Dark Supernova Remnants revealed by CO-line Bubbles in the W43 Molecular Complex along the 4-kpc Arm
aa r X i v : . [ a s t r o - ph . GA ] F e b Dark Supernova Remnants revealed by CO-line Bubbles in theW43 Molecular Complex along the 4-kpc Arm
Yoshiaki Sofue ∗ Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo 181-0015, JapanAccepted for Galaxies, 2021
Abstract
Fine structure of the density distribution in giant molecular clouds (GMC) around W43 (G31+00+90km s − at ∼ . ∗ CO-line survey at high-angular (20 ′′ ∼ . − ) resolutions ( ∗ Four-receiver-system Unbiased Galactic Imaging survey withthe Nobeyama 45-m telescope). The GMCs show highly turbulent structures, and the eddies arefound to exhibit spherical bubble morphology appearing in narrow ranges of velocity channels. Thebubbles are dark in radio continuum emission, unlike usual supernova remnants (SNR) or HII regions,and in infrared dust emission, unlike molecular bubbles around young stellar objects. The CO bubblesare interpreted as due to fully evolved buried SNRs in molecular clouds after rapid exhaustion of thereleased energy in dense molecular clouds. The CO bubbles may be a direct evidence for exciting andmaintaining the turbulence in GMCs by SN origin. Search for CO bubbles as ”dark SNRs” (dSNR)will have implication to estimate the supernova rate more accurately, and hence the star formationactivity in the Milky Way.
Keywords: galaxies: individual (Milky Way) — ISM: CO line — ISM: molecular clouds — ISM:supernova remnant
Galactic supernova remnants (SNRs) are observed as extended objects bright in radio, X-ray, and/oroptical emissions, often exhibiting shell structures expanding at high velocities [1, 2, 3]. About 300Galactic SNRs are currently catalogued in the Milky Way [4, 5]. Their feedback to the ISM via interactionwith the ambient molecular clouds has crucial implication to the interstellar physics such as the origin ofinterstellar turbulence, star formation, and cosmic ray acceleration [6, 7, 8, 9, 10].Although most of radio SNRs are supposed to be catalogued for the transparency of the Galactic discin radio, a larger number of SNRs is predicted from the estimated supernova rate of 0 . ± .
01 SNe y − [11, 12, 13, 14, 15, 16], suggesting 200 to 2000 SNRs in the Galaxy for supposed life time of a SNR of10 − y. Since remnants of core collapse SNe are expected to be located near to their birth places becauseof the short lifetime of high-mass progenitors, it is expected that their distribution is tightly correlatedwith that of HII regions. However, only a weak concentration has been found in the spiral arms for theknown SNRs [17]. In order to uncover missing SNRs, an extensive survey has been obtained by radiocontinuum observations, and a large number of candidate SNRs have been detected [18]. However, theirlongitudinal distribution does not indicate a clear correlation with that of HII regions.From these statistics we may expect that there exist a larger number of uncovered SNRs that are notdetected in the current observations in radio or in other wave lengths. A possible way of existence of suchuncovered SNRs would be buried SNRs in molecular clouds (MC). Supernovae (SNe) exploded in densegas of MCs evolve in a quite different way from that exploded in the low-density inter cloud space. They ∗ E-mail: [email protected] ∼ y reaching a small radius of a few pc after peaky infrared flash[19, 3, 20, 21]. Hence, their direct detection is difficult for the short time scale and compactness, so thatno observational evidence has been obtained yet of the buried SNRs.In spite of the short flashing phase, the molecular cavity left after the buried SNR can survive formuch longer time, which would be detectable as a cavity or a bubble of molecular gas. We recentlyreported the discovery of an almost perfect round-shaped cavity with a clear-cut boundary in a mediumsized molecular cloud at G35.75-0.25+28 in the CO ( J = 1 −
0) line emission km s − [22]. The cavityis quiet in radio emission, unlike usual SNRs or HII regions. It is also quiet in infrared emissions, unlikeSpitzer bubbles associated with molecular shells around young stellar objects (YSO) [23, 24]. The peculiarproperty of the molecular cavity G35.75 was understood as due to a relic of a fully evolved supernovaremnant (SNR) in the cloud, which may be the first evidence for the existence of a buried SNR in theGalactic disc. We called the molecular cavity a ”dark SNR” (dSNR).In this paper, we extend the search for dSNRs in the form of molecular bubbles and/or cavities inthe giant molecular clouds (GMC) surrounding the active star-forming region, W43 Main (G31+00+90km s − ), in the tangential direction of the 4-kpc molecular arm. The molecular gas properties and starformation in the W43 region have recently been extensively studied using the FUGIN CO line data[25, 26]. We adopt a distance of 5.5 kpc according to the literature, which is close to the near-sidekinematical distance of 5 . ± .
