Reinforcing the link between the double red clump and the X-shaped bulge of the Milky Way^⋆

¹ European Southern Observatory, Ave. Alonso de Cordova 3107, Casilla 19, 19001 Santiago Chile
e-mail: ogonzale@eso.org
² Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
³ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago 22, Chile
⁴ Instituto Milenio de Astrofísica, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁵ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
⁶ Unidad de Astronomía, Facultad Cs. Básicas, Universidad de Antofagasta, Avda. U. de Antofagasta 02800, Antofagasta, Chile
⁷ European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
⁸ Departamento de Ciencias Físicas, Universidad Andrés Bello, República 220, Santiago, Chile
⁹ Vatican Observatory, 00120 Vatican City State, Italy

Received: 23 September 2015
Accepted: 19 October 2015

Abstract

The finding of a double red clump in the luminosity function of the Milky Way bulge has been interpreted as evidence for an X-shaped structure. Recently, an alternative explanation has been suggested, where the double red clump is an effect of multiple stellar populations in a classical spheroid. In this Letter we provide an observational assessment of this scenario and show that it is not consistent with the behaviour of the red clump across different lines of sight, particularly at high distances from the Galactic plane. Instead, we confirm that the shape of the red clump magnitude distribution closely follows the distance distribution expected for an X-shaped bulge at critical Galactic latitudes. We also emphasize some key observational properties of the bulge red clump that should not be neglected in the search for alternative scenarios.