Spontaneous and Stimulated Star Formation in Galaxies

TL;DR

This paper reviews recent observational studies on star formation thresholds in galaxy outer disks and investigates the existence of non-luminous galaxies predicted by cosmological models, finding no evidence for either thresholds or GNL galaxies.

Contribution

It provides new observational constraints on star formation thresholds and the presence of non-luminous galaxies, challenging some predictions of galaxy formation theories.

Findings

No evidence for gas-density thresholds for star formation in outer galaxy disks.

No observational evidence for gravitating non-luminous galaxies interacting with known systems.

Nearby isolated galaxies show no signs of interactions with unseen GNL galaxies.

Abstract

We present recent results from several on-going studies: The first addresses the question of gas-density thresholds for star formation, as probed by the outer disks of normal nearby galaxies. The second concerns the observational evidence for the existence of gravitating non-luminous (GNL) galaxies, as predicted by most recent simulations of galaxy formation in Lambda-CDM cosmologies. We find that (1) If star formation is traced by far-ultraviolet light, then there is no evidence for a threshold to star formation at any gas density so far probed, and (2) there is no evidence for GNL galaxies gravitationally interacting with known optical systems based on the observations (a) that there are no ring galaxies without plausible optically visible intruders, (b) all peculiar galaxies in the Arp Atlas that are bodily distorted have nearby plausibly interacting companions, and (c) there are no convincingly distorted/peculiar galaxies within Karachentsev's sample of more than 1,000 apparently/optically isolated galaxies.