NGC 7549/7550 - Arp 99 - Hickson 93

Object: NGC 7549, 7550 and 7547 galaxy trio are tucked into the South West corner of the Great Square of Pegasus.

The group, also known as Arp 99 and the compact group Hickson 93, lies at a distance of ~190 Mly. The field of view of the image is ~13' x 9' which corresponds to a window in space of 720 x 500 kly at the galaxies distance. NGC 7549 is on the left, NGC 7550 near center and NGC 7547 at the top.

Subimages Clear: 5.6 hrs (9 x 20min + 6 x 10min + 19 x 5min)
Subimages R: 1.6 hrs (8 x 10min + 3 x 5min)
Subimages G: 1.75 hrs (9 x 10min + 3 x 5min)
Subimages B: 2.4 hrs (11 x 10min + 7 x 5min)
Scope: C11XLT, EFL=2967 mm (f/10.6 effective)
Mount: Losmandy G11 with Gemini L4
Camera: SBIG ST8XME+CFW10 (0.63"/pixel) set to between -20C and -30C
Guiding: Self-guide, exptime = 1 sec, all dithered
Acquisition and Processing: MaxIm DL/CCD v4.5, 4.6 and v5
Imaged in September and October of 2007 an 2008.

Data from NED (NASA/IPAC Extragalactic Database) compared to image integrated magnitude and size measurements:

NGC  Hickson group  V Mag. (NED)  V Mag.  B Mag. (NED)  B Mag.  Size
Size  D (Mpc)  D (Mly)  Type  Vaucouleurs Type Decoding 
7547    13.9  14.7  14.6  1.1' x 0.5'  1.8' x 0.6'  61.1  187  (R)SAB(s)0/a pec  Outer ring, weakly barred lenticular/spiral with inner s shape, peculiar 
7549    13.6  13.7  14.2  2.8' x 0.7'  3.4' x 0.7'  60.6  186  SB(s)cd pec AGN  Barred spiral with inner s shape, loose arms, peculiar, active galaxy nucleus 
7550  12.9  12.9  14.0  13.6  1.4' x 1.2'  2.0' x 2.0'  65.4  200  SA0- AGN  Unbarred lenticular, active galaxy nucleus 
7558    14.7  15.9  15.5  0.4' x 0.4'  0.7' x 0.7'  119  365  Sa  Spiral, bar state unknown, tight arms 

The odd extensions and distorted spiral shapes of the three bright members of this group indicate that all have been involved in recent galactic collisions. The brightest member NGC 7550, center right, has strange faint hammer shaped extensions to the W and SW. While the top galaxy NGC 7547 shows a simlar but much more compact extension to the E and a broader faint arc to the W. NGC 7549 on the left has no faint extensions however its spiral arms have been stretched out N-S. The 4 small clumps along the ridge of the N arm and the 2 brighter clumps due W of the nucleus are likely super star clusters formed soon after the previous close encounter. SSCs are also observed in PGC 57129 and NGC 6621 for example. The redshift data for NGC 7550 is discordant with the apparent dynamics of the group and the other members redshift. NGC 7558, the faint face on galaxy in the bottom right corner, does not seem associated with this trio, it is twice as far away and does not show any obvious distortion. The current classifcation of Sa may be upgraded to SAb, a normal unbarred spiral.

Below is highly non-linearly stretched version of the luminance channel to help in the identification of the extremely faint features.