The Aggregate Shapley I in LMC

Nebulae and stellar associations reside in this complex in the central region of the Large Magellanic Cloud. How much can we see through an amateur telescope?

It is Spring in the Southern Hemisphere, so it is time to aim a telescope to one of our satellite galaxies, the Large Magellanic Cloud (hereafter LMC), and find another interesting structure there. The Magellanic Clouds show a large variety of bubbles, superbubbles and supergiant shells among its objects, many of them being visible for the amateur observers with common telescopes.

"McKibben and Shapley (1953) were the first to distinguish the giant stellar and gaseous groupings of the LMC, the well known Shapley constellations I - V. Later, van den Bergh named (in 1981) the constellations VI - IX. These seem to be the most active star forming areas of the LMC" (paper "Morphology and Stellar Content of Complexes in the Large Magellanic Cloud" M. Kontizas et al. 1995).

This video by ESO`s VLT is a zoom-in view of the interesting group of nebulae and OB stars  named LHa120¹ N44, one of the targets you can observe with an 8-inch telescope.

This article deals with this complex which is also known as "Shapley Constellation I"  according to the LMC extended catalogue (Bica+ 1999) published in VizieR web page. N44, cataloged by Henize (1956) and named DEM² 150, 151 and 152 by Davies et al. (1976) is a luminous HII complex in LMC. It contains an assortment of compact HII regions, filaments, and shells of all sizes, as well as three OB associations³, LH⁴47, 48, and 49 (Lucke & Hodge 1970). N44 is dominated by a prominent shell around LH47 in the central region (Chu & Mac Low 1990), paper "The Multi-Phase Medium in the Interstellar Complex N44" by Sungeun Kim et. al. 1998. Situated at RA 05 22 06.9 Dec -67 56 46, N44 lies (see Figure 1) about 43 arc minutes southwest of a curious structure named "Sextant" (see the article "An enigmatic Structure in the Local Universe - part 2" in this blog). 

The observation was made from Uspallata valley, in Los Andes mountains, where the dark skies make possible to carry out this kind of projects. 

Figure 1. Position of the complex N44 in LMC. North is up.

Two main hazy regions are visible in the field of view at a first observation at 48x. I call them "northern region" and "southern region" (see Figure 4). The northern region is bigger than the other one but both look pretty similar in the sense that they show several faint stars embedded in nebulae structures. I identified this complex after recognize the T-shaped asterism formed by the stars HD 35862, GSC-9162-0453, HD 35978, and GSC-9162-0321 (visual magnitudes 9.3, 11.4, 9.7, and 11.4 respectively). The brightest star in the field of view is the 8.5 magnitude HD 35665. According with data given by the aforementioned catalogue (Bica 1999) the center of Shapley Constellation I lies at the center of the bigger region (indicated with the blue arrow in Figure 4). This aggregate has an overall dimension of 23 x 16 arc min and an angular position of 150° according with the same source (See Figure 2).

Matching with what can see on the DSS image (Figure 4), the area labelled with an ellipse is where nebulosity and stars are visible at this power. That is the area of the association LH49 and the HII region IC 2128 (also N44D). I found discrepances about the nature of IC 2128. Some sources (e.g. SIMBAD Database) state that is an HII region while others (like the revised NGC/IC catalogue and STScI DSS) say it is a stellar association. Beyond that differences, it is not a conspicuous object at all. The section easier to detect (a little better using of course averted vision) is the more contrasted patch on DSS image in Figure 4 immediately southwest of the 9.7 magnitude star HD 35978, appearing as a faint and defocused star engulfed, in turn, by very subtle nebulosity. Averted vision is necessary to get this view through an 8-inch telescope at low magnification. 

Figure 2. According with the LMC extended catalogue (Bica 1999)

Shapley constellation I is a 23x16 arc minutes wide structure with

an angular position of 150 degrees. North is up. East to the left.

Moving to the northern section of the complex, N44C (also NGC 1936, IC 2127) seems to be the most prominent nebula there, looking round and small.

A little higher magnification (63x) shows very subtle nebulosity encompassing the northern half of the "T" asterism. To the northeast, very faint and challenging is the view of the small component N44E (see Figure 4). With averted vision a ghostly nebulosity is visible. For moments, 2 or 3 stars can be discerned there. We will analize that region again using higher power.

At 63x, N44C is the most contrasted nebulosity visible in the whole complex. Stasinska et al. (1986) favour radiative radiation of N44C by a central star whereas Pakull & Motch (1989) suggest that it could be a fossil, X-ray photo ionized nebula formed by a now

Figure 3. LH49 & N44D

quiescent X-ray binary system. "The bright main body of N44C has the appearance of a normal H II region, apart from its high excitation and He II emission. The east, northeast, and northwest boundaries of N44C are relatively sharp and have low [O III]/Ha, indicating the presence of an ionization front" (paper "The He II Emitting Nebula N44C in The Large Magellanic Cloud: Optical/Ultraviolet Spectroscopy of the Nebula and its Ionizing Star" Donald R. Garnett et al. 2000). The area southwest of N44C is visible with averted vision as a rather smooth hazy spot. N44B is round in shape but slightly fainter and smaller than N44C. Even fainter is another round nebulosity, DEM L 140, to the northwest of N44B. It is smaller than the other two mentioned nebulosities. The OB association LH47 contains 40 blue stars according to Lucke & Hodge (1970) and appears, through an 8-inch telescope, as several stars embedded in a faint hazy background. Roughly north of LH47 we find another section of the complex visible like an elongated faint nebulosity (N44I) where a chain of few stars, situated in the same direction of the elongation, can be visible (surely members of the association LH48).