46 kpc at v lsr = 93 km s − (center velocity of W43) for the most recentrotation curve of the Galaxy [27].We make use of the FUGIN (Four-receiver Unbiased Galactic Imaging survey with the Nobeyama45-m telescope) survey data in the CO, CO, and C O line emissions [28]. The data are availableat the URL of FUGIN, http://nro-fugin.github.io. The full beam width at half maximum of the 45-m telescope was 15 ′′ at the CO ( J = 1 −
0) frequency and the velocity resolution was 1.2 km s − .The effective beam size in the used 3D FITS cube is 20 ′′ , rms noise level ∼ l, ∆ b, ∆ v lsr ) = (8 ′′ . , ′′ . , .
65 km s − ). In figure 8 of the Appendix, we show channel maps of the CO -line brightness temperature in the2 ◦ × ◦ region around W43. Figure 9 shows the same, but extended emissions with scale sizes greaterthan 3 ′ ( ∼ − , and the diameter is typically ∼ ◦ . ∼ ′ to ∼ ′ (3 to 24 pc). Arcs and filaments are generally fainter than completebubbles, and are more extended.Figure 1 shows typical cases from a channel map, where CO bubbles at G31.2+0.2+81 at v lsr = 81 . − are shown by a composite color-coded map of CO brightness in red ( T B from 0 to 15 K), CO ingreen (0 to 4 K), and C O in blue (0 to 2 K). Several almost empty cavities, each about 0 ◦ . CO line), the molecular gas is mildly compressed at the bubble edges.Using the original channel maps of a 2 ◦ × ◦ region around W43 taken from the FUGIN FITS cubedata (http://nro-fugin.github.io), we identified many bubbles and arcs as shown in figure 10. Theirpositions, approximate radii, center velocities, and approximate velocity widths are listed in table 1. In figure 10 we plot positions and extents of extended radio continuum sources at 5.8 GHz taken from theVLA survey (GLOSTAR) [29] with a similar angular resolution (18 ′′ ) as the present CO maps. Except2 ersion January 30, 2021 submitted to Journal Not Specified CO COC O RGB( CO , CO ,C O )
Figure 1.
Brightness temperature maps of the G31+00 region at 81.675 km s in the CO , CO , and lines, and a color-coded (RGB) map of CO (red, 0 - 15 K), CO (green,0 - 5 K), and O (blue, 0 - 3 K).
Figure 1: Brightness temperature maps of the G31+00 region at 81.675 km s − in the CO , CO , andC O ( J = 1 −
0) lines, and a color-coded (RGB) map of CO (red, 0 - 15 K), CO (green,0 - 5 K),and C O (blue, 0 - 3 K). 3or W43 Main associated with comparable sized shells in CO and radio continuum, there appear no clearcorresponding pairs.In figure 2 we enlarge the bubbles around G31.0+0.1, and compare with the radio continuum emissionat 20 cm [32, 31] (MAGPIS) and dust emission at 8 µ m (GLIMPSE ). We emphasize that the bubbles arenot associated with radio continuum emission, unlike usual SNRs or HII regions. Also, unlike molecularbubbles around young stellar objects (YSO), they are dark in thermal radio and infrared emissions.Panel (c) of the figure shows the positions of YSOs by crosses (SIMBAD) and Spitzer bubbles by circles(showing approximate extents) [24], which are not coincident with the CO bubbles. These facts indicatethat the CO bubbles are empty not only in the molecular gas, but also in warm dust, ionized thermalgas and non-thermal emitters (cosmic rays and magnetic fields). These CO bubbles are recognized onlyin the subsequent several channels (0.65 km s − increment), indicating that the velocity width of eachbubble is several km s − . Figure 3 shows a close up of the bubble at G31.2+0.2+81.675 km s − , and an averaged LV diagramacross the center made from four subsequent LV diagrams. The bubble is clearly visible as an ellipticalridge in the LV diagram as marked by an ellipse of radius of ∼ ◦ .
