THE ASTROPHYSICAL JOURNAL, 550 :301È313, 2001 March 20( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.

MULTIFIELD MOSAIC OF THE NGC 7538 REGION

X.-W. ZHENG,1,2 QIZHOU ZHANG,2 PAUL T. P. HO,2 AND PREETHI PRATAP3Received 2000 April 19 ; accepted 2000 November 8

ABSTRACTWe present mosaics of six Very Large Array images made in the (1, 1) and (2, 2) lines towardNH3NGC 7538. Both the (1, 1) and (2, 2) emissions show clumpy and Ðlamentary structures. The denseNH3clumps are found near water maser emission or near the youngest member of groups of infrared sources.

At the edges of clumps, temperature enhancements indicate external heating possibly by the nearby H II

regions. The outÑows in this region are conspicuously located in the voids between the clumps. Becausethere is more mass in the dense clumps than in the outÑows, we suggest that the dense medium shapesthe outÑows and channels the swept-up material to the low-density regions.Subject headings : H II regions È ISM: clouds È ISM: individual (NGC 7538) È

ISM: kinematics and dynamics È masers È stars : formation

1. INTRODUCTION

In the cycle of star formation, outÑows play an importantrole not only in the formation of young stellar objects butmay also in triggering the next generation of star formation(Lada 1985 ; Fukui et al. 1986 ; Bally & Lane 1991). Super-sonic winds from young stars accelerate the surroundingmolecular material, creating shocks and injecting turbulentenergy into their environments. The surrounding molecularclouds are compressed, heated, and possibly shaped anddisrupted by these violent activities. The resulting conden-sations in these regions could collapse and form new stellarobjects (e.g., Wiseman & Ho 1996). The various interactionsoccur on scales from less than 0.01 pc to tens of parsecs. Theability of the Very Large Array (VLA) to image Ðelds ofmany arcminutes with an angular resolution of arcsecondshas made it possible for us to study these activities.

The NGC 7538 molecular cloud contains a particularlyrich collection of star-forming objects. In particular, infra-red and radio observations reveal a sequence of progres-sively younger objects moving from northwest to southeast.In the optical nebula north of the cloud, several stars arepresent within a large H II region which is in the process ofclearing material from the region. The high-density coreregion associated with IRS 1, 2, and 3 is a very active star-forming region on the southern edge of the optical nebula(Werner et al. 1979 ; Henkel, Wilson, & Johnston 1984 ;Keto 1991). A high-velocity CO outÑow is associated withIRS 1, and two more outÑows are associated with IRS 9 and11 located farther south (Kameya et al. 1986, 1989 ; Daviset al. 1998). Therefore, this region provides a goodopportunity to study the di†erent stages of evolution asmanifested.

In this paper we present mosaic maps of six VLA Ðeldstoward the NGC 7538 region for both the (1, 1) andNH3(2, 2) lines. We used the density, temperature, and velocity-position maps with high resolution and high sensitivity tostudy the conditions in dense cores that form young stellarobjects and to resolve the interaction of the H II regions andoutÑows with their environments. We report the obser-

1 Department of Astronomy, Nanjing University, Nanjing 210093,China.

2 Harvard Smithsonian Center for Astrophysics, 60 Garden Street,Cambridge, MA 02138.

3 MIT Haystack Observatory, Route 40, Westford, MA 01886.

vations and data reduction in ° 2, the results in ° 3, dis-cussions of IRS sources and their environments in °° 4, 5,and 6, and the conclusions in ° 7.

2. OBSERVATIONS AND DATA REDUCTION

2.1. V L AThe observations using the VLA4 were carried out in

1990 and 1995 in the compact D conÐguration. Weobserved the (J, K) \ (1,1) and (2, 2) inversion lines of NH3at 23694.495 and 23722.633 MHz toward NGC 7538. Thebaselines ranged from 40 to 1000 m. A total of six Ðelds (seeTable 1) was observed, covering an area of about 12 squarearcminutes. Fields 1 and 2 were observed on 1990 January28, and the rest of the Ðelds were observed on 1995 May 19.The primary beam of each VLA antenna is approximately2@ FWHM at 1.3 cm wavelength. Our Ðeld centers weretherefore separated by 1@ for complete sampling. The totalon-source integration time was approximately 2 hr per Ðeld.A bandpass of 6.25 MHz was employed to cover the mainand the two inner satellite electric quadruple hyperÐnelines. The spectra were resolved into 63 independent chan-nels of 97.66 KHz (about 1.24 km s~1 at the observingfrequencies). The central channel was set to [59 km s~1(LSR velocity).

The visibility data scans were edited for gain and phasediscontinuities before calibration. The source 2229]695(1950) was used to track the instrumental and theatmospheric gain and phase. Our primary Ñux calibrationwas referred to 3C 286, assumed to be 2.43 Jy at the (1, 1)and (2, 2) frequencies. The bandpass was calibrated byobserving 3C 273.

