Peering Past the Galactic Bar: Uncovering a Hidden Spiral Arm in the Milky Way

The Milky Way is notoriously difficult to study from within, especially near its crowded, dust-filled center. In this work, Simran Joharle and collaborators set out to explore whether a newly observed group of stars, known as a secondary red clump, might belong to a spiral arm that lies just beyond the Galactic bar. By analyzing the motion and light absorption (extinction) of these stars, the team offers fresh insight into the hidden structure of our Galaxy’s inner regions.

Mapping the Milky Way with Red Clump Stars

To understand what lies beyond the Galactic bar, the authors use red clump (RC) stars, core-helium-burning stars that act as reliable distance indicators because their brightness varies only slightly with age or chemical composition. Earlier studies, including Gonzalez et al. (2018), discovered a double red clump feature at low Galactic latitudes, hinting that one group of these stars might trace a spiral arm behind the bar. Joharle’s team set out to test this by combining kinematic (motion-based) and extinction (dust-based) analyses.

Gathering the Data

The study relies on observations from the VISTA Variables in the Via Lactea (VVV) survey, which images the inner Milky Way in near-infrared light to see through dense dust clouds. Using high-precision measurements from the VIRAC astrometric catalogue, the team analyzed the proper motions, tiny shifts in position on the sky, of stars between −10° and +10° in Galactic longitude. They identified two distinct groups, the bright red clump (RC1) and the faint red clump (RC2), which appear as separate clusters of stars in a color–magnitude diagram.

Tracing Motion Beyond the Bar

By comparing the average motions of these two groups, the authors found that RC2 stars move slightly slower across the sky than RC1 stars, with a difference of −0.16 ± 0.02 milliarcseconds per year in the component parallel to the Galactic plane. This subtle offset is consistent with what would be expected if RC2 stars were farther away and participating in the Galaxy’s rotation at a greater distance. Simulations of a Milky Way–like galaxy confirmed that such a motion difference naturally arises for stars located beyond the bar, reinforcing the idea that RC2 marks a more distant structure, possibly a spiral arm.

Measuring Dust Along the Way

The authors also measured how much light was absorbed by interstellar dust along the line of sight to each red clump. Using both the J and Ks infrared filters, they determined an extinction curve ratio of AJ/AKs = 3.34 ± 0.07, consistent with other studies of the Galactic center. The extinction towards RC2 was found to be slightly higher, by about 0.05 magnitudes in Ks-band, or roughly 5% of the total extinction from Earth to the nearer RC1 stars. This small difference suggests that there is very little dust between the Galactic bar and the more distant stellar population, implying an open region of space before reaching the new structure.

A Hidden Spiral Arm Revealed

Combining the results, Joharle and collaborators conclude that the RC2 stars are located several thousand light-years beyond the Galactic bar, likely tracing a spiral arm. Their distinct kinematics and slightly greater extinction show that these stars are not merely part of the same population as RC1 (such as red giant branch bump stars) but belong to a separate Galactic feature. The extinction curve appears uniform across the surveyed region, supporting the consistency of the results along the line of sight.

Looking Ahead

This study provides some of the clearest evidence yet for a spiral arm beyond the Milky Way’s bar, visible through careful analysis of red clump stars. By combining detailed kinematic data with extinction mapping and simulation comparisons, the authors open a new window into the Galaxy’s hidden architecture. Their results not only refine our understanding of the inner Milky Way but also demonstrate how subtle stellar signatures can reveal the vast, unseen structures that shape our cosmic neighborhood.

Source: Joharle