075 and half velocity width of ∼ − . Such an elliptical LV feature can be naturally understood as due to an expanding shell of radius 7.2pc at velocity 7 km s − . It is stressed that the thus estimated expanding velocity, which is ubiquitousin other bubbles, is greater than the velocity dispersion of a few km s − in the surrounding MC. If thebubbles are dark SNRs, or the relics of buried SNRs, such increased velocity would be a direct evidencefor the acceleration of interstellar turbulence by the feedback kinetic energy of an SN explosion.In the third panel of figure 3, we present a cross section of T B across the bubble center. It revealsa clear-cut inner wall with the intensity maximum at the edge followed by an extended outskirt. Thisindicates that the bubble is a vacant cavity, and suggests that the interior gas, which had filled thecavity, is accumulated near the edge of the cavity, composing a shell structure observed as a CO bubble.Alternatively or additionally, the inner gas may have escaped from the bubble through a smaller holeor a crack in the wall. In fact, the bubble is not perfectly surrounded by the wall, but some parts aremissing, being merged with the neighbouring bubbles. In order to examine a wider area for the CO bubbles, we show channel maps of the 2 ◦ × ◦ region aroundW43 in the Appendix, along with background-filtered images of each channel map to enhance bubblyand filamentary features. Thereby, we detected many possible bubbles as indicated in the Appendix.Although the purpose of this paper is to search for CO bubbles in the molecular complex aroundW43 between 80 and 100 km s − , it may be worthwhile to look for similar objects in different regionsand velocity ranges. Figure 4 shows examples of such bubbles found at different velocities, and hence indifferent arms, at G30.4+0.4+70 km s − and G30.45+0.36+46 km s − in the same sky area as for W43.G30.4+0.4+70 is located on the lower-velocity branch of the Scutum arm, and bubble diameters areabout 0 ◦ .
12, corresponding to D ∼ . . ± . . ± . − .G30.45+0.36 +46 km s − is located on the higher-velocity branch of the Sagittarius arm. The radiusof 0 ◦ . × ◦ . . ± . . ± . − . If we adopt the near distances, the sizes are comparable to those obtained for W43 bubbles.The present search for CO bubbles was obtained only for the limited area on the sky around G31+00.However, it is expected that similar CO bubbles would be found generally in many other molecular clouds,particularly in giant molecular clouds adjacent to star forming regions. We may, therefore, reasonably ersion December 10, 2020 submitted to Journal Not Specified (cid:23414)(cid:23412)(cid:23409)(cid:23412)(cid:23416)(cid:23411) (cid:23414)(cid:23412)(cid:23409)(cid:23411)(cid:23411)(cid:23411) (cid:23414)(cid:23411)(cid:23409)(cid:23419)(cid:23416)(cid:23411) (cid:23411) (cid:23409) (cid:23414)(cid:23411)(cid:23411)(cid:23411) (cid:23409) (cid:23413)(cid:23411)(cid:23411)(cid:23411) (cid:23409) (cid:23412)(cid:23411)(cid:23411)(cid:23411) (cid:23409) (cid:23411)(cid:23411)(cid:23411) (cid:23408) (cid:23411) (cid:23409) (cid:23412)(cid:23411)(cid:23411) (cid:23434)(cid:23460)(cid:23471)(cid:23460)(cid:23462)(cid:23479)(cid:23468)(cid:23462)(cid:23395)(cid:23471)(cid:23474)(cid:23473)(cid:23466)(cid:23468)(cid:23479)(cid:23480)(cid:23463)(cid:23464) (cid:23434) (cid:23460) (cid:23471) (cid:23460) (cid:23462) (cid:23479) (cid:23468) (cid:23462) (cid:23395) (cid:23471) (cid:23460) (cid:23479) (cid:23468) (cid:23479) (cid:23480) (cid:23463) (cid:23464) (cid:23408)(cid:23412)(cid:23409)(cid:23414) (cid:23413)(cid:23409)(cid:23412) (cid:23416)(cid:23409)(cid:23417) (cid:23420) (cid:23412)(cid:23413) (cid:23412)(cid:23417) !"! ! " ! $% & " ! $%$ *+ , -./-.0 01213-.4 Figure 2. (Top) CO bubbles around G31+0.1 in CO (K) at 81.465 km s , (middle) 20 cm brightnessin red (from 0 to 10 mJy/beam; MAGPIS) and 8 m brightness (from 50 to 500 mJy/str; GLIMPSE), and(bottom) YSO positions by crosses (SIMBAD: AGAL, 2MASS; http://simbad.u-strasbg.fr/simbad/)and Spitzer bubbles by circles with names.
