Updated Analysis of a "Dark" Galaxy and its Blue Companion in the Virgo Cloud HI 1225+01

TL;DR

This study provides a comprehensive multi-wavelength analysis of the intergalactic gas cloud HI 1225+01 in the Virgo cluster, revealing a young stellar population in one clump and the absence of optical emission in the other, challenging the dark galaxy hypothesis.

Contribution

First deep optical imaging of HI 1225+01 covering the entire structure, combined with archival UV to IR data, offering new insights into its components and nature.

Findings

R1 hosts a young stellar population of 10-100 Myr age.

No optical counterpart detected in the second clump down to very faint surface brightness.

The properties of R1 resemble low-metallicity blue compact dwarf galaxies.

Abstract

HI 1225+01 is an intergalactic gas cloud located on the outskirts of Virgo cluster. Its main components are two large clumps of comparable HI masses (M_HI ~ 10^9 Msun) separated by about 100 kpc. One of the clumps hosts a blue low-surface-brightness galaxy J1227+0136, while the other has no identified stellar emission and is sometimes referred to as a promising candidate of a "dark galaxy", an optically invisible massive intergalactic system. We present a deep optical image covering the whole HI 1225+01 structure for the first time, as well as a collection of archival data from ultraviolet to far-infrared (IR) spectral region of the brightest knot "R1" in J1227+0136. We find that R1 has a young stellar population of age 10-100 Myr and mass ~ 10^6 Msun, near-IR excess brightness which may point to the presence of hot dust with color temperature ~ 600 K, and relatively faint mid- to far-IR fluxes corresponding to the dust mass of up to ~ 100 Msun. Overall, it seems to share the general properties with low-metallicity blue compact dwarf galaxies. On the other hand, no optical counterpart to the other clump is found in our deepest-ever image. Now the limiting surface brightness reaches down to R_AB > 28 mag/arcsec2 for any emission extended over 10" (comparable to R1), which is more than one hundred times fainter than the brightest part of the companion galaxy J1227+0136.