Halo Velocity Groups in the Pisces Overdensity

Abstract

We report spectroscopic observations with the Gemini South Telescope of 5 faint (V∼ 20) RR Lyrae stars associated with the Pisces overdensity. At a heliocentric and galactocentric distance of ∼ 80 kpc, this is the most distant substructure in the Galactic halo known to date. We combined our observations with literature data and confirmed that the substructure is composed of two different kinematic groups. The main group contains 8 stars and has V gsr = 50 km s −1 , while the second group contains four stars at a velocity of V gsr = −52 km s −1 , where V gsr is the radial velocity in the galactocentric standard of rest. The metallicity distribution of RR Lyrae stars in the Pisces overdensity is centered on [F e/H] = −1.5 dex and has a width of 0.3 dex. The new data allowed us to establish that both groups are spatially extended making it very unlikely that they are bound systems, and are more likely to be debris of a tidally disrupted galaxy or galaxies. Due to small sky coverage, it is still unclear whether these groups have the same or different progenitors.

The Profile of the Galactic Halo from Pan-STARRS1 3π RR Lyrae

“…In Figure 19, we plot the overdensities in an ( ) D R.A ., projection similar to Sesar et al (2007) (see their Figure 13; see also Figure 11 in Sesar et al 2010), using our full range in decl.…”

“…We compare the overdensities found by us with those discovered previously by Sesar et al (2007Sesar et al ( , 2010.…”

“…Uppercase letters denote overdensities found in the SDSS sample of Sesar et al (2007Sesar et al ( , 2010, numbers denote overdensities found in their analysis of the Ivezić et al (2000) sample (not numbered in Sesar et al 2007), and lowercase letters denote overdensities we found in regions not covered by the analysis of Sesar et al (2010).…”

“…They had used 634 RR Lyrae candidates from SDSS Stripe 82 and 296 RR Lyrae candidates from Ivezić et al (2000) in their 2007 analysis (Sesar et al 2007), and later on they cleaned the SDSS Stripe 82 sample of RR Lyrae (Sesar et al 2010), using then 366 highly probable RRab stars.…”

“…Thus, the mean luminosity of an RRab variable, and hence its distance, can be determined with knowledge of the light curve only. RRab stars were used by many previous studies, including those of Hawkins (1984), Saha (1984), Wetterer et al (1996), Ivezić et al (2000), Vivas & Zinn (2006), Jurić et al (2008), Catelan (2009), Watkins et al (2009), de Jong et al (2010, Sesar et al (2010Sesar et al ( , 2011, Deason et al (2011), Akhter et al (2012), Drake et al (2014), Torrealba et al (2015), Cohen et al (2015), Xue et al (2015), Soszyński et al (2016), Bland-Hawthorn & Gerhard (2016), Iorio et al (2017), and Cohen et al (2017).…”