An optical search for supernova remnants in the nearby spiral galaxy NGC 2903
aa r X i v : . [ a s t r o - ph . GA ] J a n Astronomy&Astrophysicsmanuscript no. ngc2903paper7 c (cid:13)
ESO 2018November 8, 2018
An optical search for supernova remnants in the nearby spiralgalaxy NGC 2903(Research Note)
E. Sonbas , , A. Akyuz , and S. Balman University of Cukurova, Department of Physics, 01330 Adana, Turkey Special Astrophysical Observatory of R.A.S., Karachai-Cherkessia, Nihnij Arkhyz, 369167 Russia Dept. of Physics, Middle East Technical University, 06531 Ankara, TurkeyReceived -; accepted -
ABSTRACT
Aims.
We present the results of an optical search for supernova remnants (SNRs) in the nearby spiral galaxy NGC 2903.
Methods.
Interference filter images and spectral data were taken in March 2005 with the f / α and continuum-subtracted[SII] λλ Results.
Five SNR candidates were identified in NGC 2903 with [SII] / H α ratios ranging from 0.41 - 0.74 and H α intensities rangingfrom 9.4 × − to 1.7 × − ergs cm − s − . This work represents the first identification of SNRs by an optical survey in NGC 2903.We present the spectrum of one of the bright candidates and derive an [SII] / H α emission line ratio of 0.42 for this source. In addition,the weak [OIII] λ / H β emission line ratio in the spectrum of this SNR indicates an old oxygen-deficient remnant with a lowpropagation velocity. Key words.
Supernova Remnants (SNRs)– Spiral Galaxies, NGC 2903
1. Introduction
Supernova remnants (SNRs) are important for many of the theo-ries of interstellar medium (ISM) because supernova explosionsand their eventual dispersion of ejected material have the e ff ectof enriching the ISM with the material processed in stellar in-teriors. In a typical HII region, the sulfur is in the form of S ++ because of the strong photoionization flux of the central hot staror stars. Therefore, the [SII] / H α ratio is typically ≈ S + and to produce forbidden-line([SII] λλ , / H α ≥ hereafter MF97);Pannuti et al. 2000, 2002) and in our Galaxy (Mavromatakis etal. 2002; Fesen et al. 1997, 2008) has already been discussed.The sample of Galactic SNRs is quite large, and interstellar ex-tinction and uncertain distances cause selection e ff ects. Theseproblems are much less significant in extragalactic samples.Assuming the distance to a galaxy is known, all the SNRs are atthe same distance from us, so some properties can be compareddirectly. Also, the foreground extinction is generally low, thusrelative positions of SNR samples are determined accurately inan extragalactic survey. By knowing the positions of the SNRs,their distributions are calculated relative to HII regions and spi-ral arms. Possible SNR progenitors have been investigated fromthese distributions (MF97; Blair & Long 1997, 2004). Send o ff print requests to : E. Sonbas Extragalactic searches for SNRs were first obtained for theMagellanic Clouds by Mathewson & Clarke (1973). They werethe first to use the [SII] / H α emission line ratios for optical iden-tification of SNRs. Blair et al. (1981), Smith et al. (1993), andBlair & Long (1997,2004) also used same method successfully.A number of nearby spiral galaxies have already beenobserved to identify SNRs using optical observations (e.gD ′ Odorico, Dopita & Benvenuti 1980; Braun & Walterbos 1993;Magnier et al. 1995; MF97; Matonick et al. 1997; Gordon et al.1998, 1999; Blair & Long 1997, 2004) and X-ray observations(Pence et al. 2001; Ghavamian et al. 2005). Radio searches forextragalactic SNRs have been conducted by Lacey et al. 1997;Lacey & Duric (2001) and Hyman et al. (2001). SNR surveyshave also been carried out at optical, radio, and X-ray wave-lengths by Pannuti et al. (2000, 2002, 2007).In this work, we searched for SNRs in the nearby spiralgalaxy NGC 2903 using the criterion [SII] / H α ratio is ≥ ◦ inclination an-gle, and 17 ◦ position angle at a distance of 9.4 Mpc to NGC2903 has been adopted for this paper (Bresolin et al. 2005). Itwas observed