Extragalactic Supernova Remnants

Published in the October 2014 issue of the Rosette Gazette Newsletter (Rose City Astronomers, Portland Oregon USA)

There is a Population of This Kind of Object in the Large Magellanic Cloud. 

Are They Accessible to Amateur Observers?A Visual Analysis.

Detailed charts are necessary to find the supernova 
remnants and identify their main features and morphology.

February 1987, the astronomical world was amazed by a powerful and wonderful phenomenon. A supernova appeared in the southern skies, within one of our satellite galaxies, the Large Magellanic Cloud. It was a top topic during that time among scientists and amateur astronomers. This kind of object, that has fascinated human beings for centuries leaves evidence, for the night sky observers, of such energetic event, a cloud of stellar material known as a “supernova remnant”.

As an observer, you are surely familiar with maybe the best example of them, the “Crab Nebula” (Messier 1) in the Milky Way galaxy, visible toward constellation Taurus (the bull). But, where can we find other examples of “dead star´s relic” in the sky? The Large Magellanic Cloud could be a good site to look for them.

I decided to carry out a short observing program containing four of these kinds of objects that belong to this nearby galaxy, which is high in the sky during the southern summer. I used the MCSNR (Magellanic Cloud Supernova Remnant Database) and the list given by C. Badenes, D. Maoz, and B. Draine in their paper ”On the size distribution of supernova remnants in the Magellanic Clouds” (2010) to look for candidates to include in my list. The main criterion to select the sample to observe with a 16-inch dobsonian reflector was obviously the brightness in optical wavelength. I have chosen supernova remnants that look seemingly brighter and more detached on DSS (Digitized Sky Survey) images and as isolated as possible from HII regions, thus avoiding a confusing identification of the target under observation. Certainly, DSS images are a very useful tool, in my opinion, to make this kind of analysis.

Several supernova remnants, like N157B1 for example, or N159 (I talk briefly about this one in the article “A Remarkable Complex in the Vicinity of 30 Doradus”, published in the May issue of the Rosette Gazette, Rose City Astronomers, 2012) lie in zones with prominent HII clouds so they are hard to identify. Both mentioned criteria drove me to compile the following short list, N49, N86, DEM2 L 316A & B, and N63A. Then, the observation at the observing field will determine how easy or hard each supernova remnant is to see.

I spent two nights (January 3 and 4, 2014) to observe the first two supernova remnants (i.e. N49 and N63A). The observations were made when the galaxy was at its highest in the sky, around its transit through the local meridian.

The N49 Supernova Remnant

Embedded in the North Blue Arm3, this is a bright X-ray and optical supernova remnant in the Large Magellanic Cloud (Long et. al. 1981). Certainly, it is the optically brightest object of this kind in this galaxy. D.S. Mathewson and J. R. Healy identified this remnant while Park et .al. (2003) state that its age is about 6,000 years. Moreover, it is almost surely the result of a type II explosion. You can see here a short video about this supernova remnant by the Chandra X-Ray Observatory.

Before observing this object, I was reading papers and watching pictures about this magellanic remnant. It shows a peculiar shape suggesting the nickname of “South America Nebula” (because of the mirror image of this part of the continent). I went to a place named Potrerillos ( lat. -32.9° long. 69.2° W) to set up a 16-inch telescope and observe N49. The night was clear and dark, so I had good conditions to study this object.

Figure 1. Picture of the Large Magellanic Cloud showing the positions
of the four supernova remnants. North is up.

I made a first observation of the field where N49 lies using low magnification (43x). This eyepiece gave me a 1.2° field of view, where the most conspicuous object is the globular cluster NGC 1978, which is clearly detected through a 16-inch telescope. The other region dominating the zone is the 10 Myr old OB association4 LH535, that lies about 10 arc minutes to the south-southwest of the remnant and is associated with it. It is also known with the entry 1948 in the more familiar NGC catalog. It was discovered in 1826 by James Dunlop from Australia. This 11.6 magnitude association appears like a roundish hazy patch, containing several faint stars of similar brightness that are better viewed using averted vision. Some of very faint nebulosity seems to be in the zone connecting LH53 and NGC 1978.