The continuum was collected from the line-free channelsand was subtracted from all 63 channels to form the linedata. The six calibrated Ðelds were then imaged in MIRIADusing the linear mosaic algorithm. All the images presentedare in 1950 coordinates.

The emission in the NGC 7538 region is moreNH3extended than the size of the primary beam of the VLAantennas. Structures larger than 1@ are not sampled in theinterferometer data because of missing short-spacing infor-mation. This e†ect is shown in the images as extendedNH3

4 The VLA is operated by the National Radio Astronomy Observatoryand the Associated Universities, Inc., under cooperative agreement withthe National Science Foundation.

301

302 ZHENG ET AL. Vol. 550

TABLE 1

POINTING CENTERS OF NGC 7538 OBSERVATIONS

POINTING CENTER

(1950)

FIELD DATE OF OBSERVATIONS a d

1 . . . . . . 1990 Jan 28 23 11 36.6 61 11 47.92 . . . . . . 1990 Jan 28 23 11 44.7 61 10 58.83 . . . . . . 1995 May 19 23 11 53.58 61 10 54.14 . . . . . . 1995 May 19 23 11 35.07 61 10 39.75 . . . . . . 1995 May 19 23 11 25.98 61 11 06.16 . . . . . . 1995 May 19 23 11 27.11 61 12 13.3

NOTE.ÈUnits of right ascension are hours, minutes, and seconds,and units of declination are degrees, arcminutes, and arcseconds.

negative emission, which is difficult to remove by the stan-dard ““ CLEANing ÏÏ technique. To alleviate the problem, weadded zero-spacing Ñux to the interferometer visibilities.

In general, measurements using the single-dish telescopewould provide the zero-spacing Ñux. A technique for com-bining interferometer and single-dish line data wasdescribed in detail by Vogel et al. (1984) and Wilner &Welch (1994). The technique generates a set of simulatedvisibilities from a model of the large-scale brightness dis-tribution derived from the single-dish maps. Our approachmodels the zero-spacing Ñux only slightly di†erently fromthat described by Wilner & Welch (1994). Here we describethe practical aspects of our procedure.

We examined the single-dish line map of NGC 7538NH3taken by the Haystack antenna (Ho, Martin, & Barret 1981)and found that the large-scale line structure in theNH3source is much larger than the size of the primary beam ofthe VLA antennas. For such an extended source, the large-scale source brightness distribution within the primarybeam will be nearly uniform. The Ðrst step in modeling thelarge-scale brightness distribution is to produce an imagecube with a uniform distribution of Ñux across the spatialand velocity axes. The Ñux densities of the channel maps forsix Ðelds were estimated from the plots of the intensity-(u, v)distance obtained from the VLA. The image cube corre-sponds to the model of the large-scale line emission havingdeconvolved the single-dish beam response from the single-dish maps. The modeled image cube was then multiplied bythe interferometer primary-beam response (D2@) at 1.3 cmwavelength. A set of simulated visibilities was generatedfrom the sky brightness model. The visibilities contain a setof (u, v) points randomly distributed in the range of 0È0.7 kjwith a density of about 10 points per (kj)2. Finally, themodeled and observed visibilities of all six Ðelds were com-bined and Fourier transformed using the MIRIADpackage, followed by CLEANing. With the natural weight-ing of the visibilities, the synthesized beam in the combinedimage is about 8@@] 6@@ at a position angle of [68¡. Thenoise level per 1.2 km s~1 channel is about 8 mJy.

2.2. Haystack ObservationsSingle-antenna observations of a large section of the

cloud were made using the Haystack 37 m antenna. The(1, 1) and (2, 2) lines were observed during severalNH3sessions during 1997 November 12È20. The spectra were

spaced by 40A ; the beam at the ammonia frequencies is 1@.4.The aperture efficiency was measured with continuum scanson Venus. It was found to be 30%. The gain of the cooled

high-electron mobility transistor (HEMT) receiver at thesefrequencies is stable with elevation. Noise tube calibrationof the system temperature was done every 5 minutes andwas found to be about 80 K. Pointing was done on Venusand was accurate to less than 5A. Online correction of thegain of the telescope was performed on the data. Both lineswere observed in a frequency-switching mode with a band-width of 17.8 MHz over 4096 lags, resulting in a uniformspectral resolution of 5.24 kHz channel~1.

The data were read into the CLASS software package.The frequency-switched data were ““ folded,ÏÏ and poly-nomial baselines were subtracted. Data from certain posi-tions where more than one 5 minute scan was taken wereaveraged before the baseline subtraction.

Figure 1 shows the (1, 1) spectra taken from theNH3Haystack 37 m telescope (top) and the VLA (bottom). TheVLA data were acquired from the position near IRS 11 andconvolved with the 90A beam. The appearance, the veloci-ties, and the intensity ratio of the main and satellite lines ofthe VLA spectrum are similar to these of the Haystack. WeÐnd that much of the single-dish Ñux is missing in the VLAdata.

