Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation

By T. Thompson, E. Quataert, E. Waxman, N. Murray, Crystal L. Martin. 2006

189 citations recorded by the source corpus.

We estimate minimum energy magnetic fields (Bmin) for a sample of galaxies with measured gas surface densities, spanning more than four orders of magnitude in surface density, from normal spirals to luminous starbursts. We show that the ratio of the minimum energy magnetic pressure to the total pressure in the ISM decreases substantially with increasing surface density. For the ultraluminous infrared galaxy Arp 220, this ratio is ~10-4. Therefore, if the minimum energy estimate is applicable, magnetic fields in starbursts are dynamically weak compared to gravity, in contrast to normal star-forming spiral galaxies. We argue, however, that rapid cooling of relativistic electrons in starbursts invalidates the minimum energy estimate. We assess a number of independent constraints on the magnetic field strength in starburst galaxies. In particular, we argue that the existence of the FIR-radio correlation implies that the synchrotron cooling timescale for cosmic-ray electrons is much shorter than their escape time from the galactic disk; this in turn implies that the true magnetic field in starbursts is significantly larger than Bmin. The strongest argument against such large fields is that one might expect starbursts to have steep radio spectra indicative of strong synchrotron cooling, which is not observed. However, we show that ionization and bremsstrahlung losses can flatten the nonthermal spectra of starburst galaxies even in the presence of rapid cooling, providing much better agreement with observed spectra. We further demonstrate that ionization and bremsstrahlung losses are likely to be important in shaping the radio spectra of most starbursts at GHz frequencies, thereby preserving the linearity of the FIR-radio correlation. We thus conclude that magnetic fields in starbursts are significantly larger than Bmin. We highlight several observations that can test this conclusion.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/f8601138a063c5058436e2b84c82232fcfa8e68b

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. P. Hopkins
2. D. M. Alexander
3. L. Hernquist
4. D. Narayanan
5. E. Quataert
6. R. Dav'e
7. A. Cooray
8. E. Daddi

In Nuclear and High Energy Physics

1. E. Quataert
2. A. Strong
3. E. Orlando
4. R. Ivison
5. Troy A. Porter
6. I. Smail
7. T. Thompson
8. T. Slatyer

Citations in the corpus

• Open Questions in Cosmic-Ray Research at Ultrahigh Energies (2019, methodology): “For tidal disruption events they are based on the prototypical jetted-TDE Swift J1644+57 (Burrows et al., 2011; Kumar et al., 2013; Senno et al., 2017 ), for starburst galaxies and normal galaxies they were estimated in Thompson et al. (2006).”
• GIANT GAMMA-RAY BUBBLES FROM FERMI-LAT: ACTIVE GALACTIC NUCLEUS ACTIVITY OR BIPOLAR GALACTIC WIND? (2010, background): “The synchrotron cooling timescale for cosmic ray electrons emitting at frequency ν is τsyn ≈ 106 B−3/2100 ν −1/2 GHz yr, where B100 = B/100µG (Thompson et al. 2006).”
• GeV OBSERVATIONS OF STAR-FORMING GALAXIES WITH THE FERMI LARGE AREA TELESCOPE (2012, background): “…(de Jong et al. 1985; Wunderlich et al. 1987; Dressel 1988; Wrobel & Heeschen 1988, 1991), covers galaxies with magnetic energy densities likely ranging at least 4 orders of magnitude (Condon et al. 1991; Thompson et al. 2006), and applies to galaxies out to at leastz ∼ 1 (Appleton et al. 2004).”
• Deep multi-frequency radio imaging in the Lockman Hole using the GMRT and VLA (2009, background): “This suggests magnetic fields are large enough to retain cosmic rays (see Thompson et al. 2006) and contradic s the idea that flatter radio SEDs – such as those thermally dominated galaxies (Hunt & Maiolino 2005) or in synchrotron selfabsorbed sources – are more appropriate SEDs for these massive…”
• THE DISRUPTION OF GIANT MOLECULAR CLOUDS BY RADIATION PRESSURE & THE EFFICIENCY OF STAR FORMATION IN GALAXIES (2009, background): “Although the magnetic fields in starburst galaxies can be large (∼ few mG for Arp 220; Thompson et al. 2006, Robishaw et al. 2008), we neglect them throughout this paper in order to focus on the competition between various processes that contribute to disrupting GMCs.”
• The far-infrared/radio correlation as probed by Herschel (2010, methodology): “…flux density ratio has been useful when estimating the redshift or Td of a distant starburst, or when defining samples of AGN (Condon 1992; Carilli & Yun 1999; Ivison et al. 2002; Bell 2003; Chapman et al. 2005; Donley et al. 2005), or probing magnetic field strength (Thompson et al. 2006).”
• The physics of galactic winds driven by active galactic nuclei (2012, background): “In starbursts, the magnetic field can reach B ∼ 1 mG (Thompson et al. 2006).”
• The Star Formation History of the Universe as Revealed by Deep Radio Observations (2008, unknown): “−1.0 (Thompson et al. 2006).”
• The Starburst Contribution to the Extragalactic γ-Ray Background (2006, methodology): “In x 2 we review Völk’s (1989) electron calorimeter model for the observed FIR-radio correlation (see Thompson et al. 2006, hereafter T06).”
• THE PHYSICS OF THE FAR-INFRARED–RADIO CORRELATION. I. CALORIMETRY, CONSPIRACY, AND IMPLICATIONS (2009, methodology): “We use the Schmidt law and the Σg from Kennicutt (1998) and Thompson et al. (2006) to calculate the emissivities of gamma rays for these systems, which we then multiply by volume (from the radii given in Thompson et al. 2006 and the scale heights in Section 2.2.2) to get total luminosities to be…”