Published in the May 2012 issue of the Rosette Gazette newsletter (Rose City Astronomers, Portland Oregon USA)
A Notable Chain of Extragalactic Nebulae is Displayed to Visual Observers to the South of the Famous “Tarantula Nebula”
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The nebulae complex near 30 Doradus (indicated with a circle in the picture).
North is up. Image from Photopic Sky Survey.
©Nick Risinger
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Summer in the Southern Hemisphere is the best season of the year to observe the Magellanic Clouds. If you are South of declination +15° take a look at the 30 Doradus region, a starburst region in the Large Magellanic Cloud usually known with the colorful name “Tarantula Nebula”. It is impressive even in small telescopes, showing dark and bright features of nebulosity.
But if you want to know a little more about the HII regions content of our satellite galaxy, just aim your telescope about 35 arc minutes due south of that bright nebula, in fact, the brightest and biggest of this kind of object in our nearby galaxy, and you will find a very interesting field displaying several nebulae and some small clusters. I am referring to a 1° field centered at R.A. 05hs 41m Dec. -69° 41' J2000.0 (see Figure 1).
The Chain N158, N160, N159
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DSS image of the three major regions in the complex.
North is up.
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Under a dark sky, a UHC filter works properly, displaying, at low magnification, an astounding field where the nebulae structures appear very detached. With the whole complex centered, extended and elongated zones of nebulosity are visible in the northwest part of the eyepiece field. Prominent nebulosity exists in the area connecting 30 Doradus with the N159/160 complex. Through this filter, NGC 2080 in the N160 region is the brightest zone of the whole complex (UHC works very well in this case). On the other hand, N159 looks more detached with a similar brightness of that of N158 (LH 101 region).
The Cloud N158
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Figure 1. 1-degree eyepiece field centered at R.A. 05h 41m
Dec. -69° 41'. The field shows the complex south of 30 Doradus. North is up. |
The structure closest to 30 Doradus is named N158 (see DSS picture below). In the paper “The OB associations LH 101 and LH 104 in the HII region N158 of the LMC”, G. Testor and V. Niemela state that the northern part of this region is composed of a superbubble around the OB association1 LH 104, while the southern part, containing the OB association LH 101 and dominated by unevolved and evolved O-type massive stars, is a diffuse HII region which is characterized by three bright zones. Using the mentioned magnification, the area coincident with the OB association LH 101 is the brightest part of the whole complex understudy, rivaling in brightness with the zone in NGC 2080, situated farther south, where the small blobs named A1 and A2 are situated (read more about this below). In the zone of the OB association LH 101 at least five stars are visible (visual magnitudes around 10 and 11) forming a sort of crown-shaped asterism that is better viewed using averted vision. Very close to this asterism, on the side opposite to the position of 30 Doradus, a small and bright nebulosity is clearly visible, N158C (also NGC 2074), an emission nebula + cluster according to the NGC/IC Project web page, where LH 101 lies. As said, one of the most prominent zones where faint stars could be hardly glimpsed using averted vision. This technique was useful to see the overall shape of the cloud N158, appearing elongated southwest-northeast. The region in the middle of N 158 looked less conspicuous, with a 10th magnitude star (GSC-9167-0702) well detached there, equidistant from both stellar associations and with some faint nebulosity present there. At the other end of the cloud, in the area coincident with the OB association LH 104, the brightness of the complex arises again becoming more obvious (little less bright than the LH 101 zone and a little bigger) showing a similar appearance, with nebulosity and stars situated on the side. The distribution of the nebulosity surrounding LH 104, evident in the DSS image, was not visible at this magnification.
Using the stars indicated with letter D in the eyepiece field as reference (see Figure 1) I tried to identify and see a very small nebulosity near LH 101, labelled with letter A in the DSS image below, but it was not visible under the observing conditions. The asterism marked with red lines was also identified.
Focusing again on N158, the view through a UHC filter was interesting. The middle zone shows more nebulosity if we compare it with the view without it. This filter allowed me to identify the overall shape of N158 better. Like the view without a filter, the zone coincident with LH 101 is the most conspicuous, a bright patch of light with some stars close to it. Helped by averted vision, the elongated central region is visible reaching the zone of LH 104 where stars with some of nebulosity are visible. This area of the complex was observed around 10:30pm local time (UT – 3hs) when the altitude of the target on the sky was around 52°.