2.3. T he Missing Flux and the Extended StructuresWe compare the total integrated Ñux from the VLA NH3maps to the single-dish Ñux as measured with the Haystack

37 m telescope (see Fig. 1) by convolving the VLA maps tothe angular resolution of the Haystack telescope (90A). WeÐnd that about 90% of the Ñux is missing in the interferome-ter maps. By adding a zero-spacing Ñux on the order of thesingle-dish Ñux, the interferometer maps can recover anadditional 25% of the Ñux. This extra Ñux appears as an

FIG. 1.ÈComparison of the (1, 1) spectra taken from the Hay-NH3stack 37 m telescope and the VLA. The VLA data are convolved to a 90Abeam and taken from the position near IRS 11.

No. 1, 2001 MULTIFIELD MOSAIC OF NGC 7538 303

extended component at approximately the Ðrst contourlevel (25 mJy beam~1) in the individual channel maps.There are many small-scale clumps at the Ðrst contour level,which is the classic signature of the incomplete response ofan interferometer to extended structures. Upon smoothingof the map, this extended structure can be seen. It is clearthat the CLEAN algorithm cannot reach the correct solu-tion on the extended structure with so much of the shortspacing information missing. What we can conclude is thatthe missing Ñux must reside in structures larger than 1@.

Before interpreting the data we are concerned with threeissues :

1. Do our VLA structures reÑect the physical struc-NH3tures in the region?2. What is the relation between these dense clumps and

the missing Ñux of the extended structures?3. How are our VLA structures important in theNH3kinematics and star formation activities in the context of the

large amount of mass which is not seen in the maps?

To check whether our VLA maps depict physical struc-tures in the cloud, we compared our VLA maps withNH3maps of the dust at wavelengths shorter than 1 mm (E.Ladd 1998, private communication). The structuresNH3are well correlated with features seen also in the dust mapssuggesting that the structures are distinct morphologi-NH3cal features. What about the extended structures whichaccount for 90% of the Ñux? We think a key may be foundin the hyperÐne lines of the (1, 1) emission and theNH3(2, 2)/(1, 1) line ratios. In Figure 1, we compare the interfer-ometer spectrum with the single-dish spectrum. It appearsthat the extended emission is less optically thick and is alsocooler. While the optical depth of the main hyperÐne com-ponent 1)D 1.4 for the interferometer spectrum,q

m(1,

1)\ 0.5 for the single-dish spectrum. We note that inqm(1,

typical conditions in molecular cloud cores, the satellitehyperÐne components of are normally optically thin.NH3Thus, the hyperÐne ratios are reliable measures of theoptical depth (cf. ° 3.5 for more detailed discussions). Sincethe mass scales with the optical depth, the interferometer isseeing roughly one-third of the total mass in the region. Ifthe missing Ñux of the extended structure comes from asheetlike geometry, then the extended Ñux is spread over atleast 10 times the area of the VLA structures, based on theirÑux ratio. The contrast in column density between theclumpy streamer-like structures seen with the VLA and theextended material is then not very great, on the order of afactor of 3, just the ratio of optical depths. In that scenario,the VLA is detecting the part of the cloud which is clumpy.Alternatively, if the extended structure is more spherical innature, with a size scale along the line of sight comparableto the 3@ size seen projected on the sky, then the extendedmass is distributed over perhaps 100 times more volumethan the clumpy streamer-like structure. In that case, themass and volume density contrast would be quite substan-tial, with the clumpy structures being typically 30 timesmore dense and more massive as compared to an equivalentvolume immediately adjacent to it.

Given the extended CO emission seen in this region, aÑu†y geometry for the extended emission seems moreNH3likely than a sheetlike geometry. Then the VLA structurespresented here are likely signiÐcant and important morpho-logical structures that are closely associated with the starformation processes.

The data also suggest that the structure detected with theVLA is very clumpy and has a small Ðlling factor. Thebrightness temperature is where f is theT

B\ f Tex(1[ e~q)

Ðlling factor. As shown in Figure 1, the brightness tem-peratures of the single-dish and VLA measurements areabout 1.7 K and 0.17 K, respectively, much smaller than theexcitation temperature in the region. Assuming that theexcitation temperature of the gas equals the rotational tem-perature, we Ðnd that the Ðlling factor of the structuredetected with the VLA is a factor of 20 smaller than thatseen with a single dish. This strengthens the argument thatthe emission not recovered by the interferometer arises fromlow-density gas.