At this magnification N49, with an angular size of 1.4 arc minute, is very difficult to see. If you look carefully using averted vision and with patience, an extremely faint, smooth, round, and small nebulosity can be glimpsed. Higher power is necessary to see this remnant better. Take a look at Figure 2.1 (lower in this article), there you will see a small nebulosity and a star very close to each other, situated to the northeast of the remnant (lower right on the picture). Actually, the nebulosity is an open cluster in that galaxy, KMHK 889. At this power, it was visible like a faint “defocused” star. In fact, it was visible more clearly than N49.

It is time now to see N49 using a little higher magnification. A 32mm eyepiece gave me 56x. Now N49 is more clearly detected. Even with direct vision, a small and rather round nebulosity is visible. However, averted vision improves the view, N49 appears then more contrasted against the background sky. The UHC filter improves the view of the remnant a lot, being clearly seen with direct vision. Using averted vision, two or three brighter spots are visible within the object. Higher magnification is necessary to try to see them better and with more detail. One of these spots looks a little brighter, being situated in the area indicated with number 1 in Figure 2.1.

Using an OIII filter the view also improves, with the bright spots being visible for moments. The view is not as sharp as that obtained with the UHC filter.

The background sky did not look too dark through the Orion Ultrablock filter. A very faint background nebulosity makes the view of N49 not so sharp. However, the view is better than that without a filter, with N49 being clearly identified in the field.

N49 is in the reach of an 8-inch telescope. This sketch, made

by the author, gives a clue of what to expect to see through

this kind of instrument at 119x and dark sky conditions.

H-Beta was not useful to see this remnant.

The view at 106x starts to reveal some of the internal structure of the remnant. The view of N49 and LH53 is very interesting, the association is a conspicuous object in the field and now several of its stars can be seen. At this magnification, N49 starts to show its peculiar shape, somewhat triangular. A small dark region seems to lie in the southwest part (blue arrow in Figure 2.1). Without a doubt, the region 1 appears a little brighter.

The UHC filter at this magnification helps to identify both the spots and the faint regions (see Figure 2.1). These regions show a smooth brightness.

I also used OIII and Orion Ultrablock filters at 106x. The first one helps to see the remnant.

On the other hand, Ultrablock filter shows a brighter image and, in this case, the fainter regions of the remnant are a little better viewed. H-Beta filter was absolutely useless. Without any filter, the small open cluster KMHK 889 and the faint star are clearly visible.

Figure 2. DSS image of N49 (center) and its surrounding

field.

I made a final observation of N49 using 200x. I think we are around the best magnification to see this supernova remnant. N49 looks just beautiful. Even if we are talking about a faint nebulosity, this magnification makes possible to see and suggest the "South America" shape, including the dark lane or zone in the west part of this object. At this magnification, the faint star (A) is visible. It is a good guide or “candle” to try to catch the narrow region of the remnant, immediately to the left of this star on the image below.

However, the “southern tip of South America” (north end of the remnant) was not possible to identify with certainty. I only used the UHC filter at this power and the view, as in the other cases, was improved. Zone 1 can be clearly viewed, appearing somewhat brighter and noticeable. The other fainter parts of the remnant were visible too, but even with this filter the north end was not very well observed.

Figure 2.1. The supernova remnant N49.

The N86 Supernova Remnant

Situated at the western end of the stellar bar we find this object. A good way to find the area is to use the stars μ (Mu) Mensae and θ (Theta) Doradus that are visible with the unaided eye from dark sky sites. The position of the target is equidistant from these stars (see Figure 1).

Figure 3. The N86 supernova remnant is the faint nebulosity
immediately to the south of the star GSC-9161-0804.

Overall, the optical morphology of N86, which has an angular size of 3.5 arc minutes, shows a relatively well-defined shell rich in internal filamentary structure, and a large jet-like feature extending to the north. It can be described (in optical morphology) in terms of four "quadrants” with the eastern quadrant being the brightest (paper “Supernova Remnants in the Magellanic Clouds. II. Supernova Remnant Breakouts from N11L and N86” R. M. Williams, Y. Chu, and J. R. Dickel, 1999). As a colorful note, we can say that N86 is also known with the nickname “Lionel Murphy Nebula” after Lionel Murphy, who was a justice of the High Court in Australia. That nickname comes from Dopita, Mathewson, and Ford, who studied the remnant and found that its shape resembles Murphy´s large nose.

