Distribution of red clump stars does not support the X-shaped Galactic bulge

Abstract

Context. Claims of an X-shaped Galactic bulge were based on the assumption of red clump stars as standard candles in some lines of sight crossing the off-plane bulge. However, some doubts have been cast on whether the two peaks in star counts along the line of sight really represent a double peak in the density distribution, or whether there is something wrong with the assumption of a unique constant absolute magnitude for all of these stars. Aims. With the advent of Gaia-DR2 parallaxes in combination with near-infrared VISTA-VVV data, we are able to check which of the hypotheses is correct. Methods. We calculated the median absolute magnitude M K corresponding to both peaks of putative red clumps in seven lines of sight with the lowest extinction in the interesting coordinates' range. Results. The difference between the absolute magnitude of the bright and the faint peak is ∆M K ≈ 0.4. The selected stars in both peaks cannot be represented by the same red clump giants with constant M K ≈ −1.6. Conclusions. The hypothesis that the bulge contains an X-shape is based on the assumption that the faint and bright peaks of the density distribution towards the bulge are dominated by standard red clump stars. However, we show that both the faint and bright peaks cannot be dominated by standard red clump stars simultaneously.

The structure of the Milky Way based on unWISE 3.4 μm integrated photometry

“…Also, based on unWISE (Lang 2014) 3.4 and 4.6 μm wide-field images, Ness & Lang (2016) identified and quantified (Ciambur et al 2017) the X/peanut(P)-shaped structure. However, López-Corredoira (2016), Lee et al (2018), andLópez-Corredoira et al (2019) do not confirm the discovery of the X-shape structure based on the population of the double red clump. In addition to that, as stressed by López-Corredoira (2017), oxygen-rich Mira variables (also used as standard candles) match a boxy, but not an X-shape bulge.…”

Age demographics of the Milky Way disc and bulge

“…Recent mappings of the inner and outer bulge regions with RC stars from the Vista Variables in the Via Lactea (VVV) survey by Wegg & Gerhard (2013) and Wegg et al (2015) have further confirmed the B/P nature of the inner bulge that proceeds to extend and flatten into a barred structure into the disk, both structures oriented at an angle of ∼ 27 • . Whilst this interpretation has been contested with López-Corredoira et al (2019) claiming that the split RC is a result of two distinct stellar populations, the spectacular WISE imaging of Ness & Lang (2016) reveals the X-shape clearly. The proper motion data from VVV as analyzed by Sanders et al (2019a) show differential motion between the peaks, which argues strongly against population effects causing the X-shape.…”

Blanco DECam Bulge Survey (BDBS) II: Project Performance, Data Analysis, and Early Science Results

“…As mentioned in § 1, these and other authors contend that the double red clumps trace out an "X"-shaped structure, and 0.0 2.0 4.0 that the two clumps reside at the near (∼ 6.5 kpc) and far (∼ 8.8 kpc) ends of the bar. However, other explanations relating to chemical composition variations, particularly involving He enhancements, have also been suggested (e.g., Lee et al 2015;Joo et al 2017;Lee et al 2018Lee et al , 2019López-Corredoira et al 2019). 17 also shows that the red clump (g − i) o dispersion is relatively small, and that the bright and faint red clumps effectively span the same color range.…”