3. RESULTS

3.1. Integrated Maps and Filamentary StructuresFigure 2 shows the integrated intensity of the main com-

ponent of the (1, 1) and (2, 2) transitions, as well as theNH3superposition of the CO (2È1) integrated intensity map(Davis et al. 1998) on the structure. The structureNH3 NH3is located immediately south of the optical H II region ofNGC 7538. While there are many obvious condensations,one complex is associated with NGC 7538-IRS 1, IRS 2,and IRS 3, and another Ðlamentary structure is associatedwith the infrared sources IRS 9 and IRS 11. Low-densitymaterial undoubtedly surrounds these condensations as canbe seen in the CO map and as inferred from the missing Ñuxin the maps. CO outÑows identiÐed by the CO obser-NH3vations (Kameya et al. 1989 ; Davis et al. 1998) are mostconspicuous in being located in the ““ holes ÏÏ within the NH3distribution.

As seen by several authors (e.g., Ungerechts & Thaddeus1987 ; Schneider & Elmegreen 1979 ; Wiseman & Ho 1998),condensations, like beads on a string, are often strungtogether to form a Ðlament. Such Ðlamentary structures areapparent in Figure 2. For instance, some eight or nine con-densations of 25A in size are linked within the elongatedÐlament toward IRS 9 and 11. While the Ðlament adjacentto the optical H II region appears less complete, it is likelythe result of the strong absorption seen toward IRS 1 andpossibly the disruption of a strong CO outÑow.

The e†ect of the optical H II region on the molecularmaterial is best seen in the CO map where a ““ cavity wall ÏÏ isapparent as a steep gradient in the CO contours (see Fig.2c). This wall not only indicates the action of a wind thathas swept out a void from the surrounding molecular cloudbut probably reÑects local heating as well. The close align-ment of the Ðlament of gas with the wall of CO molec-NH3ular gas suggests that compression of the general interstellarmedium by winds may be conducive to the formation ofdense cores.

There are 11 compact infrared sources, IRS 1È11, distrib-uted within the optical H II region and the large-scale NH3structure. Sources IRS 1È3 are associated with three ultra-compact H II regions detected in the radio continuum.Sources IRS 4È8 are located within the optical nebula andhave bright optical counterparts, suggesting that these arefully evolved H II regions which have pushed back theirmolecular environment. Source IRS 4 is projected againstthe cavity wall seen in CO and may be located in the fore-ground with respect to the molecular cloud. Sources IRS 9and IRS 11 are located farther south or east of the opticalnebula. Their positions are o†set from the emissionNH3

304 ZHENG ET AL.

FIG. 2a

FIG. 2.È(a, b) Integrated emission of the (1, 1) and (2, 2) inversion transitions toward the NGC 7538 region. The data are obtained from aNH3 NH3six-Ðeld mosaic with the VLA. The range of the integration for is from [ 52.8 to [ 60.2 km s~1. The dashed contours mark the 50% and 100% of theNH3peak sensitivity level. The black contours denote the emission and are plotted at ^(1, 3, 5, 6, 8, 10, 12, 16, 20, 24)] 0.02 Jy km s~1. The red and blueNH3contours outline the blue- and redshifted CO outÑows in the region (Davis et al. 1998) The stars mark the near-infrared point sources (see Table 2). Thecrosses represent the maser positions. The size of the synthesized beam is indicated in the upper-left corner. (c) Integrated emission of the CO (2È1) lineH2O(contours) from Davis et al. (1998) overlaid on the (1, 1) integrated emission (gray scales). The CO data are taken with JCMT at D 20A resolution. TheNH3dashed box denotes the region shown in Figs. 2a and 2b.

peaks. The source IRS 11 is located at the edge of the mostprominent Ðlament in the integrated maps. The source IRS10 is just at the margin of our map.

The spatial extent of the three CO outÑows (IRS 1, IRS 9,and IRS 11 ; Davis et al. 1998) is identiÐed in Figure 2a. Allthree outÑows are oriented in approximately the samedirection. Copious shock-excited emission is alsoH2observed throughout the NGC 7538 region. In particular, acollimated ““ jet ÏÏ associated with the IRS 9 outÑow, as wellas possible bow shocks in both the IRS 1 and IRS 9 Ñows,have been detected (Davis et al. 1998).

In order to understand the relations between the Ðlamen-tary structures and the dynamics of the region, mosaics ofsix consecutive velocity channels are shown in Figure 3 inboth the (1, 1) and (2, 2) lines. The most prominentNH3Ðlament associated with IRS 11 is clearly deÐned in velocitychannels [56.5, [55.3, and [54.1 km s~1. We will discussthe velocity structures in detail in the following section.

3.2. Absorption against IRS 1We detect absorption against IRS 1 at the northern edge

of the structure. The line proÐle, i.e., the blueshiftedNH3

absorption with respect to the redshifted emission, is consis-tent with the expansion of gas as seen in CO. Absorptionwas not found toward any other infrared sources. From themain-to-satellite line ratios, we derive an average opticaldepth toward IRS 1 of 5.4 for the (1, 1) line and 9.4 forNH3the (2, 2) line. The apparent optical depth, deÐned as theratio of the line intensity to the continuum Ñux, is 0.5 for the(1, 1) line and 0.6 for the (2, 2) line. The di†erence in opticaldepth can be explained by an incomplete coverage of themolecular gas against the background continuum. A beamÐlling factor of D0.5 suggests an angular size scale of D6Afor the molecular cloud in front of the continuum source.The coverage and size of the cloud as determined above isonly an upper limit because dense molecular material notonly is found in front of the continuum but surrounds thecompact infrared and radio continuum source. A possiblegeometry for the molecular material may be an edge-ondisk.

