High Velocity Dispersion in A Rare Grand Design Spiral Galaxy at Redshift z=2.18

TL;DR

This paper reports the discovery of a rare grand-design spiral galaxy at redshift 2.18 with high velocity dispersion, suggesting that such structures can form in dynamically hot disks possibly due to minor mergers, challenging previous assumptions about their rarity.

Contribution

It provides the first spectroscopic confirmation of a grand-design spiral galaxy at z>2 and discusses its implications for galaxy formation theories.

Findings

The galaxy has a dynamically hot, thick disk with high velocity dispersion.

Spiral structure may be transient, driven by minor mergers in hot disks.

Such spiral galaxies are likely short-lived, with a duty cycle under 100 million years.

Abstract

Although relatively common in the local Universe, only one grand-design spiral galaxy has been spectroscopically confirmed to lie at z>2 (HDFX 28; z=2.011), and may prove to be a major merger that simply resembles a spiral in projection. The rarity of spirals has been explained as a result of disks being dynamically 'hot' at z>2 which may instead favor the formation of commonly-observed clumpy structures. Alternatively, current instrumentation may simply not be sensitive enough to detect spiral structures comparable to those in the modern Universe. At redshifts <2, the velocity dispersion of disks decreases, and spiral galaxies are more numerous by z~1. Here we report observations of the grand design spiral galaxy Q2343-BX442 at z=2.18. Spectroscopy of ionized gas shows that the disk is dynamically hot, implying an uncertain origin for the spiral structure. The kinematics of the galaxy are consistent with a thick disk undergoing a minor merger, which can drive the formation of short-lived spiral structure. A duty cycle of < 100 Myr for such tidally-induced spiral structure in a hot massive disk is consistent with their rarity.