Figure 2: (Top) CO bubbles around G31+0.1 in CO T B (K) at 81.465 km s − , (bottom left) 20 cmbrightness in red (from 0 to 10 mJy/beam; MAGPIS) and 8 µ m brightness in blue/green(from 50 to 500 mJy/str; GLIMPSE), and (bottom right) YSO positions by crosses (SIMBAD: AGAL,2MASS; http://simbad.u-strasbg.fr/simbad/) and Spitzer bubbles by circles with names.5igure 3: (Top left) Molecular bubble S31.2+0.2 at 81.465 km s − . (Top right) Longitude-velocitydiagram around b = 0 ◦ .
23 across the bubble G31.2+0.2 (average of 4 channels in the latitude directionnear the bubble center). The ellipse represents bubble radius 0 ◦ .
074 (7.1 pc at 5.5 kpc distance) and halfvelocity width 6.7 km s − centered on l = 31 ◦ . v lsr = 81 . − . (Bottom) T B cross section at 81.0km s − across the bubble center at position angle of 120 ◦ .6rgue that the CO bubbles of the same property as found here are rather ubiquitous in the Galactic disc,particularly in molecular complexes near active star forming regions in dense spiral arms. It is not easy to estimate the mass of a bubble, not only because it is vacant of the gas, but also becausethe outer boundary of the molecular cloud is not definite for the extended outskirt. Instead, we maycalculate the mass of supposed exhausted molecular gas that had filled the bubble in the past, assumingthat the density was same as the ambient cloud density. The mean T B of the surrounding gas cloud aroundthe bubble G31.1+0.2+81 is about T B ∼
10 K, velocity width is δv ∼ − . The mean moleculardensity can be estimated to be n H ∼ X CO T B δv/r ∼
230 cm − , where r = 7 pc is the bubble radius and X CO ∼ × H cm − (K km s − ) − is the conversion factor for extended molecular clouds [32]. Thetotal lost mass inside the bubble is then estimated as M ∼ π/ µn H m H r ∼ . × M ⊙ , where µ = 2 . m H is the hydrogen mass. If the mass had escaped from the bubble ata velocity of v esc ∼ − , the total kinetic energy of the thus lost mass is E esc ∼ / M v ∼ erg, safely supplied by the input energy by an SN explosion. We showed that the giant molecular clouds (GMCs) composing the W43 molecular complex are filled withCO bubbles. Such bubbly structures are particularly evident in the channel maps after subtracting theextended emission (figure 9). Thus, in so far as the W43 complex is concerned, the GMCs are generallybubbly rather than exhibiting filament structures. This makes contrast to the filamentary interstellarturbulence in the local Orion clouds [33] or to that expected from simulations [34]. Hence, as will beconcluded in the next subsection, the bubbly behavior may be due to a more efficient feedback by thecurrent SF activities in W43 associated with a larger number of SNe compared to that in the local ISMin the solar vicinity.
As to the origin of the CO bubbles, we may consider several possible mechanisms.(i) The first idea is that they are fully evolved relics of buried SNRs, simply argued from the requiredtotal energy. Thereby, the shell structure is maintained by its own expanding motion, as described later.(ii) The second idea is that they are evolved Spitzer bubbles. This idea encounters the difficulties, asraised in the previous section, that the bubbles are quiet in thermal radio and infrared emissions.(iii) Stellar winds from young stars may also be excluded, because there is no signature of star formationinside the bubbles, as for the reasons against (ii), except for the core area of the W43 Main.(iv) Outflows from old population stars such as planetary nebulae would be another possibility [37]. Theresponsible mass-loss stars are distributed in the population II disc at a number density approximatelyequal to that of AGB stars n PN ∼ t AGB /t ∗ ( M disc /M ∗ ) / ( πr z disc ) ∼ − pc − , where t AGB ∼ y, t ∗ ∼ y, M disc ∼ M ⊙ is the Galactic disc mass, M ∗ ∼ M ⊙ , r disc ∼ z disc ∼
200 pc is thedisc radius and full thickness, respectively. We thus expect only one such star within ∼
100 pc volumearound W43. Moreover, the supplied energy would be too small, E PN ∼ (1 / v ˙ M t
AGB ∼ ergs fromthe wind, where ˙ M ∼ − M ⊙ y − is the mass-loss rate and v ∼ − is outflow velocity.(v) Thermal instability produces a cavity, if the heating rate by cosmic rays per molecule is constant andthe cooling rate is proportional to the square of gas density [38]. A lower-density perturbation results ina growing cavity. However, it cannot explain the observed expanding velocity of the shell at several kms − , because the perturbation grows at the sound speed of molecular gas, ∼ − .(vi) Magnetic filaments will produce perpendicular molecular filaments [39, 40]. However, in order tomake CO bubbles, the magnetic fields must be radial about the bubble centre.7 ersion December 10, 2020 submitted to Journal Not Specified
GREY: Galactic GLAT 00 08 51.25 s031cub12.lvb.1PLot file version 6 created 22-SEP-2020 15:00:24Grey scale brightness range= 0.00 10.00 K0 5 10 vii) Finally, one may attribute the bubbles to interstellar turbulence. However, this argument does notanswer the question about the origin of the CO bubbles. In fact, ideas (i) to (vi) are almost equivalentto that about the origin of turbulence in molecular clouds.
