Blowin' in the wind: both `negative' and `positive' feedback in an obscured high-z Quasar

Accreting super-massive black holes at the center of galaxies (referred to as Active Galactic Nuclei or Quasars if particularly energetic) are thought to have a significant influence on galaxy evolution, despite their much smaller size with respect to that of their host galaxy. A causality link between AGN and galaxy evolution is suggested for example by the tight correlation between the black hole mass and the host galaxy mass. In particular AGN can drive outflows that are expected to sweep away most of the gas in their host galaxy, hence depriving the galaxy of the fuel for further star formation and black hole accretion. This 'negative feedback' is often invoked as the main mechanism responsible for the red colors of elliptical galaxies and for preventing galaxies to over-grow, thus explaining the dearth of very massive galaxies compared to what expected from hierarchical galaxy formation models.

In addition to this negative feedback, some models also require AGN to provide a 'positive' feedback inducing star formation, rather than suppressing it, through enhancing the gas turbulence in the interstellar medium. Such positive feedback could explain the observed correlation between nuclear star formation activity and AGN luminosity, the black-hole mass - galaxy mass scaling relation and the enhanced star formation rate at high redshift concurrent with a higher black hole accretion. However, finding observational evidence of the effect of such AGN negative and positive feedback is still one of the main challenges in extragalactic astronomy.

A team of astronomers led by Giovanni Cresci (INAF-Arcetri Observatory) and including Alessandro Marconi (University of Florence) and Filippo Mannucci (INAF-Arcetri Observatory) has recently published a detailed study of an X-ray bright but optically obscured QSO at redshift z~1.6, the epoch where the effects of AGN feedback are expected to be more relevant for galaxy evolution. Given its properties, the Quasar was expected to be still in the dust enshrouded phase typical of the first phases of AGN activity, and therefore to represent an ideal case to look for feedback at its maximum.

The Quasar (XID2028) was observed with the near-infrared integral field spectrograph SINFONI at the VLT, that is capable of obtaining simultaneously the spectra of small regions of 0.125"x0.125" on sky, allowing to study the physical properties of the different regions of the Quasar host galaxy. The spectrum of the Quasar (Fig.1) reveals a very asymmetric and blue-shifted shape of the [OIII] emission lines: this is evidence of the presence of a high velocity gas outflowing from the QSO.  The image obtained integrating the spectra over the blue wing of the [OIII]5007 line (lower right panel) shows that the blue high velocity wing extends up to 1.5", i.e. 13 projected kpc from the QSO position (lower left panel), and reaches velocities up to 1500 km/s towards the line of sight. The estimated mass outflow rate is ~1000 M(sun)/yr, more than three times the star formation rate in the galaxy, confirming that the outflow observed cannot be sustained by young stars in the galaxy, but by the presence of the Quasar.

Fig.1: Upper panels: J-band SINFONI spectrum of the source XID2028, integrated in a region of 1"x1" around the QSO. The observed spectrum is shown in black, the different broken power-law components in the fit for each line (Hβ large due to the Broad Line Region, Hβ, [OIII]4959,5007) are shown in blue, while their sum is shown in red. The residuals of the fit, i.e. the difference between the observed and the model spectrum, are shown below. The shaded regions show the location of sky lines, that were excluded from the fit. Lower panels: [OIII]5007 channel maps obtained integrating the continuum-subtracted SINFONI datacube on the line core (1.296<λ<1.300 μm, left, see red box in the upper panel) and on the blue wing (1.294<λ<1.296 μm, right, see blue box in the upper panel). The contours on the line core, marking the position of the central QSO, are shown in black in both panels. The fully resolved, blue high velocity wing due to the outflow extends up to 1.5" from the QSO position.

Moreover, it was possible to study the effects of such energetic outflow on the host galaxy, using both H+K band SINFONI observations sampling the Hα line and HST-ACS imaging in the rest frame U band. Both the narrow component of Hα emission line map and the rest-frame U-band ACS imaging, tracing the star formation in the host galaxy, show that the outflow position lies in the center of a cavity in the star forming regions of the host galaxy (Fig.2). This suggests a  scenario in which the powerful outflow is removing the gas from the host galaxy (`negative feedback'), but is also triggering star formation through an outflow-driven shock which drives turbulent compression of the gas clouds especially at the outflow edges (`positive feedback').

Fig.2: Narrow Hα emission map with superposed the ACS U-band contours (left) and the contours of the outflow from Fig.1 (middle). The outflow lies in the cavity between the nucleus (marked with a diamond) and the two star-forming regions (A and B). The right panel shows a schematic view of the proposed scenario in which the powerful outflow originating from the AGN removes the gas from the host galaxy producing a cavity without star formation, and at the same time triggering star formation at the edges via turbulent compression of the gas.

XID2028 represents the first direct detection of outflow-induced star formation in a radio-quiet AGN, as well as the first example of both types of feedback simultaneously at work in the same galaxy. The data presented demonstrate that both `positive' and `negative' AGN feedback are crucial ingredients to shape the evolution of galaxies, by regulating the star formation in the host and driving the black hole-galaxy coevolution. The results show that these mechanisms are in action not only in powerful radio galaxies with relativistic jets, but also in less extreme objects during an obscured QSO phase that is thought to be a common step in the evolutionary sequence of star forming galaxies.

The results of this work are presented in the article "Blowin' in the wind: both `negative' and `positive' feedback in an obscured high-z Quasar", Astrophysical Journal (in press; arXiv:1411.4208)

Edited by Anna Gallazzi and Giovanni Cresci, 1/12/2014