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

Abstract

We study the structure of the inner Milky Way (MW) using the latest data release of the Vista Variables in Via Lactea (VVV) survey. The VVV is a deep near-infrared, multi-colour photometric survey with a coverage of 300 square degrees towards the Bulge/Bar. We use Red Clump (RC) stars to produce a high-resolution dust map of the VVV's field of view. From de-reddened colour-magnitude diagrams we select Red Giant Branch stars to investigate their 3D density distribution within the central 4 kpc. We demonstrate that our best-fit parametric model of the Bulge density provides a good description of the VVV data, with a median percentage residual of 5 % over the fitted region. The strongest of the otherwise low-level residuals are overdensities associated with a low-latitude structure as well as the so-called X-shape previously identified using the split RC. These additional components contribute only ∼ 5% and ∼ 7% respectively to the Bulge mass budget. The best-fit Bulge is "boxy" with an axis ratio of [1:0.44:0.31] and is rotated with respect to the Sun-Galactic Centre line by at least ∼ 20• . We provide an estimate of the total, full sky, mass of the Bulge of M Chabrier Bulge = 2.36 × 10 10 M for a Chabrier initial mass function. We show there exists a strong degeneracy between the viewing angle and the dispersion of the Red Clump (RC) absolute magnitude distribution. The value of the latter is strongly dependent on the assumptions made about the intrinsic luminosity function of the Bulge.

The pattern speed of the Milky Way bar from transverse velocities

“…In small fields of 0.2 deg × 0.2 deg across the VVV bulge footprint, the bulge giants (both red giant branch and red clump) were selected in a unextincted colour-magnitude box of 0.4 < (J − K s ) 0 < 1 and 11.5 < K s0 < 14.5 using a two-dimensional extinction map from the method of Gonzalez et al (2011). First, the density in each field was measured assuming a luminosity function (taken from Simion et al 2017) and accounting for incompleteness, and then the density distribution was used to extract the first two moments of the transverse velocity distributions as a function of distance by probabilistically considering the possible velocities for each star given its K s0 magnitude. Here we utilise the mean longitudinal transverse velocities with location v ( , b, s) and the density field ρ( , b, s) computed on a grid in log(distance), and b.…”

Transverse kinematics of the Galactic bar-bulge from VVV and Gaia

“…Red clump giants have been used as a tracer of the structure of the bulge in numerous studies (Stanek et al 1997;Saito et al 2011;Wegg & Gerhard 2013;Simion et al 2017) due to their standard candle nature. They appear as a clear peak in bulge colourmagnitude diagrams, lying at (J − K s ) 0 ≈ 0.6 and between K s0 ≈ 12 and K s0 ≈ 14 depending on Galactic longitude (subscript 0 denotes unextincted -we describe extinction correction in the following section).…”

“…metallicity) are unimportant so we use a single luminosity function for both components. The luminosity function p(M K s ) is adopted from Simion et al (2017) composed of three Gaussian peaks for the AGB, RGB and RC bumps along with a background RGB exponential: Simion et al (2017) sets a g = a g0 = 0.642. Simion et al (2017) allowed the mean magnitude of the red clump to vary in their modelling.…”

“…The luminosity function p(M K s ) is adopted from Simion et al (2017) composed of three Gaussian peaks for the AGB, RGB and RC bumps along with a background RGB exponential: Simion et al (2017) sets a g = a g0 = 0.642. Simion et al (2017) allowed the mean magnitude of the red clump to vary in their modelling. Requiring a Galactic centre distance of 8 kpc, Simion et al (2017) found M K, RC = −1.63 mag consistent with the solar neighbourhood result of M K, RC = −1.61 mag (Alves 2000;Hawkins et al 2017).…”

Galactic resonance rings: modelling of motions in the wide solar neighbourhood

“…There is a lot of evidence that our Galaxy includes a bar. Infrared observations of the inner Galactic plane (Dwek et al 1995;Benjamin et al 2005;Cabrera-Lavers et al 2007;Churchwell et al 2009;González-Fernández et al 2012), gas kinematics in the inner Galaxy (Pohl et al 2008;Gerhard 2011), X-shaped distribution of red giants in the central region derived from BRAVA, WISE and VVV data (Li & Shen 2012;Ness & Lang 2016;Simion et al 2017) confirm the presence of the bar in the Galaxy. The estimates of the length of the bar major semi-axis lie in the range 3-5 kpc which corresponds to the bar angular velocity 40-70 km s −1 .…”