Ionised outflows in z $\sim$ 2.4 quasar host galaxies

TL;DR

This study investigates AGN-driven feedback in six luminous QSOs at $z\sim2.4$ by mapping the ionised gas kinematics via the [OIII]5007 line with near-IR IFU spectroscopy. Using spectroastrometry, the authors derive sub-PSF outflow radii $R_{o}$ and characteristic velocities $v_{o}$, then estimate mass outflow rates and energetics within a simple shell-like, conical outflow model; simulations validate the robustness of the spectroastrometric approach. They find ionised outflows extending to several kpc with blue-dominated profiles, with $\dot M_{o}$ in the range $6$–$700\,M_\odot$ yr$^{-1}$ and kinetic powers around $10^{-3}$ of the AGN luminosity, generally lower than molecular counterparts. The results imply that ionised gas traces only a fraction of the total outflowing mass and energy, likely due to multiphase gas and different acceleration mechanisms, highlighting the need for multi-wavelength follow-up (e.g., ALMA) to fully characterize AGN feedback.

Abstract

AGN-driven outflows are invoked by galaxy evolutionary models to quench star formation and to explain the origin of the relations observed locally between super massive black holes and their host galaxies. This work aims to detect the presence of extended ionised outflows in luminous quasars where we expect the maximum activity both in star formation and in black hole accretion. Currently, there are only a few studies based on spatially resolved observations of outflows at high redshift, $z>2$. We analyse a sample of six luminous (${\rm L>10^{47} \ erg/s}$) quasars at $z\sim2.4$, observed in H-band using the near-IR integral field spectrometer SINFONI at VLT. We perform a kinematic analysis of the [OIII] emission line at $λ= 5007Å$. [OIII] has a complex gas kinematic, with blue-shifted velocities of a few hundreds of km/s and line widths up to 1500 km/s. Using the spectroastrometric method we infer size of the ionised outflows of up to $\sim$2 kpc. The properties of the ionised outflows, mass outflow rate, momentum rate and kinetic power, are correlated with the AGN luminosity. The increase in outflow rate with increasing AGN luminosity is consistent with the idea that a luminous AGN pushes away the surrounding gas through fast outflows driven by radiation pressure, which depends on the emitted luminosity. We derive mass outflow rates of about 6-700 M$_{\odot}$/yr for our sample, which are lower than those observed in molecular outflows. Indeed physical properties of ionised outflows show dependences on AGN luminosity which are similar to those of molecular outflows but indicating that the mass of ionised gas is smaller than that of the molecular one. Alternatively, this discrepancy between ionised and molecular outflows could be explained with different acceleration mechanisms.

Figures (11)

• Figure 1: Upper panel: The spectra of the six QSOs if our sample. Each spectrum is extracted from a nuclear region of 0.25$^{\prime\prime}$$\times$025$^{\prime\prime}$ ($2\times2$ pixel). The different components in the fit for each line (${\rm H\beta}$,[O iii] and FeII) are shown in green and the red line is the total fit. The shaded yellow regions indicate the zone affected by strong sky line residuals which are excluded from the fit. Lower panel: fit residuals, obtained as a difference between observed and model spectra.
• Figure 2: Residuals from the pixel-per-pixel fitting assuming that the QSO emission is not spatially resolved. Left panels: residual spectra extracted from a region of 0.25$^{\prime\prime}$$\times$0.25$^{\prime\prime}$, where the residual map, obtained by collapsing the [O iii]$\lambda$5007 spectral channel, shows likely the presence of a spatially resolved emission. Dashed lines indicate the wavelength of the doublet [O iii]$\lambda$5007 and the dotted line shows the ${\rm H\beta}$ position. The red shaded region denotes the wavelength range over which the residual [O iii]$\lambda$5007 emission has been integrated to produce the maps shown on the right. Right panels: residual maps obtained by collapsing the spectral channels corresponding to the residual [O iii]$\lambda$5007 emission, as shown by the red region in the left panels. In the first five maps the presence of a clear [O iii]$\lambda$5007 residual emission suggests that the emitting region is spatially resolved. The "noise" residual map of HE0251 indicates that the sources is not resolved.
• Figure 3: [O iii]$\lambda$5007 flux, median velocity, $v_{10}$, and velocity dispersion map. The maps are obtained by selecting pixels with a SNR>2. The velocity maps are characterised by blue shifted regions with a large velocity dispersion. Contours represent the total ${\rm H\beta}$ line surface brightness at 90%, 50%, and 30% of the peak value
• Figure 4: Left panels. The points show the [O iii]$\lambda$5007 velocity $v$ versus the distance $R$ of the [O iii]$\lambda$5007 photocenter from the continuum (indicated by the dashed line). The arrows indicate the velocity $v_o = v(R_o)$ corresponding to the distance $R_0$. Right panels. The [O iii]$\lambda$5007 photocenter position in the field of view. Symbols are coloured according to their velocity (the velocity scale is reported in the colour bar). The dotted line indicates the project direction of the outflow as inferred by comparing the spectroastrometry results with velocity maps (Fig. \ref{['fig:velocity_maps']}).
• Figure 5: Cartoon showing the basic structure of our model. The outflow (solid blue curve) is perpendicular to the galaxy plane. $\vec{v}$ indicates the direction of the outflow and $\vec{r}$ is the distance from the QSO. This model assumes that the outflow is marginally resolved but it is not larger than the PSF dimension (dashed purple line).