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We study the dynamics of cold molecular gas in two main-sequence galaxies at cosmic noon (zC-488879 at z  ≃ 1.47 and zC-400569 at z  ≃ 2.24) using new high-resolution ALMA observations of multiple 12 CO transitions. For zC-400569 we also reanalyze high-quality H α data from the SINS/zC-SINF survey. We find that (1) both galaxies have regularly rotating CO disks and their rotation curves are flat out to ∼8 kpc contrary to previous results pointing to outer declines in the rotation speed V rot ; (2) the intrinsic velocity dispersions are low ( σ CO  ≲ 15 km s −1 for CO and σ Hα  ≲ 37 km s −1 for H α ) and imply V rot / σ CO  ≳ 17 − 22 yielding no significant pressure support; (3) mass models using HST images display a severe disk-halo degeneracy, that is models with inner baryon dominance and models with “cuspy” dark matter halos can fit the rotation curves equally well due to the uncertainties on stellar and gas masses; and (4) Milgromian dynamics (MOND) can successfully fit the rotation curves with the same acceleration scale a 0 measured at z  ≃ 0. The question of the amount and distribution of dark matter in high- z galaxies remains unsettled due to the limited spatial extent of the available kinematic data; we discuss the suitability of various emission lines to trace extended rotation curves at high z . Nevertheless, the properties of these two high- z galaxies (high V rot / σ V ratios, inner rotation curve shapes, bulge-to-total mass ratios) are remarkably similar to those of massive spirals at z  ≃ 0, suggesting weak dynamical evolution over more than 10 Gyr of the Universe’s lifetime. 

We model the kinematics of the high- and intermediate-velocity clouds (HVCs and IVCs) observed in absorption towards a sample of 55 Galactic halo stars with accurate distance measurements. We employ a simple model of a thick disc whose main free parameters are the gas azimuthal, radial, and vertical velocities (vϕ, vR, and vz), and apply it to the data by fully accounting for the distribution of the observed features in the distance–velocity space. We find that at least two separate components are required to reproduce the data. A scenario where the HVCs and the IVCs are treated as distinct populations provides only a partial description of the data, which suggests that a pure velocity-based separation may give a biased vision of the gas physics at the Milky Way’s disc–halo interface. Instead, the data are better described by a combination of an inflow component and an outflow component, both characterized by rotation with vϕ comparable to that of the disc and vz of $50\!-\!100\, {\rm km\, s}^{-1}$. Features associated with the inflow appear to be diffused across the sky, while those associated with the outflow are mostly confined within a bicone pointing towards (l = 220°, b = +40°) and (l = 40°, b = −40°). Our findings indicate that the lower ($|z| \lesssim 10\, {\rm kpc}$) Galactic halo is populated by a mixture of diffuse inflowing gas and collimated outflowing material, which are likely manifestations of a galaxy-wide gas cycle triggered by stellar feedback, that is, the galactic fountain.

 

Intermediate- and high-velocity clouds (IVCs, HVCs) are a potential source of fuel for star formation in the Milky Way (MW), but their origins and fates depend sensitively on their distances. We search for IVCs and HVCs in HST high-resolution ultraviolet spectra of 55 halo stars at vertical heights $|z|\gtrsim \,1$ kpc. We show that IVCs (40 ≤ |$v$LSR| < 90 ${\rm km\, s}^{-1}$) have a high detection rate – the covering factor, fc – that is about constant (fc = 0.90 ± 0.04) from $z$ = 1.5 to 14 kpc, implying IVCs are essentially confined to |$z$| ≲ 1.5 kpc. For the HVCs (90 ≤ |$v$LSR| ≲ 170 ${\rm km\, s}^{-1}$), we find fc increases from fc ≃ 0.14 ± 0.10 at |$z$| ≲ 2–3 kpc to fc = 0.60 ± 0.15 at 6 ≲ |$z$| ≲ 14 kpc, the latter being similar to that found towards QSOs. In contrast, the covering factor of very high-velocity clouds (VHVCs; |$v$LSR| ≳ 170 ${\rm km\, s}^{-1}$) is $f_c \lt 0.04$ in the stellar sample compared to 20 per cent towards QSOs, implying these clouds must be at d ≳ 10–15 kpc (|$z$| ≳ 10 kpc). Gas clouds with |$v$LSR| > 40 ${\rm km\, s}^{-1}$ at |b| ≳ 15° have therefore |$v$LSR| decreasing with decreasing |$z$|. Our findings are consistent with a Galactic rain and/or fountain origin for these clouds. In the latter scenario, VHVCs may mostly serve as fuel for the MW halo. In view of their high covering factors and since all the IVCs and some HVCs are found in the thick disc, they appear good candidates as gas reservoirs to help sustain star formation in the MW.

 

ABSTRACT We study the gas kinematics of a sample of six isolated gas-rich low surface brightness galaxies, of the class called ultra-diffuse galaxies (UDGs). These galaxies have recently been shown to be outliers from the baryonic Tully–Fisher relation (BTFR), as they rotate much slower than expected given their baryonic mass, and to have a baryon fraction similar to the cosmological mean. By means of a 3D kinematic modelling fitting technique, we show that the H i in our UDGs is distributed in ‘thin’ regularly rotating discs and we determine their rotation velocity and gas velocity dispersion. We revisit the BTFR adding galaxies from other studies. We find a previously unknown trend between the deviation from the BTFR and the exponential disc scale length valid for dwarf galaxies with circular speeds ≲ 45 km s−1, with our UDGs being at the extreme end. Based on our findings, we suggest that the high baryon fractions of our UDGs may originate due to the fact that they have experienced weak stellar feedback, likely due to their low star formation rate surface densities, and as a result they did not eject significant amounts of gas out of their discs. At the same time, we find indications that our UDGs may have higher-than-average stellar specific angular momentum, which can explain their large optical scale lengths. 

We present radial velocities for five member stars of the recently discovered young (age ≃ 100−150 Myr) stellar system Price-Whelan 1 (PW 1), which is located far away in the Galactic Halo (D≃ 29 kpc, Z≃ 15 kpc), and that is probably associated with the leading arm (LA) of the Magellanic Stream. We measure the systemic radial velocity of PW 1, Vr = 275 ± 10 km s−1, significantly larger than the velocity of the LA gas in the same direction. We re-discuss the main properties and the origin of this system in the light of these new observations, computing the orbit of the system and comparing its velocity with that of the H i in its surroundings. We show that the bulk of the gas at the velocity of the stars is more than 10 deg (5 kpc) away from PW 1 and the velocity difference between the gas and the stars becomes larger as gas closer to the stars is considered. We discuss the possibilities that (1) the parent gas cloud was dissolved by the interaction with the Galactic gas, and (2) that the parent cloud is the high-velocity cloud (HVC) 287.5+22.5 + 240, lagging behind the stellar system by ≃ 25 km s−1 and ≃10 deg ≃ 5 kpc. This HVC, which is part of the LA, has metallicity similar to PW 1, displays a strong magnetic field that should help to stabilize the cloud against ram pressure, and shows traces of molecular hydrogen. We also show that the system is constituted of three distinct pieces that do not differ only by position in the sky but also by stellar content.