The X-shaped Milky Way bulge in OGLE-III★ photometry and in N-body models

Abstract

We model the split red clump of the Galactic bulge in OGLE-III photometry, and compare the results to predictions from two N-body models. Our analysis yields precise maps of the brightness of the two red clumps, the fraction of stars in the more distant peak, and their combined surface density. We compare the observations to predictions from two N-body models previously used in the literature. Both models correctly predict several features as long as one assumes an angle α Bar ≈ 30 • between the Galactic bar's major axis and the line of sight to the Galactic centre. In particular that the fraction of stars in the faint red clump should decrease with increasing longitude. The biggest discrepancies between models and data are in the rate of decline of the combined surface density of red clump stars toward negative longitudes and of the brightness difference between the two red clumps toward positive longitudes, with neither discrepancy exceeding ∼25% in amplitude. Our analysis of the red giant luminosity function also yields an estimate of the red giant branch bump parameters toward these high-latitude fields, and evidence for a high rate (∼25%) of disk contamination in the bulge at the colour and magnitude of the red clump, with the disk contamination rate increasing toward sightlines further distant from the plane.

The Galaxy in Context: Structural, Kinematic, and Integrated Properties

“…Unsharp masking results in a strong off-centered X-shape structure ( Fig. 7), similar to some galaxies in the sample of Bureau et al (2006); see also Nataf et al (2015). (Wegg & Gerhard 2013).…”

A parametric description of the 3D structure of the Galactic bar/bulge using the VVV survey

“…Table 1. Parameters describing the Galaxia generated synthetic LF modelled with three Gaussians to represent the RC, AGBB and RGBB and an exponential function for the RGB (see equation in section 3.1 of Nataf et al 2015 and the caption of Figure 3). For a 10 Gyrs old population (age in the first column), we provide the parameters for both a skew-Gaussian and a Gaussian RC distribution, in the first and second row respectively.…”

“…Nataf et al (2015) also test their model on this N-body simulation, assuming four viewing angles α = 0…”

Absence of an X-shaped Structure in the Milky Way Bulge Using Mira Variable Stars

“…One may wonder why astronomers have not seen an X-shaped bulge in the 1990s in the COBE-DIRBE images. The structure along the line of sight has also been claimed to show an X-shapeusing metal rich red clumps as standard candles (Nataf et al 2010(Nataf et al , 2015 There are also signs of a non-X-shape bulge in other populations: very old and metal poor stars like RR-Lyrae in the bulge are not X-shaped according to some authors (Dékány et al 2013;Pietrukowicz et al 2015), and young ( 5 Gyr) populations like F0-F5V stars are not X-shaped according to different interpretations (López-Corredoira 2016), interpretations that might also have doubt cast upon. For instance, the argument for non-X-shape by López-Corredoira (2016) might also raise doubts based on the possible dispersion of absolute magnitudes, which might be much larger than assumed and thus may convolve the two peaks along the line of sight into one.…”