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THE ASTROPHYSICAL JOURNAL, 510:197–204, 1999 January 1 © 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. Hα Emission 11 Kiloparsecs above M82

 DAVID DEVINE 1, 2

Laboratory for Astronomy and Solar Physics, Code 681, NASA Goddard Space Flight Center, Greenbelt, MD 20771

AND

JOHN BALLY 2, 3

Department of Astrophysical and Planetary Sciences and Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309

Received 1998 March 27; accepted 1998 August 6

ABSTRACT

     We report the discovery of Hα emission associated with the redshifted lobe of the M82 superwind extending out to a projected distance of 11 kpc from the disk of M82, which is 3 times farther than previously identified emission-line components associated with the nuclear superwind. The Hα emission can be traced nearly continuously for 10&arcmin; northwest of M82 out to an emission-line structure (the "cap") that lies at a projected distance of 11–12 kpc from the M82 nucleus. The cap has a shell-like morphology, is blueshifted by 50–200 km s-1 relative to the M82 nucleus, and is visible in a ROSAT PSPC image of the region. We consider two possible models for the Hα bright cap: it may be a bow shock formed by the impact of the superwind, either with previously emitted wind material or with ambient intergalactic material and/or tidal debris left over from the interaction between M81 and M82; or it may trace photoionized material illuminated by Lyman continuum photons leaking out of the M82 nuclear region through the hot bipolar cavity produced by the starburst-driven superwind.

Subject headings: galaxies: starburst—galaxies: nuclei—galaxies: individual (M82)

FOOTNOTES

     1 [email protected].

     2 Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under contract with the National Science Foundation.

     3 [email protected].

§1. INTRODUCTION

     M82 is the classic example of a starburst galaxy driving a superwind (e.g., Heckman 1997; Heckman, Lehnert, & Armus 1993). It is ideal for studying galactic outflows, since it is close (D∼3.63 Mpc; Freedman et al. 1994) and nearly edge-on (i∼81°; Lynds & Sandage 1963). It is also one of the brightest IR galaxies in the sky, with LIR∼3 × 10$\mathstrut{^{10}}$ L$\mathstrut{_{{\odot}}}$ (Telesco & Harper 1980; Telesco 1988). Most of the IR emission originates from warm dust located in the central 500 pc of the disk, which coincides with the starburst region (Lord et al. 1996). The starburst was most likely triggered by a close encounter with the spiral galaxy M81 some 2 × 10$\mathstrut{^{8}}$ yr ago (Cottrell 1977; Yun, Ho, & Lo 1993; Yun et al. 1994), which also led to the formation of a roughly 1 kpc long stellar bar in the central region of the M82 disk (Telesco et al. 1991; Larkin et al. 1994; Achterman & Lacy 1995). Perturbations in the disk induced by the bar may be responsible for the infall of material and the high rate of star formation and supernovae observed within 250 pc of the nucleus (e.g., Friedli & Martinet 1997). The energy injected by the supernovae, which occur at a rate of order 0.1 yr-1 (Muxlow et al. 1994; Ulvestad & Antonucci 1994; van Buren & Greenhouse 1994), heats and accelerates the surrounding material that then expands outward and escapes along the minor axis of M82, forming a bipolar superwind. The wind primarily consists of hot, tenuous gas with temperatures of 106–107 K and densities of 10-3 to 10-1 cm-3 (Bregman, Schulman, & Tomisaka 1995; Strickland et al. 1997; Moran & Lehnert 1997). X-ray emission associated with the superwind has been detected out to a projected distance of 6 kpc from the disk (Strickland et al. 1997; Moran & Lehnert 1997), although Tsuru et al. (1990) claim evidence of a faint halo of X-ray emission extending for 100 kpc. The superwind has spectacular optical emission-line filaments associated with it that are clearly visible above and below the disk, especially toward the northwest along the redshifted outflow lobe (e.g., Lynds & Sandage 1963). The filaments are believed to trace ambient material that has been swept up and accelerated to velocities of several hundreds of km s-1 by the superwind, and are photoionized and/or shock-excited (Lehnert & Heckman 1996).