at radio (Williams & Becklin 1985; Tsai et al.2006), infrared (Simons et al. 1988; Williams & Becklin 1985;Alonso - Herrero, Ryder & Knapen 2001), X - ray (Fabbiano,Trinchieri & MacDonald 1984; Mizuno et al. 1998; Junkes &Hensler 1996; Tschoke, Hensler & Junkes 2003), and opticalwavelengths (Bresolin et al. 2005). Multiwavelength observa-tions of NGC 2903 implies that it has a very complex structurewith knots in the nucleus. The knots, called “hot-spots”, containmany early type stars (Oka et al. 1974; Simons et al. 1988). Tsaiet al. (2006) report subarcsecond-resolution VLA (Very Large E. Sonbas et al.: An optical search for supernova remnants in the nearby spiral galaxy NGC 2903 (RN)
Array) imaging of NGC 2903. They found seven discrete radiosources in the central 15 ′′ × ′′ of galaxy at the wavelengthsof 6 and 2 cm to a limiting integrated flux density of 0.2 and 0.3mJy, respectively. They identified one of their sources (source D)as a candidate radio SNR. Their detected sources meet at leastone of the following criteria: 5 σ detection at the peak intensityat one wavelength and 4 σ emission detection at both 6 and 2 cm.The organization of the paper may be described as follows:.In Sect. 2 we discuss both the imaging and spectroscopic obser-vations that were conducted as part of this study, as well as theaccompanying data reduction. The SNR identification technique,search results, and discussion are described in Sect. 3.
2. Observations and data reduction
NGC 2903 was observed in 2006 March with the 1.5 m RussianTurkish Telescope (RTT150) at TUBITAK National Observatory(TUG) in Turkey. Images were taken with TFOSC (TUBITAKFaint Object Spectrograph and Camera) 2048 × ′′ .39 pixel − , giving 13 ′ .3 × ′ .3 FOV.We used narrowband interference filters centered on the linesof [SII] & H α + α and [SII] images. Characteristics of the interference filtersused in these observations are listed in Table 1. An observationlog of the imaging data is shown in Table 2 for this galaxy. Thedata were reduced using ESO–MIDAS (The European SouthernObservatory Munich Image Data Analysis System) software en-vironment. Several H α , [SII], and associated continuum imagesof each galaxy field were combined to obtain deeper field im-ages in order to increase the signal-to-noise ratio for the faintestobjects. As shown in Table 2, 20 exposures (5400 seconds intotal for two filters) were combined for two observation nights.Standard stars from the list of Oke (1974) and Stone (1977) wereobserved each night to determine the flux conversion factors.Bias frames and dome flats were also observed. Each exposurewas bias-subtracted, trimmed, and flat-fielded. The cosmic rayswere removed from each [SII] and H α image. The SNR can-didates overlaid on DSS (Digitized Sky Survey) images of thespiral galaxy can be found in Fig. 1. The spectral data of one bright SNR were obtained with the op-tical telescope BTA-6 m (Bolshoi Azimuthal Telescope, Russia)2008 April. The SCORPIO (Spectral Camera with OpticalReducer for Photometrical and Interferometrical Observations)spectrograph was used in BTA with a CCD 2048 × ′′ slit width, and a 3500 - 7200 Å spectralrange was assumed for SCORPIO with 10 Å spectral resolution.The IDL codes and IRAF packages were used to perform thebasic reductions, flux, and wavelength calibrations and interstel-lar extinction correction. Spectrophotometric standard stars fromOke (1974) and Stone (1977) catalogs were observed each night.Derived fluxes from the spectrophotometric standards were usedto find the fluxes for spectral lines in our SNR spectrum. Biases,halogen lamp flats, and FeAr or Neon calibration lamb exposureswere obtained for each observation set. Finally [SII] and H α emission line fluxes were measured using splot routine in IRAF.To obtain spectral data of SNRs, the slit position was arrangedso that both a bright star and the SNR were inside the slit. Theaim of spectral observations was to resolve [SII] λλ , λλ , α line.
