Tracing the long bar with red-clump giants

Abstract

Context. Over the last decade a series of results have lent support to the hypothesis of the existence of a long thin bar in the Milky Way with a half-length of 4.5 kpc and a position angle of around 45• . This is apparently a very different structure from the triaxial bulge of the Galaxy. Aims. In this paper, we analyse the stellar distribution in the inner 4 kpc of the Galaxy to see if there is clear evidence for two triaxial or barlike structures, or whether there is only one. Methods. By using the red-clump population as a tracer of the structure of the inner Galaxy we determine the apparent morphology of the inner Galaxy. Star counts from 2MASS are used to provide additional support for this analysis. Results. We show that there are two very different large-scale triaxial structures coexisting in the inner Galaxy: a long thin stellar bar constrained to the Galactic plane (|b| < 2• ) with a position angle of 43.• 0 ± 1.• 8, and a distinct triaxial bulge that extends to at least |b| ≤ 7.5• with a position angle of 12.• 6 ± 3.• 2. The scale height of the bar source distribution is around 100 pc, whereas for the bulge the value of this parameter is five times larger.

The long bar as seen by the VVV Survey

“…We get an average σ = 1.01 ± 0.03 degrees; hence we get a vertical thickness approximately equal to 176 ± 6 pc. This value is higher than 100 pc obtained by Cabrera-Lavers et al (2007a) using red clump stars. First, we must take into account that we are using a wider range of stellar types than red clumps, and we are also observing the bar with a different angle (at positive longitudes, we observe the bar almost perpendicularly, whereas at negative longitudes our line of sight is more tangential to the bar).…”

“…The asymmetry is also visible in star counts (e.g., Stanek et al 1994;Hammersley et al 1994), which show systematically more stars at positive Galactic longitudes (within < 30 • ) and close to the Galactic plane, compared to negative longitudes. It was also supported by the distribution of red clump stars (Hammersley et al 2000;Cabrera-Lavers et al 2007a), the dynamics of the stellar and the gaseous components in the Galactic center (e.g., Minchev et al 2007), gravitational micro-lensing toward the Galactic bulge (Paczynski et al 1994;Popowski et al 2005), and the kinematic effects on the local disk stars (Gardner & Flynn 2010;Romero-Gómez et al 2011).…”

“…While some papers refer to a thick triaxial structure (bulge or thick bar) with a semi-major axis length of 2.5 kpc and a position angle (PA) of 15 • -30 • with respect to the Sun-Galactic center direction (e.g., López-Corredoira et al 2005;Habing et al 2006;Rattenbury et al 2007;Vanhollebeke et al 2009;Robin et al 2012), other contributions suggest that there is also a long thin bar, the in-plane bar, with a half length of 4 kpc and a position angle of around 45 • : e.g., López-Corredoira et al (2007, LC07 and references therein), Cabrera-Lavers et al (2007a), Vallenari et al (2008), Churchwell et al (2009).…”

A spectroscopic survey of faint, high-Galactic-latitude red clump stars

“…They display properties that make them a primary tool to investigate Galactic structure and kinematics: (i) their absolute magnitude shows minimal dispersion at optical and infrared wavelengths; (ii) they are intrinsically bright, and thus observable throughout most of the Galaxy; (iii) in magnitude-limited surveys they count for a fairly large fraction of observed targets; (iv) their spectral types, ranging mainly from G8III to K2III, make them ideal stars to measure accurate radial velocities and atmospheric chemical abundances. Examples of recent applications of RC stars to Galaxy investigations are, among countless more, the peculiarities of Galactic rotation (Rybka et al 2008), the stellar bar in the inner Galaxy (Cabrera-Lavers et al 2007), the Galactic Bulge (Nataf et al 2010), the vertical distribution of disk stars in terms of kinematic and metallicity , the surface mass density in the Galactic plane , the origin of the Thick disk (Ruchti et al 2010), the surface mass density in the Galactic disk (Bienayme et al 2006) and age-metallicity relation (AMR), age-velocity relation (AVR) (Soubiran et al 2008), tidal streams in solar neighborhood (Famaey et al 2005;Antoja et al 2008), Galactic substructures (Correnti et al 2010;Law et al 2010). The large proportion of RC stars observed by the ongoing RAVE The catalog is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/527/A40 survey (Steinmetz et al 2006;Zwitter et al 2008) and the accurate distances derived for them (Zwitter et al 2010) support a great potential of the RAVE data base in progressing towards a better understanding of how the Galaxy formed, structured and evolved (Freeman & Bland-Hawthorn 2002;Siebert et al 2008;Veltz et al 2008;Kiss et al 2010).…”

Evidence of the Galactic outer ring R 1 R 2 ′ $R_{1}R_{2}'$ from young open clusters and OB-associations

“…Here are some examples of galaxies with the R 1 R ′ 2 morphology that can be viewed as possible prototypes of the Milky Way: ESO 245-1, NGC 1079, NGC 1211, NGC 3081, NGC 5101, NGC 5701, NGC 6782, and NGC 7098. Their images can be found in de Vaucouleurs Atlas of Galaxies by Buta et al (2007) at http://bama.ua.edu/∼rbuta/devatlas/ There is extensive evidence for the existence of the bar in the Galaxy derived on the basis of infra-red observations (Blitz & Spergel 1991;Benjamin et al 2005;Cabrera-Lavers et al 2007;González-Fernández et al 2012;Churchwell et al 2009) and gas kinematics in the central region (Binney et al 1991;Englmaier & Gerhard 1999;Weiner & Sellwood 1999). The general consensus is that the major axis of the bar is oriented in the direction θ b = 15-45 • in such a way that the end of the bar closest to the Sun lies in quadrant I, where θ b is the position angle between the line connecting the Sun and the Galactic center and the direction of the major axis of the bar.…”