Ionized Gas Kinematics at High Resolution. II: Discovery of a Double Infrared Cluster in II Zw 40

TL;DR

This study uses high-resolution ionized gas kinematics to discover a double infrared cluster in the dwarf galaxy II Zw 40, revealing complex internal structures and potential merger remnants.

Contribution

It provides the first high-resolution kinematic evidence for a double embedded cluster in II Zw 40, highlighting the galaxy's complex starburst region.

Findings

Detected two distinct ionized gas emission features with different velocities.

Estimated masses of the two clusters are approximately 3x10^5 and 1.5x10^5 solar masses.

Suggested the velocity shift may be due to a kinematic feature from a past galaxy merger.

Abstract

The nearby dwarf galaxy II Zw 40 hosts an intense starburst. At the center of the starburst is a bright compact radio and infrared source, thought to be a giant dense HII region containing ~14,000 O stars. Radio continuum images suggest that the compact source is actually a collection of several smaller emission regions. We accordingly use the kinematics of the ionized gas to probe the structure of the radio-infrared emission region. With TEXES on the NASA-IRTF we measured the 10.5um [SIV] emission line with effective spectral resolutions, including thermal broadening, of ~25 and ~3 km/s and spatial resolution ~1". The line profile shows two distinct, spatially coextensive, emission features. The stronger feature is at galactic velocity and has FWHM 47 km/s. The second feature is ~44km/s redward of the first and has FWHM 32 km/s. We argue that these are two giant embedded clusters, and estimate their masses to be ~3x10^5Mo and ~1.5x10^5 Mo. The velocity shift is unexpectedly large for such a small spatial offset. We suggest that it may arise in a previously undetected kinematic feature remaining from the violent merger that formed the galaxy.