ALMA discovers an outflow of molecular gas in a nearby spiral galaxy
16 octobre 2013
Every galaxy hosts in its nucleus a supermassive black hole, which becomes very luminous when it swallows some gas : it is called then an Active Galactic Nucleus or AGN. The ways AGN are fueled so that black holes can grow, then eject some gas until the star formation is quenched, are still not very well known. In a nearby barred spiral galaxy, NGC 1433, a team of astronomers has mapped the molecular gas in the very center, within a box of roughly 1000pc in size (1pc 3 light-yr), with a spatial resolution of 24pc. They have discovered a nuclear spiral structure in the molecular gas, which explains how the gas is driven towards the center to fuel the AGN. And also about 7% of the molecular gas shows a peculiar velocity, blue and re-shifted with respect to the rest of the disk, revealing an outflow from the center. The star formation is already quite low in this galaxy, and it is likely that the AGN itself is driving the outflow, through its radio jets.
See also the ESO press release.
Figure 1
Image dans le visible de la galaxie spirale barrée NGC 1433 (gauche), montrant la barre et vers le centre le champ de vue de ALMA, où le gaz moléculaire a été détecté (zoom à droite. L’échelle de couleur indique l’intensité de la molécule CO, proportionnelle à la densité du gaz
How the supermassive black holes in the nuclei of spiral galaxies get their gas ? It can be shown that the stellar bar, which is strong in this particular galaxy NGC1433, can drive the gas to the center if its distribution is non-symmetric, and fuel the nucleus. This is what is observed in this CO(3-2) line map of the galaxy obtained during early observations with ALMA (in Cycle 0, in 2012) : the gas reveals a spiral structure, which winds up in nearly a ring, which leaks towards the center.
In addition of revealing the fueling process, these observations have provided a surprise : about 4 millions solar masses of molecular gas are outflowing from the center, entrained by the AGN jets. The extent of the jet is only 150 light-years, it is the smallest such molecular outflows observed in an external galaxy.
Figure 2
Détection dans la raie de CO de deux composantes à haute vitesse, l’une venant vers nous (décalée vers le bleu), l’autre s’éloignant (décalée vers le rouge. Le schéma en encart indique leur position par rapport à la galaxie.
The discovery of the molecular outflow confirms the current ideas that the AGN activity is able to remove gas, even temporarily, from the center and stop the star formation there. The AGN then moderates and regulates the growth of the bulge in galaxy centers, which could explain the observed relation between mass of bulges and mass of black holes.
Figure 3
L’émission du gaz moléculaire (rouge/orange) est superposée à l’image visible obtenue avec le télescope spatial Hubble, en bleu (l’image a été tournée par 120 degrés par rapport à la Figure 1)