3. Results and discussion
To identify the SNR candidates in NGC 2903, we used the tech-nique where continuum-subtracted H α and [SII] λλ , / H α image ratios weremade. Finally, regions that have image ratio values ≥ α subfield images. Weonly displayed a region about 2 ′ on a side at one time for vi-sual inspection of the fields to search for candidates. Any brightfeature in the continuum-subtracted [SII] image was checkedagainst the continuum-subtracted H α to make sure the stars werenot poorly subtracted. If the feature in the [SII] image lookedbrighter than it was in the H α image, we marked it as an SNRcandidate. Each preliminary SNR candidate was a possible tar-get for follow-up spectral observation. In the latter step [SII] andH α , continuum-subtracted images were used to measure the totalcounts for each SNR candidate. We selected a circular aperturein continuum-subtracted images to sum the ADU (Analogue toDigital Units) counts. Afterwards a concentric annulus was se-lected to determine the background counts to subtract from theaperture sum. The aperture sizes used to measure fluxes wereconstrained by the seeing that was attained during our inter-ference filter imaging observations (namely 1.9 ′′ , which corre-sponds to ∼
87 pc for the assumed distance to NGC 2903 of 9.4Mpc). Because of the di ff erence between seeing and pixel scales,we did not include radii calculations for the SNR candidates thatwe detected. To correct flux values for interstellar extinction, weused data from Cardelli, Clayton, and Mathis (1989).Using the SNR identification technique described above, 5SNR candidates were detected in NGC 2903 with [SII] / H α ≥ α flux of SNR4 is smaller thanthe other four optically-identified SNRs by factors of approxi-mately 2-3. We caution that calculations involving the H α linemay have slight contamination from the [NII] lines. We carriedout these calculations in an environment with a limiting flux sen-sitivity level of 3.1 × − erg cm − s − for our imaging obser-vations of NGC 2903. This sensitivity limit was determined bychoosing a structure that has minimum magnitude in the galaxyfield. Then the total counts from the same circular aperture ofSNRs used are converted to galaxy flux value using the spectro-scopic standard star flux mentioned above.Only one field was observed in NGC 2903. It covers a totalfield of 12 ′ × ′ . In Figs. 2 - 3 we show 4 ′ × ′ subfields of NGC2903. No SNRs were found in the southern half of the galaxy. Wenote that the south field of the galaxy consists of mainly brightHII regions compared with the north field (Bresolin et al. 2005).This might be one of the reasons for the absence of SNRs in thisregion. The detected SNR distribution in NGC 2903 resemblesthe skewed distribution of SNRs seen in NGC 2403 by Matonicket al. (1997). They also explain the absence of SNRs in the northfield of the image of NGC 2403 in a similar fashion.Among the five candidates detected in NGC 2903, we wereable to observe SNR4 spectroscopically and derive a specificline ratio of [SII] / H α of 0.42. The optical spectrum of SNR4is shown in Fig. 4. Line intensities relative to H β , the E ( B − V ) , andH α intensity value for this spectrum given in Table 4. We usedthis spectrum to calculate line ratios for [SII] λ / [SII] λ . Sonbas et al.: An optical search for supernova remnants in the nearby spiral galaxy NGC 2903 (RN) and [OIII] λ / H β . We calculated electron density, N e , us-ing [SII] λ / [SII] λ nebular . temden .When an electron temperature value is given, this task calculatesthe electron density, based on the five-level atom approximationexplained in the task. The line ratio of [SII] λ / [SII] λ > N e ≤
10 cm (Osterbrock 1989). Assuming an electron temperature of T = K, the calculated N e value is 360, which is not so atypicalfor such galaxies (for example, SNR 19 in NGC 2403, Matonicket al. 1997). For SNR4, we detected only a weak [OIII] λ / H β emission line ratio of ∼ ≤