The rotational temperature between the (1, 1) and (2, 2)levels can be derived in terms of the (1, 1) and (2, 2) opticaldepths. Assuming equal excitation temperatures and usingthe previously determined values of the line opacities, we

FIG. 2b

FIG. 2c

FIG. 3.ÈChannel maps of the (1, 1) and (2, 2) lines. The contour levels are drawn at 25 mJy beam~1] ([4, [3, [2, [1, 1, 2, 3, 6, 8, 10, 12, 16, 20).NH3The synthesized beam is indicated at the lower-left corner of the Ðrst panel.

MULTIFIELD MOSAIC OF NGC 7538 307

obtained a rotational temperature for the absorbing gas, TR,

of 50^ 3 K.

3.3. Moment Maps and Velocity StructuresFigure 4 shows the Ðrst moment map (intensity-weighted

central velocity) of the (1, 1) transition, over the sameNH3spatial region as in Figure 2a, integrated over the velocityrange [60.2 to [52.8 km s~1. The integrated intensitymap of the (1, 1) transition is shown in contours, while thevelocity is shown in false colors. The most blueshifted emis-sion with the velocity [60 km s~1 is located immediatelyadjacent to the optical H II region. The most redshiftedvelocity is about [53 km s~1, which is found in the Ðla-ment associated with IRS 11. Figure 5 shows the secondmoment map (velocity dispersion) over the same spatialregion as in Figure 2a. The line widths are wider in gas nearthe sources IRS 1, IRS 9, and IRS 11. We Ðnd that thewidest line widths do not coincide with the emission peaks.This suggests that there is no necessary relationshipbetween the enhancement in line widths and the opticaldepth in the molecular cores. These are possibly due to theinteraction of the ambient molecular material with the COoutÑows seen toward these sources (Kameya et al. 1989 ;Davis et al. 1998). There are also several emission peaks in

Figure 5 which are also peaks in line widths. These caseswould be indications of radial motions of di†erent clumps.

3.4. Ratio Map and HeatingFigure 6 shows the (2, 2)/ (1, 1) ratio of the inte-NH3grated intensity of the main hyperÐne component. This

ratio reÑects the level population and is indicative of thetemperature. We Ðnd that the molecular gas around IRS 1,IRS 2, and IRS 3 appears to be centrally heated by theembedded young stars. The average rotational temperaturenear IRS 1 is 50 K. The temperature away from IRS 1 isestimated to be around 20 K. The decrease in temperaturenear IRS 1 scales roughly as R~3@5. We note that the rota-tional temperature is directly proportional to the line inten-sity ratios when the lines are optically thin. As shown in thenext section, the (1, 1) line emission is optically thin in mostof the region except for the Ðlaments toward IRS 11. There-fore, the (2, 2)/(1, 1) line ratios reÑect the true temperature. Ifthe (1, 1) line becomes optically thick, the (2, 2)/(1, 1) ratioswill overestimate the temperature. Carefully examiningFigure 6, we see that the emission peaks in the southernÐlament tend to be minimal in line ratios. Considering thecorrections for the optical depth e†ect, the contrast with thewarmer gas in between emission peaks will likely be

FIG. 4.ÈCentroid velocity (moment 1, in color) of the (1, 1) superposed on the integrated (1, 1) emission (contours). The contour levels are theNH3 NH3same as those plotted in Fig. 2.

308 ZHENG ET AL. Vol. 550

FIG. 5.ÈLine width of the (1, 1) line (color scale) superposed on the integrated (1, 1) emission (contours). The contour levels are the same asNH3 NH3those plotted in Fig. 2.

enhanced. This suggests external heating, similar to the caseof Orion (Wiseman & Ho 1998).

3.5. Optical Depth and MassFigure 7 shows the ratio of the satellite hyperÐne com-

ponent as compared to the main hyperÐne component forthe (1, 1) line. This ratio reÑects the optical depth of thetransition. A ratio of 0.4 corresponds to q\ 1 for the mainhyperÐne component, a ratio of 0.6 corresponds to q\ 4,while a ratio of 0.3 corresponds to q\ 0.2. We see, there-fore, from Figure 7 that the Ðlament associated with IRS 11clearly dominates the column density and, hence, the massdistribution in the cloud.