From the above consideration, we here conclude that idea (i) is most plausible as the origin of theobserved CO bubbles. We here try to explain the CO bubbles by well evolved and radio quiet SNRs,which exploded inside molecular clouds and had evolved as buried SNRs. We assume that the responsibleenergy sources are mostly core-collapsed (type II) SNe, because most of the catalogued SNRs, mainlyfrom radio observations by their shell structures, are of type II SN origin. Type Ia SNe would make SNRsof filled center morphology, while rarely produce shell structures. Also, kinetic energy released by thistype is not sufficient to explain the expanding kinetic energy of the CO bubbles.Massive stars produce cavities in the ambient gas by the stellar winds for some My. The wind-drivenshells evolves into shocked SNRs soon after SN explosions [21, 20]. By scaling the current SNR modelsfor ambient density of ∼ − to a case of ∼ H cm − in a MC, both the radius and velocity canbe scaled down by a factor of 100 − / = 0 .
16 for the same time scale unit. When the shock wave reachesthe wind’s boundary, the molecular gas is compressed and evolves as a buried SNR. Here, we consider acase that massive stars are distributed over the GMC, and they end their lives as individual SNe.If high-mass stars compose a dense cluster ending by multiple SNe, they will disrupt the ambientclouds [21]. In this case, the SNe may not leave such a bubbly GMC as observed around W43, but willblow off the surrounding MCs, from which they formed, leaving a naked stellar cluster.The expansion velocity v and radius r of a spherical adiabatic shock wave in a uniform-densitygas are related to the input kinetic energy E and gas density ρ as E ∼ (1 / π/ r ρ v , where ρ = µn H , m H is the ambient gas density. Most of the released energy by core-collapse SN explosion, ∼ erg, in a dense gas cloud is exhausted by the initial infrared flash within ∼ years [19, 3, 20].After the initial radiation phase, the kinetic energy given to the gas expansion may be assumed to be onthe order of E ∼ erg, an order of magnitude smaller than the total released energy.We here introduce a parameter, ED , defined by ED = log( E /n H ) , where E is the input energy bythe explosion in ergs, and n H is the number density of hydrogen atoms in cm − . The hydrogen numberdensity in a molecular cloud is related to the H density through n H = µn H with µ = 2 .
