Resolved Schmidt-Kennicutt relation in a binary hyperluminous infrared galaxy at $z=2.41$

TL;DR

This work harnesses ALMA to map CO J:7--6, [CI] 2--1, H2O, and rest-frame submillimeter continua in the four galaxies of the z$=2.41$ HyLIRG system HATLAS J084933.4+021443, achieving ~2.5 kpc resolution that reveals rotation-dominated gas in W and T and lensing-induced minor-axis extensions in T. By combining multi-line tracers with resolved dust SEDs, the authors derive a 2.5-kpc Schmidt-Kennicutt relation with a steep slope of $n\sim1.7$ and depletion times of ~50–500 Myr, depending on the galaxy and region. They further validate [CI] 2--1 as a tracer of warm/dense molecular gas in extreme systems, though its utility depends on excitation conditions. Overall, the paper provides critical, high-resolution constraints on star formation laws and ISM structure in the most intensely star-forming environments at cosmic noon, with implications for merger-driven starbursts and the choice of gas tracers at high redshift.

Abstract

Hyperluminous infrared galaxies (HyLIRGs; SFRs up to about 1000 Msun yr-1), though rare, provide key constraints on galaxy evolution. H-ATLAS J084933.4+021443, a z = 2.41 binary HyLIRG (galaxies W and T) with two additional luminous companions (C and M), offers an ideal laboratory for studying star formation during "cosmic noon". We use ALMA to obtain resolved imaging and kinematics of CO J:7-6, [C I] 2-1, H2O, and rest-frame 340-1160 GHz continuum emission in all four galaxies. Each system is spatially resolved within ~0.3 arcsec (2.5 kpc) apertures. Gas kinematics in W and T are rotation-dominated, with galaxy T showing emission extended along its kinematic minor axis due to lensing magnification. Spatially resolved SEDs indicate that W is well fitted by single-temperature greybody dust despite hosting a luminous AGN, while T requires an additional hot-dust component and extra millimetre emission. We confirm [C I] J:2-1 as a tracer of warm/dense molecular gas in these extreme systems, though its luminosity ratio with CO J:7-6 rises sub-linearly. We derive resolved (2.5 kpc-scale) Schmidt-Kennicutt (SK) relations for W and T using both cold and warm/dense gas, finding depletion times of about 50-100 Myr (W) and about 100-500 Myr (T). Both galaxies follow a steep SK relation with power-law index n ~ 1.7, significantly above the n ~ 1 observed in normal star-forming galaxies.

Figures (16)

• Figure 1: Left panel: Illustration of the method used to separate the CO J:7--6 and [C i] 2--1 lines in individual channels of the datacube (see text). The dashed red line shows our choice for the dividing frequency (based on the fitted gas rotation model) and the solid yellow line shows the systemic velocity of the galaxy (see Sect. \ref{['sec: Galaxy-integrated spectral properties']}). Middle and right panels: Observed (filled grey regions) and rotation-model-separated profiles (see text) of the CO J:7--6 (red) and [C i] 2--1 (blue) emission lines in galaxy W (middle panel) and galaxy T (right panel).
• Figure 2: Maps of the observed-frame 340-GHz (corresponding to rest-frame 260 $\mu$m) continuum emission in (left to right) galaxies W, T, C and M. Continuum fluxes are in mJy $\mathrm{beam^{-1}}$ following the colour bar above each panel. Each panel is 3$^{\prime\prime}$$\times$ 3$^{\prime\prime}$ in size and the axes, in arcsec, are centred on the kinematic centre of each galaxy, as obtained from Kinemetry. These kinematic nuclear positions, hereafter used as the galaxy positions, are: W: 08:49:33.685, +02:14:44.680; T: 08:49:32.947, +02:14:39.696; C: 08:49:33.908, +02:14:44.860; M: 08:49:33.795, +02:14:45.595. For galaxies W and T, the kinematic major and minor axes are shown with thick and thin grey lines, respectively.
• Figure 3: Galaxy W. From left to right: Maps of the integrated flux (moment 0; units of Jy km s$^{-1}$ beam$^{-1}$), intensity-weighted average velocity (moment 1; km s$^{-1}$ relative to systemic), velocity dispersion (moment 2; km s$^{-1}$), and the galaxy-integrated line profile, of the detected emission lines. From top to bottom the lines are: CO J:3--2, CO J:7--6, [C i] $^3P_2 \longrightarrow ^3P_1$ , and H$_2$O 2$_{11} - 2_{02}$. In the three left-most columns, the colours follow the respective colour bars, the synthesised beam is shown at the lower left, and axis units are arcseconds with the same central position used in all panels. For ease, the kinematic major and minor axes are shown in solid black lines and a single specific flux contour from the moment 0 image is shown in all panels of the same row. All moment maps were made from natural-weighted data cubes. The right-most column shows the corresponding galaxy-integrated line profile extracted within the square aperture shown in the corresponding left-most panel. The line profiles are shown both at the observed spectral resolution (grey histograms; spectral resolutions of -- top to bottom -- 46 km s$^{-1}$, 19.7 km s$^{-1}$, 19.7 km s$^{-1}$, and 21.2 km s$^{-1}$, per channel), and at a smoothed resolution (blue solid lines in the lower three panels) of $\sim$ 100 km s$^{-1}$. The line profiles of CO J:7--6 and [C i] $^3P_2 \longrightarrow ^3P_1$ have been de-blended as is explained in Sect. \ref{['sec: Galaxy-integrated spectral properties']}.
• Figure 4: Same as Fig. \ref{['fig : w-moments & profiles']} but for galaxy T. Here the relatively narrow line profiles, as compared to W, allow for a cleaner separation of the galaxy-wide CO J:7--6 and [C i] $^3P_2 \longrightarrow ^3P_1$ line profiles.
• Figure 5: Similar to Fig. \ref{['fig : w-moments & profiles']} but for galaxies M and C. The top row shows the results for the CO J:3--2 line in both M and C together. The middle and bottom rows show the results for the CO J:7--6 line separately for M and C, respectively. In the right-most panels of the middle and bottom rows (the galaxy-integrated profiles of the CO J:7--6 lines in M and C, respectively) the CO J:7--6 line covers a velocity range of approximately $\pm 300$ km s$^{-1}$ and 500 km s$^{-1}$, respectively; for both galaxies the [C i] $^3P_2 \longrightarrow ^3P_1$ line is also clearly visible at lower ($\sim$1000 km s$^{-1}$ to the blue) velocities. We do not show the equivalent moment maps for the [C i] $^3P_2 \longrightarrow ^3P_1$ line in M and C as they are significantly noisier.