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The Cosmic Jellyfish in Norma seen by ALMA

30 septembre 2019

A galaxy may have all its gas swept out by ram-pressure when entering a galaxy cluster at high velocity. The result looks like a jelly-fish, where the head is the galaxy, and the tail, the stripped gas. In the Norma cluster of galaxies, one of the nearest jellyfish galaxies, ESO137-001, has been mapped in molecular gas with high resolution with the Atacama Large Millimeter/submillimeter Array (ALMA) and compared with the ionized gas mapped by MUSE on ESO’s Very Large Telescope (VLT) : a number of knotty streams of molecular gas has been detected extending 60kpc in length and 25kpc in width.

Most of the visible mass in a rich galaxy cluster consists in the very hot diffuse and ionized gaseous atmosphere emitting in X-rays. This gas is in hydrostatic equilibrium in the potential well of the cluster, and may be 10 times more massive than all the ensemble of galaxies. When a spiral galaxy enters in the cluster at high speed, this atmosphere creates like a strong wind in the moving galaxy, that can blow the internal gas of the galaxy away. This is the ram-pressure stripping phenomenon. The stripped gas forms a wake behind the galaxy motion, that creates beautiful, intricate systems such as that seen around the spiral galaxy ESO 137-001, which travels in the Norma galaxy cluster. The direction and position of the tail shed light on the way in which the galaxy is moving.

Fig. 1 : La méduse cosmique est représentée ici avec de superbes détails. La galaxie spirale qui compose la tête a été imagée par le télescope spatial Hubble (NASA/ESA). La queue, constituée de filaments d’hydrogène, est représentée ici par le gaz ionisé cartographié avec MUSE en rose-violet et par le gaz moléculaire (émission de CO) cartographié avec ALMA en rouge-orangé.

The image of the CO emission obtained with ALMA (see Figure 1) offers the first high-resolution map of the cold molecular gas lurking within a ram-pressure stripped system. ESO 137-001 is one of the nearest jellyfish galaxies, and is particularly interesting because its long, extended tails of gas contain features known as ‘fireballs’ : bursts of star formation. The precise mechanisms governing how stars form within jellyfish tails are mysterious, and this map thus provides a new window onto the conditions needed for new stars to form in such intense, changeable environments.

Reference

  • ALMA unveils widespread molecular gas clumps in the ram-pressure stripped tail of the Norma jellyfish galaxy, Jachym, P., Kenney, J.D.P., Sun, M., Combes, F., Cortese, L. et al. 2019, ApJ in press

Scientific contact

  • Francoise Combes
    Observatoire de Paris - LERMA, PSL Univ., Collège de France, Sorbonne Univ.
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