Clustered Cepheid Variables 90 kiloparsec from the Galactic Center

TL;DR

This study reports the discovery of four Cepheid variables at 90 kiloparsecs from the Galactic Center using near-infrared data, suggesting a possible dwarf galaxy at this large distance, which challenges current understanding of the Galactic disk.

Contribution

First detection of Cepheid variables at such a large distance from the Galactic Center, indicating a potential dwarf galaxy and demonstrating the effectiveness of near-infrared surveys in dust-obscured regions.

Findings

Four Cepheid variables discovered at 90 kpc from Galactic Center.

Cepheids are highly clustered in position and distance.

Suggests presence of a dwarf galaxy at this distance.

Abstract

Distant regions close to the plane of our Galaxy are largely unexplored by optical surveys as they are hidden by dust. We have used near-infrared data (that minimizes dust obscuration) from the ESO Public survey VISTA Variables of the Via Lactea (VVV) (Minniti et al. 2011; Saito et al. 2012; henceforth S12) to search for distant stars at low latitudes. We have discovered four Cepheid variables within an angular extent of one degree centered at Galactic longitude of $l = -27.4^\circ$ and Galactic latitude of $b = -1.08 ^\circ$. We use the tightly constrained period-luminosity relationship that these pulsating stars obey (Persson et al. 2004; Matsunaga et al. 2011) to derive distances. We infer an average distance to these Cepheid variables of 90 kpc. The Cepheid variables are highly clustered in angle (within one degree) and in distance (the standard deviation of the distances is 12 kpc). They are at an average distance of $\sim 2~\rm kpc$ from the plane and their maximum projected separation is $\sim 1~ \rm kpc$. These young ($\sim$ 100 Myr old), pulsating stars (Bono et al. 2005) are unexpected at such large distances from the Galactic disk, which terminates at $\sim$ 15 kpc (Minniti et al. 2011). The highly clustered nature in distance and angle of the Cepheid variables suggests that the stars may be associated with a dwarf galaxy, one that was earlier predicted by a dynamical analysis (Chakrabarti \& Blitz 2009).