Assuming a relative abundance we[NH3/H2]\ 10~7,estimate the mass for the small clumps to be a few TheM

_.

most mass is concentrated in the Ðlament toward IRS 11.For an average q\ 2, T \ 25 K, and *V \ 5 km s~1, weestimate a total of 600 in the Ðlament.M

_

4. NATURE OF THE SOURCES IN THE NGC 7538 REGION

The NGC 7538 region has several luminous sourcesembedded in the molecular cloud and in the visible H II

region. Table 2 outlines the various methods and wave-lengths by which the sources were initially discovered and

by which the nature of the individual sources was deter-mined. The ease of detection at the various wavelengths andthe luminosity of the source at a particular wavelength canprovide clues to the evolutionary state of the star. The moreevolved stars are less embedded and are more likely to haveidentiÐable optical components. The younger stars aremore embedded and are strong emitters of radio and infra-red emission. There are sources which are so deeply embed-ded and at such an early stage of star formation that theyhave not broken through their parent molecular cloud. Thequestion can then be raised : have all the stellar or protostel-lar sources in the NGC 7538 molecular cloud been identi-Ðed? If not, what is the best way to identify possible sites ofearly star formation? We would like to show that theammonia observations described in this paper play a criti-cal role in making this identiÐcation.

4.1. IRS 4È8We start with the stars embedded in the visible H II

region. These stars appear to be the most evolved ones.Wynn-Williams, Becklin, & Neugebauer (1974) determinedthat IRS 5 is at the center of the optical nebula and has Ñuxdensities which agree with MartinÏs (1973) radio data forthermal emission from ionized hydrogen. IRS 6 is an O7star which is also associated with the nebula. IRS 7 has been

No. 1, 2001 MULTIFIELD MOSAIC OF NGC 7538 309

FIG. 6.ÈRatio of the integrated Ñux densities of the Ðrst satellite hyperÐne component and the main hyperÐne component of the (1, 1) line (colorNH3scale) superposed on the integrated (1, 1) emission (contours).NH3

identiÐed as a K-type star, and IRS 8 does not coincide withany optical or radio feature. IRS 7 and 8 are most probablynot associated with the NGC 7538 nebula. No molecularemission (CO or ammonia) has been detected toward thenebula. The CO map (Fig. 2c) shows a cavity that corre-sponds to the position of the nebula. The stars associatedwith it most probably indicate the Ðrst stage of star forma-tion in this region. IRS 4, in view of its signiÐcant 20 km Ñuxand its coincidence with a weak peak in the 2.7 GHz map(Martin 1973), is most probably a compact H II region. IRS4 is also situated close to a knot in the ammonia emission(Fig. 2a) and appears to be close to the interface between thenebula and the molecular cloud. This is consistent with theconclusion that its evolutionary state is similar to that ofIRS 2 and IRS 3.

4.2. IRS 1È3The infrared cluster consisting of IRS 1, 2, and 3 lies at

the edge of the molecular cloud with strong radio contin-uum emission and maser emission. This region has beenextensively studied in the radio. IRS 1 is identiÐed as anultracompact H II region with an associated molecularoutÑow. Several di†erent molecular masers, such as OH,

and have been detectedH2O, CH3OH, H2CO, 15NH3,toward this source. Interferometric observations of these

masers identify their positions to be close to the IRS 1 H II

region. An optical source has been associated with the radiosource which is o†set by about from the radio and2A.2infrared positions (Campbell & Persson 1988). This o†set isattributed to the scattering of light from the core sourcewhich is embedded in a dense column of dust with a calcu-lated extinction of greater than 60 mag. These data identifythe star to be an O6.5 zero-age main-sequence (ZAMS) star.The ammonia data presented here also conÐrm the largegas column density toward this source.

IRS 2 is a compact H II region with an extended sphericalstructure. The optical morphology shows a bright core sur-rounded by faint nebulosity. The properties of the core indi-cate that it is an O9.5 V star (Campbell & Persson 1988).They also conclude that the extinction toward the source isabout 16 mag, indicating that the H II region is at the edgeof the molecular cloud and located in less dense gas. Thereis no detectable ammonia emission toward this source, indi-cating that the source was formed at the very edge of thecloud and has broken out of the molecular material.

IRS 3 is an optically thin compact H II region. Campbelland Persson conclude on the basis of its luminosity andradio Ñux that it is a B1 ZAMS or a luminosity class V star.The optical and radio positions of IRS 3 also appear todisagree at about the 1A level, again suggesting the possi-

310 ZHENG ET AL.

FIG. 7.ÈRatio of the integrated Ñux densities between the (1, 1) main hyperÐne component and the (2, 2) main hyperÐne component (colorNH3 NH3scale) superposed on the integrated (1, 1) emission (contours).NH3

bility of scattering from a deeply embedded source. IRS 3sits at the edge of a dense ammonia clump at the interface ofthe molecular cloud with the visible nebula. There is alsoevidence of high ammonia temperatures at this edge (Fig. 7)and lower velocities than the material to the south (Fig. 4).