100 km / s (Smith et al. 1993), which shuts o ff the nebular shocks. There are many examples of such weak lineratios with poor oxygen content in a number of nearby galaxies(Blair & Long 2004; MF97).We searched for X-ray counterparts to the SNRs thatwere found by our optical observations. Twenty one X-ray point sources were determined in the position andextension of NGC 2903 in the Master X-Ray Catalog(http: // heasarc.nasa.gov / W3Browse / all / xray.html). We foundonly one positional coincidence with the Master X-ray cata-log sources taking a 30 ′′ positional error circle around the ob-jects. SNR3 falls in the error circle of 1RXS J093210.2 + ′′ , which was lower than our assumed error, once the error cir-cle diminished in size, there we found no correlation of our can-didate SNR3 with the source. This catalog contains data fromthe Position Sensitivity Proportional Counter (PSPC) onboardROSAT (Rontgen Satellite) and Imaging Proportional Counter(IPC) onboard Einstein observatories. IPC provides an angularresolution of ∼ ′′ (FWHM) at ∼ ∼ × − to ∼ × − erg cm − s − in the 0.3- 3.5 keV energy band (Gioia et al. 1990). And ROSAT PSPCminimum sensitivity lies around a few times 10 − - 2 × − erg cm − s − in the energy band 0.1-2 keV (Morley et al. 2001).PSPC has provided ∼ ′′ (FWHM) on-axis angular resolution at1 keV (Trumper, 1984). These limiting fluxes set an upper limiton the luminosity of sources that would be detected in this cat-alog as a few times 10 erg s − at the distance of 9.4 Mpc (forNGC 2903). Given that a maximum radiative X-ray luminosityof an SNR (with a shock temperature of 0.1 keV or above) willbe a few times 10 − erg s − (see Panutti, Schlegel, Lacey 2003;Schlegel & Panutti 2003; Holt et al. 2003) , it is only normal thatthere are no SNRs among the Master X-ray Catalog sources inthe vicinity of NGC 2903. Tschoke et al. (2003) have also found18 sources in the vicinity of NGC 2903. We checked for anypositional coincidence with these sources using an error circleof 30 ′′ , but found no correlation. Given our results for SNR4,since we derive no strong [OIII] emission and our [OIII] λ / H β ratio indicates that nebular shocks are around or below 100km / s (i.e., an old remnant), this yields an upper limit on the elec-tron temperatures of about 10 K, which would greatly reducethe X-ray emission that will be detected from this remnant. It isexpected that X - ray imaging with Chandra, with improved an-gular resolution and sensitivity, could provide valuable improve-ment for SNR detections in nearby galaxies especially for NGC2903.We also checked for an overlap between our optically iden-tified SNRs and the candidate radio SNR in NGC2903 reportedby Tsai et al (2006). However, there is no overlap (within 2 ′′ positional accuracy) among these sources. As noted by MF97, Braun & Walterbos (1993) have esti-mated that about half of all SNe are of Type Ib / c or Type II ( thatis, produced by the deaths of massive stars); in turn, only half ofall of these SNe are located in regions with enough ambient den-sity to produce a detectable SNR. This means that, only about aquarter of all SN events may leave easily detectable optical rem-nants. In our case we have detected five SNRs, indicating that thetotal number of SNe in NGC 2903 is ∼
20, about half of which( ∼
10) would leave remnants. This number could also be takenas an upper limit for observations with much more improved vis-ibility and seeing conditions. However, only about half of them( ∼
5) would occur in easily detectable regions. All these provideacceptable explanations for our observations. If the optically vis-ible lifetime of a typical SNR is about 20,000 years (Braun et al.1989), it would also give us an SN occurrence rate of about 1per 1000 yrs. When we compared this rate to MF97 SN rates,we find quite good overlap for the case of NGC 5585, which hasthe same number of SNRs. In the same work, for galaxies withlower and higher SNR numbers, this rate goes proportionallyhigher and lower.In their analysis, MF97 also present the mode values of themeasured H α intensities for the detected SNR samples from nu-merous galaxies to see evidence of any selection e ff ects and bi-ases (see their Table 19). Using the galaxy distances they showthe log of the mode of the H α luminosity, L(H α ) mode , is larger formore distant galaxies. This means that, if the distance of galaxiesincreases, it is much more di ffi cult to detect the fainter SNRs. Inour case, we calculated that the L (H α ) mode value could be takenas ∼ × erg s − (since we have detected a few SNRs, wewere only able to calculate an average value for H α intensitiesand considered this as our mode value. This was also the prac-tice by MF97 for galaxies with a low number of SNRs). With adistance of 9.4 Mpc, NGC 2903 follows the same trend towardhigher L(H α ) mode values to go with greater distances. Acknowledgements.