I observed this object under very good seeing conditions. A first view of the field around N86, at 43x, shows stars with rather similar brightness, without very bright stars. The most conspicuous object there is the cluster with nebulosity NGC 1770, discovered in 1826 by James Dunlop. Look at Figure 3, the stars forming a triangle were very useful to reach the accurate zone of N86 (which is at the center of the field in the image) with its vertex pointing to that area. The star GSC-9161-08046 was also visible at this power, helping to scan the area looking for some nebulosity of this remnant.

Using some higher magnification (56x), a very faint nebulosity can be barely glimpsed using averted vision. The small nebulosity is coincident with the brighter region indicated by the DSS image of this remnant (see Figure 3-1). This brighter area lies in the eastern tip of N86, in the “eastern quadrant” mentioned above. A UHC filter brings the remnant to the view, but still very faint and hard to see. A smooth nebulosity lies close to GSC-9161-0804. Averted vision and a very slight movement of the telescope were necessary to improve the challenging view. This filter seems to work a little better than an OIII, which makes it possible to see this part of the remnant for a moment. On the other hand, an H-Beta filter is useless. Higher magnification is necessary for a better analysis of this supernova remnant.

Figure 3.1. DSS image of the N86 supernova remnant.

I observed again this remnant in a different night, using 106x. I must say that the galaxy was low in the sky at the moment of this observation (29°) so the results of seeing N86 under this condition could be not the best. At this power, some of extremely faint and subtle nebulosity seems to lie in the zone of the stars indicated with A in Figure 3.1. Using a UHC filter, I was able to glimpse the nebulosity a little better close to the star GSC-9161-0804 (indicated with the arrow in Figure 3.1), and embedding the aforementioned stars. This area is the brightest portion of the remnant as appears in the DSS image. The view through the Orion Ultrablock filter was not as good as that through the UHC but it was possible to glimpse some nebulosity embedding the stars A. I also observed this part of the remnant with an OIII filter. Through this filter, the faint nebulosity was visible using averted vision, with a sharper view for a very brief moment.

Even higher magnification (200x) makes possible to see the field of the remnant with more detail with the three stars A and the pair B identified. Averted vision is necessary to see them. I think the view at 200x was not better than that at lower power (106x). At 200x, none of nebulosity is visible in the field. Using the UHC and Orion Ultrablock filters the faint nebulosity between the star GSC and the stars A was barely visible for moments.

The N63A Supernova Remnant

Figure 4. The small supernova remnant N63A. Even if the DSS image shows
a"bright" and compact spot within a cluster with nebulosity, the view through 
a 16-inch telescope is quite different, being a faint and elusive spot to catch. 
North is down, East to the right.

I should call it “the high-magnification-remnant”. It is embedded in a larger H II region, N63, and appears to be located within the OB association NGC 2030 (LH83) (Chu & Kennicutt 1988). The supernova remnant is expanding within a bubble produced by its progenitor within the N63 H II complex. N63A is believed to be the product of the explosion of a massive star in a dense and complex environment (Shull 1983; Hughes, Hayashi, & Koyama 1998) and is the first confirmed supernova remnant in an HII region (Shull 1983), see “Revealing New Physical Structures in the Supernova Remnant N63A Through Chandra Imaging Spectroscopy”, Jessica S. Warren and John P. Hughes (2003).

Let´s start saying that this is a very challenging object even for a 16-inch telescope. Although it is in an HII region (it doesn`t match one of my selection criteria) I thought this would be an easier object for a visual observation taking into account the “brightness” of the remnant (showed in Figure 4 above). If you compare it with the brightness of N49 on DSS images, you should think that it would be relatively easy to see…but that was not the case. I observed this object during a very good seeing sky on January 4, 2014. The observing site, Potrerillos, offers good dark skies to see deep sky objects. My friend Carlos Gutierrez, who was observing Jupiter that night, stated that the view of the planet was really astounding, a good indicator of a night with excellent seeing. Moreover, the transparency was good enough.

