Revisiting the Milky Way stellar long bar and the 3 kpc arm

Abstract

CONTEXT. One of the most difficult and unexplored regions of the Milky Way is the highly extincted in-plane central region within the Galactic coordinates $10^\circ \lesssim |\ell |\lesssim 30^\circ $, $|b|\lesssim 3^\circ $, where we have the long-bar and 3 kpc arm with intermediate-age stellar population, whose morphological properties are still unclear. AIMS. We aim to advance our knowledge of the morphology of these two components. METHODS. We examined star counts of bright M giants in WISE-4.6$μ$m and its distribution of distances derived from spectroscopic parallaxes with APOGEE-DR17. We also examined the distribution of distances of young OGLE-O-rich Mira variable stars, and reviewed the literature on red clump distance determination within that area. RESULTS. We corroborate the asymmetry between positive and negative longitudes in in-plane regions, thus confirming the necessity to include a long bar. We obtain an average angle between the major axis of the long bar and the line Sun-Galactic centre of $α=27.4^\circ \pm 1.5^\circ $, aligned with the triaxial bulge and a semi-major-axis length $\approx 4$ kpc. The tips of the long bar are in contact with the elliptical 3 kpc arm, with the major axis again aligned with the bulge and the long bar's major axes, whose tangential lines of sight correspond to $\ell =-22^\circ $ and $\ell=+27^\circ $. In the range of 50 degrees in the sky between these two longitudes, the stellar near 3 kpc arm is clearly detected at heliocentric distances around 5 kpc, and the stellar far 3 kpc arm is tentatively detected at heliocentric distances of 9-12 kpc.

Figures (13)

• Figure 1: WISE star counts. Top: As a function of $\ell$, $b$ within $|\ell |<40^\circ$, $|b|<5^\circ$, $m_{4.6\mu m}<8.0$, $\Delta \ell =1^\circ$, $\Delta b=0.1^\circ$. Bottom: As a function of $\ell$ within $|\ell |<40^\circ$, $|b|<0.5^\circ$, for $m_{3.4\mu m}<8.2$, $m_{4.6\mu m}<8.0$, $m_{12\mu m}<7.0$, $m_{22\mu m}<4.5$, $\Delta \ell =1^\circ$.
• Figure 2: Colour-magnitude diagram in low extincted areas ($30^\circ <|\ell |<40^\circ$, $4^\circ <|b|<5^\circ$; $\langle E(J-K)\rangle =0.4$; Sch98) for sources with $m_{4.6\mu m}<8.0$.
• Figure 3: WISE star counts as in Fig. \ref{['Fig:WISEcounts']} (top panel) but with stars with $J-K<1.0$ removed (dwarfs and red clumps from Galactic disc) and with the correction of extinction through Eq. (\ref{['extcorr']}). We observe a prominent asymmetry between positive and negative Galactic longitudes and a very slight asymmetry between positive and negative Galactic latitudes.
• Figure 4: Extinction $A_{4.6\mu m}$ as a function of $\ell$, $b$ within $|\ell |<40^\circ$, $|b|<5^\circ$; $\Delta \ell =1^\circ$, $\Delta b=0.1^\circ$, derived from Eq. (\ref{['extcorr']}).
• Figure 5: Top: APOGEE-2/DR17 M5-6III ($-6.5<M_H<-5.5$, $1.0<(J-K)_0<1.5$) star counts, in the range of Galactic coordinates $|\ell |<40^\circ$, $|b|<5^\circ$. These counts are highly in incomplete (and white areas are unobserved regions) due to the observing strategy of APOGEE-2, but for each line of sight the completeness is constant, with a magnitude within $7.0<H<12.2$ when $A_H<2.7$. Bottom: Distribution of distances of these stars for $-6^\circ <\ell <30^\circ$, $1.0^\circ <|b|<2.5^\circ$; dashed line shows the best fit to a long bar with an angle of $\alpha =25.6^\circ$ for $\ell>10^\circ$.