BST1047+1156: An Extremely Diffuse and Gas-rich Object in the Leo I Group

Abstract

We report the detection of diffuse starlight in an extragalactic HI cloud in the nearby Leo I galaxy group. We detect the source, dubbed BST1047+1156, in both broadband optical and GALEX ultraviolet light. Spanning ∼ 2 kpc in radius, it has a peak surface brightness of µ B =28.8 mag arcsec −2 , making it the lowest surface brightness object ever detected via integrated light. Although the object is extremely gas-rich, with a gas fraction of f g = 0.99, its peak HI column density is well below levels where star formation is typically observed in galaxies. Nonetheless, BST1047+1156 shows evidence for young stellar populations: along with the detected UV emission, the object is extremely blue, with B − V = 0.14 ± 0.09. The object sports two tidal tails and is found embedded within diffuse gas connecting the spiral galaxy M96 to the group's extended HI Leo Ring. The nature of BST1047+1156 is unclear. It could be a disrupting tidal dwarf, recently spawned from star formation triggered in the Leo I Group's tidal debris. Alternatively, the object may have been a pre-existing galaxy -the most extreme example of a gas-rich field LSB known to date -which had a recent burst of star formation triggered by encounters in the group environment.

The Dragonfly Nearby Galaxies Survey. V. HST/ACS Observations of 23 Low Surface Brightness Objects in the Fields of NGC 1052, NGC 1084, M96, and NGC 4258

“…Telescopes such as Dragonfly have the capacity to quickly and efficiently detect extended LSB objects, and HST followup can then be used to study them in exquisite detail. Similar synergies exist with H I observations; these are complementary to LSB searches, as illustrated in, e.g., Watkins et al (2014) and Mihos et al (2018). A particular advantage of H I is that it provides radial velocity information, something that is exceedingly difficult to do for the faintest galaxies in the optical.…”

Galactic cirri in deep optical imaging

“…The current generation of deep optical surveys allows us, for the first time, to unravel critical issues of the hierarchical evolution of galaxies that are intrinsically connected with the low surface brightness Universe. For instance, the detection of galaxies of decreasing surface brightness (e.g., Sandage & Binggeli 1984;Impey et al 1988;Dalcanton et al 1997;Blanton et al 2005;Mihos et al 2015;Javanmardi et al 2016;Mihos et al 2018), the stellar halos and tidal features surrounding nearby galaxies (e.g., Bullock & Johnston 2005;Abadi et al 2006;Johnston et al 2008;Martínez-Delgado et al 2010;Duc et al 2015; or the intracluster light in galaxy clusters (e.g., Uson et al 1991;Mihos et al 2005;Rudick et al 2010;Giallongo et al 2014;Montes & Trujillo 2014 are all crucial observational pillars for testing the current Λ-Cold Dark Matter (ΛCDM) cosmological paradigm (e.g., Moore et al 1999;Klypin et al 1999;Cooper et al 2010Cooper et al , 2015.…”

Discovery of a large H i ring around the quiescent galaxy AGC 203001

“…It has no diffuse large-scale optical counterpart even in deep optical imaging (Michel-Dansac et al 2010;Watkins et al 2014). Only faint star-forming knots and diffuse structures were found within the ring in the UV (Thilker et al 2007) and in the optical (Stierwalt et al 2009;Michel-Dansac et al 2010;Mihos et al 2018) suggesting insitu star-formation. None of the galaxies in the Leo i group show any signs of recent interactions, except for NGC 3384 which might have undergone a minor merger (Watkins et al 2014).…”

“…The metallicity of the Leo ring was found to be pre-enriched (Michel-Dansac et al 2010;Rosenberg et al 2014) suggesting a galactic origin. Recently, a very diffuse stellar object (BST1047+1156) was found in the region connecting the Leo ring and M96 which could either be a tidal dwarf galaxy or a pre-existing extremely diffuse low surface brightness (LSB) galaxy (Mihos et al 2018). Targeted simulations showed that the Leo ring could also have a collisional origin (Michel-Dansac et al 2010).…”