I observed this remnant the night after the observation of N49. At 43x, I scanned the surrounding field where N63A lies. The field shows rather faint stars and some of the brightest ones form a pattern I could easily identify to be sure I was in the accurate zone of this remnant. As I said before, this remnant is situated in the association NGC 2030. The Wolfgang Steinicke's Revised NGC and IC Catalog shows this object like a diffuse nebula, or a supernova remnant, while the NGC/IC Project web page states that NGC 2030 is a cluster with nebulosity. It was discovered by John F. W. Herschel in 1826, who recorded it as "bright, large, gradually brighter in the middle”. It looks like a round nebulosity embedding several faint stars of similar brightness that are better detected using averted vision. One of the stars looks a little brighter.

Unlike N49, which is visible even at low magnification, N63A is not detected at 56x. Several stars embedded in the nebulosity of NGC 2030 are obviously better detected at this power and the brighter star is visible again when you see the stellar swarm using averted vision.

At 56x, using a UHC filter, the nebulosity of the association is better viewed, appearing more detached and round in shape. Other hazy areas jump to the view in the field of view using this filter. I got a similar image using an OIII filter. With an H-Beta filter, I could also see a smooth and faint nebulosity. However, this filter did not help very much to try to identify the supernova remnant within the complex.

106x. At this magnification the detailed structure of the association is visible, making possible to identify the accurate position of the supernova remnant there. The stars indicated with blue circles in Figure 4 were identified, and a faint nebulosity was visible engulfing that stars and the nearby area.

Using the UHC filter, that nebulosity looks more detached. In the zone where N63A lies, I could barely glimpse, using averted vision, a very small spot slightly brighter (the remnant?). The view was very hard so higher magnification was necessary to try to get a better view. I decided then to ignore the other filters and apply higher power to the telescope.

Using 288x, without any filter, I could see the remnant! But it is still a very challenging and elusive object. Averted vision is necessary to see it, suggesting for moments a sort of roughly triangular shape, very tight and small.

The view at this magnification with UHC was a little worse. N63A is situated in a nebulae area so the view was always difficult.

The Obscure DEM L 316 Supernova Remnant

I observed this faint object on February 1 from Uspallata valley (lat. -32.5° long. 69.3° W) starting at 10:30pm local time (UT – 3hs) with the Magellanic Cloud at its highest (53°). The seeing was not perfect that night though to try to see which I guess is the faintest remnant of the list.

This peanut-shaped nebula was first noted by Mathewson & Clarke (1973) to have a high [S ii]/H_ ratio, a typical signature of supernova remnants (SNRs). The authors designated the two lobes of this system shells A (the northeastern shell) and B (to the southwest), paper “Supernova Remnants in the Large Magellanic Clouds VI. The DEM L316 Supernova Remnants” R. M. Williams and Y.-H. Chu (The Astrophysical Journal, 635:1077–1086, 2005 December 20).

Regarding to their nature, these two shells are, a) two independent remnants superposed along the line of sight (Mathewson & Clarke, 1973), b) a single supernova that exploded into an interconnected bubble formed by a stellar wind or a previous supernova (Lasker 1981; Mills et. Al. 1984), or c) “colliding remnants” (Williams et. al. 1997) (paper “ASCA Observations of the Twin Supernova Remnants in the Large Magellanic Cloud, DEM L316” by M. Nishiuchi, J. Yokogawa and K. Koyama, 2000).

Figure 5. DEM L 316 (center) and the surrounding field.

At 43x, the field of view shows the brightest stars grouped in the eastern region, being visual magnitude 7.6 HD 40156 the brightest one. The stellar patterns (stars linked with blue lines) in Figure 5 were easily recognized, thus confirming that it is the accurate area of the remnant. The most conspicuous object in the field is a bright and slightly northeast-southwest elongated nebulosity, NGC 2122. If you pay attention to the opposite zone (180° from the stellar pattern, right in Figure 5) a very faint and extended nebulosity can be glimpsed. Just a small movement of the telescope toward the west makes possible to see more nebulosity with several faint stars and some small hazy spots embedded there, all of this a result of our proximity to the amazing complex 30 Doradus, which lies at only 1° to the northwest of DEM L 316. At this magnification the target was not visible at all, only the stars labeled A, B, and C in Figure 5.1 were identified (C appearing a little dimmer). Higher magnification is necessary to try to glimpse this object.