We thank the TUBITAK National Observatory (TUG) andSpecial Astrophysical Observatory (SAO) for their support with observing timeand equipment. Also we would like to thank to IGPP (Institute of GeophysicalPlanetary Physics) at UCR (University of California Riverside) for providing ussome of the interference filters. We also thank an anonymous referee and M.E.Ozel for their valuable comments and discussions.
References
Alonso-Herrero, A., Ryder, S. D., Knapen, J. H. 2001, MNRAS, 322, 757Blair, W. P., Kirshner, R. P. & Chevalier, R. A. 1981 ApJ, 247, 879Blair, W. P. & Long, K. S. 1997, ApJS, 108, 261Blair, W. P. & Long, K. S. 2004, ApJS, 155,101,121Braun, R., Goss, W. M., Lyne, A. G. 1989, ApJ, 340, 355Braun, R. & Walterbos, R. A. M. 1993, A&AS, 98, 327Bresolin, F., Schaerer, D., Gonzlez Delgado, R. M., Stasinska, G. 2005, A&A,441, 981Cardelli, J. A., Clayton, G. C. & Mathis, J. S. 1989, ApJ, 345, 245D ′ Odorico, S., Dopita, M. A. & Benveuti, P. 1980, A&AS, 40, 67Fabbiano, G., Trinchieri, G. & MacDonald, A. 1984, ApJ, 284, 65Fesen, R. A., Winkler, F., Rathore, Y., Downes, R. A. & Wallace, D. 1997, AJ,113, 767Fesen, R. A., Rudie, G., Hurford, A. & Soto, A. 2008, ApJS, 174, 379Ghavamian, P., Blair, W. P., Long, K. S., Sasaki, M., Gaetz, T. J., Plucinsky, P. P.2005 AJ, 130, 539Gioia, I. M., Maccacaro, T., Schild, R. E., Wolter, A., Stocke, J. T., Morris, S. L.,Henry, J. P. 1990, ApJS, 72, 567Gordon, S. M. , Kirshner, R. P., Long, K. S., Blair, W. P., Duric, N., Smith, R. C.1998, ApJS, 117, 89Gordon, S. M., Duric, N., Kirshner, R. P., Goss, W. M., Viallefond, F. 1999,ApJS, 120, 247Holt, S. S, Schlegel, E. M., Hwang, U., Petre, R. 2003, ApJ, 588, 792Hyman, S. D., Calle, D., Weiler, K. W., Lacey, C. K., Van Dyk, S. D. & Sramek,R. 2001, ApJ, 551, 702
E. Sonbas et al.: An optical search for supernova remnants in the nearby spiral galaxy NGC 2903 (RN)
Junkes, N. & Hensler, G. 1996, International Conference on X-ray Astronomyand Astrophysics: R¨ontgenstrahlung from the Universe, 459Kilgard, R. E. et al. 2002, American Astronomical Society, 201, 1416Lacey, C., Duric, N., Goss, W. M. 1997, ApJS, 109,417Lacey, C. K. & Duric, N. 2001, ApJ, 560, 719Magnier, E. A. et al. 1995, A&A, 114, 215Mathewson, D. S. & Clarke, J. N. 1973, ApJ, 180, 725Matonick, D. M. & Fesen, R. A. 1997, ApJS, 112, 49 (MF97)Matonick, D. M., Fesen, R. A., Blair, W. P. & Long, K. S. 1997, ApJS, 113, 333Mavromatakis, F., Boumis, P. & Paleologou, E. V. 