     In this paper we report the discovery of Hα emission associated with the redshifted lobe of the M82 superwind extending out to a projected distance of 11 kpc from the disk of M82, which is 3 times farther than previously identified emission-line components associated with the nuclear superwind. The Hα emission can be traced nearly continuously for 10&arcmin; northwest of M82 out to an emission-line structure (the "cap") that lies at a projected distance of 11–12 kpc from the M82 nucleus. The cap has a shell-like morphology, is blueshifted by 50–200 km s-1 relative to the M82 nucleus, and is clearly visible in a ROSATPSPC image of the region (E. C. Moran 1998, private communication). We consider two possible models for the Hα bright cap: it may be a bow shock formed by the impact of the superwind with previously emitted wind material, ambient intergalactic material, and/or tidal debris left over from the interaction between M81 and M82; or it may trace photoionized material illuminated by Lyman continuum photons leaking out of the M82 nuclear region through the hot bipolar cavity produced by the starburst-driven superwind.

§2. OBSERVATIONS

     Our initial images of M82 were obtained on the nights of 1995 December 17–19 with the Kitt Peak National Observatory (KPNO) 0.6 m Burrell Schmidt telescope, which was equipped with a SITE 2048 × 2048 pixel CCD. The field of view was roughly 1 deg2 with a plate scale of 2&arcsec; pixel-1. We obtained six 1000 s exposures of the region through narrowband filters centered on Hα (KP 1563: 6560 Å/67 Å) and [S II] (KP 1566: 6709 Å/71 Å), and a single 1000 s Gunn z exposure to check for continuum emission.

     We obtained follow-up high-resolution (R ≈ 6000) spectra based on the Schmidt images with the 4 m Mayall reflector at KPNO on the nights of 1996 January 7–11. We used a 2$\mathstrut{^{{\prime}{\prime}}}$ × 5$\mathstrut{^{{\prime}}}$ long slit on the Ritchey-Chrétien spectrograph with grating KPC24 operating in the second order with the T2KB 2K × 2K CCD. This arrangement gave a pixel scale of 0.54 Å pixel-1 in the red near Hα, which corresponds to a velocity scale of about 24 km s-1 pixel-1. The scale along the slit was 0&farcs;6 pixel-1. We used a quartz lamp to obtain flat fields and a HeNeAr comparison lamp for wavelength calibration. Our spectra covered the wavelength range from [O I] λ6300 to [S II] λ6731. The data were reduced in the standard fashion following theprocedures for long-slit spectra outlined by P. Massey, F. Valdes, & J. Barnes 4 and were flux calibrated relative to the standard Feige 34.

     The deep, high-resolution Hα images presented in this paper were obtained on the nights of 1997 October 23–30 at the Mayall 4 m reflector using the engineering grade MOSAIC 8192 × 8192 detector. The field of view with the imaging correctors is approximately 36$\mathstrut{^{{\prime}}}$ × 36$\mathstrut{^{{\prime}}}$ with 0&farcs;26 pixel-1. 5 In order to minimize the effects of bad columns and pixels, we obtained a dithered set of five 600 s exposures through an Hα narrowband filter centered on 6563 Å with a bandpass of 75 Å. The data were overscanned, trimmed, dark-subtracted and flat-fielded using the IRAF package MSCRED. Dark subtraction was based on a median-combined set of 11 dark exposures of 600 s taken at various times throughout the observing run. A comparison of darks taken at different times during the run did not show any significant variations. The Hα flat was formed by median-combining nine twilight sky flats obtained during the observing run.

     After reduction, the multifit format images were combined into a single-fit image within IRAF following the guidelines set forth in Valdes (1997). An internal reflection persists at the few percent level after flat-fielding and median-combining; this is a result of the corrective optics currently employed at the 4 m telescope in combination with the use of narrowband filters. The amount of reflection depends critically on the presence of bright objects near the imaging field and can vary between dithered images. The reflection is less noticeable in regions with high background emission.

FOOTNOTES

     4 See A User's Guide to Reducing Slit Spectra with IRAF, available electronically at http://iraf.noao.edu/docs/spectra.html.