At 56x, the stars 1 in Figure 5.1 can be glimpsed for a moment, appearing very faint. Even if the remnant is not visible at this power, the identification of the stars A, C, and 1, whose positions delineate (in some sense) the remnant, makes it possible to concentrate the view on this small area. At this time of the observation, I guessed that to try to see DEM L 316 would be a “retinal torture”. Using a UHC filter, I could see for a brief moment, and in a very hard way (using averted vision of course), a very dim and small nebulosity which position I have indicated with the blue ellipse in Figure 5.1. I can certainly not affirm I saw part of DEM L 316 because the nebulosity was in the very threshold of visibility (bigger scopes are welcome!). I got a similar view using an OIII filter, the ghostly nebula seems to be the south-east edge of the bigger lobule (DEM L 316 B). Like with the other remnants, the H-beta filter did not help to see this object.

Figure 5.1. Close up view of DEM L 316

At 106x, the view starts to be more interesting. The pair of stars 1 is viewed much more clearly, showing both stars a similar brightness. Once again, the extremely faint hazy area within the ellipse is visible with difficulty. I only used a UHC filter with this eyepiece but the view did not show an evident nebulosity.

Even at high magnification (200x) this remnant, or parts of it, are not easy to catch. DEM L 316 is a target for astrophotographers or maybe owners of bigger aperture telescopes, always remembering that good conditions of transparency and seeing are a must for a successful observation.

CONCLUSIONS

This kind of obscure objects can be a good target for those observers who want to test their capability of using averted vision and extreme visual conditions. Most of these supernova remnants were virtually invisible, at least in my observing conditions, i.e. a 16-inch well-collimated mirror, the performance of my eyes, clean optics, a site offering usually a value of 6.2 as a mean limiting magnitude sky (which is not the best limiting magnitude certainly!), etc. Surely, other/s observer/s using bigger mirrors, or the same aperture but under a darker sky, can be able to see these objects a little better, at least in some cases. The cases of DEM  L 316 and DEM L 299 (I observed this last remnant but I have not included a report in this article) are good examples of that situation where you must use averted vision and observe the zone for several minutes, then take a couple of minutes of resting and observe it again. I am sure that is the situation where you are undoubtedly in the “threshold of the imagination”. Are you really seeing something like a tiny and very dim filament, or your brain is making a trick to you?

Beyond all the elusive and challenging objects, N49 is, by far, the brightest and easier remnant to see. You will be able to see it even in smaller telescopes, like 8-inch for example, being obvious even without filters in bigger instruments. Well, if you see at least N49 you can be sure that you were able to add an extragalactic supernova remnant to your observing log files.


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1_ N refers to the Henize Catalogue. The complete name of an object is LHa120-N1. "L" refers to the Lamont-Hussey Observatory of the University of Michigan; "Ha" means the Hydrogen-alpha emission line, the key signature line used in the survey; "120" refers to the plate number (objective prism plate) for the LMC; "N" labels the object as a nebula, as distinct from a star (label "S"). Some of the more complex nebulae are subdivided, using the letters A,B,C, etc to indicate related knots of nebulosity within the complex.

2_ DEM is a catalog of 357 nebulae in the Large Magellanic Cloud and 167 nebulae in the Small Magellanic Cloud that was published in 1976 by R.D. Davies, K.H. Elliot and J. Meaburn. The LMC catalog is DEM L and the SMC catalog is DEM S.

3_ This is the name of one of the structures extending off the ends of the LMC bar that are morphologically similar to spiral arms. The blue arm is still remarkable as the remains of a major star formation event in the outer disk of the LMC 100–160 Myr ago.

4_ OB Association: The concept of a stellar association was originally introduced in 1949 by V. A. Ambartsumian, who later separated them into OB and T associations (Ambartsumian 1968). Morgan, Sharpless, & Osterbrock (1952) considered as a stellar association any loose group of stars within an area where bright OB stars exist and with evidence of a common origin.
A recent definition of a stellar association (Kontizas et al. 1999) refers to it as a single, unbound concentration of early-type luminous stars, embedded in a very young star-forming region.

5_ LH is a catalog of OB associations in the Large Magellanic Cloud published by Lucke & Hodge (1970).

6_ GSC stands for Guide Star Catalogs, created by the staff of the Catalogs and Surveys Branch of the Space Telescope Science Institute.