A view of N158 using higher magnification (78x) shows the region of LH 101 bright, with a few stars embedded in the brightest zone. A lane of nebulosity seems to follow the shape formed by the distribution of the brightest stars there, suggesting a crown or arc shape. Scanning the rest of this region with this magnification, the middle area looks (like with the lower power observation) less conspicuous. However, some of faint nebulosity could be seen there. At the end, brighter nebulosity is visible embedding the OB association LH 104 where the use of averted vision makes possible the identification of a faint stellar swarm.
At 78x with a UHC filter, the view of this cloud was very interesting because its detailed shape started to be revealed (see DSS image to the right). The OB association LH 101 looks elongated approximately northwest-southeast, the middle zone shows the nebula lane. Observing carefully with averted vision the faint nebulosity surrounding the association LH 104 could be fairly glimpsed.
The region in LH 101 looked like a C-shaped nebulosity at higher power (106x), harbouring a few stars brighter than the others in the nearby surrounding field at both ends of the “C”. The association LH 104 looked like a roundish group of faint stars of similar brightness and nebulosity. HD 38489, an extreme supergiant star, is located in this association and its ultraviolet spectrum is similar to the spectrum of the peculiar Eta Carinae star.
The small and elusive nebula, indicated by A in the DSS image above, could be glimpsed at this magnification using averted vision.
At this magnification and a UHC filter, N158 looked prominent in two sections, one of them coincident with LH101 suggesting again the mentioned “C” shape, with two stars named Sk -69 249 and Sk -69 247 in its northern side, well detached from the hazy background (see lower panel in DSS image above). The other section is situated between the central star of the cloud reaching the zone of LH104, appearing like a pretty faint and smooth lane of nebulosity.
N158 is the third richest area of the Large Magellanic Cloud for WR stars2.
N158 is the third richest area of the Large Magellanic Cloud for WR stars2.
The Complex N159/N160
According to a team of researchers from Universidad de la Plata and Casleo Observatory in Argentina, Carnegie Institute, and the Space Telescope Institute, the field south of 30 Doradus is particularly rich in clusters, associations, and nebulae, including the N159/N160 complex. This field also contains the most massive CO concentrations in the LMC, so it may be expected that in a few million years 30 Doradus successor will appear there (you can read the paper “Spectroscopic Study of the N159/N160 Complex in the Large Magellanic Cloud”, Cecilia Fariña et. al. 2009).
The HII Region N160 and the “Ghost Head Nebula”
Moving the telescope away from N158, in the opposite direction to 30 Doradus, we find another region of the complex under observation, N160. Yasushi Nakajima et. al. in their paper “Near-infrared Imaging Observations of the N159/N160 Complex in the Large Magellanic Cloud: Large Clusters of Herbig Ae/Be Stars and Sequential Cluster Formation”, The Astronomical Journal (2005), divide N160 in two parts, N160-north and N160-south. The first one (older) is situated in the zone coincident with an asterism I could easily identify and I indicate with a red line in Figure 1. On the southern side of the asterism, and indicated with an ellipse, faint and smooth nebulosity can be seen at 42x even without a filter, reaching the N160-south region which shows some prominent features.
At 78x, very faint nebulosity was visible with faint stars present there. The nebulosity was also visible through an UHC filter but the view was not improved.
At 106x and UHC filter the nebulosity was very hard to see, so lower magnification was better in this case.
What Happen in N160?
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N160 region containing NGC 2080, the "Ghost Head Nebula". A1 and A2 blobs
are the bright dots at the ends of the chainnear the center of the picture. Photo obtained by the author using a 16-inch remote-controlled telescope installed at La Punta Observatory in San Luis Province (Argentina). |
The more interesting and conspicuous feature in this part of the complex, appearing at 42x like a small and round nebulosity (in my opinion the brightest feature of the whole complex under study) is N160A. Actually, it is an NGC object (NGC 2080) discovered by John Frederick William Herschel in 1834. It is the nebular patch in the middle of the HII regions under observation, nicknamed the “Ghost Head Nebula”. NGC 2080 requires higher magnification for more detailed observation. To the southwest and very close (bounded) to NGC 2080 a small, faint, and round nebula were visible. The star HDE 269953, the brightest one in the area (visual magnitude 9.9), was visible immersed in faint nebulosity in the zone where the nebulae NGC 2085, 2086, and IC 2145 lie (see Figure 2). At this magnification, these nebulae were not visible at all.
