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How to Blow a Bubble in a Galaxy

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Arp 220
Hubble Heritage image of Arp 220. A bubble has recently been discovered in the center of this galaxy. [NASA/ESA/Hubble Heritage Team]

When two galaxies merge, the event often produces enormous galactic outflows. Though we’ve been able to study these on large scales, resolution limits in the past have prevented us from examining the launch sites, propagation, and escape of these outflows.

But recent high-resolution observations of Arp 220, a galaxy merger located a mere 250 million light years away from us, have finally provided a closer look at what’s happening in the center of this merger — and spotted something interesting.

A Curious Find

Arp 220 is an object clearly in the late stage of a galaxy merger; it has tidal tails, two distinct nuclei at its center (heavily-obscured by dust), lots of star formation, and a large-scale outflow that extends far from the galaxy.

While using Hubble observations to construct the first high-spatial-resolution optical emission line maps of Arp 220, a team led by Kelly Lockhart (Institute for Astronomy, Hawaii) discovered something unusual: evidence of a bubble-like structure, visible in the Hα+[N ii] emission. The bubble is slightly offset from the two nuclei at the galactic center, and measures ~600 pc across.

Origin Explanations

Large-scale (top) and zoomed-in (bottom) three-color Hubble observations of Arp 220: blue is optical, red is near-infrared, and green is Hα+[N ii] line emission. The bubble and the western nucleus (nuclei are marked by white circles) lie along the axis of the large-scale outflows (white vector). [Lockhart et al. 2015]

Large-scale (top) and zoomed-in (bottom) three-color Hubble observations of Arp 220: blue is optical, red is near-infrared, and green is Hα+[N ii] line emission. The bubble and the western nucleus (nuclei are marked by white circles) lie along the axis of the large-scale outflows (white vector). [Lockhart et al. 2015]

The authors propose several explanations for how the bubble was created, and examine the implications to determine which is the most likely. The explanations fall into two categories:

  1. The bubble is centered around its source.
    It could be produced by an outflow from an accreting black hole or a massive star cluster located at the bubble center.
  2. The bubble’s source is located near the two nuclei in the galactic center, but outside the bubble.
    It could be produced by a jet originating from one of the two galactic nuclei, or by a collimated outflow from a startburst concentrated near the nuclei. Either of these outflows could blow a bubble as it first interacts with the interstellar medium.

The authors show that the first category is disfavored based on observational and energetics arguments. In addition, the western-most nucleus and the bubble both align exactly with the axis of the large-scale outflows of the galaxy. Unlikely to be due to chance, this alignment is strong support in favor of the second category.

Thus, it’s probable that the bubble is blown by an outflow that originates from the inner ~100pc around one of the nuclei, either due to a jet or a starburst wind. Further observations should be able to differentiate between these two mechanisms.

Citation

Kelly E. Lockhart et al 2015 ApJ 810 149. doi:10.1088/0004-637X/810/2/149