Two Ultra-faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey

Abstract

We report the discovery of two ultra-faint stellar systems found in early data from the DECam Local Volume Exploration survey (DELVE). The first system, Centaurus I (DELVE J1238−4054), is identified as a resolved overdensity of old and metal-poor stars with a heliocentric distance of D = 116.3 +0.6 −0.6 kpc, a half-light radius of r h = 2.3 +0.4 −0.3 arcmin, an age of τ > 12.85 Gyr, a metallicity of Z = 0.0002 +0.0001 −0.0002 , and an absolute magnitude of M V = −5.55 +0.11 −0.11 mag. This characterization is consistent with the population of ultra-faint satellites, and confirmation of this system would make Centaurus I one of the brightest recently discovered ultra-faint dwarf galaxies. Centaurus I is detected in Gaia DR2 with a clear and distinct proper motion signal, confirming that it is a real association of stars distinct from the Milky Way foreground; this is further supported by the clustering of blue horizontal branch stars near the centroid of the system. The second system, DELVE 1 (DELVE J1630−0058), is identified as a resolved overdensity of stars with a heliocentric distance of D = 19.0 +0.5 −0.6 kpc, a half-light radius of r h = 0.97 +0.24 −0.17 arcmin, an age of τ = 12.5 +1.0 −0.7 Gyr, a metallicity of Z = 0.0005 +0.0002 −0.0001 , and an absolute magnitude of M V = −0.2 +0.8 −0.6 mag, consistent with the known population of faint halo star clusters. Given the low number of probable member stars at magnitudes accessible with Gaia DR2, a proper motion signal for DELVE 1 is only marginally detected. We compare the spatial position and proper motion of both Centaurus I and DELVE 1 with simulations of the accreted satellite population of the Large Magellanic Cloud (LMC) and find that neither is likely to be associated with the LMC.

Milky Way Satellite Census. I. The Observational Selection Function for Milky Way Satellites in DES Y3 and Pan-STARRS DR1

“…During this sampling, the age and metallicity were Figure D2. DELVE 1 is a candidate faint outer-halo star cluster discovered in our search of PS1 DR1 data and independently in DELVE (Mau et al 2020). The small physical size ( = r 3.7 pc 1 2…”

“…DES covers only one-sixth of the sky accessible to DECam, and the past several years have seen an active community campaign to complete contiguous DECam coverage of the entire southern sky. Programs like MagLiteS (Drlica-Wagner et al 2016;Torrealba et al 2018), BLISS (Mau et al 2019), DECaLS (Dey et al 2019), and DELVE (Mau et al 2020) 57 are actively collecting, processing, and mining these data for fainter and more distant satellites. Meanwhile, HSC SSP on the 8.2 m Subaru telescope will achieve r∼26 over ∼1400 deg 2 , thereby extending the search for the faintest galaxies to unprecedented distances (Homma et al 2016(Homma et al , 2018(Homma et al , 2019.…”

“…Isochrones were generated using the models of Bressan et al (2012) with an IMF from Chabrier (2001). Martin et al 2015;Homma et al 2016Homma et al , 2018Homma et al , 2019Torrealba et al 2016aTorrealba et al , 2016bTorrealba et al , 2018Torrealba et al , 2019bLuque et al 2017;Mau et al 2020). In addition, maximum-likelihood-based algorithms have been developed to simultaneously combine morphological and photometric information to increase sensitivity (Bechtol et al 2015;Drlica-Wagner et al 2015.…”

“…Beginning with the Sloan Digital Sky Survey (SDSS), digitized wide-area multi-band optical imaging surveys-combined with automated search algorithms-have greatly increased the known population of Milky Way satellites (Willman et al 2005a(Willman et al , 2005bBelokurov et al 2006Belokurov et al , 2007Belokurov et al , 2008Belokurov et al , 2009Belokurov et al , 2010Grillmair 2006Grillmair , 2009Sakamoto & Hasegawa 2006;Zucker et al 2006aZucker et al , 2006bIrwin et al 2007;Walsh et al 2007;. More recently, searches using data from the Dark Energy Survey (DES; Bechtol et al 2015;Drlica-Wagner et al 2015;Koposov et al 2015;Luque et al 2016), other DECam surveys (e.g., SMASH, MagLiteS, and DELVE;Martin et al 2015;Drlica-Wagner et al 2016;Koposov et al 2018;Torrealba et al 2018;Mau et al 2020), ATLAS (Torrealba et al 2016a(Torrealba et al , 2016b, Pan-STARRS1 (PS1; Laevens et al 2015aLaevens et al , 2015b, and Gaia (Torrealba et al 2019b) have further increased the sample of confirmed and candidate satellites to more than 50 ( Figure 1).…”

“…), lowluminosity (M V =0.6) star cluster residing at a heliocentric distance of = D 15.6 kpc. We note that this system was independently discovered in deeper data from the DECam Local Volume Exploration survey (DELVE), where it was investigated in more detail and named DELVE 1 (Mau et al 2020). We use the same name here to avoid confusion.…”