Using a UHC filter, NGC 2080 looks very bright and small suggesting a slight elongation. Again, the faint nebulosity close to this object was detected. To the east-southeast, two “stars” were visible embedded in a zone of nebulosity.
The view of NGC 2080 at 78x and without a filter starts to suggest two small patches of nebulosity with similar brightness. They are very close to each other and to discern them was very hard, so higher magnification was necessary for a better view. Faint nebulosity was viewed very close to those patches and at this power it seems to engulf them. The view of NGC 2080 and the nearby nebulosity through a UHC filter was similar.
About 2.7 arc minutes to the east-southeast of NGC 2080 a group of small objects lies (see Figure 2). A pretty faint and small hazy dot was visible as a little defocused star. According to the position given by the STScI DSS web page it is IC 2145. The zone connecting this object with NGC 2080 displays faint nebulosity that was better viewed using averted vision.
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| Figure 2. The N160 region. North is up. |
The view of the “Ghost Head Nebula” at 106x was very interesting and worked very well to see the details of this region in the middle of the complex. Averted vision suggests the presence of three bright and small patches (see the picture I took with the remote telescope). These patches are surrounded by faint nebulosity. This nebulosity is also visible toward the southwest where a faint star is visible within it, near the center. Through a UHC filter, it was not easy to discern the three patches of NGC 2080, the zone showed a sort of “granularity” and appeared bright.
The two hazy objects, IC 2145 and NGC 2085, close to the star HDE 269953 (Figure 2), looked very small resembling distant and compact open clusters. The fainter one (NGC 2085) is situated close to the star, the other one (IC 2145) is a little brighter and was easier to see.
A final observation of NGC 2080 using high magnification (360x) in a 16-inch telescope, to try to see “the eyes”, showed a blurry image. However, a sort of chain of small nebulous objects embedded in nebulosity could be seen.
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NGC 2080, the Ghost Head Nebula.
Copyright: ESA, NASA, & Mohammad Heydari-Malayeri
(Observatoire de Paris, France) |
The “Eyes of the Ghost”
Heydari-Malayeri & Testor (1986) discovered two compact HII regions embedded in N160A, identified as N160A1 and N160A2, the “eyes of the ghost” (bright spots in the picture to the left). These objects belong to the special class of so-called High Excitation Blobs (HEBs3) in the Magellanic Clouds. HEBs are very dense small regions usually 5” to 10” in diameter. A1 (left) is powered by a single massive star. A2 (right) harbors several exciting stars enshrouded by large quantities of dust. The best part of this story is they can be glimpsed through a common amateur telescope like 8 inch using high magnification and good seeing conditions!
The N159 Cloud
Interesting! Roughly round in shape, this cloud has maybe the most striking appearance of the three patches in the complex. The line of three stars (with visual magnitudes between 9.4 and 11) labeled B in Figure 1 was used as a guide to identify this HII region.
At 42x, N159 looks like a round and smooth nebula with a few bright spots on the periphery (I talk about them below) distributed in the form indicated by arrows in a sketch I made. Through an 8-inch telescope and without a nebular filter, the dark feature across the “disk” in N159 was glimpsed for moments using averted vision and observing carefully (see DSS image – Figure 3).
At 42x, N159 looks like a round and smooth nebula with a few bright spots on the periphery (I talk about them below) distributed in the form indicated by arrows in a sketch I made. Through an 8-inch telescope and without a nebular filter, the dark feature across the “disk” in N159 was glimpsed for moments using averted vision and observing carefully (see DSS image – Figure 3).
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Sketch of N159 made by the author showing the four patches
seen through an 8-inch telescope. Two are nebular in appearance (below in the sketch)and the other two look stellar (up in the picture). |
The brightest patch, and easier to see, is N159A (also NGC 2079), situated on the southwest corner of N159. The “patch” labeled A in Figure 3 looked like a star and it is coincident with the position of LMC X-1 (read about this object in N159´s Features section below). A higher power was necessary to observe and identify the east region, where NGC 2084 and a supernova, named SNR 0540-697, are situated.
