Mapping the Tidal Destruction of the Hercules Dwarf: A Wide-field DECam Imaging Search for RR Lyrae Stars

Abstract

We investigate the hypothesized tidal disruption of the Hercules ultra-faint dwarf galaxy (UFD). Previous tidal disruption studies of the Hercules UFD have been hindered by the high degree of foreground contamination in the direction of the dwarf. We bypass this issue by using RR Lyrae stars, which are standard candles with a very low field-volume density at the distance of Hercules. We use wide-field imaging from the Dark Energy Camera on CTIO to identify candidate RR Lyrae stars, supplemented with observations taken in coordination with the Beijing-Arizona Sky Survey on the Bok Telescope. Combining color, magnitude, and light-curve information, we identify three new RR Lyrae stars associated with Hercules. All three of these new RR Lyrae stars lie outside its published tidal radius. When considered with the nine RR Lyrae stars already known within the tidal radius, these results suggest that a substantial fraction of Hercules' stellar content has been stripped. With this degree of tidal disruption, Hercules is an interesting case between a visibly disrupted dwarf (such as the Sagittarius dwarf spheroidal galaxy) and one in dynamic equilibrium. The degree of disruption also shows that we must be more careful with the ways we determine object membership when estimating dwarf masses in the future. One of the three discovered RR Lyrae stars sits along the minor axis of Hercules, but over two tidal radii away. This type of debris is consistent with recent models that suggest Hercules' orbit is aligned with its minor axis.

Search for RR Lyrae stars in DES ultrafaint systems: Grus I, Kim 2, Phoenix II, and Grus II

“…The location of Phoenix II in the vicinity of the HI Magellanic Stream (see Figure 1 in Jerjen et al 2018), its kinematics, and photometry, may all indicate that this galaxy is (or was) a satellite of the Magellanic Clouds. This hypothesis is supported by the following studies, -Fritz et al (2018) claim the possible prior association with the LMC due to the fact that its orbital pole (∼16 • ) is close to the orbital pole of the LMC.…”

“…Following the procedure described in § 4.1, we determined the distance modulus using this RRL. Adopting [Fe/H]=-2.75 dex (Fritz et al 2018) 7 and [α/Fe]=+0.2 dex (Jerjen et al 2018), we obtain Z=0.00004. Thus, the distance modulus of Phoenix II is µ 0 =20.01±0.08 mag (D =100±3 kpc).…”

“…The location of this system in the luminosity-half light radius plane makes it a strong candidate to be a dwarf galaxy, supported by spectroscopic measurements. Fritz et al (2018) found five potential members in this galaxy combining proper motions and photometry from Gaia together with intermediate resolution spectra from VLT/FLAMES. They obtained a velocity dispersion of 7.1 +1.5 −1.1 km/s, a mean [Fe/H]=-2.75±0.17 dex, and an intrinsic metallicity spread of 0.34 dex.…”

“…DE-Cam observations, in particular those of the Dark Energy Survey (DES; The Dark Energy Survey Collaboration 2005) and MagLites (Drlica-Wagner et al 2016) surveys, have contributed to the discovery of more than 20 ultra-faint stellar systems undetectable in the past (e.g., Martin et al 2015;Bechtol et al 2015;Koposov et al 2015;Drlica-Wagner et al 2015;Martin et al 2016a;Luque et al 2016Luque et al , 2017Torrealba et al 2018;Koposov et al 2018;Mau et al 2019). The fact that many of them are close to the Magellanic Clouds suggests a possible association (e.g., Jethwa et al 2016;Erkal et al 2018;Fritz et al 2018;Jerjen et al 2018;Kallivayalil et al 2018). This scenario of satellites of satellites is predicted by cosmological simulations at the time of infall (e.g., Sales et al 2011;Deason et al 2015;Wheeler et al 2015;Pardy et al 2019).…”

“…The RRL numbers here are also based on previous studies:Joo et al (2018);Monelli et al (2018). (e) We updated the number of RRL stars in Hercules including the outer RRL stars discovered byGarling et al (2018).…”