§3. RESULTS

     Figure 1 shows a 4096 × 4096 subfield of the MOSAIC Hα image of M82 highlighting the faint emission in the region. The doughnut-shaped reflection mentioned earlier can be seen in the upper left corner. Although the emission near the galaxy is burned out in this figure, the outer portions of the Hα bright filaments are clearly visible 2–3 kpc from the disk, especially toward the northwest along the redshifted outflow lobe. The emission fades considerably beyond 3 kpc, but remains visible along the minor axis of M82 for at least 10&arcmin;, or 11 kpc at our assumed distance of 3.63 Mpc to M82. The Hα bright "cap" is clearly visible 10&arcmin;–11&arcmin; (11–12 kpc) northwest of M82. The cap is positioned above the brightest filamentary emission associated with the superwind and extends for roughly 3&arcmin; (3 kpc) toward the northeast from the minor axis of M82. The cap subtends an angle of about 15° relative to the M82 nucleus.

Fig. 1

     A close-up of the Hα cap is shown in Figure 2. The cap appears as a collection of bright knots connected by diffuse emission and is morphologically similar to supernova remnants and Herbig-Haro objects. The brightest knots have angular sizes of order 4&arcmin;&arcmin;, with corresponding linear sizes of order 70 pc. The coordinates for several of the bright knots are listed in Table 1. Most of the emission lies within a roughly 215$\mathstrut{^{{\prime}{\prime}}}$ × 30$\mathstrut{^{{\prime}{\prime}}}$ (∼3800 pc × 530 pc) box at a position angle of about 45°. For comparison, the position angle of the M82 disk is roughly 65°. Faint Hα emission can be traced nearly continuously from the disk out to 30&arcmin;&arcmin; beyond the cap. Several faint galaxies are also visible in the field.

Fig. 2

     Figure 3 shows the location and orientation of the long slits that we used for obtaining spectra in the M82 region. The placement of the long slits was based on the Schmidt images of M82. The results from LS2 are consistent with those from LS4 and are therefore not presented. The region sampled by LS3 has been discussed in detail by several authors (e.g., Axon & Taylor 1978; Gotz et al. 1990; McKeith et al. 1995; Lehnert & Heckman 1996) and will also not be presented, except to mention that our results agree with those previous observations. Figure 4 shows the spectrum of a portion of the cap obtained through LS4. The cap is Hα-bright, with no other line emission detected in our spectrum. The spike shortward of 6300 Å is an artifact of the background subtraction. Figure 5a shows a velocity contour plot of the cap emission relative to the systemic M82 LSR velocity of 210 km s-1 (Beck et al. 1978; McKeith et al. 1993), and Figure 5b shows a velocity contour plot of the filamentary emission observed through LS1. The Hα cap is blueshifted by 50–200 km s-1 relative to M82. Most of the emission shown in Figure 5b is redshifted relative to M82; however, blueshifted velocities similar to those observed in the cap are also present.

Fig. 3 Fig. 4 Fig. 5

§4. DISCUSSION

     Our observations show that the redshifted lobe of the superwind driven by M82 is at least 11 kpc long. Faint Hα emission can be traced nearly continuously for 10&arcmin; along the minor axis of M82 out to an Hα bright emission-line feature (the cap), which lies at a distance of 11–12 kpc from the nucleus of M82. The cap lies directly above the brightest Hα emission associated with the M82 superwind and hence is most likely associated with M82 and not Galactic. It is not clear whether the cap is part of the superwind or ambient material that is being shocked by the wind and/or photoionized by radiation escaping from the nucleus of M82 along the path excavated by the wind.

     The cap may be related to the extensive H I present in the M81, M82, NGC 3077 system; however, an inspection of the VLA maps of the region presented by Yun et al. (1993) does not show any emission associated with the cap above a column density N(H I)=1.8 × 10$\mathstrut{^{20}}$ cm-2. This implies that the average density of atomic hydrogen in the cap must satisfy n(H)≲0.06/lη$\mathstrut{^{1/3}}$ cm-3, where l is the linear dimension of the cap along the line of sight in kpc and η is the volume-filling factor of the cap material. As shown in the Appendix, the cap may be photoexcited by radiation from the nucleus of M82. If we assume this to be the case and l ∼ 0.5–5, then equations (A2) and (A3) imply that η∼5×10$\mathstrut{^{-2}}$ to 10$\mathstrut{^{-3}}$ and ne∼0.1 cm-3. The volume of the cap is ∼(3800)(530)(103 l)η pc3, and therefore, under the assumption of photoexcitation, the mass of the cap is approximately 5 × 10$\mathstrut{^{7}}$(n$\mathstrut{_{e}}$lη/χ) M$\mathstrut{_{{\odot}}}$, where χ is the ionization fraction of the cap material. If χ∼1 and ne∼0.1 cm-3, the mass of the cap is of order 104–105 M⊙, which is similar to the mass of a giant molecular cloud and below the detection limit of the H I map presented by Yun et al. (1993). The cap is probably not related to the M82 tidal tails, since the northern H I streamers lie several arcminutes east of the cap and are redshifted relative to M82, while the cap is blueshifted (Yun et al. 1993).

