JWST's PEARLS: A z=6 quasar in a train-wreck galaxy merger system

TL;DR

JWST NIRSpec IFU spectroscopy of the $z=5.89$ quasar NDWFS J1425+3254 reveals a complex, train-wreck merger: two companion galaxies are actively merging with the quasar host, connected by a luminous gas bridge, while the quasar drives an exceptionally fast ionised outflow. The analysis combines integrated and spatially resolved spectroscopy with stellar-population modelling to constrain black hole mass ($M_{BH}\approx(1.4^{+3.1}_{-1.0})\times10^{9}$ M$_⊙$) and accretion rate ($L_{bol}/L_{Edd}\approx0.3$), the host and companion stellar populations, and the gas-phase properties of the system. A foreground lensing candidate at $z\approx1.135$ may magnify the quasar by $1<\mu<2$, adding to the system’s complexity. The findings support merger-driven growth of black holes in the early universe and demonstrate the power of JWST for dissecting high-z quasar environments, though higher spectral resolution is needed to refine kinematics and outflow energetics.

Abstract

We present JWST NIRSpec integral field spectroscopy observations of the z=5.89 quasar NDWFS J1425+3254 from 0.6-5.3 microns, covering the rest-frame ultraviolet and optical at a spectral resolution of R~100. The quasar has a black hole mass of $M_{\rm{BH}}=(1.4\substack{+3.1\\-1.0})\times10^9 M_\odot$ and an Eddington ratio of $L_{\rm{Bol}}/L_{\rm{Edd}}=0.3\substack{+0.6\\-0.2}$, as implied from the broad Balmer H$α$ and H$β$ lines. The quasar host has significant ongoing obscured star formation, as well as a quasar-driven outflow with velocity $6050\substack{+460\\-630}$ km/s and ionised outflow rate of $1650\substack{+130\\-1230}M_\odot$yr$^{-1}$. This is possibly one of the most extreme outflows in the early Universe. The data also reveal that two companion galaxies are merging with the quasar host. The north-eastern companion galaxy is relatively old and very massive, with a luminosity-weighted stellar age of $65\substack{+9\\-4}$ Myr, stellar mass of $(3.6\substack{+0.6\\-0.3})\times10^{11} M_\odot$, and star-formation rate (SFR) of ~15-30 $M_\odot$yr$^{-1}$. A bridge of gas connects this companion galaxy and the host, confirming their ongoing interaction. A second merger is occurring between the quasar host and a much younger companion galaxy to the south, with a stellar age of $6.7\pm1.8$ Myr, stellar mass of $(1.9\pm0.4)\times10^{10} M_\odot$, and SFR of ~40-65 $M_\odot$yr$^{-1}$. There is also another galaxy in the field, likely in the foreground at z=1.135, which could be gravitationally lensing the quasar with magnification $1<μ<2$, and, thus, <0.75 mag. Overall, the system is a 'train-wreck' merger of three galaxies, with star formation and extreme quasar activity that were likely triggered by these ongoing interactions.

Figures (12)

• Figure 1: Flux from the background-subtracted data cube integrated across 0.82--5.2$\mu$m (left) and the [O$\;$iii] $\lambda5007$ flux map after quasar subtraction (right). The fluxes in each spaxel are as indicated in the respective colour bars. Purple to white contours show the quasar-subtracted HST/WFC3 F125W surface brightness Marshall2019c. Dark purple, light purple, and white correspond to 24.75, 24.00, and 23.25 mag arcsec$^{-1}$, respectively. Four regions are marked by green elliptical apertures: region 1 is the north-eastern companion galaxy, region 2 is a bridge of connecting gas, the quasar host-galaxy is region 3, and region 4 is the south-eastern companion galaxy. The maps are aligned along the cardinal directions, with north upwards and east to the left.
• Figure 2: Integrated quasar spectrum (black curve) showing the full wavelength range covered by the NIRSpec prism. The spectrum is measured within an aperture of radius 035 centred on the quasar emission peak and has an aperture correction of 1.119$\times$ applied. The green region shows the $\pm5\sigma$ uncertainty level of the spectrum, showing that $S/N\gg5$ for $\lambda>0.65$$\mu$m. Key quasar and galaxy emission lines are marked with dotted vertical lines and regions of strong Fe$\;$ii emission, where the numerous transitions form a pseudo-continuum, are shaded.
• Figure 3: Integrated quasar spectrum in an aperture centred on the peak of the quasar emission: the region around H$\beta$--[O$\;$iii] with an aperture radius of 025 (left) and the region around H$\alpha$--[N$\;$ii] with an aperture radius of 035 (right). The solid black line in the upper panel shows the observations, and the red line shows the best model fit from QubeSpec. The individual components of the model fit are also shown as indicated in the legend: the power-law continuum emission, Gaussian narrow and outflow lines, Fe$\;$ii template Park2022, and double-Gaussian BLR model. The emission-line model components are offset by $0.4\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ Å$^{-1}$ in the left panel and $0.3\times10^{-18}$erg s$^{-1}$ cm$^{-2}$ Å$^{-1}$ in the right panel to aid in visualisation. The lower panels show the residual between the observed flux and the full model. No aperture correction is applied to this figure, but an aperture correction of 1.119 $\times$ for H$\alpha$ and 1.149 $\times$ for H$\beta$ is applied to all calculations from these spectral fits.
• Figure 4: Black hole mass and Eddington ratio estimates for NDWFS J1425+3254 from single-epoch mass-scaling relations (Equation \ref{['eq:MBH']}) using the 5100 Å continuum luminosity and H$\beta$ line FWHM (blue; second-left panel), the H$\beta$ line luminosity and FWHM (yellow; middle panel), and the H$\alpha$ line luminosity and FWHM (purple; right panel). Each distribution shows estimates using various calibrations to these relations: Greene2005, Vestergaard2006, DallaBonta2020, DallaBonta2024 and Cho2023. The green curves show the combined probability distributions across each of the calibrations from each equation, and from all equations combined (left-most panel), giving each calibration and equation equal weighting. The horizontal black lines mark the minimum, median, and maximum for each distribution. The probability distribution reflects the posterior distribution from the MCMC fit, i.e. only the uncertainty in the fitting parameters is included and not the ${\sim}0.4$ dex scatter from the scaling-relation conversion to a black hole mass. The top panels show the black hole mass estimates, and the bottom panels show the resulting Eddington ratios.
• Figure 5: Maps of the [O$\;$iii] $\lambda5007$ emission line regions surrounding NDWFS J1425+3254. From top to bottom, the panels show the line flux, velocity, and line width. The left column shows the data cubes containing both the quasar and extended emission, while the right column shows the cubes after the quasar has been subtracted. The location of the quasar is marked as a black cross, and the emission line region apertures from Figure \ref{['fig:HST']} are included for reference. The line velocities and widths are measured from Gaussian fits to the lines in each spaxel; the velocity is measured as the line centre relative to the quasar host redshift of $z=5.901$, and the width is the FWHM corrected for the instrumental resolution of 1966 km s$^{-1}$.