HCG 027 – galaxy line that couldn’t decide its redshift

Hickson Compact Group (HCG) number 27 is a line of five or six galaxies. There is only 5 objects within HCG 027 with redshift available in NED. Hickson et al. (1992) reported a sixth redshift for the system. They have there an object called “27f” with a redshift of 26100 km/s, while originally group had only five members 27a-e. This is object 6 in Figure 1 and table below. This additional galaxy is somewhat off from the line of five other galaxies.

HCG 027 was included to the sample of Mendes de Oliveira (1995) who studied discordant redshift objects in Hickson Compact Groups, and concluded that the results of the study supported the view of a cosmological origin for galaxy redshifts. However, Mendes de Oliveira found that “the distribution of positions for the discordant galaxies in quintets, however, is more centrally concentrated than that predicted for a uniform distribution of field interlopers,” and offered gravitational lensing as an explanation for the observation. Here we don’t explore this general compact group situation much further, and we concentrate on the HCG 027, but for another view of the general situation, an interested reader might take a look at Sulentic (1997).

So far, I haven’t found any paper that discusses HCG 027 as an individual discordant redshift system. The discordant redshift situation in the system is such that in the line there are two galaxies having radial velocity of cz ~ 18500 km/s, while three galaxies (and the additional sixth galaxy) have cz ~ 26000. In the traditional view this system is interpreted so that a pair of galaxies at cz ~ 18500 and group of background galaxies at cz ~ 26000 are accidentally aligned so that they seem to form a line (or a chain).

Figure 1 shows the objects with measured redshifts in the field of HCG 027, and data for the objects is given in the table below.

Figure 1. Objects with measured redshifts in/near HCG 027. Image is from Simbad.

Objects and their data

1 PGC 014863 SBc 0.061756 (18514 km/s) 16.44 0
2 2MASX J04192158-1142395 S0 0.086873 (26044 km/s) 18.74 0.910
3 PGC 014861 S0a 0.087581 (26256 km/s) 18.02 1.010
4 PGC 014870 Sa 0.087901 (26352 km/s) 17.50 1.813
5 PGC 014873 Sb 0.061176 (18340 km/s) 17.01 2.796
6 APMUKS(BJ) B041656.98-114846.3 S0 (26100 km/s) 19.46 0.519

NED objects within 10′ from HCG 027


Hickson et al., 1992, ApJ, 399, 2, 353-367, “Dynamical properties of compact groups of galaxies”

Mendes de Oliveira, C., 1995, MNRAS, 273, 1, 139-145, “The nature of discordant redshift galaxies in compact groups”

Sulentic, Jack W., 1997, ApJ, 482, 640, “The Twin Paradoxes of Compact Groups: Discordant Excess Muted but the Dynamical Puzzle Persists”