Star formation in the circumgalactic high-velocity cloud Complex H

Abstract

The accretion of metal-poor gas sustains galactic star formation. In the Milky Way, this process is fueled by high-velocity clouds (HVCs), yet their fundamental properties have remained elusive in the absence of stellar tracers. Here we report a binary open cluster within HVC Complex H. With an age of 11.2 +- 0.6 Myr and a subsolar metallicity of 0.05(+0.05-0.02) Zsun, the clusters provide a direct stellar distance anchor to the cloud at 13.8 +- 0.6 kpc. Their proper motions indicate Complex H is on a prograde, south-to-north orbit through the outer Galactic disk. The resulting interaction produces a 'slow-fast-slow' velocity gradient, with the cloud's outer layers decelerating as they merge into the disk. Orbit integration suggests the clusters formed from an internal cloud-cloud collision. This triggering mechanism implies other HVCs could similarly produce high-velocity stars. The scarcity of previous stellar detections in HVCs is explained by the rapid escape of young stars (< 20 Myr), while CO non-detections may stem from weak emission due to low metallicity and gas dispersal. This work reveals that the circumgalactic medium can sustain star formation, offering a tangible laboratory to probe the physical conditions of accreting gas before it merges with the Galactic disk.

Figures (10)

• Figure 1: Gaia detection of young binary cluster in cold cloud C1. The figure presents astrometric and photometric properties of the Emei star clusters and their massive members (E-S1 and E-S2). (a) Proper motion distribution of candidate member stars. The cluster exhibits tightly grouped kinematics with a mean proper motion ($\bar{\mu}_{\mathrm{\alpha^*}}$, $\bar{\mu}_\mathrm{\delta}$) = (-0.58 $\pm$ 0.01, 0.30 $\pm$ 0.02) mas yr$^{-1}$. (b) Magnitude versus parallax for the same stars, confirming a common distance with a mean parallax $\bar{\varpi}$ = 0.05 $\pm$ 0.02 mas. (c) Colour–magnitude diagram showing a clear blue main sequence. The distribution matches an isochrone-derived age of 11.2 Myr at a distance of 13.8 kpc (black dashed line), which corresponds to a Galactocentric radius of 20.0 kpc. Cluster members are provided in Extended Data Fig. 2. (d) Spatial density distribution of those blue stars, revealing two compact and distinct clusters Emei-1 and Emei-2. The scale bar corresponds to 5 pc; and the cyan cross marks the pixel position of the peak HI emission in C1, with the arm lengths representing the maximum sampling interval of the EBHIS survey.
• Figure 1: Neutral hydrogen emission from Complex H in EBHIS. (a) Integrated intensity map of HI emission from the VHVC component ($v_\mathrm{lsr}$ = -220 to -170 km s$^{-1}$; color scale). Contours show the lower-velocity component ($v_\mathrm{lsr}$ = -170 to -120 km s$^{-1}$), starting at 5 K km s$^{-1}$ with steps of 10 K km s$^{-1}$. The upward/downward arrow indicates the direction toward the north/south Galactic pole. The inset offers a detailed view of the cold compact core C1 in SCH, which is spatially and kinematically linked to the young clusters Emei-1 and Emei-2. (b) HI spectrum at the peak emission pixel of C1. The inset highlights the high-velocity emission ($v_\mathrm{lsr} < -120$ km s$^{-1}$) with a Gaussian fit, revealing both warm (velocity dispersion $\sigma_v$ = 15.2 km s$^{-1}$) and cold ($\sigma_v$ = 2.1 km s$^{-1}$) neutral gas components. These kinematic properties are characteristic of Complex H Lockman2003Kalberla2006.
• Figure 2: HI channel maps of the HVC complex surrounding the Emei clusters. Each panel shows the neutral hydrogen distribution within a different velocity channel, with the central $v_\mathrm{lsr}$ indicated at the bottom. The grey circles show the angular resolution of the EBHIS survey. Cluster positions are marked by triangle (Emei-1) and square (Emei-2). Contours start at 1 K km s$^{-1}$ and increase in steps of 2 K km s$^{-1}$. The contours in Channel 2 exhibit a clear cometary morphology. An arrow indicates the clusters' proper motion direction, which passes through the center of C1. The proper motion is closely aligned with the elongation of the head-tail structure. This distinctive geometry is also visible for core C2 and across Channels 2 to 4. Star symbols mark the present-day locations of potential progenitor cloud candidates at different $v_{\mathrm{lsr}}$ ranges that may have collided with C1 11.2 Myr ago. They are closely associated with a dense clump in SCH at ($l,b$$\sim$ 132.0$^{\circ}$, -5.6$^{\circ}$) within SCH.
• Figure 2: Isochrone fitting for each cluster. (a) Spatial distribution of member stars in Galactic coordinates for Emei-1 (blue) and Emei-2 (cyan), showing compact and concentrated stellar populations. The scale bar corresponds to 5 pc. The two clusters are spatially separated by approximately 20 pc. (b) Color-magnitude diagram displaying the main sequences of both clusters. Isochrone fits (dashed lines) correspond to an age of log(age/yr) = 7.05. Emei-2 exhibits higher extinction than Emei-1, consistent with its closer proximity to the dense cloud core C1. Grey dashed lines indicate possible binary sequences. Some stars deviate from the best-fit isochrones, likely due to significant differential extinction within the cloud core causing variations in local extinction.
• Figure 3: Kinematic structure of SCH. Color map indicates the velocity at the peak intensity in the high-velocity component, including only pixels with peak brightness temperature above 5$\sigma_\textrm{rms}$ (Methods). Black contours represent the integrated intensity of the warm gas ($v_\mathrm{lsr}$ = -170 to -120 km s$^{-1}$), starting from 15 K km s$^{-1}$ in steps of 10 K km s$^{-1}$. The upward/downward arrow indicates the direction toward the north/south Galactic pole. Prominent wings are visible: W1-W3 originate in the warm gas and exhibit velocities lower than the main body, while the northern cold component C3 also shows an elongated, lagging tail oriented similarly to the wings. In contrast, cold cores C1 and C2 display narrow tails aligned with the direction of motion, suggesting a different formation mechanism.