \I, , . 411...,........ -\ -. .' ! .. ~ , -. ... .... - - a_. '\ 1 •• ,""~, I : .. t \ -. _. II, : :, A-..... \ e.... ~ " i + N W J : : I: -.\ -•• \ , • I I • • t. e •• ,. ~ '_fill' : I , : :. ~ ~ . • • _, -,. • : : I I 

: ::, 9. ~ : ::.' ~ ~:, .". ~ : : I I e.. e.. ~. " \. \ : ._ I I 

• e.. • 

_.. -., _. " ·c· ,.... , , : • ..,; •••• -.' •• !'II. .- , , . -. ' :: : ...... , .- " -. -.. -.' ... ",. " -. ... -. '.... .-.-: + .: " -. ... . ... ~.. ~ , •••••• ••• •••••••• ·-. ... .; ... S E :~ ,: -. : ... , : I " ... : \:, ' , •• • '., I , 

• -. \:, ' I .. -. ,. \ ' I : -.. ': , __ ,' I 

• ............. e •• : : I : • ••• I · : e.... ~ I :: ~\. , · . . ~ -. " :':. :.'lIt-.' 

: -. : : ~~--'- · -. . : ~ •..•.. ~:.:.,,;, ...... , ... 

T 

I arcmin 

..l 

a RA (aremin) 

Fig. 8 
> :3-

3.6 

~ 

u.. 

c:n 3.4 

,g 

G25.4 

TOTAL 74 

/ 

I 

G25.4NW I 

PEAK 1 

200 

r 

~ 

37 

It-I ----l 

100 

A (pm) 

Fig. 9 

G25.4-0.2 

T = 61K e = CONST 
38 

W43 

5.8 

FAR·IR FLUXES 

vs. APERTURE AREA -I5.4 

-

E 

::L 

0 

0 

.... :::,. 

LL 

Cl 

..!2 

5.0 

4.6 

o 1 2 3 

log A (arcmin 2) 

Fig. 10 
AUTHORS ADDRESSES: 

R. L. BROWN 

National Radio Astronomy Observatory 

Edgemont Road 

Charlottesville, VA 22901 

H. L. DINER STEIN 

N. J. EVANS II 

P. M. HARVEY 

D.F.LESTER 

Department of Astronomy 

UniversIty of Texas at Austin 

Austin TX 78712 

M. W. WERNER 

39 

Astrophysical Expenments Branch MS245-6 

NASA Ames Research Center 

Moffett Field CA 94035 
1 Report No I 2 Government Accession No 3 Recipient's Catalog No 

NASA TM-88184 

4 Title and Subtitle 5 Report Date 

STAR FORMATION IN THE INNER GALAXY: A FAR-INFRARED AND RADIO December 1985 

STUDY OF TWO H I! REGIONS 6 Performing Organization Code 

7 Author(s) 8 Performing Organization Report No 

t t D. F. Lester*.tH. L. Dinersteini. M. W. Werner. P. M. Harvey • 86008 

N. J. Evans II. and R. L. Brown 10 Work Unit No 

9 Performing Organization Name and Address 

*University of Texas at Austin. Austin. TX 78712 

tAmes Research Center. Moffett Field. CA 94035 11 Contract or Grant No 

tUniversity of Texas at Austin. Austin. TX 78712 

§Nationa1 Radio Astronomy Observatory. Charlottesville. VA 22901 13 Type of Report and Penod Covered 

12 Sponsonng Agency Name and Address Technical Memorandum 

National Aeronautics and Space Administration Sponsoring Agency Code Washington. DC 20546 14 

352-02-03 

15 Supplementary Notes 

Preprint Series 1138. Supported by NASA grants. 

Point of Contact: L. C. Haughney. Ames Research Center. MS 211-12. Moffett Field. CA 94035. 

(415) 694-5339 or FTS 464-5339 

16 Abstract 

Far-infrared and radio continuum maps have been made of the central 61 of the inner-galaxy 

HI! regions G30.8-0.0 (in the W43 complex) and G25.4-0.2. along with radio and molecular l~ne 

measurements at selected positions. The purpose of this study is an effort to understand star 

formation in the "molecular ring" at 5 kpc in galactic radius. Measurements at several far 

infrared wavelengths allow the dust temperature structures and total far infrared fluxes to be 

determined. Comparison of the radio and infrared maps shows a close relationship between the 

ionized gas and the infrared-emitting material. There is evidence that parts of G30.8 are 

substantially affected by extinction. even at far-infrared wavelengths. 

For G25.4-0.2. our radio recombination line and CO line data allow us to resolve the distance 

ambiguity for this source. The large distance previously ascribed to the entire complex is found 

to apply to only one of the two main components. The confusion in distance determination is found 

to result from an extraordinary near-superposition of two bright H II regions. Using our revised 

distances of 4.3 kpc for G25.4SE and 12 kpc for G25.4NW. we find that the latter. which is 

apparently the fainter of the two sources. is actually the more luminous. Though it is not seen 

on the Palomar Sky Survey. G25.4SE is easily visible in the 9532A line of [S III] and was mapped 

in this line. 

The ratio of total luminosity to ionizing luminosity is very similar to that of H II regions 

in the solar circle. Assuming a coeval population of ionizing stars. a normal initial mass 

function is indicated. 

17 Key Words (Suggested by Author(s) I 18 Distribution Statement 

Far-infrared: two H II regions Unlimited 

Star formation: inner galaxy 

Subject category - 89 

19 Secunty Oasslf (of thiS report I 20 Security Classlf (of thiS pagel 21 No of Pages 22 Price" 

Unclassified Unclassified 41 A03 

'For sale by the National Technical Information Service, Springfield, Virginia 22161 
End of Document