Hunting the Parent of the Orphan Stream: Identifying Stream Members from Low-Resolution Spectroscopy

TL;DR

This study identifies probable members of the Orphan Stream using low-resolution spectroscopy, revealing its distance, velocity, and metallicity, and suggesting its parent is a dwarf spheroidal galaxy.

Contribution

It introduces a method to select stream members from low-resolution spectra and provides new measurements of the stream's properties, including metallicity and kinematics.

Findings

Stream distance of 22.5 kpc with uncertainties

Kinematic signature peaks at 82.1 km/s

Metallicity of -1.63 dex with dispersion 0.56 dex

Abstract

We present candidate K-giant members in the Orphan Stream which have been identified from low-resolution data taken with the AAOmega spectrograph on the Anglo-Australian Telescope. From modest S/N spectra and independent cuts in photometry, kinematics, gravity and metallicity we yield self-consistent, highly probable stream members. We find a revised stream distance of 22.5 +/- 2.0 kpc near the celestial equator, and our kinematic signature peaks at Vgsr = 82.1 +/- 1.4 km/s. The observed velocity dispersion of our most probable members is consistent with arising from the velocity uncertainties alone. This indicates that at least along this line-of-sight, the Orphan Stream is kinematically cold. Our data indicates an overall stream metallicity of [Fe/H] = -1.63 +/- 0.19 dex which is more metal-rich than previously found and unbiased by spectral type. Furthermore, the significant metallicity dispersion displayed by our most probable members (0.56 dex) suggests that the unidentified Orphan Stream parent is a dSph satellite. We highlight likely members for high-resolution spectroscopic follow-up.