A Deep, Wide-Field Study of Holmberg II with Suprime-Cam: Evidence for Ram Pressure Stripping

TL;DR

This study provides a detailed optical analysis of Holmberg II, revealing a regular stellar component and evidence of ram pressure stripping likely caused by a hot intragroup medium, affecting the galaxy's gas and stellar morphology.

Contribution

It offers the first deep, wide-field optical profile of Holmberg II and links its stellar and gas distributions to environmental effects like ram pressure stripping.

Findings

Old stars are more centrally concentrated than young stars.

The stellar component is smaller and more regular than the HI gas cloud.

Evidence suggests ram pressure stripping influences galaxy morphology.

Abstract

We present a deep, wide-field optical study of the M81 group dwarf galaxy Holmberg II (HoII) based on Subaru/Suprime-Cam imaging. Individual stars are resolved down to I~25.2, i.e. about 1.5 mag below the tip of the red giant branch (RGB). We use resolved star counts in the outskirts of the galaxy to measure the radial surface brightness profile down to \mu_V~32 mag arcsec^-2, from which we determine a projected exponential scalelength of 0.70'+-0.01' (i.e. 0.69+-0.01 kpc). The composite profile, ranging from the cored centre out to R=7', is best fit by an EFF profile which gives a half-light radius of 1.41'+-0.04' (i.e. 1.39+-0.04 kpc), and an absolute magnitude M_V=-16.3. The low surface-brightness stellar component of HoII is regular and symmetric and has an extent much smaller than the vast HI cloud in which it is embedded. We compare the spatial distribution of the young, intermediate age, and old stellar populations, and find that the old RGB stars are significantly more centrally concentrated than the young stellar populations, contrary to what is observed in most dwarf galaxies of the Local Universe. We discuss these properties in the context of the comet-like distribution of HI gas around HoII, and argue for the presence of a hot intragroup medium in the vicinity of HoII to explain the contrasting morphologies of the gas and stars.