Revealing the nature of the starburst galaxies in the $z=2.4$ overdensity HATLAS J0849

Abstract

Today's most massive ellipticals are proposed to originate from starbursting galaxies in $z\gtrsim2$ overdensities. To discern what triggers these starbursts, and their $z=0$ descendants, we performed a detailed case study of five gas-rich galaxies in the $z=2.41$ overdensity, HATLAS J084933.4+021443. Using 0.15" resolution CO(4-3), [C I] 1-0, and dust-continuum observations, we characterised their cold gas morphology and kinematics. We find two rotating discs, W and C, both exhibiting non-axisymmetric radial gas motions (consistent with bars). Of the two extreme starbursts, W is a lopsided, rotation-dominated disc with a rotation velocity of $\sim520$ km s$^{-1}$, whereas T is most likely a late-stage merger. Combined with recent studies, we find that $\gtrsim42\%$ of gas-rich, massive starbursts in overdensities are rotation-dominated discs, a fraction not yet systematically reproduced by galaxy evolution models. Beyond $z=1$, disc galaxies with rotation velocities of $>400$ km s$^{-1}$ reside almost exclusively in overdensities, consistent with early mass assembly in dense environments. By comparing to local early-type galaxies with cold gas discs, we confirm that these systems already reside in halos comparable to the most massive $z\sim0$ ellipticals at the centres of groups and clusters. Despite their extreme star-formation rates, these discs lie on the same $σ-$SFR locus as lower-SFR field galaxies, implying that stellar feedback remains the dominant turbulence driver. We postulate that this is because inflowing gas is effectively transported through ordered streaming, such that only a small fraction of kinetic energy feeds disc-wide turbulence.

Figures (12)

• Figure 1: $9\farcs5 \times 9\farcs5$ cutout of HATLAS J084933.4+021443 comparing the HST/110W stellar emission (background colour map) vs ALMA/Band 4 dust-continuum emission (white contours, in steps of $2^n\sigma$, where $n=2,3,4,5,...$). The five known member galaxies are labelled in white. The ALMA beam is shown in the bottom left of the image.
• Figure 2: Columns (from left to right): unmasked CO(4--3) moment-0, [C i] 1--0 moment-0, underlying dust-continuum emission maps, represented in terms of the signal-to-noise ratio (S/N). The moment-0 maps were made by imaging a single channel over 90% of the line emission.
• Figure 3: Each row: CO(4--3) moment-0, 1, 2 maps, major-axis PVD, and minor-axis PVD for the galaxy labeled at left. For HyLIRGs-W and T, we show the CO(4--3) moment maps generated from the emission lines cubes with 4$\times$ the native resolution ($\sim17$ km s$^{-1}$), whereas for the ULIRGs, we show the moment maps generated from the cubes with 8$\times$ the native resolution ($\sim35$ km s$^{-1}$). The colormap in the left panel shows the moment-0 maps generated from the masked cubes, whereas the contours show the 1-channel-image moment-0 maps also shown in Fig. \ref{['fig:lines_and_dust']}, in steps of 3$\sigma$ (grey) and $4n\sigma$ (black, where $n=1,2,3...$). For consistency, we show the PVDs extracted along the axis shown in the left panels (black line, with centre marked), from the 8$\times$native channel resolution cube, with contours at $3n\sigma$.
• Figure 4: PV diagrams along the potential bar axis of HyLIRG-W (right) and perpendicular to it (left). The left-hand PV exhibits the X-shape typical of a barred galaxy.
• Figure 5: Best-fit kinematic model for the CO(4--3) emission of HyLIRG-W, imaged at $\sim35$ km s$^{-1}$. a) Moment-0, 1, and 2 maps (left), best-fit model (centre), and residual (right). b) Position-velocity diagrams for the CO(4--3) emission of HyLIRG-W, extracted along the major (top) and minor (bottom) kinematic axes. The data is shown by the red-blue colour map and the mask is outlined in transparent grey. Contours for the data and best-fit model are shown in red and black respectively, in levels of $\pm3n\sigma$, with the rotation curve depicted by the overlaid black points. c) Best-fit (with error bars) and fixed parameters for the example model fit showing the 2nd iteration with the PA fixed to the mean of the initial fit. d) Comparison of the data and model for every second 35 km s$^{-1}$ channel. The top panels in each row depict the data (red-blue colour map), whereas the bottom panels depict the best-fit model. Contours are shown in steps of $3+2n\sigma$ (red lines, steps labelled at bottom), with the mask applied to the data outlined in the top panels (thick grey line).