CORE Metadata, citation and similar papers at core.ac.uk Provided by Dartmouth Digital Commons (Dartmouth College) Dartmouth College Dartmouth Digital Commons Open Dartmouth: Faculty Open Access Articles 3-2001 Hubble Space Telescope Images of the Ultraluminous Supernova Remnant Complex in NGC 6946 William P. Blair Johns Hopkins University Robert A. Fesen Dartmouth College Eric M. Schlegel Harvard-Smithsonian Center for Astrophysics Follow this and additional works at: https://digitalcommons.dartmouth.edu/facoa Part of the Stars, Interstellar Medium and the Galaxy Commons Recommended Citation Blair, William P.; Fesen, Robert A.; and Schlegel, Eric M., "Hubble Space Telescope Images of the Ultraluminous Supernova Remnant Complex in NGC 6946" (2001). Open Dartmouth: Faculty Open Access Articles. 2227. https://digitalcommons.dartmouth.edu/facoa/2227 This Article is brought to you for free and open access by Dartmouth Digital Commons. It has been accepted for inclusion in Open Dartmouth: Faculty Open Access Articles by an authorized administrator of Dartmouth Digital Commons. For more information, please contact [email protected]. THE ASTRONOMICAL JOURNAL, 121:1497È1506, 2001 March ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. HUBBL E SPACE T EL ESCOPE IMAGES OF THE ULTRALUMINOUS SUPERNOVA REMNANT COMPLEX IN NGC 69461 WILLIAM P. BLAIR Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686; wpb=pha.jhu.edu ROBERT A. FESEN Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755-3528; fesen=snr.dartmouth.edu AND ERIC M. SCHLEGEL Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; eschlegel=cfa.harvard.edu Received 1999 September 4; accepted 2000 November 20 ABSTRACT We present Hubble Space Telescope (HST ) narrow-passband Ha and [S II] images and broadband continuum images of the region around an extremely luminous optical and X-ray supernova remnant complex in the spiral galaxy NGC 6946. These images, obtained with the PC1 CCD of the Wide Field Planetary Camera 2, show a circular, limb-brightened shell of diameter0A.35 [9 d/(5.1 Mpc) pc] super- posed on the edge of a larger, lower surface brightness elliptical shell(1A.4] 0A.8, or ^34 pc ] 20 pc). The HST images allow us to see that the [S II]:Ha ratio remains high across both shells, indicating that both are collisionally heated. A brightening of the Ha and [S II] line emission arises on the eastern side of the smaller shell, where it is apparently interacting with the western edge of the larger shell. Our HST V image includes the nebulaÏs strong [O III] j5007 emission in the blue wing of the Ðlter, providing a glimpse at the [O III] nebular morphology. The smaller shell looks similar, but the extended structure looks sharper than in Ha and [S II] images, reminiscent of a cavity wall. The HST and ground-based continuum images show the brightest members of the underlying and adjacent stellar population, indi- cating the presence of massive OB stars in and near the region. A new optical ground-based spectrum conÐrms that the [N II]:Ha ratio is enhanced in the region, consistent with mass loss from massive stars. These data show an average ([S II] jj6716, 6731):Ha ratio across both shells of D1andamean electron density of D400 cm~3, indicating preshock densities of order 10 cm~3. We interpret this nebular morphology and supporting information as an indication of multiple supernova explosions in relatively close temporal and spatial proximity. We discuss possible scenarios for this complex region and the reasons for its extreme luminosity. Key words: galaxies: individual (NGC 6946) È supernova remnants INTRODUCTION 1. In the course of an imaging survey of NGC 6946, an The face-on spiral galaxy NGC 6946 [SAB(rs)cd, de Vau- extraordinarily bright optical SNR was discovered (Blair & couleurs, de Vaucouleurs, & Corwin 1976; d \ 5.1 Mpc, de Fesen 1994) within the error circle of a bright ROSAT Posi- Vaucouleurs 1979] has generated at least six supernovae tion Sensitive Proportional Counter source having a lumi- ] 39 ~1 (SNe) in the last 85 years. This rate is rivaled only by M83, nosity ofL X(0.2È3 keV) D 3 10 ergs s (Schlegel also a face-on Scd galaxy, which has had six recorded SNe 1994). This object is listed as No. 16 in a subsequent optical (see Cowan & Branch 1985 and references therein; also SNR survey of NGC 6946 (Matonick & Fesen 1997; here- Liller 1990). Both NGC 6946 and M83 show evidence of after the object will be designated MF 16) and is several active star formation and starburst activity, with at least times brighter than any of the other 26 SNRs they identi- some of the observed SNe presumably arising from young, Ðed. VLA observations show MF 16 to be a powerful radio massive stars. source, several