Revealing the nature of central emission nebulae in the dwarf galaxy NGC 185^⋆

¹ Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
e-mail: mandjelic@matf.bg.ac.rs
² Special Astrophysical Observatory, Russian Academy of Sciences, Nizhny Arkhyz 369167, Russia
³ Lomonosov Moscow State University, Sternberg Astronomical Institute, Universitetsky pr. 13, Moscow 119234, Russia
⁴ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow 117997, Russia
⁵ Department of Physics, Earth Science and Space Systems Engineering, Morehead State University, 235 Martindale Drive, Morehead, KY 40351, USA
⁶ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shosse Blvd, 1784 Sofia, Bulgaria

Received: 1 May 2019
Accepted: 15 July 2019

Abstract

Aims. In this paper we present new optical observations of the galaxy NGC 185 intended to reveal the status of supernova remnants (SNRs) in this dwarf companion of the Andromeda galaxy. Previously, it was reported that this galaxy hosts one SNR.

Methods. Our deep photometric study with the 2m telescope at Rozhen National Astronomical Observatory using narrow-band Hα and [S II] filters revealed complex structure of the interstellar medium in the center of the galaxy. To confirm the classification and to study the kinematics of the detected nebulae, we carried out spectroscopic observations using the SCORPIO multi-mode spectrograph at the 6m telescope at the Special Astrophysical Observatory of the Russian Academy of Science, both in low- and high-resolution modes. We also searched the archival X-ray and radio data for counterparts of the candidate SNRs identified by our optical observations.

Results. Our observations imply the presence of one more SNR, one possible H II region previously cataloged as part of an SNR, and the presence of an additional source of shock ionization in one low-brightness planetary nebula. We detected enhanced [S II]/Hα and [N II]/Hα line ratios, as well as relatively high (up to 90 km s⁻¹) expansion velocities of the two observed nebulae, motivating their classification as SNRs (with diameters of 45 pc and 50 pc), confirmed by both photometric and spectral observations. The estimated electron density of emission nebulae is 30–200 cm⁻³. Archival XMM-Newton observations indicate the presence of an extended, low-brightness, soft source in projection of one of the optical SNRs, whereas the archival VLA radio image shows weak, unresolved emission in the center of NGC 185.