382 research outputs found
Stellar feedback sets the universal acceleration scale in galaxies
It has been established for decades that rotation curves deviate from the Newtonian gravity expectation given baryons alone below a characteristic acceleration scale gâ âŒ10â»âž cm sâ»ÂČâ , a scale promoted to a new fundamental constant in MOND. In recent years, theoretical and observational studies have shown that the star formation efficiency (SFE) of dense gas scales with surface density, SFE âŒ ÎŁ/ÎŁ_(crit) with ÎŁ_(crit)âŒâšpË/mââ©/(ÏG)âŒ1000 M_â pcâ»ÂČ (where âšpË/mââ© is the momentum flux output by stellar feedback per unit stellar mass in a young stellar population). We argue that the SFE, more generally, should scale with the local gravitational acceleration, i.e. that SFE âŒg_(tot)/g_(crit) ⥠(GM_(tot)/RÂČ)/âšpË/mââ©â , where M_(tot) is the total gravitating mass and g_(crit) = âšpË/mââ© = ÏGÎŁ_(crit) â 10â»âž cm sâ»ÂČ â gâ . Hence, the observed gâ may correspond to the characteristic acceleration scale above which stellar feedback cannot prevent efficient star formation, and baryons will eventually come to dominate. We further show how this may give rise to the observed acceleration scaling g_(obs) ⌠(g_(baryon)gâ )^(1/2) (where g_(baryon) is the acceleration due to baryons alone) and flat rotation curves. The derived characteristic acceleration gâ can be expressed in terms of fundamental constants (gravitational constant, proton mass, and Thomson cross-section): gâ âŒ0.1Gmp_/Ï_Tâ
A Flat Photoionization Rate at 2<z<4.2: Evidence for a Stellar-Dominated UV Background and Against a Decline of Cosmic Star Formation Beyond z~3
We investigate the implications of our measurement of the Lyman-alpha forest
opacity at redshifts 2<z<4.2 from a sample of 86 high-resolution quasar spectra
for the evolution of the cosmic ultraviolet luminosity density and its sources.
The derived hydrogen photoionization rate is remarkably flat over this redshift
range, implying an increasing comoving ionizing emissivity with redshift.
Because the quasar luminosity function is strongly peaked near z~2,
star-forming galaxies likely dominate the ionizing emissivity at z>~3. Our
measurement argues against a star formation rate density declining beyond z~3,
in contrast with existing state-of-the-art determinations of the cosmic star
formation history from direct galaxy counts. Stellar emission from galaxies
therefore likely reionized the Universe.Comment: 5 pages, including 1 figure, published by Ap
When Feedback Fails: The Scaling and Saturation of Star Formation Efficiency
We present a suite of 3D multi-physics MHD simulations following star
formation in isolated turbulent molecular gas disks ranging from 5 to 500
parsecs in radius. These simulations are designed to survey the range of
surface densities between those typical of Milky Way GMCs (\sim 10^2
M_\odot\,pc^{-2}}) and extreme ULIRG environments (\sim 10^2
M_\odot\,pc^{-2}}) so as to map out the scaling of the cloud-scale star
formation efficiency (SFE) between these two regimes. The simulations include
prescriptions for supernova, stellar wind, and radiative feedback, which we
find to be essential in determining both the instantaneous per-freefall
() and integrated () star formation
efficiencies. In all simulations, the gas disks form stars until a critical
stellar surface density has been reached and the remaining gas is blown out by
stellar feedback. We find that surface density is a good predictor of
, as suggested by analytic force balance arguments from
previous works. SFE eventually saturates to at high surface density.
We also find a proportional relationship between and
, implying that star formation is feedback-moderated even over
very short time-scales in isolated clouds. These results have implications for
star formation in galactic disks, the nature and fate of nuclear starbursts,
and the formation of bound star clusters. The scaling of with
surface density is not consistent with the notion that is
always on the scale of GMCs, but our predictions recover the value for GMC parameters similar to those found in sprial galaxies,
including our own.Comment: 21 pages, 7 figures. Accepted to MNRA
Can magnetized turbulence set the mass scale of stars?
Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the GIZMO code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs (â MâŒ10â50â ). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass Mâ
â becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find Mâ
âââŒMâMâ»ÂłÎ±_(turb)SFE^(1/3) for GMC mass Mâ, sonic Mach number Mâ , virial parameter α_(turb), and star formation efficiency SFE = Mâ/Mâ. This fit agrees well with previous IMF results from the RAMSES, ORION2, and SPHNG codes. Although Mâ
â has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict Mâ
â to be >20Mââ , an order of magnitude larger than observed (â âŒ2Mââ ), together with an excess of brown dwarfs. Moreover, Mâ
â is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF
Unravelling the physics of multiphase AGN winds through emission line tracers
Observations of emission lines in active galactic nuclei (AGNs) often find fast (âŒ1000 kmâsâ1) outflows extending to kiloparsec scales, seen in ionized, neutral atomic and molecular gas. In this work we present radiative transfer calculations of emission lines in hydrodynamic simulations of AGN outflows driven by a hot wind bubble, including non-equilibrium chemistry, to explore how these lines trace the physical properties of the multiphase outflow. We find that the hot bubble compresses the line-emitting gas, resulting in higher pressures than in the ambient interstellar medium or that would be produced by the AGN radiation pressure. This implies that observed emission line ratios such as [OâIV]25ÎŒm / [NeâII]12ÎŒmâ , [NeâV]14ÎŒm / [NeâII]12ÎŒmâ , and [NâIII]57ÎŒm / [NâII]122ÎŒm constrain the presence of the bubble and hence the outflow driving mechanism. However, the line-emitting gas is under-pressurized compared to the hot bubble itself, and much of the line emission arises from gas that is out of pressure, thermal and/or chemical equilibrium. Our results thus suggest that assuming equilibrium conditions, as commonly done in AGN line emission models, is not justified if a hot wind bubble is present. We also find that âł50 per cent of the mass outflow rate, momentum flux, and kinetic energy flux of the outflow are traced by lines such as [NâII]122ÎŒm and [NeâIII]15ÎŒm (produced in the 104K phase) and [CâII]158ÎŒm (produced in the transition from 104K to 100 K)
On The Nature of Variations in the Measured Star Formation Efficiency of Molecular Clouds
Measurements of the star formation efficiency (SFE) of giant molecular clouds
(GMCs) in the Milky Way generally show a large scatter, which could be
intrinsic or observational. We use magnetohydrodynamic simulations of GMCs
(including feedback) to forward-model the relationship between the true GMC SFE
and observational proxies. We show that individual GMCs trace broad ranges of
observed SFE throughout collapse, star formation, and disruption. Low measured
SFEs (<<1%) are "real" but correspond to early stages, the true "per-freefall"
SFE where most stars actually form can be much larger. Very high (>>10%) values
are often artificially enhanced by rapid gas dispersal. Simulations including
stellar feedback reproduce observed GMC-scale SFEs, but simulations without
feedback produce 20x larger SFEs. Radiative feedback dominates among mechanisms
simulated. An anticorrelation of SFE with cloud mass is shown to be an
observational artifact. We also explore individual dense "clumps" within GMCs
and show that (with feedback) their bulk properties agree well with
observations. Predicted SFEs within the dense clumps are ~2x larger than
observed, possibly indicating physics other than feedback from massive (main
sequence) stars is needed to regulate their collapse.Comment: Fixed typo in the arXiv abstrac
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