Warm and cold molecular gas in the cluster center of MACS 1931-26 with JWST and ALMA

TL;DR

MACS1931's BCG in a cool-core cluster hosts a massive H$_2$ reservoir and an AGN-driven starburst, providing a unique laboratory for multiphase gas in a cluster core. By combining JWST/MIRI MRS maps of warm H$_2$ rotational lines with ALMA CO(3-2) imaging, the study reveals that warm and cold molecular gas are co-spatial over ~30 kpc, with similar redshifted CGM tails and line kinematics. The excitation analysis yields warm temperatures of about $T_{ex}\approx 516$–$535$ K and warm gas fractions around $1.4$–$1.9\%$, with a total warm mass near $2.3\times10^8\,M_\odot$ and a total molecular gas mass consistent with CO-based estimates under reasonable $\alpha_{CO}$ assumptions. X-ray heating cannot explain the H$_2$ emission, while dissipation of kinetic energy from the warm H$_2$ gas likely fuels the CO-emitting reservoir, highlighting a dynamic baryon cycle linking the BCG to its CGM. These findings advance our understanding of heating processes, gas phase balance, and the role of the CGM in fueling star formation and AGN activity in cluster cores, and motivate future Cycle 4 JWST observations to refine the excitation and origin scenarios.

Abstract

We perform one of the first spatially resolved studies of warm ($>$100 K) and cold (10-100 K) molecular gas in the circumgalactic medium (CGM), focusing on the brightest cluster galaxy (BCG) of a cool-core galaxy cluster, MACS1931-26 at z=0.35. This galaxy has a massive H$_2$ reservoir and a radio-loud active galactic nucleus (AGN) and is undergoing a starburst event. We present new JWST observations of this system, revealing warm H$_2$ gas that is co-spatial with the cold molecular gas traced by CO, extending over 30 kpc around the BCG in a tail-like structure reaching into the circumgalactic medium of this galaxy. Analysis of the mid-infrared pure H$_2$ rotational lines H$_2$S(1), H$_2$S(5), and H$_2$S(9) indicate warm gas temperatures of $515.6 \pm 0.8$ K and $535.2\pm 1.9$ K in the BCG and tail regions, respectively. We compare cold gas, traced by the CO(3-2) observed with ALMA, to the warm gas traced by JWST. The warm-to-cold molecular gas fraction is comparable in the BCG ($1.4\%\pm0.2\%$) and the CGM tail ($1.9\%\pm0.3\%$). Our analysis suggests that the dissipation of the kinetic energy of the H$_2$-emitting gas is sufficient to lead to the formation of the CO gas. This observation provides new insights into the molecular gas distribution and its potential role in the interplay between the central galaxy and its circumgalactic environment.

Figures (4)

• Figure 1: Continuum-subtracted spectra toward the three $\rm H_2$ S(9), S(5) and S(1) lines in the first three columns. Spectra are extracted from two apertures centered on the BCG (top row) and the CGM (bottom row), respectively. The bottom-right panel illustrates the spatial extraction regions of the BCG and CGM, overlaid on a stacked moment zero map of $\rm H_2\,S(1)$ and $\rm H_2\,S(5)$. The grey circle in the bottom left of the panel illustrates the size of the MIRI PSF FWHM of the lower spatial resolution map, $\rm H_2\,S(1)$, to which the $\rm H_2\,S(5)$ map was convolved to match. The extracted spectra are shown as colored step plots as a function of velocity relative to the systemic redshift. The Gaussian models (solid grey) are shown for all lines other than $\rm H_2\,S(9)$ in the CGM as there is no distinguishable line profile to fit. The 1$\sigma$ uncertainties on the flux are shown as black vertical bars (see \ref{['sec:ferr_est']} for more information). The dashed horizontal and dotted vertical grey lines represent the $3\sigma$ flux upper limits and systemic velocity of the BCG, respectively. The 3$\sigma$ flux upper limits are computed as three times the RMS noise (per velocity channel) over the velocity range [$-2500$,$+2500$]$~\mathrm{km\,s^{-1}}$, excluding the interval used for line flux integration (defined as the mean velocity of the Gaussian fit $\pm 3\times\mathrm{FWHM}$ of the fitted profile). Residuals of the fits in mJy are displayed beneath each spectral plot. The top-right panel presents a normalized comparison of all line models, with BCG extractions in blue and CGM extractions in red in order to illustrate the velocity offset of $\rm H_2$ emission line from the CGM tail.
• Figure 2: Moment maps of the molecular gas tracers in MACS1931 BCG, showing similar spatial distribution and kinematics for warm and cold gas. Left: Intensity maps of the $\rm H_2\,S(5)$, $\rm H_2\,S(1)$, and $\rm CO(3-2)$ emission lines from top to bottom, respectively. ALMA $\rm CO(3-2)$ contours are overplotted in cyan shown at $[0.5,1,2,3.6]\sigma$ with $\sigma=0.46 \, \rm Jy \, arcsec^{-2} \, km \, s^{-1}$. Middle: Velocity offset maps of the same lines. Right: Velocity dispersion maps of the same lines. The moment one and two maps only show pixels with SNR$>3$ in the moment zero map. The velocities are defined relative to the systemic redshift of the BCG. The grey circle in the bottom left of the $\rm H_2$ plots illustrates the size of the MIRI PSF FWHM, and the cyan ellipse is the synthesized beam of ALMA $\rm CO(3-2)$ line. The grey ellipse in the CO maps in the last row shows the ALMA synthesized beam. The black crosses indicate the centroid of the 1.4 GHz emission Giacintucci14. The black scale bar indicates 5 proper kpc at the systemic redshift of the BCG or equivalently 1 arcsecond on the sky. We spatially bin the moment one and two maps of $\rm H_2\,S(5)$ by $2\times2$ pixels in order to improve SNR.
• Figure 3: (a) The line ratio map of $\rm H_2\,S(5)$ to $\rm H_2\,S(1)$, calculated using the unbinned extinction-corrected intensity maps with extinction corrected. (b) The excitation temperature calculated under the LTE assumption using the $\rm H_2\,S(5)$ and $\rm H_2\,S(1)$ lines through Equation \ref{['eq:Tex']}. (c) Surface density of warm molecular gas mass. (d) Ratio of the warm-to-cold molecular gas mass. This map is calculated using the warm mass map shown in the left panel of Figure \ref{['fig:Tex']} and cold gas masses calculated based on ALMA CO(3-2) emission. As in Figure \ref{['fig:mom1-2']}, the black crosses are the VLA 1.4 GHz radio sources. The black scale bar indicates 5 kpc in space or equivalently 1 arcsecond on the sky. The dark blue circle shows the size of the MIRI PSF FWHM at the wavelength $\rm H_2\,S(1)$ and the cyan ellipse shows the ALMA Band 6 beam.
• Figure 4: The $\rm H_2$ excitation diagram for the BCG region of MACS1931. The crosses show the observed values. The error bars are very small compared to the marker size. The blue curve shows the continuous temperature fit using the Togi16 model. The magenta lines show single temperature fits to the $\rm H_2\,S(1)$-$\rm H_2\,S(5)$ transitions (solid) and the $\rm H_2\,S(5)$-$\rm H_2\,S(9)$ transitions (dashed) and the best-fitting temperatures are shown in the legend. The inset shows the distribution of the continuous temperature model parameters drawn from the MCMC sampling. The best model is shown with a star, and the contours show the [68, 95, 99] percentiles.