     The filamentary emission close to the disk is due to a combination of photoionization and shock excitation (Lehnert & Heckman 1996; Shopbell & Bland-Hawthorn 1998) and subtends an angle of order 30°–60° relative to the nucleus. If we assume that the ionizing radiation and the superwind originate in the nuclear starburst region, then the radiation and the superwind should be able to escape toward the northwest from the disk to "light up" additional ambient material over a region larger than that covered by the cap. The placement of the cap might be coincidental, but its location and relatively limited extent suggest either that ionizing photons can escape along the minor axis of M82 out to 10 kpc only over an angle of order 15° or that the cap traces a well-collimated component of the M82 superwind. As shown in Figure 5b, blueshifted velocities similar to those observed in the cap are also observed in filaments associated with the redshifted lobe of the M82 superwind. Since the cap is blueshifted by 50–200 km s-1 relative to M82, its affiliation with the redshifted portion of the superwind is somewhat puzzling, but may be explained by assuming that the cap lies along the near side of the redshifted outflow lobe (e.g., McKeith et al. 1995). If so, then the cap could trace shock-heated gas resulting from the collision between a well-collimated (opening angle of order 15°) component of the superwind with slower moving gas. If the cap is shock-excited, then the shock velocities must be consistent with Hα/[S II] and Hα/[N II] ratios greater than 4 and the atomic densities implied by the H I observations of Yun et al. (1993). Shock velocities of order 100 km s-1 could heat the gas to temperatures of order 106 K, resulting in the X-ray emission detected by ROSAT. Unfortunately our limited data do not allow us to make a more detailed study of shock emission from the cap, which will be presented in a future paper.

     The cap could also trace a shell of material that has either been swept up by the superwind or ejected from the M82 disk. In the latter case we can estimate an upper bound to the time since the cap material was ejected by the dynamical age: τ ∼ (1 × 10$\mathstrut{^{10}}$)/(100/$\mathstrut{{\rm tan}}$i) yr, where i is the inclination of the flow to the plane of the sky and an average velocity of 100 km s-1 has been assumed. With i∼15° we get τ∼ 3 × 10$\mathstrut{^{7}}$ yr; this is comparable to the starburst age of (3–6) × 10$\mathstrut{^{7}}$ yr for M82 derived by Doane & Matthews (1993). This is a factor of 10 smaller than the cooling times for the X-ray–emitting gas associated with the superwind (Strickland et al. 1997), and therefore it does not seem likely that the cap traces hot wind material that has cooled and condensed. It is also interesting to note that models of the X-ray and optical emission along the minor axis of M82 suggest a two-component wind: a jetlike component with an opening angle of 5°–20° within 500–1000 pc of the disk and a fanlike component with an opening angle of 20°–60° at larger distances (Suchkov et al. 1996; Shopbell & Bland-Hawthorn 1998). Thus the cap may trace material that was lifted from the disk by a collimated wind component during the early stages of starburst activity in M82.

     The relative velocity of the cap with respect to M82 is given by v ∼100/$\mathstrut{{\rm sin}}$i km s-1, where an average radial velocity of 100 km s-1 has been assumed for the cap. M82 has a mass of 2.7 × 10$\mathstrut{^{10}}$ M⊙ (Lynds & Sandage 1963), and hence the escape velocity at a height of 10 kpc above the disk is of order 150 km s-1. Therefore, if the inclination angle is less than roughly 45°, most of the material in the cap is moving faster than the escape velocity from the galaxy and hence is most likely not "falling" back toward M82. This can also be deduced from the rotation curve of M82, which is flat with a rotation speed of about 112 km s-1 both in carbon monoxide, which traces the inner disk of the galaxy (Sofue et al. 1992), and in optical spectra, which trace the outer portions of the disk (O'Connell & Mangano 1978). The radial velocity of the cap is of the order of the rotation speed of the M82 disk at its outermost parts, which are about 5 kpc from the nucleus. Thus, the blowout of the nuclear superwind and the cap of Hα-emitting plasma that delineates its most distant optically visible component is likely to inject metal-enriched matter into the intergalactic medium of the M81/M82/NGC 3077 system.

