by Steve Gottlieb
Interestingly, redshift surveys of the cluster reveal a distinct bi-modal distribution. The main group is dominated by the cD galaxy NGC 4696 with a mean recessional velocity of ~3400 km/sec (referred to as the Cen 30 group). A plume of hot gas, radiating strongly in X-rays appears to spiral away from NGC 4696 (see http://chandra.harvard.edu/photo/2002/0146/index.html). In addition, there is a smaller subcluster about 15' east that is dominated by NGC 4709 with a radial velocity of ~4750 km/sec (Cen 45 group). A simple explanation would be that we are looking at two unrelated clusters along the same line of sight but a number of studies suggests there is indeed one cluster here with the Cen 45 group falling towards and merging with the main Cen 30 group.
At -41 degrees declination, Abell 3526 is a bit low to view from northern California though I have tracked down a couple of dozen of the brighter NGC members from Fiddletown (see Sky & Tel, April '02), spread out over a few degrees. Nevertheless, I've been itching for a better view for quite awhile and brought along to Australia a finder chart of the central core which passes within 10 degrees of the zenith on early July evenings!
As I mentioned above, the core of the Centaurus cluster is dominated by NGC 4696, a massive elliptical with an extensive halo. From Australia I was quickly able to nab 18 additional members within just 22' of NGC 4696, most of these listed in the ESO catalogue. Here are notes on the brighter NGC members and a sampling of the fainter ESO galaxies.
NGC 4696 is fairly bright, moderately large, elongated 3:2 WNW-ESE, 1.8'x1.2'. Gradually increases to a brighter 30" core. The halo reaches a mag 13.5 at NW edge. This galaxy is the brightest member of the Centaurus cluster and the surrounding rich star field is littered with faint galaxies.
NGC 4677 is a moderately bright and large glow, elongated 5:2 NNW-SSE, 1.0'x0.4'. Gradually increases to a small bright core and quasi-stellar nucleus. Located 25' SW of N4696.
NGC 4683 is comparable to many of the ESO galaxies and appeared fairly faint, fairly small, elongated 3:2 NW-SE, 0.5'x0.3', moderate concentration with a 10" core. A mag 14.5 star is off the SE end and a mag 13 star lies 1.0' SW. Located 18' SW of N4696.
NGC 4706 is similar to NGC 4683; fairly faint, fairly small, oval 3:2 SSW-NNE. Fairly well concentrated with a bright 15" core. The halo increases in size to 0.7'x0.4' with averted vision. Located 6.4' NNW of brighter N4709. A faint companion, PGC 43402, lies 3.2' NNW.
NGC 4709 is the second brightest galaxy in the core and appeared moderately bright, fairly small, irregularly round, 0.8'x0.7' diameter. This galaxy is the brightest member in the Cen 45 subgroup and has a symmetrical appearance, steadily increasing to a small, bright core and quasi-stellar nucleus. The subgroup has a significantly higher recessional velocity, although these galaxies may be streaming towards and merging with the main cluster. Located 14' ESE of N4696 with many faint galaxies within 10'.
ESO 322-077 is the first of the fainter ESO galaxies missed by John Herschel when he surveyed the cluster from the Cape of Good Hope. In the 18-inch, it was faint, fairly small, elongated 5:2 N-S. Appears like a low surface brightness version of N4677 which is 5' to the south.
ESO 322-081 was picked up 17' WNW of NGC 4696. I recorded this member as fairly faint, fairly small, elongated 3:2 SW-NE, gradually increases to center. Adjacent to a small triangle of mag 13.5 stars with one of the stars just off the SW edge and a 4th mag 14 star near the star at the southwestern vertex.
ESO 322-099 is 13' SE of N4696 and forms a 1.5' pair with ESO 322-100. Fairly faint, very small, round, only 20" diameter but has a fairly high surface brightness. A mag 10.3 star lies 3.2' SW. The companion was faint, very small, round, 20" diameter and weak concentration. A mag 14.5-15 star is close following. With averted vision the core has faint extensions E-W roughly in the direction of faint star increasing the overall size to 30"x20".
ESO 322-101 is located 18' NE of N4696 and was easily visible as a fairly faint glow, fairly small, slightly elongated E-W, with a weak, even concentration to the center.
ESO 322-102 was found 5' W of NGC 4709 and appeared faint, fairly small, very elongated 3:1 SW-NE, 0.6'x0.2', very weak concetration.
ESO 323-005 is the brightest in a trio and logged as fairly faint, very small, oval 3:2 N-S, 0.4'x0.25', fairly high surface brightness. A mag 13 star lies 0.9' SW of center. Located 8' S of N4709 and 20' SE of N4696. Two galaxies, ESO 323-008 and 323-009 lie 5' and 7.5' NE, respectively. ESO 323-008 appeared faint, small, elongated 2:1 NW-SE, 0.5'x0.25', with a weak or no concentration. A mag 10.8 star lies 2.3' NE. ESO 323-009 is the faintest of the trio and appeared very faint, very small, slightly elongated E-W, 0.4'x0.3'.
Posted on sf-bay-tac Aug 14, 2005 14:28:54 PT
Converted by report.pm 1.2 Mar 07, 2006 20:04:26 PT