Using the same magnification with a UHC filter the view was clearer because N159 looked more contrasted against the background sky and definitively round in shape. The dark lane across N159 was glimpsed with averted vision. Only two of the four spots in N159 were clearly visible through this filter. N159A is, by far, the more obvious, being visible like a defocused star o very small spot of nebulosity. Fainter and less contrasted is the other patch situated inner in N159 (see Figure 3). Its position seems to match that of NGC 2084. However, higher magnification was necessary in order to have a better view.
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Figure 3. North is up.
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At 78x, NGC 159 was clearly visible. It is still necessary to use averted vision to discern the black feature inside. The four “patches” are again visible; two of them are undoubtedly hazy in appearance, somewhat similar in apparent size. The other two clearly look like stars.
At the same magnification but this time using a UHC filter, the cloud looked round and smooth in brightness. The two hazy patches were detected immediately, especially N159A which is the brightest one. In fact, N159A is brighter than the rest of the N159 cloud. Using the configuration offered by the two stars indicated with A and B in the lower panel in Figure 3, and the nebula N159A, the other patch seems to be NGC 2084. This was confirmed later having an observation of the zone with a 16-inch telescope that made possible the identification of other features situated between both patches, indicated with white arrows on the lower panel in Figure 3. According to the NGC/IC Project web page, NGC 2084 is a bright nebula. Once again, the black feature was barely visible using averted vision.
An observation of this object at 106x (a good magnification) showed the two round hazy patches. They look similar, with N159A slightly smaller and brighter.
As usual, for most of the nebulae, the H-beta filter did not work to observe N159, being the image totally blocked.
N159´s Features
A Supernova Remnant and an X-ray source
A recently identified supernova remnant lies within N159. I am talking about SNR 0540-697, which is situated at RA 05h 40m 00s Dec. -69° 45' 10” (J2000.0) according to MCELS (The UM/CTIO Magellanic Cloud Emission Line Survey). On the northeast corner of N159 a small zone, slightly brighter than the rest of the cloud, was visible through an 8-inch telescope at 78x and nebular filter in the zone coincident with the position of the supernova remnant. However, 0540-697 is not an object for amateur astronomers. The supernova remnant, discovered by Chu et al. (1997), is difficult to observe in optical and radio due to emission from the surrounding H II region N159.
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The 14.8 magnitude star #32 is the optical
counterpart of LMC X-1, a radio source in our satellite galaxy. This photo appears in Figure 2 in the paper "Determination of the Optical Counterpart of LMC X-1" (A.P. Cowley et. al. 1995). |
Not too far from this remnant, indicated in the DSS image with letter A, we find LMC X-1, which is (with Cyg X–1) one of only two known persistently luminous x-ray binaries consisting of a black hole accreting the wind of a massive blue star (“The nature and cause of spectral variability in LMC X–1” L. Ruhlen et. al. 2011). I think the star visible there through my 8-inch telescope at different magnifications is surely R148, a B5 type supergiant star in LMC with a visual magnitude of about 12.5, one of the four “patches” in N159. The fainter star very close to R148 labeled #32 in the photo to the right, is probably the optical counterpart of the x-ray source LMC X-1.
The “Papillon Nebula”, an elusive object
Situated in the HII region N195, this small nebula is also the prototype of the HEB family, “High-Excitation Blobs” (Heydari-Malayeri & Testor 1982) that constitute a rare class of ionized nebulae in the Magellanic Clouds, according to the paper “Stellar Populations Associated with the LMC Papillon Nebula” by F. Meynadier et. al. (2004),. It is known with the name N195-5 and its angular dimension is about just 2x2.8 arc seconds. It is not visible in an 8-inch telescope.
If current researches are right… will generations in a remote future see maybe “a new 30 Doradus” in this part of our neighbor galaxy?
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1_ 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.
2_ WR Star: Wolf-Rayet stars are hot massive stars (20+ solar mass) with a high rate of mass loss. Strong, broad emission lines arise from the winds of material being blown off the stars.
3_ HEB: Stands for High Excitation Blobs, compact HII regions which constitute a rare class of ionized nebulae in the Magellanic Clouds. They are characterized by high excitation, small size, high density, and large extinction compared to typical Magellanic Cloud HII regions. These objects are tightly linked to the early stages of massive star formation.