Spiral arms, bar shape and bulge microlensing in the Milky Way

By Nicolai Bissantz, O. Gerhard. 2001

207 citations recorded by the source corpus.

A new model for the luminosity distribution in the inner Milky Way is found, using a non-parametric penalized maximum-likelihood algorithm to deproject a dereddened COBE/ DIRBE L-band map of the inner Galaxy. The model isalso constrained by the apparent magnitude (line-of-sight) distributions of clump giant stars in certain bulge fields. An important new feature is the inclusion of a spiral arm model in the disc. Spiral arms make the model appear broader on the sky; thus our bar is more elongated than in previous eight-fold symmetric models. They also lead to a smoother disc model interior to the Sun. The bar length is 3.5 kpc, and its axis ratios are 1: (0.3-0.4): 0.3, independent of whether the spiral arm model is four-armed or two-armed. The larger elongation in the plane makes it possible to reproduce the observed clump giant distributions as well. With only the surface brightness data, a small model degeneracy is found even for fixed orientation of the bar, amounting to about ′0.1 uncertainty in the in-plane axial ratio. Including the clump giant data removes most of this degeneracy and also places additional constraints on the orientation angle of the bar. We estimate 15° ≤ φ b a r ≤ 30°, with the best models obtained for 20° ≤ φ b a r ≤ 25°. We use our reference model to predict a microlensing optical depth map towards the bulge, normalizing its mass by the observed terminal velocity curve. For clump giant sources at (l,b) = (3°.9, -3°.8) we find τ - 6 ≡ τ/10 - 6 = 1.27, within 1.8σ of the new MACHO measurement given by Popowski et al. The value for all sources at (l, b) = (2°.68, -3°.35) is τ-6 = 1.1, still >3σ away from the published MACHO DIA value. The dispersion of these τ - 6 values within our models is ≃ 10 per cent. Because the distribution of sources is well fitted by the near-infrared model, increasing the predicted optical depths by >20 per cent will be difficult. Thus the high value of the measured clump giant optical depth argues for a near-maximal disc in the Milky Way.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/584c1d8670cf780c90c9d2696f5fa97bace356e8