8. The observedradius and velocity for the CO bubble G31.2+0.2 of ∼ ∼ − is realized, when ED = 46 . t ∼ . E = 10 erg, the density is required to be n H = 5 × H cm − , or n H ∼ × H cm − . If the cooling is significant so that the input energyis equivalently decreased to E = 10 erg, the density may be an order of magnitude lower, consistentwith the measured density of ∼
230 H cm − in the GMC around W43. The presently identified molecular bubbles exhibit close resemblance to that reported in our earlier paperon G34.75-0.2 [22]. We here try to explain the molecular bubbles as due to dark SNRs, which had evolvedin the molecular clouds as buried SNR in W43 molecular clouds, ceased their expansion, and faded outof the thermal and high-energy radiation phase.The evolutionary time scale in the radiation phase of buried SNRs is two orders of magnitude shorterthan the usual SNRs exploded in inter-cloud low-density regions because of the extremely higher ambientdensity, so that the luminous phase ends in ∼ −
100 y [19, 3, 20, 21]. For the short lifetime, they havelittle chance to be observed and catalogued as radio or optical SNRs, but can be recognized by molecularbubbles as dark SNRs in their latest phases.Figures 5 illustrates the evolutionary scenario along a flow line of the Galactic rotation. It schemat-ically explains the spiral arm structures of molecular clouds, HII regions and of SNRs, according to theevolutionary scenario under the galactic shock wave theory. We may summarize the evolution from corecollapse SNe to dSNR as follows. 9
CW43 MainW43 South dSNR Mol. bub, SNR - k p c M o l . A r m G a l a c t i c S ho ck S F a r m S NR a r m Galactic rotation
Figure 5: Face-on view of the 4-kpc arm from the north Galactic pole, illustrating the evolution of amolecular cloud and dSNRs along the Galactic flow line by rotation through a galactic shock. t ∼ − My)
Diffuse ISM as well as molecular clouds are strongly compressed by the galactic shock wave along the4-kpc molecular arm [25]. Due to both the galactic shock and orbital condensation in the bar-end, cloudcollisions are strongly enhanced [26]. Accordingly, intense star formation is activated at cloud interactionfronts, and OB stars are formed and HII regions are produced, emitting thermal radio and far infrareddust emissions. A significant fraction of the formed OB stars and clusters develop inside the giantmolecular cloud. Frequent cloud collisions cause not only star formation, but also growth of molecularclouds by merger, resulting in formation of larger scale molecular complex. t = 0 y), buried, and cool SNR ( ∼ − y) The OB stars explode as supernovae, and their significant fraction are still embedded inside the molecularcomplex along the molecular arm. Most of the released energy of SNe is exhausted by radiation ofneutrinos, γ rays, and hard-X rays. The ejecta of SNe and snow-plowed gas in the molecular cloud formexpanding buried SNRs, which evolve rapidly due to strong cooling by the dust and thermal emissionsin infrared and mm waves. The buried SNRs end their shining phase in a life time as short as ∼ y,leaving cool SNRs. ∼ − y) The evolved buried SNRs remain as dark SNRs, which expand as an almost adiabatic shock wave in thedense molecular gas. They are observed as the molecular cavities and bubbles in their expanded phase,as reported in this paper. 10 (pc)N (cid:16092) (cid:16097) (cid:16093)(cid:16092) (cid:16093)(cid:16097) (cid:16094)(cid:16092) (cid:16094)(cid:16097) (cid:16095)(cid:16092)(cid:16094)(cid:16096)(cid:16098)(cid:16100)(cid:16093)(cid:16092)
Figure 6: Frequency of bubble radii for assumed distance of 5.5 kpc to W43.
As readily shown in figures 1, 8 and 9, the analyzed region is full of molecular bubbles. The bubbles havea typical radius of r ∼ − . The rate of SNe in the Galaxy has been estimated to be on the order of 2 ± γ -ray spectroscopy [12, 14, 15],yielding 1 . ± . ∼
200 shell type SNRs in the Galaxy for an assumed lifetime of a shell of ∼ y, or ∼ ∼ y, strongly dependent on the adopted life time ofa shell. Estimation of the exact life time of a shell is difficult from observations of SNRs expandinginto the turbulent ISM with significant deformation. Furthermore, the Galactic plane is observed to befull of unidentified radio filaments [30, 23, 31], suggesting the presence of a large number of debris ofun-catalogued old SNRs.We may thus argue that the density of existing shell type SNRs in the Galaxy is on the order of or11 ersion January 30, 2021 submitted to Journal Not Specified
12 of 19 !"! / + , ) (cid:16092)(cid:16095)(cid:16092)(cid:16098)(cid:16092)(cid:16101)(cid:16092)(cid:16093)(cid:16093)(cid:16092)(cid:16093)(cid:16092)(cid:16092) !"! / + , ) /’12 (cid:16094)(cid:16092)(cid:16094)(cid:16097)(cid:16095)(cid:16092)(cid:16095)(cid:16097)(cid:16096)(cid:16092)(cid:16093)(cid:16093)(cid:16092)(cid:16093)(cid:16092)(cid:16092) Figure 7. [Left] Longitudinal number density per one degree of Green’s catalog SNRs (black andgrey circles in the 1st and 4th Galactic quadrants, respectively), HII regions (triangles), and CO bubbles(dSNR) (big circle with error bars). [right] Same, but enlarged around the Scutum Arm.
