More Like this

1. Forming young and hypervelocity stars in the Galactic Centre via tidal disruption of a molecular cloud

   https://doi.org/10.1093/mnras/stac419  

   Generozov, A. ; Nayakshin, S. ; Madigan, A. M. ( March 2022 , Monthly Notices of the Royal Astronomical Society)

   The Milky Way Galaxy hosts a four million solar mass black hole, Sgr A*, that underwent a major accretion episode approximately 3–6 Myr ago. During the episode, hundreds of young massive stars formed in a disc orbiting Sgr A* in the central half parsec. The recent discovery of a hypervelocity star (HVS) S5-HVS1, ejected by Sgr A* five Myr ago with a velocity vector consistent with the disc, suggests that this event also produced binary star disruptions. The initial stellar disc has to be rather eccentric for this to occur. Such eccentric discs can form from the tidal disruptions of molecular clouds. Here, we perform simulations of such disruptions, focusing on gas clouds on rather radial initial orbits. As a result, stars formed in our simulations are on very eccentric orbits ($\bar{e}\sim 0.6$) with a lopsided configuration. For some clouds, counterrotating stars are formed. As in previous work, we find that such discs undergo a secular gravitational instability that leads to a moderate number of particles obtaining eccentricities of 0.99 or greater, sufficient for stellar binary disruption. We also reproduce the mean eccentricity of the young disc in the Galactic Centre, though not the observed surface density profile. We discussmore »missing physics and observational biases that may explain this discrepancy. We conclude that observed S-stars, HVSs, and disc stars tightly constrain the initial cloud parameters, indicating a cloud mass between a few × 104 and $10^5\, {\rm M}_{\odot }$, and a velocity between ∼40 and 80 km s−1 at 10 pc.

   « less
2. Hot-mode accretion and the physics of thin-disc galaxy formation

   https://doi.org/10.1093/mnras/stac1603  

   Hafen, Zachary ; Stern, Jonathan ; Bullock, James ; Gurvich, Alexander B. ; Yu, Sijie ; Faucher-Giguère, Claude-André ; Fielding, Drummond B. ; Anglés-Alcázar, Daniel ; Quataert, Eliot ; Wetzel, Andrew ; et al ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   We use FIRE simulations to study disc formation in z ∼ 0, Milky Way-mass galaxies, and conclude that a key ingredient for the formation of thin stellar discs is the ability for accreting gas to develop an aligned angular momentum distribution via internal cancellation prior to joining the galaxy. Among galaxies with a high fraction ($\gt 70{{\ \rm per\ cent}}$) of their young stars in a thin disc (h/R ∼ 0.1), we find that: (i) hot, virial-temperature gas dominates the inflowing gas mass on halo scales (≳20 kpc), with radiative losses offset by compression heating; (ii) this hot accretion proceeds until angular momentum support slows inward motion, at which point the gas cools to $\lesssim 10^4\, {\rm K}$; (iii) prior to cooling, the accreting gas develops an angular momentum distribution that is aligned with the galaxy disc, and while cooling transitions from a quasi-spherical spatial configuration to a more-flattened, disc-like configuration. We show that the existence of this ‘rotating cooling flow’ accretion mode is strongly correlated with the fraction of stars forming in a thin disc, using a sample of 17 z ∼ 0 galaxies spanning a halo mass range of 1010.5 M⊙ ≲ Mh ≲ 1012 M⊙ and stellarmore »mass range of 108 M⊙ ≲ M⋆ ≲ 1011 M⊙. Notably, galaxies with a thick disc or irregular morphology do not undergo significant angular momentum alignment of gas prior to accretion and show no correspondence between halo gas cooling and flattening. Our results suggest that rotating cooling flows (or, more generally, rotating subsonic flows) that become coherent and angular momentum-supported prior to accretion on to the galaxy are likely a necessary condition for the formation of thin, star-forming disc galaxies in a ΛCDM universe.

