Intergalactic Medium Tomography with the Sunburst Arc

TL;DR

The paper addresses the challenge of constraining small-scale gas structures in the CGM and IGM at $z\approx 2$ by performing 2D tomography with the Sunburst Arc as a multiply-imaged LyC background source. It applies BPASS-based stellar population fits to the non-ionizing continuum and adds foreground absorbers to model the LyC sightlines, identifying three recurring pLLS/LLS absorbers across multiple knots. The de-lensed absorber lengths are found to be $\lesssim 2$ kpc with HI masses around $10^3\,M_\odot$, and the two C IV absorbers likely reside in the CGM while the Lyα absorber traces the IGM, highlighting multiphase, collisionally-ionized gas. This study demonstrates the first 2D tomography of pLLSs/LLSs at $z\approx 2$, offering empirical constraints on absorber coherence lengths that will inform simulations and motivate sub-kpc gas investigations with future lensing surveys and JWST capabilities.

Abstract

Gravitational lensing has transformed the field of gas tomography in the intergalactic medium (IGM) and circumgalactic medium (CGM). Here we use the brightest lensed galaxy identified to date, the Sunburst Arc ($z$$\approx$2.37), to constrain the physical size of foreground absorbers at $z$$\approx$2 in 2D. This galaxy is a confirmed Lyman continuum leaker, where its single leaking region is imaged 12 times over four separate arcs. The separations between the arcs allows for large scale tomography, while the distances between the images along an arc allow for small scale tomography. Using HST/WFC3 UVIS G280 grism observations, we extracted the spectra of the leaking region and fit for absorbers detected along these lines of sight using a binary population and spectral synthesis (BPASS) model for the galaxy. We identified two partial Lyman limit systems (pLLSs) and one Lyman limit system (LLS) across the different spectra and measured their physical sizes. We find consistent HI column densities across $\lesssim$2 kpc and an average HI mass of $\approx$10$^3$ ${\rm M}_\odot$ for the absorbers. Given the strong CIV lines associated with two of the absorbers, they are likely located within the CGM of foreground galaxies. The third absorber has no associated metal lines, so it is most likely within the IGM. This study provides the first tomography measurements of pLLSs/LLSs in the CGM and IGM at $z$$\approx$2.

Figures (15)

• Figure 1: HST/ACS F814W image of the Sunburst Arc (left, rivera-thorsen2019) and HST/WFC3 UVIS G280 Beam A 2D spectra of each arc (labelled in red) with the 12 knot traces labelled in blue (right). Knot 7 is not detectable due to several bright foreground stars. Each knot is an image of the same leaking region in the galaxy.
• Figure 2: Convolved high-resolution BPASS model including the effect of dust attenuation used for absorber fits. We have interpolated the model onto the UVIS wavelength grid, convolved it with a Gaussian 1D filter with a FWHM of 10Å, and corrected for Milky Way dust extinction. No emission lines are included in the model because it is only made from the star light.
• Figure 3: Magellan/MagE combined spectrum of the leaking region in the Sunburst Arc (knots 1, 4, 5, 8, 9, 10; Rigby et al. in prep). Strong galaxy interstellar medium (ISM) lines are shown in gray. Intervening foreground absorbers are marked by their redshift. All of the lines are either confirmed Ly$\,\alpha$ associated with a C$\,$ IV absorber or LLS (blue and green dotted lines) or a potential Ly$\,\alpha$ absorber (red dotted lines). We checked for these seven absorbers in each of the individual knot spectra; those in bold occurred repeatedly.
• Figure 4: HST/WFC3 UVIS G280 spectra of each knot for the different arcs at rest wavelengths. Ly$\,\alpha$, Ly$\,\beta$, Ly$\,\gamma$, and the Lyman break are prominent features in these spectra. Regions that were masked due to contamination appear as breaks in the spectral flux. We do not plot the left-most end of the spectra due to an exponential increase in the sensitivity function; this increase in flux is not real. Many of the knots exhibit LyC flux, which can be used to probe foreground IGM gas.
• Figure 5: BPASS model fit to knot 4 spectrum. The galaxy Lyman break is denoted by the dashed and dotted line. The Lyman break of two intervening absorbers that appear in more than one knot spectrum are shown with the colored dotted lines. The Lyman break of a singly-occurring absorber is shown with the gray dotted line. We do not fit the model to the data, but rather only add to the model the absorbers that are identified in the MagE spectrum if they improve the fit.