The FIR-Radio Correlation in Rapidly Star-Forming Galaxies: The Spectral Index Problem and Proton Calorimetry

Thompson, Todd A.; Lacki, Brian C.

doi:10.1007/978-3-642-35410-6_20

Cited by 5 publications

(3 citation statements)

References 48 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We show that under a variety of assumptions, CRs are dynamically weak with respect to gravity in the starburst core and along the minor axis, limiting their ability to accelerate large-scale, heavily mass-loaded winds. Although we substantiate this conclusion here with a suite of GALPROP models that demonstrate the resilience of these results to changes in our model assumptions, the basic conclusion that CRs are weak with respect to gravity in the cores of dense, gas-rich, rapidly star-forming galaxies follows from earlier work (see, e.g., Thompson & Lacki 2013a). In short, the shallow radio spectral indices of starbursts imply that relativistic bremsstrahlung and ionization losses dominate the cooling of the CRe population, implying that the CRe must interact with gas at nearly the mean density of the ISM.…”

Section: A Multi-dimensional Modelsupporting

confidence: 77%

“…In this section, we use the observed synchrotron spectral index to understand the physics of the degeneracy between B 0 vs. n 0 . A zeroth order approximation for the magnetic field-gas density relation assumes that the synchrotron and bremsstrahlung cooling time scales are approximately equal to each other (see Thompson & Lacki (2013b)). In short, for galaxies to have shallow radio spectral indices at ∼GHz frequencies, bremsstrahlung or ionization cooling must be comparable to synchrotron and IC cooling to keep the far-infrared radio correlation consistent over many orders of magnitude in magnetic field strength and gas density Lacki et al (2010).…”

Section: B Analytic Spectral Index Analysismentioning

confidence: 99%

See 1 more Smart Citation

Cosmic rays and magnetic fields in the core and halo of the starburst M82: implications for galactic wind physics

Buckman

Linden

Thompson

2020

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

Cosmic rays (CRs) and magnetic fields may be dynamically important in driving large-scale galactic outflows from rapidly star-forming galaxies. We construct twodimensional axisymmetric models of the local starburst and super-wind galaxy M82 using the CR propagation code GALPROP. Using prescribed gas density and magnetic field distributions, wind profiles, CR injection rates, and stellar radiation fields, we simultaneously fit both the integrated gamma-ray emission and the spatially-resolved multi-frequency radio emission extended along M82's minor axis. We explore the resulting constraints on the gas density, magnetic field strength, CR energy density, and the assumed CR advection profile. In accord with earlier one-zone studies, we generically find low central CR pressures, strong secondary electron/positron production, and an important role for relativistic bremsstrahlung losses in shaping the synchrotron spectrum. We find that the relatively low central CR density produces CR pressure gradients that are weak compared to gravity, strongly limiting the role of CRs in driving M82's fast and mass-loaded galactic outflow. Our models require strong magnetic fields and advection speeds of order ∼1000 km/s on kpc scales along the minor axis in order to reproduce the extended radio emission. Degeneracies between the controlling physical parameters of the model and caveats to these findings are discussed.

show abstract

Section: A Multi-dimensional Modelsupporting

confidence: 77%

Section: B Analytic Spectral Index Analysismentioning

confidence: 99%

Cosmic rays and magnetic fields in the core and halo of the starburst M82: implications for galactic wind physics

Buckman

Linden

Thompson

2020

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

show abstract

“…This raises the question of whether CRs should be able to spread out through all the neutral gas in a starburst galaxy at all, or whether they will be confined primarily to a small volume of the galaxy. It is therefore of interest to compute the CR filling factor Q CR , which Lacki (2013) defines as the overlap fraction of the total volume of all CR 'bubbles' around individual SNRs -which act as short-lived accelerators -and the starburst that CR electrons undergo bremsstrahlung or ionisation losses at a rate that is at least comparable to their rate of inverse Compton losses, but this would only be expected if those CRs were interacting primarily with dense, cold, neutral gas, rather that diffuse, hot, and ionised material -see Lacki et al (2010) and Thompson & Lacki (2013) for further discussion.…”

Section: Cosmic Rays Pervade the Neutral Gas Of Starburstsmentioning

confidence: 99%

Cosmic ray transport in starburst galaxies

Krumholz

Crocker

Xu³

et al. 2020

Monthly Notices of the Royal Astronomical Society

137

View full text Add to dashboard Cite

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.

show abstract

Cosmic rays across the star-forming galaxy sequence – I. Cosmic ray pressures and calorimetry

Crocker

Krumholz

Thompson

2021

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

In the Milky Way (MW), cosmic rays (CRs) are dynamically important in the interstellar medium (ISM), contribute to hydrostatic balance, and may help regulate star formation. However, we know far less about the importance of CRs in galaxies whose gas content or star formation rate (SFR) differ significantly from those of the MW. Here, we construct self-consistent models for hadronic CR transport, losses, and contribution to pressure balance as a function of galaxy properties, covering a broad range of parameters from dwarfs to extreme starbursts. While the CR energy density increases from ∼1 eV cm−3 to ∼1 keV cm−3 over the range from sub-MW dwarfs to bright starbursts, strong hadronic losses render CRs increasingly unimportant dynamically as the SFR surface density increases. In MW-like systems, CR pressure is typically comparable to turbulent gas and magnetic pressure at the galactic mid-plane, but the ratio of CR to gas pressure drops to ∼10−3 in dense starbursts. Galaxies also become increasingly CR calorimetric and gamma-ray bright in this limit. The degree of calorimetry at fixed galaxy properties is sensitive to the assumed model for CR transport, and in particular to the time CRs spend interacting with neutral ISM, where they undergo strong streaming losses. We also find that in some regimes of parameter space hydrostatic equilibrium discs cannot exist, and in Paper II of this series we use this result to derive a critical surface in the plane of star formation surface density and gas surface density beyond which CRs may drive large-scale galactic winds.

show abstract

The FIR-Radio Correlation in Rapidly Star-Forming Galaxies: The Spectral Index Problem and Proton Calorimetry

Cited by 5 publications

References 48 publications

Cosmic rays and magnetic fields in the core and halo of the starburst M82: implications for galactic wind physics

Cosmic rays and magnetic fields in the core and halo of the starburst M82: implications for galactic wind physics

Cosmic ray transport in starburst galaxies

Cosmic rays across the star-forming galaxy sequence – I. Cosmic ray pressures and calorimetry

Contact Info

Product

Resources

About