The rate of SNe in the Galaxy has been estimated to be on the order of per 100 y by various observations (see table 1 of [14]). Particularly, the rate of core collapse SNe supposed to be responsible for shell type SNRs has been rather accurately determined from the -ray spectroscopy [12 14 15], yielding 1.9 1.1 per 100 y. This predicts 200 shell type SNRs in the Galaxy for an assumed life time of a shell of 10 y, or 2000 for 10 y, strongly dependent on the adopted life time of a shell. Estimation of the exact life time of a shell is difficult from observations of SNRs expanding into the turbulent ISM with significant deformation. Furthermore, the Galactic plane is observed to be full of unidentified radio filaments [23 30 31], suggesting the presence of a large number of debris of un-catalogued old SNRs.
We may thus argue that the density of existing shell type SNRs in the Galaxy is on the order of or greater than 200 2000. Furthermore, from the Al -ray spectroscopy and intensity distribution, the SNe have been shown to be concentrated around the Galactic Centre within | ∼≤
30 14].
This means that we may expect a longitudinal number density of shell-type SNRs is still greater in the present studied region, on the order of 200 2000 60 or 3 to 30 per degree of longitude.
Furthermore, if the SNRs are spatially correlated with the SF arms, they must be more concentrated in the tangential direction of the spiral arms. Thus, we may expect a much higher, or the highest longitudinal SNR density in the tangential direction of the Scutum arm (4-kpc molecular ring) at
30 nesting W43, the most active SF site in the first quadrant of the Galaxy.
If the bubbles are relic of buried SNRs, the radii and expanding velocities suggest that their ages are on the order of 0.4 My. This is an upper limit to the age, and the real dSNRs would have evolved a bit rapider due to cooling effects. So, we here assume that their ages are on the order of 10 y. As counted in the Appendix, the number of CO bubbles in the longitude range from 30 to 32 is
27 per 2 degrees in longitude, or 13.5 per degree. On the other hand, the catalogued
SNRs yields per degree in the same direction. In figure we plot the thus estimated counts in comparison with those of the catalogued SNRs [ ] as well as with the number of HII regions per degree [36].
Although the longitudinal number density of the catalogued SNRs shows enhancement in the tangential directions of the spiral arms, it is significantly weaker than that of the HII regions. It may be
Figure 7: [Left] Longitudinal number density per one degree N of Green’s catalog SNRs (black and greycircles in the 1st and 4th Galactic quadrants, respectively), HII regions (triangles), and CO bubbles(dSNR) (big circle with error bars). [right] Same, but enlarged around the Scutum Arm.greater than ∼ − Al γ -ray spectroscopy and intensity distribution, theSNe have been shown to be concentrated around the Galactic Centre within | l | ∼≤ ◦ [14]. This meansthat we may expect a higher density of shell-type SNRs in the here studied region than N ∼ − / ◦ or 3 to 30 per degree of longitude. Furthermore, if the SNRs are spatially correlated with the SF arms,they must be more concentrated in the tangential direction of the spiral arms. Thus, we may expecta much higher, or the highest longitudinal SNR density in the tangential direction of the Scutum arm(4-kpc molecular ring) at l ∼ ◦ nesting W43, the most active SF site in the first quadrant of theGalaxy.If the bubbles are relic of buried SNRs, the radii and expanding velocities suggest that their ages areon the order of ∼ . y. As counted inthe Appendix, the number of CO bubbles in the longitude range from l = 30 ◦ to 32 ◦ is N ± √ N = 27 ± . ± N ∼ Summary
Numerous round-shaped bubbles and cavities of CO-line emission with radii of ∼ −
10 pc were foundin the molecular complex around W43 (G31+00+90 km s − ) in the tangential direction of the 4-kpcstar-forming arm.The bubbles are quiet in radio continuum emission, unlike usual supernova remnants (SNR) or HIIregions, and are dark in infrared dust emission, unlike molecular bubbles around YSOs. The CO bubblesare interpreted as due to dSNR, or fully evolved SNRs buried in dense molecular clouds after rapidexhaustion of released energies by SNe. Increased velocity width in the bubbles as seen in the LVdiagrams compared with that in the ambient molecular gas may be a direct evidence for the accelerationof interstellar turbulence by SN explosions.From the number count of the ”dark” SNRs in W43 complex, we argue that the supernova rate cur-rently estimated from the catalogued SNRs has been significantly under-estimated. Such correction ofthe SN rate in the Galactic disc would affect the star formation history in the Milky Way. We proposed touse the CO bubbles to search for a more number of dSNRs. Taking advantage of simultaneously obtainedradial velocities, and hence kinematic distances to the dSNRs, we will be able to perform a more accuratestatistical analyses of the correlation between SNR and HII regions in the Galaxy. Acknowledgments : The author is indebted to the FUGIN team for the archival data base of the COline survey of the Galactic plane using the Nobeyama 45-m telescope. The data analysis was partiallycarried out at the Astronomy Data Center of the National Astronomical Observatory of Japan. The dataunderlying this article are available in the URL http://nro-fugin.github.io.