times brighter than the young, Galactic SNR With so many historical SNe seen in these two galaxies, a Cas A, indicating a probable interaction with dense circum- large number of young supernova remnants (SNRs), stellar or interstellar material (Van Dyk et al. 1994). In fact, perhaps older than the historical record but still relatively the large observed X-ray, optical, and radio luminosities young, might be expected to be present. Some of these initially suggested a relatively youthful, ejecta-dominated young SNRs might still be in the relatively brief (¹2000 yr) remnant similar to the NGC 4449 SNR mentioned above. ““ ejecta dominated ÏÏ phase, like the luminous SNR in NGC However, a moderate-resolution optical spectrum (Blair & 4449 (Blair, Kirshner, & Winkler 1983). Such considerations Fesen 1994) showed no evidence of either high velocities or motivated us to undertake deep ground-based CCD peculiar abundances. Furthermore, the VLA observation imaging surveys of NGC 6946 and M83 using interference- resolved the source at 6 cm at about 1A,or25d/(5.1 Mpc) Ðlter images to search systematically for young optical SNR pc. Collectively, these observations indicated that the object candidates. is an unusually luminous but otherwise normal, average- size, interstellar mediumÈdominated SNR. ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ We have used the Wide Field Planetary Camera 2 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the (WFPC2) on the Hubble Space Telescope (HST ) to obtain Association of Universities for Research in Astronomy, Inc., under NASA high-resolution images of the region surrounding the contract NAS 5-26555. unusually luminous SNR in NGC 6946. We have also 1497 1498 BLAIR, FESEN, & SCHLEGEL Vol. 121 obtained deeper ground-based optical spectra of the SNR. luminous SNR (left below center of each panel) and its These new observations are reported in the next section, environs in all four bandpasses. In Figures 1È5, north is to followed by a discussion of possible interpretations of this the upper right and east to the upper left, as in the original remarkable region. Our conclusion, based primarily on the HST data sets. An OB association and its H II emission can observed morphology of the emission, is that MF 16 is be seen at the lower right in Figure 1, and another SNR actually a small region, containing multiple SNRs, in which (MF 15) identiÐed by Matonick & Fesen (1997) and men- a relatively youthful SNR is currently interacting with the tioned by Blair & Fesen (1994) is at the upper right. shell of an older, neighboring SNR. However, the tremen- Although the stretch in Figure 1 is performed to show the dous luminosity of the region arises not only from the faintest emissions detected, the tremendous relative surface interaction region between the two SNRs but also from brightness of MF 16 (about 10 times as bright as MF 15 at the dense surroundings into which the shock fronts are Ha) is obvious. expanding. For comparison with the HST data and to provide context, in Figure 2 we show ““ postage stamps ÏÏ of ground- 2. OBSERVATIONS AND REDUCTIONS based CCD images of the region, from the imaging data 2.1. HST Imaging reported by Blair & Fesen (1994). The four bandpasses shown are Ha,[SII], [O III], and a red continuum band CCD images of NGC 6946Ïs northeastern section, which centered at 6100A . The images are rotated to match the contains the unusually luminous SNR complex, were HST data, and the region shown is21A.6 square, slightly obtained on 1996 January 27 with WFPC2 aboard HST . larger than the region shown in Figure 1. The left panels The SNR was positioned near the center of the PC1 CCD show a ““ soft stretch,ÏÏ making it easy to correlate features to provide both good resolution and sufficient Ðeld cover- against the HST images, and the right panels are contrast- age to assess its local environs. Filters and exposure times enhanced to show fainter structures not visible in the HST ] II ] were Ha F656N (2 700 s), [S ] F673N (2 1100 s), images. Although MF 16 is saturated, note the appearance ] V -band F555W (400 s), and B-band F439W (2 400 s). of the second SNR (MF 15) at upper right, the brightness There was only one F555W exposure because the exposure and extent of the H II emission associated with the OB was shorter than the default exposure time for the CR- association at lower right, and the presence of distinct con- SPLIT optional parameter to be invoked automatically. tinuum emission at and adjacent to the location of MF 16 Unfortunately, having only a single exposure makes itself. At least the brightest of the stars contributing to this cosmic-ray removal problematic. emission are visible in the HST images.