Authors: M. Perna, S. Arribas, M. Marshall, et al.
First Author’s Institution: Centro de Astrobiología (CAB), CSIC-INTA, Madrid, Spain
Status: Submitted to A&A [open access]
Although theories suggest that one galaxy can host multiple supermassive black holes (SMBHs), actually discovering and modeling these cosmic giants remain difficult tasks. The authors of today’s bite used the data from James Webb Space Telescope (JWST) to study LBQS 0302-0019, one of the very few such systems that have been confirmed observationally to date.
About the Dual AGNs
At the heart of massive galaxies, there are SMBHs with masses ranging from a million to a billion times that of our Sun. According to the currently favored model, galaxies regularly merge as they evolve, meaning they form hierarchically from smaller galaxy building blocks (see this exciting bite about the simulations of these mergers!). The gravitational perturbations caused by the merging of two galaxies can lead to the flow of matter towards their center, triggering the accretion activity of SMBHs and ultimately turn them into active galactic nuclei (AGN). Consequently, dual AGNs, typically separated by a few tens of kiloparsecs, are expected to arise as the result of galaxy mergers.
Dual AGNs have significant astrophysical implications for our understanding of the formation and growth of SMBHs, and can be used to study the merger-induced accretion process on SMBHs. Dual AGNs at high-redshifts (z > 2; e.g. at the cosmological redshift 3, we are “looking” at objects from when the universe is ~2.2 Gyr old!) are of particular interest, as they can help to test and constrain predictions of the merger process from cosmological models in the early universe. Although dual AGNs are the expected outcome of galaxy mergers, detecting them can be challenging due to their close proximity to each other. The sensitivity and spatial resolution of most current observational instruments are limited, making the task even more difficult. Only a handful of these objects have been observed so far, and most of them were discovered serendipitously.
The JWST/NIRSpec Integral Field Unit Spectroscopy Data
Today’s authors studied LBQS 0302-0019, an extremely luminous AGN or quasi-stellar object (QSO; or quasar), using NIRSpec onboard the JWST, in integral field unit spectroscopy (IFS) mode. IFS instruments such as NIRSpec allow imaging and spectroscopy observations to be done at the same time, generating a final data product (called a datacube) that contains spectral information at each spatial pixel (spaxel). Alternatively, a datacube can be viewed as a stack of monochromatic images of continuous wavelength or velocity slices, called channels. Figure 1 below shows a monochromatic image of a wavelength channel, highlighting the brightness distribution of the object across the field of view (left), and single-spaxel spectra from three different spaxel locations (right).
The prominent gas emission lines in LBQS 0302-0019 (such as hydrogen Balmer lines, oxygen lines, nitrogen lines, and sulfur lines) have rest wavelengths in the optical regime, but due to its redshift of ~3.3, these lines are shifted towards the redder end of the spectrum. As a result, they are observable in the near-infrared band instead. To study these lines, the LBQS 0302-0019 data was acquired using a filter that covers the near-infrared wavelength range of 1.7 – 3.1 micron.
Beware of the Wiggles
When examining individual spectra from spaxels near bright point sources, the authors noticed apparent oscillations or wiggles (the blue spectrum in Figure 2). These wiggles are a result of spatial undersampling, which occurs when the pixel on the detector sensor does not capture enough photons, and are inherent to the datacube construction process. Although these wiggles would disappear when looking at the integrated spectrum (the sum of spectra of several spaxels), they become significant when analyzing the single-spaxel spectra. This is particularly important for untangling the fluxes from a point source that overlaps with extended emission, such as the QSO and its close environment in this case. Currently, there is no correction for this effect on the JWST/NIRSpec data pipeline.
To remove the wiggles present in the single-spaxel spectra, the authors developed a new technique that involves modeling them with sinusoidal functions and subtracting them from the spectra. Upon doing so, they found that the frequency of the wiggles changes smoothly as a function of the wavelength (see Figure 2). This correction has allowed for a more precise spatial analysis of the underlying extended emission surrounding LBQS 0302-0019, which in turn led the authors to discover five previously unknown companions.
The New Jils on the QSO Block
After correcting for the wiggles, the authors subtracted the bright emission from the QSO to reveal the fainter ionized gas emission hiding beside it. To derive the kinematic and physical properties of the ionized gas, the spectrum of each spaxel was modeled (or “fitted”) with multiple Gaussian distributions. This spaxel-by-spaxel spectrum modeling also enabled the construction of brightness distribution channel maps, similar to the one shown in the left panel of Figure 1. From these brightness distribution maps, the authors identified multiple clumps and irregular structures within the close surroundings of LBQS 0302-0019 (Figure 3).
LBQS 0302-0019 has been one of the go-to objects to study the intergalactic medium properties, especially due to its high redshift and high brightness. In 2018, a group led by an astronomer at Max-Planck-Institut für Astronomie (who is also one of the authors of today’s bite paper) serendipitously discovered a point-like source with high-ionization narrow emission lines within the immediate environment of LBQS 0302-0019. They named this source Jil, the Klingon word for neighbor. The group argued that the emission of Jil is best explained as an obscured AGN, which made LBQS 0302-0019 one of the few detected dual AGNs systems. Then in 2021, the group identified three additional Jils within the close environment of LBQS 0302-0019 by combining the archival and newly obtained observational data. Their analysis further confirmed that Jils are indeed obscured AGNs, and these four are associated with LBQS 0302-0019, based on the distance, mass and age calculation.
Through their Gaussian modeling to the spectra and analysis of the brightness distribution maps, the authors were able to identify Jils 1, 2, and 3, which had been previously reported in the literature, as well as five new Jils in the area (Jil 4, that was identified in previous literature, is located outside of the NIRSpec field of view). Their analysis further revealed that the emissions from these Jils were consistent with those from AGNs. With the addition of these five new Jils, the total number of Jils in the vicinity of LBQS 0302-0019 “Block” is now nine, all within a distance of only about 20 kpc! These new discoveries highlight the exceptional capabilities of JWST/NIRspec IFS for studying the close-up environment of dual AGN in the early universe and open up new promising avenues for further study of dual AGNs!
Astrobite edited by Ben Cassese
Featured image credit: Adapted from New Kids On The Block Official Instagram Account and ESA/Webb, NASA & CSA, D. Wylezalek, A. Vayner & the Q3D Team