Authors: Rebecca L. Larson, Steven L. Finkelstein, Dale D. Kocevski, Taylor A. Hutchison, Jonathan R. Trump, Pablo Arrabal Haro, Volker Bromm, Nikko J. Clerie and the CEERS team
First Author’s Institution: The University of Texas at Austin, Department of Astronomy, Austin, TX, United States
Status: submitted to ApJ [open access]
When we obtain the light from a galaxy far away onto our telescopes, one way in which we can study the properties of the galaxy is by splitting the light into its constituent wavelengths. This technique is known as spectroscopy. Stars, gas, and dust all form a part of the galaxy spectrum through unique fingerprints called spectral lines. Studying these lines can help us understand the nature of the galaxy as a whole. The James Webb Space Telescope (JWST) is pioneering the next generation of studying galaxies based on their spectrum, specifically in the infrared regime of the electromagnetic spectra. In this paper, a galaxy spectrum has been obtained using JWST observations for the CEERS project. It is taken with the NIRSpec spectrograph, which covers the near-infrared wavelengths from 1-5 microns. There are some strong emission lines in the spectra. What exotic astrophysical object is causing these emission lines? A good old-fashioned astronomy mystery is brewing!
The astronomical fingerprints!
To find the source of the emission lines, the authors of today’s paper first need to identify what elements are causing the lines. The emission lines of any element or molecule at rest always occur at the same wavelengths, which helps us identify the elements. However, this is not the case for elements in astronomical objects. Due to the expansion of the universe, they are not at rest, and the position of the lines gets shifted towards the redder end of the spectrum (i.e., longer wavelengths). Suppose one does not have any prior information regarding the age of the galaxy. In that case, they have little idea when a particular feature was emitted, how much the universe has expanded since, and how much the feature has shifted in the spectrum.
To determine the emission line features in this spectrum systematically and without any bias from previous measurements of the redshift calculated for this galaxy, the authors use a line fitting code that can fit a Gaussian profile to any emission line that looks significant and is not noisy. They then determine the properties of the fitted profile to identify the element that created the line. The most robust line with the best Signal to Noise Ratio (SNR) was identified as the [OIII] line. Finally, they performed customized fits to determine the rest of the lines based on different spectral shapes and the relative positions to the [OIII] line (Figure 1).
What do these lines tell us?
Once the emission lines have been identified, their positions on the spectrum compared to their position at rest can be used to calculate the cosmological redshift – or how far back in cosmological time we are observing this galaxy. It was determined to be around 8.7, which agreed with previous ground-based measurements of the galaxy. Based on its cosmological redshift, this galaxy exists in the very early universe, only 570 million years after the universe began. Some emission lines display exciting features. The H-beta emission line is broad, indicating the presence of hot and energetic sources. (Figure 2 on the left). Broad components are also detected in some other permitted and semi-forbidden lines such as Lyman-alpha, CIII], and NIV] but are conspicuously missing from the forbidden lines such as [OIII] (Figure 2 on the right).
The pieces fall into place
There is plenty of evidence to indicate that the lines are caused by a powerful source. A broad component implies gas that is moving at very high velocities. It could be due to high temperatures caused by the source. It could be an outflow; however, for an outflow, the broad component is usually found in most, if not all, of the emission lines. The central region shows a point source. It must be, it has to be, an accreting black hole, an AGN!
This is a surprising result; a massive object is found when the universe is young. There simply is no time for it to have gained so much mass. Further measurements show that the black hole mass is 107 Mⵙ. The black hole is experiencing rapid growth and is accreting at higher levels than the physically allowed limits. By fitting the Spectral energy distribution (SED), which uses templates that indicate how the continuum should look based on the source, the authors also find that stars are the strongest contributor to the continuum, as opposed to the black hole, indicating that the galaxy is also forming stars at a significant rate.
An accreting black hole at such high redshifts could help us understand how the first black holes in the early universe were formed. The two significant possibilities are that the first black holes could have been created by the collapse of some of the earliest formed stars, or they could have formed when the gas in the early universe collapsed (also called a Direct Collapse Black Hole, DCBH). The authors show that it is likely to be the latter, but one cannot rule out the possibility of a star collapsing followed by highly rapid accretion processes (Figure 3).
The earliest black hole?
In this work, the broad component was detected significantly only in one line that could arise from a possible broad line region (BLR) of the AGN. The authors expect to soon get more information from another JWST instrument, the Mid-Infrared Instrument (MIRI). This instrument will cover wavelengths in the mid-infrared range, from 5-28 microns in wavelength. It will contain another important line, H-alpha. Detecting a broad component in H-alpha will be conclusive evidence for the presence of an AGN. Stay tuned to find out if there is more to this exciting mystery or if the authors of today’s paper have genuinely found one of the earliest black holes in the universe!
Astrobite edited by Keighley Rockcliffe
Featured image credit: Larson et al. 2023