ABSTRACT We revisit the question of ‘hot mode’ versus ‘cold mode’ accretion on to galaxies using steady-state cooling flow solutions and idealized 3D hydrodynamic simulations. We demonstrate that for the hot accretion mode to exist, the cooling time is required to be longer than the free-fall time near the radius where the gas is rotationally supported, Rcirc, i.e. the existence of the hot mode depends on physical conditions at the galaxy scale rather than on physical conditions at the halo scale. When allowing for the depletion of the halo baryon fraction relative to the cosmic mean, the longer cooling times imply that a virialized gaseous halo may form in halo masses below the threshold of $\sim 10^{12}\, {\rm M_{\odot }}$ derived for baryon-complete haloes. We show that for any halo mass there is a maximum accretion rate for which the gas is virialized throughout the halo and can accrete via the hot mode of ${\dot{M}}_{\rm crit}\approx 0.7(v_{\rm c}/100\, \rm km\ s^{-1})^{5.4}(R_{\rm circ}/10\, {\rm kpc})(Z/\, {\rm Z_{\odot }})^{-0.9}\, {\rm M_{\odot }}\, {\rm yr}^{-1}$, where Z and vc are the metallicity and circular velocity measured at Rcirc. For accretion rates $\gtrsim {\dot{M}}_{\rm crit}$ the volume-filling gas phase can in principle be ‘transonic’ – virialized in the outer halo but cool and free-falling near the galaxy. We compare ${\dot{M}}_{\rm crit}$ to the average star formation rate (SFR) in haloes at 0 < z < 10 implied by the stellar-mass–halo-mass relation. For a plausible metallicity evolution with redshift, we find that ${\rm SFR}\lesssim {\dot{M}}_{\rm crit}$ at most masses and redshifts, suggesting that the SFR of galaxies could be primarily sustained by the hot mode in halo masses well below the classic threshold of $\sim 10^{12}\, {\rm M_{\odot }}$.
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A high-resolution investigation of the multiphase ISM in a galaxy during the first two billion years
We have carried out the first spatially resolved investigation of the multiphase interstellar medium (ISM) at high redshift, using the z = 4.24 strongly lensed submillimetre galaxy H-ATLASJ142413.9+022303 (ID141). We present high-resolution (down to ∼350 pc) ALMA observations in dust continuum emission and in the CO(7–6), $\rm H_2O (2_{1,1} - 2_{0,2})$, [C i] (1–0), and [C i] (2–1) lines, the latter two allowing us to spatially resolve the cool phase of the ISM for the first time. Our modelling of the kinematics reveals that the system appears to be dominated by a rotationally-supported gas disc with evidence of a nearby perturber. We find that the [C i] (1–0) line has a very different distribution to the other lines, showing the existence of a reservoir of cool gas that might have been missed in studies of other galaxies. We have estimated the mass of the ISM using four different tracers, always obtaining an estimate in the range of $\rm 3.2{\!-\!}3.8 \times 10^{11}\ M_{\odot }$, significantly higher than our dynamical mass estimate of $\rm 0.8{\!-\!}1.3 \times 10^{11}\ M_{\odot }$. We suggest that this conflict and other similar conflicts reported in the literature is because the gas-to-tracer ratios are ≃4 times lower than the Galactic values used to calibrate the ISM in high-redshift galaxies. We demonstrate that this could result from a top-heavy initial mass function and strong chemical evolution. Using a variety of quantitative indicators, we show that, extreme though it is at z = 4.24, ID141 will likely join the population of quiescent galaxies that appears in the Universe at z ∼ 3.
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- 10361487
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 510
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 3734-3757
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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