2002, A&A, 387, 635Mizuno, T., Ohbayashi, H., Iyomoto, N. & Makishima, K. 1998, IAUS, 188, 284Morley, J. E., Briggs, K. R., Pye, J. P., Favata, F., Micela, G., Sciortino, S. 2001,MNRAS, 326, 1161Niklas, S., Klein, U., Braine, J. & Wielebinski, R. 1995, A&AS, 114, 21Oka, S., Wakamatsu, K., Sakka, K., Nishida, M., & Jugaku, J. 1974, PASJ, 26,289Oke, J. B. 1974, ApJS, 27, 21Osterbrock, D. E. 1989, S&T, 78, 491Pannuti, T. G., Duric, N., Lacey, C., Goss, W. M., Hoopes, C. G., Walterbos, R.A. M., Magnor, M. A. 2000, ApJ, 544, 780Pannuti, T. G., Swartz, D. A., Duric, N., Urosevic, D. 2002, AmericanAstronomical Society, 200, 3505Pannuti, T. G, Schlegel, E. M. & Lacey, C. K. 2007, AJ, 133, 1361Pence, W. D., Snowden, S. L., Mukai, K. & Kuntz, K. D. 2001, ApJ, 561, 189Schlegel, E. M., Holt, S. S., Petre, R. 2003, ApJ, 598, 982Schlegel, E.M., Panutti, T.G. 2003, AJ, 125, 3025Sersic, J. L. 1973, PASP, 85, 103Simons, D. A., Depoy, D. L., Becklin, E. E., Capps, R. W., Hodapp, K.-W. &Hall, D. N. B. 1988, ApJ, 335, 126Smith, R. C., Kirshner, R. P., Blair, W. P., Long, K. S., & Winkler, P. F. 1993,ApJ, 407, 564Stone, R. P. S. 1977, ApJ, 218, 767Trumper, J. 1984, PhyS, 7, 209Tsai, Chao-Wei, Turner, J. L., Beck, S. C., Crosthwaite, L. P., Ho, P. T. P., Meier,D. S. 2006, AJ, 132, 2383Tschoke, D., Hensler, G. & Junkes, N. 2003, A&A, 411, 41Wynn-Williams, C. G. & Becklin, E. E. 1985, ApJ, 290, 108
Table 1.
Characteristics of the interference filters used in ourobservations
Name λ FWHMWavelength (Å) Å[SII] 6728 54continuum 6964 350H α Table 2.
An observation log of imaging data for our target galaxyobtained with RTT150 at TUG.
Galaxy Name Date Filter Exposure(s)NGC 2903 2006 March 4 / × / α × / × / × / × / α × / × / × Table 3.
New optical SNR candidates detected in NGC 2903.
SNR Name RA DEC [SII] / H α I(H α )(J2000.0) (J2000.0) (erg cm − s − )SNR1 9:32:12.5 + + + + + Table 4.
Relative line intensities and observational parametersfor SNR4
Line SNR4H β ( λ λ λ λ λ λ λ λ α ( λ λ λ λ ( B − V ) α ) 5.4E-15 erg cm − s − [ S II ] / H α (RN) Fig. 1.
All the SNRs that are detected in our study are indicatedon the images extracted from Digital Sky Survey (DSS). Figureshows NGC 2903 with the 5 new SNR candidates found in thiswork labeled with circles.
Fig. 2.
The figure indicates the SNR candidates (SNR1 andSNR2) detected in this work within NGC 2903 overlayed on a ∼ ′ s subfield of continuum-subtracted [SII] image. Fig. 3.