   « less
3. The lifecycle of molecular clouds in nearby star-forming disc galaxies

   https://doi.org/10.1093/mnras/stz3525  

   Chevance, Mélanie ; Kruijssen, J M ; Hygate, Alexander P ; Schruba, Andreas ; Longmore, Steven N ; Groves, Brent ; Henshaw, Jonathan D ; Herrera, Cinthya N ; Hughes, Annie ; Jeffreson, Sarah M ; et al ( April 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT It remains a major challenge to derive a theory of cloud-scale ($\lesssim100$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (∼100 pc) CO-to-H α flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically $10\!-\!30\,{\rm Myr}$, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $\Sigma _{\rm H_2}\ge 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $\Sigma _{\rm H_2}\le 8\,\rm {M_\odot}\,{{\rm pc}}^{-2}$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H α (75–90 per cent of the cloud lifetime), GMCs disperse within just $1\!-\!5\,{\rm Myr}$ oncemore »massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4–10 per cent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and H ii regions are the fundamental units undergoing these lifecycles, with mean separations of $100\!-\!300\,{{\rm pc}}$ in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles.« less
4. Swirls of FIRE: spatially resolved gas velocity dispersions and star formation rates in FIRE-2 disc environments

   https://doi.org/10.1093/mnras/staa1619  

   Orr, Matthew E ; Hayward, Christopher C ; Medling, Anne M ; Gurvich, Alexander B ; Hopkins, Philip F ; Murray, Norman ; Pineda, Jorge L ; Faucher-Giguère, Claude-André ; Kereš, Dušan ; Wetzel, Andrew ; et al ( August 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We study the spatially resolved (sub-kpc) gas velocity dispersion (σ)–star formation rate (SFR) relation in the FIRE-2 (Feedback in Realistic Environments) cosmological simulations. We specifically focus on Milky Way-mass disc galaxies at late times (z ≈ 0). In agreement with observations, we find a relatively flat relationship, with σ ≈ 15–30 km s−1 in neutral gas across 3 dex in SFRs. We show that higher dense gas fractions (ratios of dense gas to neutral gas) and SFRs are correlated at constant σ. Similarly, lower gas fractions (ratios of gas to stellar mass) are correlated with higher σ at constant SFR. The limits of the σ–ΣSFR relation correspond to the onset of strong outflows. We see evidence of ‘on-off’ cycles of star formation in the simulations, corresponding to feedback injection time-scales of 10–100 Myr, where SFRs oscillate about equilibrium SFR predictions. Finally, SFRs and velocity dispersions in the simulations agree well with feedback-regulated and marginally stable gas disc (Toomre’s Q = 1) model predictions, and the simulation data effectively rule out models assuming that gas turns into stars at (low) constant efficiency (i.e. 1 per cent per free-fall time). And although the simulation data do not entirely exclude gas accretion/gravitationally powered turbulence as a driver of σ,more »it appears to be subdominant to stellar feedback in the simulated galaxy discs at z ≈ 0.« less
5. 3D elemental abundances of stars at formation across the histories of Milky Way-mass galaxies in the FIRE simulations

   https://doi.org/10.1093/mnras/stac1637  

   Bellardini, Matthew A. ; Wetzel, Andrew ; Loebman, Sarah R. ; Bailin, Jeremy ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   We characterize the 3D spatial variations of [Fe/H], [Mg/H], and [Mg/Fe] in stars at the time of their formation, across 11 simulated Milky Way (MW)- and M31-mass galaxies in the FIRE-2 simulations, to inform initial conditions for chemical tagging. The overall scatter in [Fe/H] within a galaxy decreased with time until $\approx 7 \, \rm {Gyr}$ ago, after which it increased to today: this arises from a competition between a reduction of azimuthal scatter and a steepening of the radial gradient in abundance over time. The radial gradient is generally negative, and it steepened over time from an initially flat gradient $\gtrsim 12 \, \rm {Gyr}$ ago. The strength of the present-day abundance gradient does not correlate with when the disc ‘settled’; instead, it best correlates with the radial velocity dispersion within the galaxy. The strength of azimuthal variation is nearly independent of radius, and the 360 deg scatter decreased over time, from $\lesssim 0.17 \, \rm {dex}$ at $t_{\rm lb} = 11.6 \, \rm {Gyr}$ to $\sim 0.04 \, \rm {dex}$ at present-day. Consequently, stars at $t_{\rm lb} \gtrsim 8 \, \rm {Gyr}$ formed in a disc with primarily azimuthal scatter in abundances. All stars formed in amore »vertically homogeneous disc, Δ[Fe/H]$\le 0.02 \, \rm {dex}$ within $1 \, \rm {kpc}$ of the galactic mid-plane, with the exception of the young stars in the inner $\approx 4 \, \rm {kpc}$ at z ∼ 0. These results generally agree with our previous analysis of gas-phase elemental abundances, which reinforces the importance of cosmological disc evolution and azimuthal scatter in the context of stellar chemical tagging. We provide analytic fits to our results for use in chemical-tagging analyses.

   « less