§5. CONCLUSIONS

     Faint Hα emission associated with the redshifted lobe of the superwind driven by the starburst galaxy M82 can be traced nearly continuously for 11 kpc from the disk out to an Hα-bright emission-line structure (the "cap"). The cap has a shell-like morphology that is similar to supernova remnants or Herbig-Haro objects and is blueshifted by roughly 100 km s-1 relative to M82. The cap may trace ambient material that is being shocked by the superwind and/or photoionized by radiation escaping from the nucleus of M82 along the path excavated by the wind, or it could trace material that has been swept up by the superwind or possibly ejected from M82 on the order of 3 × 10$\mathstrut{^{7}}$ yr ago. A more thorough analysis of the cap excitation mechanisms will be presented in a future paper.

ACKNOWLEDGMENTS

     We would like to thank the staff of KPNO for their excellent support during our observing runs, and we especially thank Taft Armandroff, Frank Valdes, and the rest of the MOSAIC team for their splendid help both on and off the mountain. We also thank Ralph Shuping for his assistance in obtaining images of M82 and the referee for directing us to the archival ROSAT PSPC image of M82. This research was supported in part by NASA grant NAGW-4590 (Origins) and NASA grant NAGW-3192 (LTSA).

APPENDIX

CAP IONIZATION PARAMETERS

     We first verify that the cap can be photoionized by radiation from M82. With Lc∼6×1053 photons s-1 (Carlstrom & Kronberg 1991), we get r$\mathstrut{_{s}}$∼2.8n$\mathstrut{^{-2/3}_{{\rm H}}}$ kpc for M82. Therefore the cap lies within a Strömgren length of M82 provided that nH is less than ∼0.1 cm$\mathstrut{^{-3}}$. The electron density in the hot wind decreases as you move outward from the disk and has a value of about 10-2 cm-3 (assuming a filling factor of 1 for the hot gas) at a projected height of about 5 kpc above the disk (Strickland et al. 1997). The optical filaments are thought to lie along the surface of the outflow lobes and hence will not absorb a significant fraction of the Lyman continuum photons emitted by the nucleus of M82. Therefore the average density of absorbing material in the region between the cap and M82 is probably less than 0.1 cm-3, and the cap may be photoionized by radiation from M82.

     The rate at which ionizing photons pass through the cap (Qcap) is related to Lcap(Hα), the observed Hα luminosity of the cap, by where E(Hα) is the energy of an Hα photon, and the case B approximation has been used (Osterbrock 1989). If we assume negligible absorption by the medium between M82 and the cap, then Q$\mathstrut{_{{\rm cap}}}$=$\mathstrut{\left(L_{c}{/}4{\pi}d^{2}\right)}$A$\mathstrut{_{{\rm cap}}}$ s$\mathstrut{^{-1}}$, where d is the distance from the M82 nucleus to the cap and Acap is the surface area of the cap relative to the M82 nucleus. Based on our observations, d∼10 kpc and A$\mathstrut{_{{\rm cap}}}$∼4lη$\mathstrut{^{2/3}}$ kpc2, where l is the length of the cap along the line of sight measured in kpc and η is the volume-filling factor of the cap material. We can estimate Lcap(Hα) from our observations by L$\mathstrut{_{{\rm cap}}}$(Hα)=4πD$\mathstrut{^{2}}$F$\mathstrut{_{{\rm obs}}}$ΔΩ$\mathstrut{_{{\rm cap}}}$/ΔΩ$\mathstrut{_{{\rm obs}}}$ ergs s$\mathstrut{^{-1}}$ cm$\mathstrut{^{-2}}$ , where D ∼ 1.1 × 10$\mathstrut{^{25}}$ cm is the distance to M82, F$\mathstrut{_{{\rm obs}}}$∼ 10$\mathstrut{^{-15}}$ ergs s$\mathstrut{^{-1}}$ cm$\mathstrut{^{-2}}$ is the observed Hα line flux from the cap, ΔΩ$\mathstrut{_{{\rm cap}}}$∼ 215 × 32 arcsec$\mathstrut{^{2}}$ is the surface area of the cap, and ΔΩ$\mathstrut{_{{\rm obs}}}$∼ 30 × 2 arcsec$\mathstrut{^{2}}$ is the surface area that we observed through the long slit. Then Lcap (Hα) ∼ 2 × 1038 ergs s-1 cm-2 , and

