The First RELHIC? Cloud-9 is a Starless Gas Cloud ^*

Abstract Five-hundred-meter Aperture Spherical Telescope observations have recently identified a compact H i cloud (hereafter Cloud-9) in the vicinity of the spiral galaxy M94. This identification has been confirmed independently by Very Large Array and Green Bank Telescope observations. Cloud-9 has the same recession velocity as M94, and is therefore at a similar distance (∼4.4 Mpc). It is compact ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> <mml:mo accent="false">′</mml:mo> <mml:mtext/> <mml:mspace width="0.1em"/> <mml:mtext/> <mml:mspace width="0.1em"/> </mml:math> radius, or ∼1.4 kpc), dynamically cold ( W 50 = 12 km s −1 ), nonrotating, and fairly massive, with an H i mass of ∼10 6 M ⊙ . Here we present deep Hubble Space Telescope/Advanced Camera for Surveys imaging designed to search for a luminous stellar counterpart. We visually rule out the presence of any dwarf galaxy with stellar mass exceeding 10 3.5 M ⊙ . A more robust color–magnitude diagram-based analysis conservatively rules out a 10 4 M ⊙ stellar counterpart with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>99</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>8.2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> % confidence. The nondetection of a luminous component reinforces the interpretation that this system is a reionization-limited H i cloud (RELHIC); i.e., a starless dark matter halo filled with hydrostatic gas in thermal equilibrium with the cosmic ultraviolet background. Our results make Cloud-9 the leading RELHIC candidate of any known compact H i cloud. This provides strong support for a cornerstone prediction of the Lambda cold dark matter model, namely the existence of gas-filled starless dark matter halos on subgalactic mass scales, and constrains the present-day threshold halo mass for galaxy formation.