KK 242, a faint companion to the isolated Scd galaxy NGC 6503

Abstract

Using Hubble Space Telescope imaging of the resolved stellar population of KK~242 = NGC6503-d1 = PGC~4689184, we measure the distance to the galaxy to be $6.46\pm 0.32$ Mpc and find that KK~242 is a satellite of the low-mass spiral galaxy NGC~6503 located on the edge of the Local Void. Observations with the Karl G. Jansky Very Large Array show signs of a very faint HI-signal at the position of KK~242 within a velocity range of $V_{hel}=-80\pm 10$ km\,s${}^{-1}$. This velocity range is severely contaminated by HI emission from the Milky Way and from NGC6503. The dwarf galaxy is classified as the transition type, dIrr/dSph, with a total HI-mass of $<10^6{M}_{\odot }$ and a star formation rate SFR(H$\hat{I}\pm$) = --4.82 dex ($M\odot$/yr). Being at a projected separation of 31 kpc with a radial velocity difference of -- 105 km\,s${}^{-1}$ relative to NGC~6503, KK~242 gives an estimate of the halo mass of the spiral galaxy to be $\log (M/M_{\odot }$) = 11.6. Besides NGC~6503, there are 8 more detached low-luminosity spiral galaxies in the Local Volume: M~33, NGC~2403, NGC~7793, NGC~1313, NGC~4236, NGC~5068, NGC~4656 and NGC~7640, from whose small satellites we have estimated the average total mass of the host galaxies and their average total mass-to-K-band-luminosity $⟨M_T/M_{\odot }⟩=(3.46\pm 0.84)\times 10^{11}$ and $(58\pm 19)M_{\odot }/L_{\odot }$, respectively.

Accepted for publication in the Astronomical Journal