     We can also use our observations to calculate the cap emission measure and estimate the average electron density in the cap. Spitzer (1978) gives I(Hα) = 8.7 × 10-8 EM ergs s-1 cm-2 sr-1 , where EM = n$\mathstrut{^{2}_{e}}$ L cm-6 pc. With I(Hα) = Fobs/ΔΩobs  ergs s-1 cm-2 sr-1 , we get I(Hα) ∼ 10$\mathstrut{^{-15}}$/(1.4 × 10$\mathstrut{^{-9}}$) ∼7 × 10$\mathstrut{^{-7}}$ ergs s-1 cm-2 sr-1, and EM∼10∼n$\mathstrut{^{2}_{e}}$η$\mathstrut{^{1/3}}$(103 l ). Then If the Hα emission from the cap is primarily due to photoionization, combining with relation (A2) yields n$\mathstrut{_{e}}$∼0.1(0.07l)$\mathstrut{^{-1/4}}$ cm$\mathstrut{^{-3}}$.

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FIGURES

Full image (303kb) | Discussion in text     FIG. 1.—A 4096×4096 section of our KPNO 4 m MOSAIC Hα image of M82 highlighting the faint emission in the region. The "doughnut" visible in the upper left corner is an artifact of the corrective optics. The 2 μm center of M82 [α(2000) = 09h55m52&farcs;3, δ(2000) = 69°40&arcmin;46&arcsec;} is marked by a white circle. The image has been 2 × 2 boxcar averaged and 2 × 2 subsampled. Full image (393kb) | Discussion in text     FIG. 2.—Close-up showing the Hα "cap," with some of the brighter knots labeled Full image (306kb) | Discussion in text     FIG. 3.—Locations and orientations of the long slits used for obtaining spectroscopy. The placement of the slits was based on our original Burrell Schmidt images of the region. Full image (27kb) | Discussion in text     FIG. 4.—Spectrum of the region of the "cap" covered by LS4. The emission near 6300 Å is an artifact of the sky subtraction and not affiliated with the cap. Flux calibration is relative to the standard Feige 34 and is accurate to approximately 20%. The flux units are ergs s-1 cm-2 Å-1. Full image (218kb) | Discussion in text     FIG. 5.—Left: Gray-scale long-slit image of the Hα emission from the region of the "cap" covered by LS4 overlaid with a velocity contour plot of the Hα emission. The linear scale assumes a distance of 3.63 Mpc to M82. The contours begin at roughly 3 σ above the noise and increase multiplicatively by 21/3. Right: Velocity contour plot of the Hα emission from the filamentary region covered by LS1. The distance measures the approximate projected height above the M82 disk. The contours begin at roughly 3 σ above the noise and increase multiplicatively by 21/2.

TABLES

TABLE 1 POSITION OF BRIGHT KNOTS IN M82 "CAP"
Object α(J2000) a δ(J2000) a Angular Size (arcsec) Linear Size b (pc)
A... 09 54 53.5 69 49 37 3.3 58
B... 09 54 57.4 69 49 39 3.6 63
C... 09 55 00.5 69 50 13 4.6 81
D... 09 55 05.4 69 50 36 2.3 40
E... 09 55 05.5 69 50 28 4.6 81
F... 09 55 06.5 69 50 48 2.6 46
G... 09 55 08.4 69 51 13 5.1 90
H... 09 55 11.9 69 51 12 4.6 81
I... 09 55 20.6 69 51 48 6.1 107
     NOTE.— Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds.      a The coordinates are based on GSC stars located within the field and are accurate to about 3&arcsec; .      b Assumes a distance of 3.63 Mpc to M82 (Freedman et al. 1994). Image of typeset table | Discussion in text

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