HALOGAS: Strong Constraints on the Neutral Gas Reservoir and Accretion Rate in Nearby Spiral Galaxies

TL;DR

This study uses sensitive HI observations of 22 nearby spiral galaxies to constrain the neutral gas accretion rates, finding them significantly lower than the galaxies' star formation rates, thus challenging the idea that HI accretion sustains ongoing star formation.

Contribution

It provides the first systematic, sensitive constraints on HI accretion rates in local galaxies, showing that neutral gas inflow is insufficient to explain star formation.

Findings

HI accretion rate is about 0.05 Msun/yr, below star formation rates

Detected HI clouds are fewer and less massive than around the Milky Way

Extended neutral gas reservoirs are generally absent around nearby spirals

Abstract

Galaxies in the local Universe are thought to require ongoing replenishment of their gas reservoir in order to maintain the observed star formation rates. Cosmological simulations predict that such accretion can occur in both a dynamically hot and cold mode. However, until now observational evidence of the accretion required to match the observed star formation histories is lacking. This paper attempts to determine whether galaxies in the local Universe possess a significant reservoir of HI and what would be the accretion rates derived from such reservoirs. We search the vicinity of 22 nearby galaxies for isolated HI clouds or distinct streams in a systematic and automated manner. The HALOGAS observations represent one of the most sensitive and detailed HI surveys to date. These observations typically reach column density sensitivities of 10^19 cm^-2 over a 20 km/s width. We find 14 secure HI cloud candidates without an observed optical counterpart. These cloud candidates appear to be analogues to the most massive clouds detected around the Milky Way and M31. However, on average their numbers seem significantly reduced. We constrain upper limits for HI accretion in the local Universe. The average HI mass currently observed amounts to a rate of 0.05 Msun/yr with a stringent upper limit of 0.22 Msun/yr, confirming previous estimates. This is much lower than the average star formation rate in this sample. Our best estimate, based on GBT detection limits of several galaxies, suggests that another 0.04 Msun/yr could be accreted from undetected clouds and streams. These results show that in nearby galaxies HI is not being accreted at the same rate as stars are currently being formed. Our study can not exclude that other forms of gas accretion are at work. However, these observations also do not reveal extended neutral gas reservoirs around most nearby spiral galaxies.