Once in a blue stream

Detection of recent star formation in the NGC 7241 stellar stream with MEGARA^⋆

¹ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía, 18080 Granada, Spain
e-mail: dmartinez@iaa.es
² Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 43 221 00 Lund, Sweden
³ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, 28040 Madrid, Spain
e-mail: sroca01@ucm.es
⁴ Instituto de Física de Partículas y del Cosmos (IPARCOS), Fac. CC. Físicas, Universidad Complutense de Madrid, Plaza de las Ciencias, 1, 28040 Madrid, Spain
⁵ Department of Physics, University of Surrey, Guildford GU2 7XH, UK
⁶ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz 369167, Russia
⁷ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁸ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria
⁹ Fundación G. Galilei – INAF (TNG), Rambla J. A. Fernández Pérez 7, 38712 Breña Baja, La Palma, Spain
¹⁰ Center for Cosmology and Particle Physics, Department of Physics, NYU, 726 Broadway, New York, NY 10003, USA
¹¹ UAI – Unione Astrofili Italiani /P.I. Sezione Nazionale di Ricerca Profondo Cielo, 72024 Oria, Italy
¹² Sternberg Astronomical Inst., M.V. Lomonosov Moscow State University, Universitetsky Prospect 13, Moscow 119234, Russia
¹³ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC, Plaza San Juan 1, 44001 Teruel, Spain
¹⁴ Department of Physics, Yazd University, University Blvd, Safayieh, Yazd, Iran
¹⁵ Perimeter Institute for Theoretical Physics, 31 Caroline St N, Waterloo, Canada
¹⁶ Steward Observatory, Department of Astronomy, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85748, USA
¹⁷ Instituto Nacional de Astrofísica, Óptica y Electrónica, Calle Luis Enrique Erro 1, Tonantzintla, Puebla, Mexico
¹⁸ FRACTAL S.L.N.E. Calle Tulipán 2, Portal 13, 1A, 28231 Las Rozas de Madrid, Spain

Received: 24 June 2022
Accepted: 13 November 2023

Abstract

Aims. In this work we study the striking case of a narrow blue stream with a possible globular cluster-like progenitor around the NGC 7241 galaxy and its foreground dwarf companion. We want to figure out if the stream was generated by tidal interaction with NGC 7241 or if it first interacted with the foreground dwarf companion and later both fell together toward NGC 7241.

Methods. We used four sets of observations, including a follow-up spectroscopic study of this stream based on data taken with the MEGARA instrument at the 10.4 m Gran Telescopio Canarias using the integral field spectroscopy mode, the Mount Lemmon 0.80 m telescope, the Telescopio Nazionale Galileo, the DESI Imaging Legacy surveys, and GALEX archival data. We also used high-resolution zoomed-in cosmological simulations.

Results. Our data suggest that the compact object we detected in the stream is a foreground Milky Way halo star. Near this compact object we detect emission lines overlapping a less compact, bluer, and fainter blob of the stream that is clearly visible in both ultraviolet and optical deep images. From its heliocentric systemic radial velocity derived from the [O III]λ5007 Å lines (V_syst = 1548.58 ± 1.80 km s⁻¹) and new UV and optical broadband photometry, we conclude that this overdensity could be the actual core of the stream, with an absolute magnitude of M_g ∼ −10 and a g − r = 0.08  ±  0.11, consistent with a remnant of a low-mass dwarf satellite undergoing a current episode of star formation. From the width of the stream and assuming a circular orbit, we calculate that the progenitor mass can be typical of a dwarf galaxy, but it could also be substantially lower if the stream is on a very radial orbit or if it was created by tidal interaction with the companion dwarf instead of with NGC 7241. These estimates also suggest that this is one of the lowest mass streams detected to date beyond the Local Group. Finally, we find that blue stellar streams containing star formation regions are commonly predicted by high-resolution cosmological simulations of galaxies lighter than the Milky Way. This scenario is consistent with the processes explaining the bursty star formation history of some dwarf satellites, which are followed by a gas depletion and a fast quenching once they enter within the virial radius of their host galaxies for the first time. Thus, it is likely that the stream’s progenitor is undergoing a star formation burst comparable to those that have shaped the star formation history of several Local Group dwarfs in the last few gigayears.