4.3. IRS 9, 10IRS 9 was discovered by Werner et al. (1979) in the far-

infrared. There is a strong 2.2 km source at this positionsurrounded by extended emission. No radio continuumemission (at 5 and 15 GHz) has been detected toward thissource, but several outÑows have been detected in thisregion, suggesting that IRS 9 is a rather young stellar objectpossibly at an earlier evolutionary stage than IRS 1. Theinfrared source sits at the edge of an ammonia core (Fig. 2a).The CO outÑow appears to originate from IRS 9, and thereis also extremely high velocity CO emission associated withthis source (Kameya et al. 1989 ; Mitchell & Hasegawa1991).

There appears to be a peak in the 2.2 km map about 20Ato the southwest of IRS 9. It is interesting to note that thereis an ammonia peak at this position. This ammonia peakhas velocities that are more negative than the rest of theclumps, has increased line widths, and also appears to be

more optically thick than the gas around IRS 9 (Figs. 4, 5,and 6). Two other peaks in the 2.2 km map situated about 1@to the west of IRS 9 were noted by Werner et al. Campbell& Persson observe an optical Herbig-HaroÈtype object atthe position of the northern of these two sources, which isdirectly west of IRS 9. They speculate that it may be associ-ated with an outÑow from IRS 9. High-velocity gas hasbeen detected toward IRS 9 (Kameya et al. 1989 ; Mitchell& Hasegawa 1991), although the extent of the high-velocityemission (as seen in CO) is much less than the 1@ separationbetween IRS 9 and the optical HH object. There is, again,an ammonia clump at this position. The velocity of thisclump is not very di†erent from the rest of the gas in thevicinity although the gas does seem to be at a higher opticaldepth than the gas around IRS 9. It is not clear from themorphology of the ammonia gas whether this clump isassociated with the gas around IRS 9 or with the ridge ofmaterial that extends to the southwest.

There is a possibility that IRS 10 is a foreground object.No radio continuum has been detected toward it. We donot detect any compact ammonia emission in that region.

4.4. IRS 11IRS 11, a compact 2.2 km source found by Werner et al.,

is situated about 75A south of IRS 1. It is situated about 10A

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312 ZHENG ET AL. Vol. 550

away from a center of OH and maser activity. There isH2Ono optical source within 5A of IRS 11. Maps at 450, 800, and1100 km (E. Ladd 1998, private communication) show thatthe peak of the emission is at the maser position rather thanthe IRS 11 position. The strongest ammonia peak in ourmaps also coincides with the OH and maser positionsH2Oand the far-IR. The ammonia peaks are part of a ridge ofemission extending for about 2@. This ridge appears to havesome of the most interesting ammonia characteristics in thecloud. This is the region of highest optical depth and mostvelocity dispersion. Although the line-center velocities arenot very di†erent from the rest of the cloud, the line widthsvary quite a bit. There is strong water maser emissionassociated with the clumps in the ridge. All the evidencepoints to this ridge as being the region where the currentstar formation activity is going on. The lack of any opticalor radio continuum source toward this active region is notsurprising given the high optical depths and strong far-infrared emission, which points to this being a region ofhigh extinction and at an early evolutionary stage. The dataalso suggest that IRS 11 is independent from the star forma-tion occurring in the ridge and could be indicative of anearlier and lower mass phase of star formation. This inter-pretation also lends itself to the suggestion that the cloud asa whole has several epochs of star formation.

There are several other peaks in the ammonia emissionthat do not have any known infrared sources associatedwith them. Some of these sources contain water masers,which will be discussed in the next section.

5. KINEMATICS AND WATER MASER EMISSION

The velocity structure in the cloud as a whole does notseem to show any evidence of bulk motions of the dense gas(Fig. 4). There do appear to be some localized trends in thegas, such as in the region at the interface between the molec-ular cloud and the optical H II region. The ammonia veloci-ties at the edge appear to be consistently at more negativevelocities than those in the rest of the cloud. Other small-scale e†ects can be seen in the clump to the south of IRS 9,which appears to have more negative velocities than the restof the gas around IRS 9. The extended ridge of gas near IRS11 appears to be mostly at the same velocities, althoughthere are some gradients in the direction across the ridge.

We would like to examine the ammonia emission and thekinematics in relation to the water maser emission that hasbeen detected in the region (Kameya et al. 1990). Thestrongest water masers as well as most of the masers are inthe IRS 1 region. Moving southward through the cloud,two other sites of maser emission were discovered byKameya et al. These are situated in the ridge of ammoniaemission extending southwest of IRS 1, about 30A and 1@southwest of the H II region. There are ammonia conden-sations associated with both groups of masers. The maserluminosities are rather low (of the order of a few Jy). Themaser velocities are about 5È8 km s~1 more negative thanthe ammonia velocities. However, the ammonia clumps atthese positions have some interesting properties. The clump30A south of IRS 1 (maser at this position was labeled ““ E ÏÏby Kameya et al.) has line widths that are broader thanthose along the ridge as a whole (Fig. 5). The clump also sitsat the edge of a warmer region than the rest of the ridge(Fig. 7). This e†ect could be an extension of the activity inthe interface region. The other clump farther south (maser““ F ÏÏ) appears to have narrow line widths and low tem-

peratures but does have enhanced optical depths (Fig. 6).The coincidence of the water maser with this optical depthpeak suggests that these regions mark the sites of mostrecent star formation.

