NGC/IC Project Restoration Effort

===== NGC 5236 = M 83. The center of this galaxy is dominated by a complex of star-forming regions with several forming an apparent arc (see Diaz et al, ApJ 652, 1122, 2006). The optical/IR center is a bright, red object well seen in Figure 1 (from HST images) of the Diaz paper. The position of this object is well-represented by the 2MASS PSC entry for the brightest IR knot in the 2MASS J-band image. Diaz et al also note that the center of rotation for the inner part of the galaxy is offset by 8 arcsec from this optical/IR object. However, it is within about 3 arcsec of the position of the presumed nuclear radio source as given in the position table from three 6-cm observations. It's clear that the position of M 83 is wavelength dependent. I've chosen to represent the galaxy by a mean value of five of the 2MASS point sources in the center. But many other positions are listed in the table, so feel free to pick the one that appeals to you. Also see NGC 6634 where M 83 figures in the identification of La Caille's object now bearing that NGC number.

===== NGC 5236 48" (4/7/13): During this observation of M83, I focused on the HII regions that light up portions of the remarkable spiral arms that emanate from the 3'x1' central bar. The bar is sharply concentrated with a small, intensely bright, 1' round core. At the northeast end of the bar a high contrast arm begins to sweep counterclockwise along the east side, ending up directly south of the core. Several knotty clumps were visible in the region where the arm is attached. First, at the northeast end of the central bar [1.2' NE of center] is NGC 5236:[dPD83] 42, a 10" HII knot. This designation is from a 1983 paper by de Vaucouleurs, Pence and Davoust that includes a map of the 60 brightest HII regions. Close east of this knot is #46, a 20"x10" elongated patch, situated where the arm begins to unfurl to the south [1.7' NE of center]. HII region #54 is another 12" knot a bit further southeast [0.6'] along the arm [1.9' ENE of center]. On the opposite southwest end of the bar a prominent second arm emerges and spirals out counterclockwise along the west side of the galaxy heading north and then spreading out as it curves east. The arm dims noticeably on the northeast side of the halo near a mag 13 star and has a low surface brightness as it continues south in the outer halo, heading towards h 4599, an 8" pair of mag 8.2/10.7 stars. Several knots are visible in this arm. As the arm emerges at the southwest end is #22 and #18, a small 10" knot [1.8' SW of center]. Close north is an elongated clump [2.0' WSW of center], ~25"x10", containing #13 and #15. Another elongated patch, 30"x10", containing #12 and #16, is 1' further north along the arm [1.9' WNW of center]. Additional HII regions were visible at the northern side of the arm; #39 and #43 are a close pair of small knots ~2.5' NNE of center. Further east along the arm [3.3' NE of center] is #56, another elongated patch, 20"x10". A third, wider and more diffuse arm begins on the south side of the bar. It extends below the brighter arm on the west side, and sweeps more gradually, forming an outer western arm. This arm passes just north of a mag 12 star and ends about 4' W of center at a brighter, elongated patch that includes #2 and #3, as well as a mag 15 star. 48" (5/15/12): jaw-dropping view of M83, with the galaxy filling about 2/3 of the 375x field. I didn't take detail notes as we were looking for a recently discovered ultraluminous X-ray source (ULX), in which a companion star is orbiting a neutron star or black hole. We found a stellar or quasi-stellar object at the north edge of the central core, but this was likely a compact HII region (the ULX is ~1' E of the nucleus). 24" (4/11/08 - Magellan Observatory, Australia): M83 resembled the photographic appearance in the 24" at 200x. The bright "bar" was elongated SW-NE and roughly 3'x1' in size with a well-defined bright, round core, 1' in diameter. The first prominent arm is attached at the northeast end of the bar and sweeps south (counter-clockwise) on the east side of the core, wrapping around the southeast side and spreading out a bit as it terminates to the south of the core (~3' from the center). This arm has a high contrast along its outer edge and a couple of faint stars or HII knots are visible near where it attaches to the bar. On the southwest end of the bar a second prominent inner arm emerges and abruptly wraps counter-clockwise around the galaxy on the west side as it heads north. This arm continues to wrap around the north side before spreading out on the northeast side and merging into the outer halo ~3.5' from center on the northeast edge of the halo. A third, more ill-defined arm, also emerges from the core on the south but sweeps more gently to the west (instead of heading north) on the outside of the second arm. It spreads out and fades into the general glow about 3.5' SW of center near a superimposed mag 12 star. Offshoots of the main arms are difficult to trace and contribute to the general background glow of the halo. 18" (7/11/05 - Magellan Observatory, Australia): in addition to the complex 3-armed spiral structure I was surprised to see a fairly obvious linear bar that extended through the small, intense core in SW-NE orientation. The fairly tightly wound spiral arm that wraps from the east side of the core around the south side in a counter-clockwise orientation clearly emerges from the NE end of this bar. The spiral arms that begin on the south and west side more vaguely emerge from the general glow near the SW end of the bar. 13.1" (2/20/04 - Costa Rica): beautiful view with easy spiral structure in excellent seeing conditions. The main central portion of the galaxy appears to be in motion, due to the embedded spiral structure and darker ribbons add to this impression. The three principal arms extending from the galaxy were well seen although they are fairly tightly wound to the main body. 13.1" (2/19/04 - Costa Rica): this impressive galaxy was viewed carefully at both 105x and 166x. The overall size including the spiral arms are ~8'x6'. The halo is broadly concentrated then rises sharply to an intense 25" core that increases to the center. Complex spiral structure is quite obvious. A spiral arm is attached on the east side of the core and wraps around the south side of the galaxy in a counter-clockwise direction. A second arm is attached at the south side of the core and winds to the west a bit on the south side. Finally, an arm is attached on the west side and shoots north before gently bending east along the north side of the outer halo. 12" (6/29/02 - Bargo, Australia): this was my best view to date of M83 with a beautiful spiral structure clearly evident and multiple knotty arms. Well concentrated with a prominent core and very small nucleus. A very long, spiral arm is attached on the west side of the central core or bar but quickly bends to the north, becoming more spread out and diffuse. It continues to wind along the entire east side of the halo and fades out near a close double star, which is the middle of three collinear stars to the SE of the galaxy. Two other principal arms are visible - one is attached on the following end of the core and heads south, wrapping clockwise around the core towards the west. A third arm emerges from the core on the west side and winds clockwise towards the north. Offshoots of the main arms are difficult to trace and contribute to the general background glow of the halo. 17.5" (5/10/86) : brighter arm or arc visible north of the core. 13.1" (3/24/84): very bright, large, very bright core, brighter along the central "bar". The shape of the spiral arms and central bar form the Greek letter "Theta" surrounded by a faint halo. 8": very bright, bright core, elongated, impressive.