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

Abstract

This work presents the first search for RR Lyrae stars (RRLs) in four of the ultra-faint systems imaged by the Dark Energy Survey (DES) using SOAR/Goodman and Blanco/DECam imagers. We have detected two RRLs in the field of Grus I, none in Kim 2, one in Phoenix II, and four in Grus II. With the detection of these stars, we accurately determine the distance moduli for these ultra-faint dwarf satellite galaxies; µ 0 =20.51±0.10 mag (D =127±6 kpc) for Grus I and µ 0 =20.01±0.10 mag (D =100±5 kpc) for Phoenix II. These measurements are larger than previous estimations by Koposov et al. 2015 andBechtol et al. 2015, implying larger physical sizes; 5% for Grus I and 33% for Phoenix II. For Grus II, out of the four RRLs detected, one is consistent with being a member of the galactic halo (D =24±1 kpc, µ 0 =16.86±0.10 mag), another is at D =55±2 kpc (µ 0 =18.71±0.10 mag), which we associate with Grus II, and the two remaining at D =43±2 kpc (µ 0 =18.17±0.10 mag). Moreover, the appearance of a subtle red horizontal branch in the color-magnitude diagram of Grus II at the same brightness level of the latter two RRLs, which are at the same distance and in the same region, suggests that a more metal-rich system may be located in front of Grus II. The most plausible scenario is the association of these stars with the Chenab/Orphan Stream. Finally, we performed a comprehensive and updated analysis of the number of RRLs in dwarf galaxies. This allows us to predict that the method of finding new ultra-faint dwarf galaxies by using two or more clumped RRLs will work only for systems brighter than M V ∼ −6 mag.

Birds of a Feather? Magellan/IMACS Spectroscopy of the Ultra-faint Satellites Grus II, Tucana IV, and Tucana V*

“…Moreover, the stream stars are ∼10kpc closer than GruII (Koposov et al 2019). The difference in distance and velocity between GruII and the Orphan Stream likely rules out any connection between the two (Martínez-Vázquez et al 2019). In our spectroscopic data set, we find two stars close to the expected velocity and proper motion of the stream, DESJ220441.57−462244.8 and DESJ220458.78−462710.6.…”

Gaia RR Lyrae Stars in Nearby Ultra-faint Dwarf Satellite Galaxies

“…In Figure 11 we show the number of known RRLs in each UFD as a function of their M V . The red line is the power-law fit that Martínez-Vázquez et al (2019) obtained using data for all dwarf satellite galaxies around the Milky Way and Andromeda in addition to isolated galaxies and two Sculptor group dwarfs, spanning near 17 magnitudes in M V . The fit was done using only galaxies in which a search for RRLs has been carried out in areas enclosing > 2 r h (i.e.…”

“…Similarly, the Chenab stream (Shipp et al 2018) cohabits the same region of the sky as Grus II. Martínez-Vázquez et al (2019) found two stars which seem to be associated with the Chenab stream when looking for RRLs in that galaxy. With Gaia we recovered one of these stars (Chenab-V4), and we found another one (Gaia ID 6561477651849190912) with similar magnitude to Chenab-V4 but farther away from the center of Grus II, at 54 .…”

“…To date there has been a large effort to obtain a census of RRLs in the satellites of the Milky Way, and Martínez-Vázquez et al (2019) presents an updated table of satellites with known RRLs, which contains 23 Galactic UFDs. Except for two cases, Carina III (Car III) and Willman I, all satellites for which a proper search has been conducted contain RRLs, although for the fainter systems the numbers can be very small.…”

“…For those not in their list, we assumed the structural parameters in the discovery papers or in photometric follow-up papers. The number of known RRLs in UFDs was taken from the recent compilation by Martínez-Vázquez et al (2019). Table 1 contains in column (12) the number of RRLs found in Gaia for each galaxy.…”