The masers associated with the southern ridge are foundin two main groups. One group is centered on the NH3clump 10A south of IRS 11, while the other group is associ-ated with the clump 30A to the southwest of IRS 11. Theclump closer to IRS 11 appears to have more activity, withthe masers distributed in two main velocity groups,H2Ofrom [51 to [56 km s~1 and from [61 to [71 km s~1.The velocities in this clump are around [56 km s~1.NH3The red- and blueshifted nature of the maser velocities withrespect to the may be an indication of outÑow activity.NH3There is a weak outÑow detected in this region, but thespatial extent of the outÑow is much larger than the spatialextent of the water maser distribution. The ridge ofammonia emission also has some interesting features ; theoptical depths are consistently high all along the ridge.The line widths show some enhancements along one edgeof the clump associated with the strong water masers, andthe temperatures appear to be warmer than most otherregions in the cloud.

The only other water maser emission that has beendetected in the cloud is toward IRS 9. This group of maseremission is right at the position of the infrared source andagain has velocities that are more negative than theammonia velocities in this region.

In general, we Ðnd that the water maser velocities aremore negative than the ammonia velocities, and the masersappear to be located at peaks of ammonia optical depth. Amore sensitive search for water masers toward the otherammonia clumps with high optical depth, as well as sensi-tive J, H, and K maps of the 2 km emission in this region,may be useful in Ðnding deeply embedded young stars in theregion.

6. CLOUD FILAMENTS

The emission shows that the gas in the NGC 7538NH3region is highly structured. The elongated Ðlament towardIRS 11 has an aspect ratio of at least 5 :1 with a major axisof 2@, or 2 pc in linear scale. At about 8A resolution, thisÐlament is resolved into Ðve clumps. Two of them areassociated with water masers, indicating the birth of youngstars. The mass in each clump is about 100 muchM

_,

smaller than the virial mass of 500 With the uncer-M_

.tainty in the relative abundance, this di†erence mayNH3not be signiÐcant. However, if the di†erence is real, thissuggests that the clumps are not overall gravitationallybound. There are subcondensations in the clumps that arenot resolved in our observations. Some of those sub-condensations are probably gravitationally bound and willform young stars. The existence of subcondensations is con-sistent with the small Ðlling factor of the emission.NH3If the clumps in the Ðlament toward IRS 11 are notNH3gravitationally bound, the Ðlament may not result fromfragmentation of a self-gravitating molecular cloud. Onealternative explanation is external triggering : the wind/shock front from the early-generation OB stars compressesthe molecular gas and induces the formation of the nextgeneration of stars (Elmegreen & Lada 1977). Although theexistence of the optical H II region toward IRS 6 and 7 inthe north and the compressed low-density gas seen in COare suggestive of such a scenario, we do not Ðnd evidence of

No. 1, 2001 MULTIFIELD MOSAIC OF NGC 7538 313

systematic heating or line broadening in the typicalNH3,signatures of shock or compressed dense gas, toward thesouthern region.

7. CONCLUSION

We imaged NGC 7538 with the VLA in the (1, 1)NH3and (2, 2) transitions. The detailed structures, includingdensity, temperature, and velocity distributions, in themolecular cloud region of a 6@] 2@ mosaic provide valuableinformation for understanding the interaction of outÑowswith their environments. We summarize the main results inthe following :

1. We found dense condensations of located justNH3outside of the boundary of the optical H II region towardIRS 4È8. The temperatures of these condensations arehigher than those of the internal Ðlament toward IRS 11.Both CO and emissions indicate that the material isNH3compressed at the interface. It is plausible that the windsfrom the H II region toward IRS 4È8 provide externalheating and compression of the dense gas.

2. Our observations showed that the outÑows in theregion are conspicuously located in the voids between the

clumps. The outÑow mass estimated from the CONH3(2È1) is far smaller than that in the dense clumps. It ispossible that the preexisting dense material in the regionshapes and redirects the outÑowing gas to the low-densityspace.

3. As pointed out by Wiseman & Ho (1998), conden-sations in a molecular cloud are often strung together toform a Ðlament, like beads on a string. Such Ðlamentarystructures are apparent in our wide Ðeld maps. Some eightor nine condensations of size 25A are linked within the elon-gated Ðlament toward IRS 9 and 11. The Ðlament appearsto be the region with highest optical depth and greatestvelocity dispersion while accompanied by strong watermaser emission. All the evidence suggests that the Ðlamentharbors the most current star formation activity. It appearsthat is a good tracer of dense gas, which marks theNH3early phases of star formation.

We are grateful to C. J. Davis for providing the CO data.We also thank D. J. Wilner and E. Keto for helpful dis-cussions.

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