Equipartition and cosmic ray energy densities in central molecular zones of starbursts

Yoast-Hull, Tova M.; Gallagher, John S.; Zweibel, Ellen G.

doi:10.1093/mnrasl/slv195

Cited by 53 publications

(51 citation statements)

References 37 publications

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“…We evaluate τ eff , Φ 0 , Υ 0 , and the midplane CR energy density U(0) = −Φ 0 /u (0) for β = 1/4 and the fiducial parameters for each of our galaxies in Table 1, using E CR,SN = 10 50 erg and M SN = 100 M , i.e., assuming 1 supernova per 100 M of stars formed, with that supernova putting ≈ 10% of its energy into CRs. For our fiducial parameters, CR midplane energy densities are of order 1 − 5 keV cm −3 , similar to previously-published estimates (e.g., Yoast-Hull et al 2016); however, we remind readers that we have explicitly assumed that advective escape of CRs is negligible, so if advective escape is significant the true energy densities may be somewhat lower. These energy densities are roughly a factor of 10 3 − 10 4 larger than in the Milky Way.…”

Section: Solutionssupporting

confidence: 85%

Cosmic ray transport in starburst galaxies

Krumholz

Crocker

Xu³

et al. 2020

Monthly Notices of the Royal Astronomical Society

137

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Starburst galaxies are efficient γ-ray producers, because their high supernova rates generate copious cosmic ray (CR) protons, and their high gas densities act as thick targets off which these protons can produce neutral pions and thence γ-rays. In this paper we present a first-principles calculation of the mechanisms by which CRs propagate through such environments, combining astrochemical models with analysis of turbulence in weakly ionised plasma. We show that CRs cannot scatter off the strong large-scale turbulence found in starbursts, because efficient ion-neutral damping prevents such turbulence from cascading down to the scales of CR gyroradii. Instead, CRs stream along field lines at a rate determined by the competition between streaming instability and ion-neutral damping, leading to transport via a process of field line random walk. This results in an effective diffusion coefficient that is nearly energyindependent up to CR energies of ∼ 1 TeV. We apply our computed diffusion coefficient to a simple model of CR escape and loss, and show that the resulting γ-ray spectra are in good agreement with the observed spectra of the starbursts NGC 253, M82, and Arp 220. In particular, our model reproduces these galaxies' relatively hard GeV γ-ray spectra and softer TeV spectra without the need for any fine-tuning of advective escape times or the shape of the CR injection spectrum.

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Section: Solutionssupporting

confidence: 85%

Cosmic ray transport in starburst galaxies

Krumholz

Crocker

Xu³

et al. 2020

Monthly Notices of the Royal Astronomical Society

137

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show abstract

“…4-a,c show a combined effect from CREs and B, i.e, from the nonthermal ISM. We also stress that γ-ray observations confirm the validity of the CRE-B equipartition for Σ SFR < 100 M ⊙ yr −1 kpc −2 [41], and hence for the nuclear ring of NGC 1097 (with Σ SFR ≃ 2 M ⊙ yr −1 kpc −2 ).…”

Section: What Controls Nuclear Star Formationsupporting

confidence: 73%

Discovery of massive star formation quenching by non-thermal effects in the centre of NGC 1097

et al. 2017

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Observations show that massive star formation quenches first at centers of galaxies. To understand quenching mechanisms, we investigate the thermal and nonthermal energy balance in the central kpc of NGC 1097-a prototypical galaxy undergoing quenching-and present a systematic study of the nuclear star formation efficiency and its dependencies. This region is dominated by the nonthermal pressure from the magnetic field, cosmic rays, and turbulence. A comparison of the mass-to-magnetic flux ratio of the molecular clouds shows that most of them are magnetically critical or supported against gravitational collapse needed to form cores of massive stars. Moreover, the star formation efficiency of the clouds drops with the magnetic field strength. Such an anti-correlation holds with neither the turbulent nor the thermal pressure. Hence, a progressive built up of the magnetic field results in high-mass stars forming inefficiently, and it may be the cause of the low-mass stellar population in the bulges of galaxies.

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“…There is also the possibility of the system being immersed in a strong cosmic ray background, however such environmental interactions are beyond the scope of this work. However, Yoast-Hull et al (2016) have found that the cosmic ray energy in nuclear starbursts tends to be considerably smaller than the magnetic field energy, suggesting that even in the full picture with a realistic galactic environment cosmic rays should not greatly influence the overall dynamics of a collapsing GMC.…”

Section: Metallicitymentioning

confidence: 99%

When feedback fails: the scaling and saturation of star formation efficiency

Grudić

Hopkins

Faucher-Giguère

et al. 2018

Monthly Notices of the Royal Astronomical Society

190

236

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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 (∼ 10 2 M pc −2 ) and extreme ULIRG environments (∼ 10 4 M 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 ( f f ) and integrated ( int ) 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 int , as suggested by analytic force balance arguments from previous works. SFE eventually saturates to ∼ 1 at high surface density. We also find a proportional relationship between f f and int , 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 f f with surface density is not consistent with the notion that f f is always ∼ 1% on the scale of GMCs, but our predictions recover the ∼ 1% value for GMC parameters similar to those found in sprial galaxies, including our own.

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Equipartition and cosmic ray energy densities in central molecular zones of starbursts

Cited by 53 publications

References 37 publications

Cosmic ray transport in starburst galaxies

Cosmic ray transport in starburst galaxies

Discovery of massive star formation quenching by non-thermal effects in the centre of NGC 1097

When feedback fails: the scaling and saturation of star formation efficiency

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