The gradient of the brightest star age and density across the spiral arm S4 in the Andromeda galaxy

Abstract

On a plate obtained with the 2-m RC telescope at the Bulgarian National Observatory about 1400 stars in the spiral arm S4 of the Andromeda galaxy were measured. The limit of completeness is 20 ^m_. 2 (B magnitudes). In the central part of S4 (Figure 3) a pronounced gradient of star luminosity and density is found (Figures 6 and 7a). Here the stars become fainter at about 2^m and their surface density decreases tenfold at the distance ∼1 kpc from the inner edge of the arm. We have interpreted the decline of star maximum brightness from this edge as age gradient and have evaluated from it the velocity of star formation propagation across the arm, which is about 60 km s⁻¹. If the Andromeda galaxy has trailing spiral arms and the pitch angle of S4 is about 25° in its central part, the pattern velocity Ωp≈7–14 km s⁻¹ kpc⁻¹. This value is close to that obtained earlier with the help of the Cepheids in the same part of S4 (Efremov, 1980). The absence of a pronounced asymmetry in the star distribution across the arm in the OB82 region may be connected with the position of the strongest dust lanes in front of the stellar spiral arm here. We have stressed that in one part of the same spiral arm there may be a pronounced age gradient, and there may be no such gradient in the near-by one. In spite of the known difficulties in understanding the structure of the Andromeda galaxy it is possible to draw some conclusions which are important for the theory of spiral arms. The detailed investigations of the nearest galaxies are, therefore, most useful for understanding the spiral structure nature.