Curious Case of a Stripped Elliptical Galaxy

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MUSE fields of view

MUSE fields of view (1′ × 1′ for each square) are superimposed on a pseudo-color image of the elliptical galaxy in Abell 2670. The blue blobs lie in the opposite direction to the galactic center. [Sheen et al. 2017]

An elliptical galaxy in the cluster Abell 2670 has been discovered with some unexpected features. What conditions led to this galaxy’s unusual morphology?

Unexpected Jellyfish

We often see galaxies that have been disrupted or reshaped due to their motion within a cluster — but these are usually late-type galaxies like our own. Such gas-rich galaxies are distorted by ram pressure as they fall into the cluster center, growing long tails of stripped gas and young stars that earn them the name “jellyfish galaxies”.

But early-type, elliptical galaxies have long since used up or cleared out most of their gas, and they correspondingly form very few new stars. It’s therefore unsurprising that they’ve never before been spotted to have jellyfish-like features.

blobs

Panels a and b show zoomed-in observations of some of the star-forming blobs with tadpole-like morphology. Panel c shows a schematic illustration of how ram-pressure stripping causes this shape. [Adapted from Sheen et al. 2017]

New deep observations of an elliptical galaxy in the cluster Abell 2670, however, have revealed some unexpected structures for an early-type galaxy. Led by Yun-Kyeong Sheen (Korea Astronomy and Space Science Institute), a team of scientists now reports on the optical and spectroscopic observations of this galaxy, made with the MUSE instrument on the Very Large Telescope in Chile.

Tadpole Blobs

These observations reveal a number of features, including starbursts at the galactic center, 80-parsec-long tails of ionized gas, disturbed halo features, and several blue star-forming blobs with tadpole-like morphology in the surrounding region. The blobs have stellar tails that point in the direction of motion of the galaxy (toward the cluster center) and streams of ionized gas that point in the opposite direction.

All of these features are signs that this galaxy is being ram-pressure stripped as it falls into the center of the cluster. The star-forming blobs, for example, are exhibiting classic ram-pressure-stripping behavior: as a galaxy falls into the cluster center, streams of ionized gas blow downwind, and stars (which don’t respond as easily to the force of the wind) are left behind in a stream pointing upwind.

Gas from a Merger?

late-type galaxy

An example of a tidal tail drawn out from a disrupted late-type galaxy. The disrupted galaxy in Abell 2670 is, in contrast, an early-type, elliptical galaxy that should be gas-poor. [H. Ford, JHU/M. Clampin, STScI/G. Hartig, STScI/G. Illingworth, UCO, Lick/ACS Science Team/ESA/NASA]

But if this is an elliptical galaxy, where did the gas come from for the tails and the galactic-center star formation? To rule out the obvious, the authors first check that this galaxy really is an early-type elliptical. The galaxy’s color (reddened), morphology (elliptical and no sign of a stellar disk), and stellar velocities (no sign of stellar rotation) all confirm this.

The authors therefore speculate that the galaxy recently underwent a “wet merger” — a merger with a companion galaxy that was gas-rich. Much of this gas was driven to the center of the elliptical galaxy in the merger, and it’s now responsible for the starbursts there.

We’ll hopefully be able to draw stronger conclusions about this unusual galaxy after additional investigation into the amount of gas it contains and the galaxy’s star formation rate. In the meantime, this stripped elliptical makes for an intriguing puzzle!

Citation

Yun-Kyeong Sheen et al 2017 ApJL 840 L7. doi:10.3847/2041-8213/aa6d79

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