Using classical Cepheids to study the far side of the Milky Way disk

II. The spiral structure in the first and fourth Galactic quadrants^⋆

¹ Pontificia Universidad Católica de Chile, Instituto de Astrofísica, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
e-mail: jhminniti@uc.cl
² Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
³ Departamento de Física, Facultad de Ciencias Exactas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁴ Vatican Observatory, 00120 Vatican City State, Italy
⁵ European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile
⁶ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
⁷ INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Naples, Italy
⁸ Departamento de Física, Universidade Estadual de Feira de Santana, Av. Transnordestina, s/n, 44036-900 Feira de Santana, BA, Brazil
⁹ Centro de Nanotecnología Aplicada, Facultad de Ciencias, Universidad Mayor, Santiago, Chile
¹⁰ Centro para el Desarrollo de la Nanociencia y la Nanotecnología (CEDENNA), Santiago, Chile

Received: 23 September 2020
Accepted: 9 July 2021

Abstract

In an effort to improve our understanding of the spiral arm structure of the Milky Way, we use classical Cepheids (CCs) to increase the number of young tracers on the far side of the Galactic disk with accurately determined distances. We used a sample of 30 CCs that were discovered using near-infrared photometry from the VISTA Variables in the Vía Láctea survey (VVV) and classified based on their radial velocities and metallicities. We combined them with another 20 CCs from the literature for which VVV photometry is available. We used the compiled sample of CCs with homogeneously computed distances based on VVV infrared photometry as a proof of concept to trace the spiral structure in the poorly explored far side of the disk. Although the use of CCs has some caveats, these variables are currently the only available young tracers on the far side of the disk for which a numerous sample with accurate distances can be obtained. Therefore, a larger sample could allow us to make a significant step forward in our understanding of the Milky Way disk as a whole. We present preliminary evidence that CCs favor the following: a spiral arm model with two main arms (Perseus and Scutum-Centaurus) that branch out into four arms at Galactocentric distances, R_GC ≳ 5−6 kpc; the extension of the Scutum-Centaurus arm behind the Galactic center; and a possible connection between the Perseus arm and the Norma tangency direction. The current sample of CCs on the far side of the Galaxy are in the mid-plane, which argues against the presence of a severely warped disk at small Galactocentric distances (R_GC ≲ 12 kpc) in the studied area. The discovery and characterization of CCs at near-infrared wavelengths appears to be a promising tool to complement studies based on other spiral arm tracers and extend them to the far side of our Galaxy.