Using UHC filter...

Figure 4. The N44 complex

The view of the small nebula southwest of the star HD 35978 improves clearly using this nebular filter, being visible even with direct vision. It looks smooth without stars present there. Fainter nebulosity is visible toward the west, part of the N44D section of the complex. The shape seems to be a kind of triangle.

Through this filter N44C is, by far, the most contrasted nebulosity. On the other hand, N44B looks a little smaller and less contrasted but easy to see however. Beyond the view of the bright patches N44C and N44B, the nebulosity connected to N44C from the southwest is the most obvious section of the bigger area when using averted vision. The whole big section is visible even with direct vision however. The nebula southwest of N44C is slightly more prominent that the small and round patch DEM L 140. A dark lane crosses the zone between N44C and N44B. It seems to be situated closer to N44B. The view of N44I and LH48 improves using this filter.

Using averted vision a small hazy spot, the zone of BSDL 1363, is visible among the subtle nebulosity right west of N44B.

N44E is, without a doubt, the faintest area of the whole complex visible through a UHC filter at this magnification. It is hard to see, visible for moments using averted vision.

Figure 5. LH48 & N44I

118x, without filter, is a good magnification to observe N44. The zone of IC 2128 and N44D is visible involving three stars situated there. At this magnification the region N44K comes to the view being visible as a round, faint, and smooth hazy spot. Averted vision is necessary to see it. The stellar association BSDL 1449 is visible with averted vision as a short and narrow hazy strip. N44C and N44B look round in shape with the first one a little more contrasted and bigger. The dark area crossing between both objects is again visible. DEM L 140 is, as aforementioned, fainter, so averted vision must be used to see it better. A star or point-like core is visible within it. A region that looks similar to DEM L 140 at this power is the one immediately northwest of N44B, maybe slightly wider and showing also a central star. LH47 is a conspicuous association in the field of view. In the bigger section, the whole stellar and nebulae structure there seems to connect to the region LH48-N44I which appears as a faint nebulosity with a few stars associated.

Figure 6

Using a UHC filter N44D is visible with direct vision. N44K can be barely glimpsed also, while averted vision makes possible to see the whole southern region better. Moving to the northern region, the patches N44C, N44B, and DEM L 140 stand out. N44C looks smooth in brightness along with its extension to the southwest that also looks smooth and fainter than N44C. The nebulosity in LH48-N44I is a little more conspicuous through this filter.

At 160x the section N44H is visible using averted vision, looking rather irregular. N44K is, undoubtely, a challenging object for an 8-inch telescope. At this magnification a hint of nebulosity seems to be present among the faint stars there. The view of N44C, N44B, and DEM L 140 along with the association LH47 is interesting. N44C and N44B appear very similar in size, with N44C slightly brighter. Different than N44C that looks smooth, N44B shows a few stars embedded there. A short observation of N44C a few days later using a 24-inch telescope at 192x showed the nebulosity labeled with the arrow in Figure 6 as a smooth "cometary tail" or fan-shaped hazy structure emerging from the bright and round structure of N44C. Another dark lane, less prominent than that between N44C and N44B, is visible between N44B and the more elusive DEM L 140. The structure surrounding LH47 and connecting to the bright spots in the northern region (i.e. Shell 1) was not visible at any magnification. Using a 24-inch (61cm) telescope the loop Shell 1 was not visible neither.

Which section is NGC 1929?

As with IC 2128, I found some discrepances about the object in the complex which is named NGC 1929. This situation is outlined in Figure 7. While sources like STSCI DSS web page

Figure 7

give the object indicated on the left panel, other good sources like SIMBAD Database indicate that NGC 1929 is the object on the right panel. An historical identification by Dreyer is the following, NGC 1929 (= GC 1135 = JH 2840, 1860 RA 05 22 02, NPD 158 03.6) is "faint, preceding of group", the latter comment presumably meaning it is the western member of a group of objects. According to the Revised NGC/IC project web site, NGC 1929 is a diffuse nebula or a supernova remnant. The ESO`s web page talks about a cluster of stars when NGC 1929 is mentioned. Have in mind these differences if you search for this object using these sources. In this article I adopted the designation DEM L 140.

The LMC, a surprising nearby galaxy to explore with your telescope from the Southern Hemisphere in spring and summer.

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1_ LHa-120 N is the full name of an entry in the Henize catalogue of LMC emission nebulae. "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").

2_ DEM is a catalogue 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 catalogue is DEM L and the SMC catalogue is DEM S.

3_ 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.

4_ LH is a catalogue of OB associations in the Large Magellanic Cloud compiled by Lucke & Hodge.