[Paper Review] A group of galaxies around the giant spiral NGC 6946

И. Д. Караченцев, М. Е. Шарина+1 more|arXiv (Cornell University)|Oct 7, 2000

Galaxies: Formation, Evolution, Phenomena3 citations

TL;DR

This study identifies a previously undetected group of eight late-type galaxies, including seven dwarf irregulars, gravitationally bound to the giant spiral NGC 6946 at a distance of 5.9 ± 0.4 Mpc. Using optical CCD imaging and HI line observations, the authors determine the group's virial mass-to-luminosity ratio of ~56 M⊙/L⊙, confirming it as a typical small galaxy group with a projected diameter of 420 kpc and velocity dispersion of 95 km s⁻¹.

ABSTRACT

We present large-scale CCD images of a dozen low surface brightness objects in the vicinity of giant Sc galaxy NGC 6946, which have been found recently on POSS-II copies. Six of them: UGC 11583, kk251, kk252, kkr55, kkr56, and kkr59 are dwarf irregular galaxies resolved into stars. We determined their distances from the luminosity of the brightest blue stars. The mean estimated distance to the NGC 6946 group is 5.9 +- 0.4 Mpc. Together with Cepheus 1, discovered before, the group consist of 8 late type galaxies. Almost all of them are detected or marginally detected in the HI line. The group is characterized by a linear projected diameter of 420 kpc, a radial velocity dispersion of 95 km/s, and a virial mass-to-luminosity ratio about 56 M_sun/L_sun.

Motivation & Objective

• To resolve the apparent isolation of the giant spiral NGC 6946, which lies near the Local Void and suffers from high galactic extinction.
• To search for low surface brightness (LSB) galaxies in the vicinity of NGC 6946 using POSS-II sky survey copies.
• To confirm the physical association of candidate galaxies with NGC 6946 through distance estimates from brightest blue stars and HI line data.
• To characterize the dynamical and structural properties of the newly discovered galaxy group, including its mass, size, and velocity dispersion.
• To determine the group's mass-to-luminosity ratio and assess its consistency with typical small galaxy groups like the Local Group.

Proposed method

• Acquired large-scale CCD images of 18 LSB galaxy candidates near NGC 6946 using the 6-m telescope (Russia) and the 2.5-m Nordic telescope (Spain), with Johnson-Cousins filters.
• Measured morphological types, major and minor axes, surface brightness, and galactic extinction (from Schlegel et al. 1998) for each candidate from POSS-II images.
• Estimated distances using the luminosity of the brightest blue stars, corrected for extinction, and derived distance moduli with a mean of 28.86 ± 0.14 mag.
• Performed HI line observations with the Effelsberg 100-m telescope to detect neutral hydrogen emission and determine radial velocities.
• Calculated hydrogen mass from HI flux and distance using the formula: log[M(HI)/M⊙] = log F + 2 log D + 5.37.
• Computed the virial mass from the velocity dispersion (95 km s⁻¹) and mean projected separation (210 kpc), assuming random orbits with eccentricity e = 0.5.

Experimental results

Research questions

• RQ1 Are the low surface brightness galaxies detected near NGC 6946 physically associated with the giant spiral, or are they background contaminants?
• RQ2 What is the true distance to the NGC 6946 group, and how does it compare to the distance of NGC 6946 itself?
• RQ3 What is the dynamical mass-to-luminosity ratio of the group, and how does it compare to other known small galaxy groups?
• RQ4 How do the kinematic and structural properties of the NGC 6946 group compare to those of other nearby galaxy groups, such as the Local Group or groups around M101?
• RQ5 What is the extent and mass distribution of the group, and does it exhibit signs of virial equilibrium?

Key findings

• The NGC 6946 group consists of eight late-type galaxies, including seven dwarf irregulars (UGC 11583, kk251, kk252, kkr55, kkr56, kkr59, and Cepheus 1), with a mean distance of 5.9 ± 0.4 Mpc.
• The group has a projected diameter of 420 kpc and a radial velocity dispersion of 95 km s⁻¹, indicating a gravitationally bound system.
• The virial mass-to-luminosity ratio is estimated at ~56 M⊙/L⊙, consistent with typical values for small galaxy groups like the Local Group.
• The hydrogen mass-to-luminosity ratio for group members ranges from 0.18 to 3.0 M⊙/L⊙, typical for irregular galaxies.
• Seven of the eight members are detected in the HI line, confirming their gas-rich nature and physical association with the group.
• The most distant member, kkr59, lies at 483 kpc from NGC 6946, comparable to the extent of companion systems around other giant spirals like M101 and M31.