The Hubble Flow around the Centaurus A/M83 Galaxy Complex

Abstract

We present HST/ACS images and color-magnitude diagrams for 24 nearby galaxies in and near the constellation of Centaurus with radial velocities V LG < 550 km s −1 . Distances are determined based on the luminosities of stars at the tip of the red giant branch that range from 3.0 Mpc to 6.5 Mpc. The galaxies are concentrated in two spatially separated groups around Cen A (NGC 5128) and M 83 (NGC 5236). The Cen A group itself has a mean distance of 3.76±0.05 Mpc, a velocity dispersion of 136 kms −1 , a mean harmonic radius of 192 kpc, and an estimated orbital/virial mass of (6.4 − 8.1) · 10 12 M ⊙ . This elliptical dominated group is found to have a relatively high mass-to-light ratio: M/L B = 125 M ⊙ /L ⊙ . For the M 83 group we derived a mean distance of 4.79±0.10 Mpc, a velocity dispersion of 61 km s −1 , a mean harmonic radius of 89 kpc, and estimated orbital/virial mass of (0.8 − 0.9) · 10 12 M ⊙ . This spiral dominated group is found to have a relatively low M/L B = 34 M ⊙ /L ⊙ . The radius of the zero-velocity surface around Cen A lies at R 0 = 1.40 ± 0.11 Mpc. implying a total mass within R 0 of M T = (6.0 ± 1.4) · 10 12 M ⊙ . This value is in good agreement with the Cen A virial/orbital mass estimates and provides confirmation of the relatively high M/L B of this elliptical-dominated group. The centroids of both the groups, as well as surrounding field galaxies, have very small peculiar velocities, < 25 km s −1 , with respect to the local Hubble flow with H 0 = 68 km s −1 Mpc −1 .

A close look at the Centaurus A group of galaxies

“…Despite this low latitude, most of its members are not highly contaminated by Galactic foreground extinction (see, e.g., Schlegel et al 1998), making it a very popular target of research. It is one of the closest groups to our own, with a mean Galactocentric distance of ∼3.8 Mpc (Hui et al 1993;Karachentsev et al 2002Karachentsev et al , 2007Harris et al 2009) and with a variety of morphological types among its member galaxies. The group is dominated by the giant radio-loud elliptical galaxy Centaurus A (NGC 5128), which shows a very perturbed morphology and which has probably undergone several mergers in its recent past (e.g., Meier et al 1989;Mirabel et al 1999;Karachentsev 2005).…”

“…Côté et al 1997;Jerjen et al 2000a;Bouchard et al 2007Bouchard et al , 2009Côté et al 2009), in some cases even with the same dataset considered in our work (Karachentsev et al 2007;Makarova & Makarov 2008;Sharina et al 2008). However, most of these studies concentrated on large samples of objects and did not investigate their physical properties individually.…”

“…Karachentsev et al 2007). The search for and study of dwarf members in this group have been pursued at different wavelengths for more than a decade (Côté et al 1997;Karachentseva & Karachentsev 1998;Banks et al 1999;Côté et al 2000;Jerjen et al 2000a,b;Karachentsev et al 2002Karachentsev et al , 2004Karachentsev 2005;Rejkuba et al 2006;Bouchard et al 2007;Grossi et al 2007;Karachentsev et al 2007;Lee et al 2007;Bouchard et al 2009;Côté et al 2009). Now there are more than 50 confirmed dwarf galaxies known in the Centaurus A group, of which about 3/5 are early-type dwarfs.…”

Highly efficient star formation in NGC 5253 possibly from stream-fed accretion

“…NGC 5253 is a dwarf spheroidal galaxy of the Cen A/M83 galaxy complex 36 with stellar mass of ~1.5 x 10 8 M ¤ 37 and estimated 38 total mass, including dark matter, about ten times higher. It is close to the large spiral galaxy M83 in projection.…”

The very massive star content of the nuclear star clusters in NGC 5253

“…The triggering process for the current starburst is not clear. An encounter with M83 has been suggested (van den Bergh 1980;Caldwell & Phillips 1989) or Cen A (Karachentsev et al 2007;Tully et al 2015). The distance to NGC 5253 is uncertain with values of 3.1-4.0 Mpc quoted in the literature.…”