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Citations in the corpus

The 6-GHz methanol multibeam maser catalogue – II. Galactic longitudes 6° to 20° (2010, background): “However, the bar descriptions fall into two main categories: a short, bulge-like bar with an orientation of 20◦-25◦ (e.g. Bissantz & Gerhard 2002; Babusiaux & Gilmore 2005) and/or a long, thin bar with an orientation of ∼45◦ (e.g. Hammersley et al. 2000; Benjamin et al. 2005).”
The milky way total mass profile as inferred from Gaia DR2 (2019, methodology): “We model the MW bulge using the McMillan (2017) profile (which is an axisymmetric form of the model proposed by Bissantz & Gerhard 2002) given by, ρbulge = ρ0,bulge (1 + r′/r0)α exp [ − ( r′/rcut )2] , (1) where, r′ represents a combination of the cylindrical coordinates (R, z) (whereR is in the…”
ARGOS – IV. The kinematics of the Milky Way bulge (2013, background): “15◦ to 30◦ (Stanek et al. 1997; Binney, Gerhard, & Spergel 1997; Bissantz & Gerhard 2002) did not significantly improve the match.”
Galactic Metallicity Gradients Derived from a Sample of OB Stars (2004, background): “In particular, these flatter gradients are generally in line with the fact that the Milky Way seems to be a spiral galaxy having a bar of ∼3.5 kpc in its Galactic center (Weiner & Sellwood 1999; Bissantz & Gerhard 2002).”
The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, background): “The disk has local stellar surface density of 38 M / pc2 (Bovy & Rix 2013) and scale lengths Rd = 2.15 kpc (Bissantz & Gerhard 2002, Bovy & Rix 2013), 2.6 kpc (Robin et al. 2003, Jurić et al. 2008), and 3.0 kpc (Kent et al. 1991, Gould et al. 1996) and includes a gas disk with surface density of…”
The all-sky distribution of 511 keV electron-positron annihilation emission (2005, background): “The B/D luminosity ratio of 3−9 is considerably larger than the B/D mass ratio of 0.3−1.0 of our Galaxy (e.g. Caldwell & Ostriker 1981; Freudenreich 1998; Bissantz & Gerhard 2002; Robin et al. 2003).”
VISTA Variables in the Via Lactea (VVV): The public ESO near-IR variability survey of the Milky Way (2009, background): “In particular, they miss the inner regions of the bulge where the optical depth is higher, providing poor constraints to current models (Bissantz & Gerhard, 2002; Bissantz et al., 2004).”
The local dark matter density (2014, background): “The very presence of a bar and spiral arms in the Milky Way, along with evidence for ‘moving groups’ in the Solar neighbourhood all point towards disequilibria (e.g. Blitz & Spergel, 1991; Dehnen, 1998; Bissantz & Gerhard, 2002; Antoja et al., 2011).”
Mass models of the Milky Way (2011, methodology): “The posterior pdf on bulge masses (Figure 3, top left) is close to our prior from Bissantz & Gerhard (2002).”
A three-dimensional Galactic extinction model (2003, background): “…in the Galactic plane and revealing its nonaxisymmetric structure, including the barred nature of the Galactic bulge (Blitz & Spergel 1991; Weiland et al. 1994), the Galactic warp (Djorgovski & Sosin 1989; Freudenreich et al. 1994) and the spiral arms (Drimmel 2000; Bissantz & Gerhard 2002).”
Constraining the Galaxy's dark halo with RAVE stars (2014, methodology): “Following McMillan (2011), we use a model similar to that constructed by Bissantz & Gerhard (2002).”
Mapping the three-dimensional density of the galactic bulge with VVV red clump stars (2013, methodology): “Most works have generally either fitted parametric c© 2012 RAS, MNRAS 000, 1–14 models of the bulge (e.g. Dwek et al. 1995; Rattenbury et al. 2007; Cao et al. 2013) or attempted to non-parametrically deproject the bulge from surface brightness data (e.g. Binney et al. 1997; Bissantz & Gerhard 2002).”
OUR MILKY WAY AS A PURE-DISK GALAXY—A CHALLENGE FOR GALAXY FORMATION (2010, result): “Other studies converged on a similar bar angle (Stanek et al. 1997; Freudenreich 1998; Fux 1997, 1999; Bissantz & Gerhard 2002).”
On the mass-to-light ratio of the local Galactic disc and the optical luminosity of the Galaxy (2006, methodology): “The infrared structure of the Milky Way has been extensively studied in the 1990s, mostly taking advantage of the COBE/DIRBE experiment, and is presently well understood (Kent, Dame & Fazio 1991; Dwek et al. 1995; Binney, Gerhard & Spergel 1997; Freudenreich 1998; Bissantz & Gerhard 2002).”
The Milky Way’s stellar disk (2013, background): “By now the strength of the Galactic bar and its pattern speed seem reasonably well established, both on the basis of photometry (Binney et al. 1997; Bissantz & Gerhard 2002) and on the basis of dynamics (Dehnen 1999; Minchev et al. 2007).”
Triaxial orbit based galaxy models with an application to the (apparent) decoupled core galaxy NGC 4365 (2007, methodology): “This has already been done in the axisymmetric case by Romanowsky & Kochanek (1997) and in the triaxial case by Bissantz & Gerhard (2002).”
Gas flow models in the Milky Way embedded bars (2008, background): “…Sellwood 1999; Sevenster 1999; López-Corredoira et al. 2001) with a number of works giving values in the range 15◦–30◦ (Mulder & Liem 1986; Binney et al. 1997; Fux 1999; Englmaier & Gerhard 1999; Bissantz & Gerhard 2002; Bissantz et al. 2003; Babusiaux & Gilmore 2005; López-Corredoira et al. 2005).”