Hyper Wide Field Imaging of the Local Group Dwarf Irregular Galaxy IC 1613: An Extended Component of Metal-poor Stars

Abstract

Stellar halos offer fossil evidence for hierarchical structure formation. Since halo assembly is predicted to be scale-free, stellar halos around low-mass galaxies constrain properties such as star formation in the accreted subhalos and the formation of dwarf galaxies. However, few observational searches for stellar halos in dwarfs exist. Here we present gi photometry of resolved stars in isolated Local Group dwarf irregular galaxy IC 1613 (M ∼ 10 8 M ). These Subaru/Hyper Suprime-Cam observations are the widest and deepest of IC 1613 to date. We measure surface density profiles of young mainsequence, intermediate to old red giant branch, and ancient horizontal branch stars outside of 12 (∼ 2.6 kpc; 2.5 half-light radii) from the IC 1613 center. All of the populations extend to ∼ 24 (∼ 5.2 kpc; 5 half-light radii), with the older populations best fit by a broken exponential in these outer regions. Comparison with earlier studies sensitive to IC 1613's inner regions shows that the density of old stellar populations steepens substantially with distance from the center; we trace the g-band effective surface brightness to an extremely faint limit of ∼ 33.7 mag arcsec −2 . Conversely, the distribution of younger stars follows a single, shallow exponential profile in the outer regions, demonstrating different formation channels for the younger and older components of IC 1613. The outermost, intermediate-age and old stars have properties consistent with those expected for accreted stellar halos, though future observational and theoretical work is needed to definitively distinguish this scenario from other possibilities.

Solo dwarfs IV: comparing and contrasting satellite and isolated dwarf galaxies in the Local Group

“…IC 1613 is another massive dwarf in Solo and has been well studied, including by Bernard et al (2007) and recently by Pucha et al (2019). Pucha et al (2019) found strong age gradients, and the young stellar population has a considerably smaller scale radius than the older stars. The SFH they determined is consistent with this picture, with a decreasing star formation rate with increasing radius.…”

“…The allure of detecting halos around dwarfs lies in our desire to better understand their formation, whether they are a result of mergers or in-situ star formation. Pucha et al (2019) claim they identify a "halo" in IC 1613, finding the older, extended stellar component exhibits a different density profile to the more compact stellar component. However, the stellar densities in question are very low and they caution against over interpretation as a halo.…”

“…In contrast, we could consider these extended stars as part of a more complex dwarf structure (like Pucha et al 2019for IC 1613or Vansevičius et al 2004 for Leo A). In this section, I expand on this previous analysis, but now specifically focus on searches for satellites of these dwarfs i.e., coherent stellar systems that otherwise appear detached from the main dwarf (in contrast to "wings" or extra-tidal stars).…”

“…Of course, isolated Local Group galaxies have been studied for many years. This includes individual dedicated studies for some of the most prominent members e.g., WLM (Wolf, 1909;Melotte, 1926;Leaman et al, 2012), IC 1613 (Hodge et al, 1991;Pucha et al, 2019), andDDO 210 (McConnachie et al, 2006); as part of catalogues like Mateo (1998); Karachentsev et al (2013); van den Bergh (1999); McConnachie (2012); as inclusion in HI Surveys like LITTLE THINGS (Hunter et al, 2012) or FIGGS2 (Patra et al, 2016); as part of Local Group-wide studies like the star formation history analysis of Weisz et al (2014); and as targets to understand their stellar dynamics (e.g., Kirby et al 2014;Kacharov et al 2017 among numerous other works). However, homogeneous, systematic studies focused on this population are generally lacking but with exceptions, such as Local Group Galaxies Survey Project (Massey et al 2006(Massey et al , 2007.…”

The In-situ Origins of Dwarf Stellar Outskirts in FIRE-2

“…It has long been observed that dwarf galaxies host extended, round old stellar populations reminiscent in structure to the stellar halos of massive galaxies (Lin & Faber 1983;Minniti & Zijlstra 1996;Minniti et al 1999;Zaritsky et al 2000;Hidalgo et al 2003;Demers et al 2006;Bernard et al 2007;Stinson et al 2009;Strader et al 2012;Nidever et al 2019a,b;Pucha et al 2019;Kado-Fong et al 2020). Recently, it has been shown that high-mass dwarfs (10 8.5 < M < 10 9.6 M ) can form a thick stellar & HI disk (Roychowdhury et al 2013;van der Wel et al 2014;Nath Patra 2020) in conjunction with a round stellar halo (Kado-Fong et al 2020).…”