A New Distance to The Antennae Galaxies (NGC 4038/39) Based on the Type Ia Supernova 2007sr

TL;DR

This paper presents a new distance measurement to the Antennae Galaxies using Type Ia supernova observations, which aligns well with flow model estimates and challenges previous shorter distance estimates based on TRGB photometry.

Contribution

The study introduces a supernova-based distance estimate that refines previous measurements and addresses issues with TRGB-based distances, providing a more reliable value for the Antennae Galaxies.

Findings

Supernova-based distance: 22.3±2.8 Mpc.

Flow model distance: 22.5±2.8 Mpc.

TRGB distance revised to 20.0±1.6 Mpc.

Abstract

Traditionally, the distance to NGC 4038/39 has been derived from the systemic recession velocity, yielding about 20 Mpc for H_0 = 72 km/s/Mpc. Recently, this widely adopted distance has been challenged based on photometry of the presumed tip of the red giant branch (TRGB), which seems to yield a shorter distance of 13.3+-1.0 Mpc and, with it, nearly 1 mag lower luminosities and smaller radii for objects in this prototypical merger. Here we present a new distance estimate based on observations of the Type Ia supernova (SN) 2007sr in the southern tail, made at Las Campanas Observatory as part of the Carnegie Supernova Project. The resulting distance of D(SN Ia) = 22.3+-2.8 Mpc [(m-M)_0 = 31.74+-0.27 mag] is in good agreement with a refined distance estimate based on the recession velocity and the large-scale flow model developed by Tonry and collaborators, D(flow) = 22.5+-2.8 Mpc. We point out three serious problems that a short distance of 13.3 Mpc would entail, and trace the claimed short distance to a likely misidentification of the TRGB. Reanalyzing Hubble Space Telescope (HST) data in the Archive with an improved method, we find a TRGB fainter by 0.9 mag and derive from it a preliminary new TRGB distance of D(TRGB) = 20.0+-1.6 Mpc. Finally, assessing our three distance estimates we recommend using a conservative, rounded value of D = 22+-3 Mpc as the best currently available distance to The Antennae.