ALMA detection of the dusty object silhouetted against the S0 galaxy NGC 3269 in the Antlia cluster^⋆

¹ Instituto de Astronomía y Ciencias Planetarias de Atacama, Universidad de Atacama, Copayapu 485, Copiapo, Chile
e-mail: lauri.haikala@uda.cl
² Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
³ Departamento de Astronomía, Universidad de Concepción, Concepción, Chile
⁴ Departamento de Ciencias Físicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernández Concha 700, Las Condes 7591538, Chile
⁵ Stockholm Observatory, AlbaNova University Centre, Stockholm University, 106 91 Stockholm, Sweden
⁶ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland

Received: 21 July 2020
Accepted: 30 October 2020

Abstract

Context. An intriguing silhouette of a small dust patch can be seen against the disk of the S0 galaxy NGC 3269 in the Antlia cluster in optical images. The images do not provide any clue as to whether the patch is a local Jupiter mass-scale cloudlet or a large extragalactic dust complex.

Aims. We aim to resolve the nature of this object: is it a small Galactic cloudlet or an extragalactic dust complex?

Methods. ALMA and APEX spectroscopy and Gemini GMOS long-slit spectroscopy were used to measure the velocity of the patch and the NGC 3269 disk radial velocity curve.

Results. A weak 16  ±  2.5 km s⁻¹ wide ¹²CO(2 − 1) T_MB 19  ±  2.5. mK line in a 2.^″2 by 2.^″12 beam associated with the object was detected with ALMA. The observed heliocentric velocity, V_r, hel = 3878  ±  5.0 km s⁻¹, immediately establishes the extragalactic nature of the object. The patch velocity is consistent with the velocity of the nucleus of NGC 3269, but not with the radial velocity of the NGC 3269 disk of the galaxy at its position. The ∼4″ angular size of the patch corresponds to a linear size of ∼1 kpc at the galaxy’s Hubble distance of 50.7 Mpc. The mass estimated from the ¹²CO(2 − 1) emission is ∼1.4 × 10⁶(d/50.7 Mpc)² M_⊙, while the attenuation derived from the optical spectrum implies a dust mass of ∼2.6 × 10⁴(d/50.7 Mpc)² M_⊙. The derived attenuation ratio A′_B/(A′_B − A′_R) of 1.6  ±  0.11 is substantially lower than the corresponding value for the mean Milky Way extinction curve for point sources (2.3).

Conclusions. We established the extragalactic nature of the patch, but its origin remains elusive. One possibility is that the dust patch is left over from the removal of interstellar matter in NGC 3269 through the interaction with its neighbour, NGC 3268.