Disentangling the Virgo Overdensity With Rr Lyrae Stars

Abstract

We use a combination of spatial distribution and radial velocity to search for halo sub-structures in a sample of 412 RR Lyrae stars (RRLS) that covers a ∼ 525 square degrees region of the Virgo Overdensity (VOD) and spans distances from the Sun from 4 to 75 kpc. With a friends-of-friends algorithm we identified six high significance groups of RRLS in phase space, which we associate mainly with the VOD and with the Sagittarius stream. Four other groups were also flagged as less significant overdensities. Three high significance and 3 lower significance groups have distances between ∼ 10 and 20 kpc, which places them with the distance range attributed by others to the VOD. The largest of these is the Virgo Stellar Stream (VSS) at 19 kpc, which has 18 RRLS, a factor of 2 increase over the number known previously. While these VOD groups are distinct according to our selection cirteria, their overlap in position and distance, and, in a few cases, similarity in radial velocity are suggestive that they may not all stem separate accretion events. Even so, the VOD appears to be caused by more than one overdensity. The Sgr stream is a very obvious feature in the background of the VOD at a mean distance of 44 kpc. Two additional high significant groups were detected at distances > 40 kpc. Their radial velocities and locations differ from the expected path of the Sgr debris in this part of the sky, and they are likely to be remnants of other accretion events.

The Virgo Overdensity Explained

“…This N-body simulation is only a proof of concept, however; if this simulation were to be plotted on Figure 2, the bodies would significantly deviate from the positions of the orbit due to the size of the progenitor, and would not reproduce the correct sky positions for the moving groups. The simulation does show that it is possible to populate the range of distances and velocities of the Vivas et al (2016) RR Lyraes with one dwarf galaxy progenitor.…”

“…However, the single progenitor idea did not last. Duffau et al (2014); Vivas et al (2016) found several halo substructures along the same line of sight in Virgo. Zinn et al (2014) noted that the VSS was at the most distant part of the much larger VOD structure, but pointed to the large number of identified moving groups as evidence that the structure might not arise from a common origin.…”

“…In the top left panel of Figure 4, we plot the heliocentric distance against V GSR for all of the stars in the Vivas et al (2016) catalog and the stars in our moving groups from Duffau et al (2014). Also plotted over this data are the calculated orbits for the Perpendicular Stream (black), the Parallel Stream (green), and Group D (red).…”

RR Lyrae stars as tracers of halo substructures

“…Hya 2 is a new detection. Crv 1 seems to be the southern extension to the Virgo Stellar Stream and G4 velocity complex discovered in [5]. Although at a similar location as the expected orbit of the Orphan Stream, Cen 3's distance and velocity do not correspond with the expected values in the area, representing then another new detection as well.…”

“…Although at a similar location as the expected orbit of the Orphan Stream, Cen 3's distance and velocity do not correspond with the expected values in the area, representing then another new detection as well. Confirmation of the results will be obtained by applying a group finding algorithm to the data, considering both the radial velocity of the stars and the separation between them, and simulating the Galactic contribution to assess the significance of these detections, as done in [1] and [5].…”