Conflicts of interest : The author declares no conflict of interest.
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In figure 8 we present channel maps of the CO brightness temperature, T B , in a 2 ◦ × ◦ regionaround W43 Main centered on G31+00+90 km s − every 4 original channels, or every = 4 × .
65 = 2 . − velocity interval. W43 Main is located at l = 30 ◦ . , b = 0 ◦ embedded in the giant molecu-lar cloud at v lsr ∼
93 km s − . The used FITS cube data are available on the FUGIN web pages athttp://nro-fugin.github.io. Figure 9 shows the same, but extended structures with scale sizes greater15han ∼ ′ have been subtracted in order to enhance smaller scale clouds and filaments. The figuresexhibit numerous bubbles, arcs and filaments.Using the original channel maps between v lsr = 80 and 100 km s − at velocity increment of 0.65 kms − , we have traced CO bubbles by eye estimate. Thereby, each bubble was identified as a loop of T B ridge that can be traced over a couple of neighboring channels. Their positions and radii are shown infigure 6, and are listed in table 1 along with radial velocities, v lsr . Half velocity widths of the bubbles, δv / , defined as half the velocity range in which a bubble can be traced on the neighboring channelmaps, were also measured and listed in the table. It is shown that the expanding velocity of a bubblemeasured on the LV diagram is about three times the here listed half velocity width. We also list typicalbrightness temperature of each bubble edge as read on the relieved channel maps.Table 1: Parameters of CO bubbles l b r r v lsr δv / T B (deg) (deg) (deg) (pc) (km s − ) (km s − ) (K)30.11 -0.17 0.071 6.8 101.2 3 630.23 -0.03 0.071 6.8 82.3 3 430.24 -0.34 0.100 9.6 86.6 2 730.29 -0.13 0.071 6.8 94.7 2 930.32 -0.54 0.197 18.9 88.8 3 930.45 0.35 0.110 10.6 94.6 2 1230.58 -0.02 0.064 6.1 83.0 3 630.62 -0.54 0.071 6.8 91.4 2 730.67 0.51 0.128 12.3 83.6 2 430.75 -0.10 0.296 28.4 84.9 4 630.77 0.03 0.107 10.3 79.2 2 630.77 -0.46 0.126 12.1 96.6 3 530.87 -0.31 0.071 6.8 99.2 3 430.88 -0.02 0.134 12.8 79.7 4 930.88 -0.13 0.057 5.5 98.6 2 430.88 0.27 0.088 8.4 79.1 1 530.97 -0.40 0.241 23.1 99.3 3 330.98 0.08 0.167 16.3 81.7 3 531.09 -0.28 0.076 7.3 91.4 4 431.11 0.13 0.067 6.4 81.0 3 531.18 0.22 0.080 7.7 81.7 3 631.19 -0.09 0.107 10.3 81.7 2 731.41 0.08 0.130 12.5 96.6 2 631.68 0.20 0.069 6.6 99.2 3 431.70 0.05 0.130 12.5 96.0 3 1131.79 -0.38 0.170 16.3 98.0 2 231.92 0.26 0.078 7.5 96.6 2 316igure 8: Channel maps of CO brightness temperature of the W43 complex from 78 to 100 km s − every2.6 km s − (every 4 original channels). Radial velocities (VRAD) of the channels are indicated in m s − in the individual panels. 17igure 9: Same as Fig.8, but extended structures have been subtracted in order to enhance shells andarcs. 18 !" ! " % ! - ) * + ( , Figure 10: Open circles with thin bars indicate positions of CO bubbles traced in the channel mapsbetween 80 and 100 km s −1