Weighing the Local Interstellar Medium using Gamma Rays and Dust

The ratio between the gas and dust densities in galactic environments is among the most important parameters in understanding star formation and galaxy evolution. Typical constraints on this ratio stem from radio observations of dust emission and absorption, as well as radio line emission from standard gas tracers. We develop an entirely new method based on gamma-ray observations, which trace the convolution of the well-measured local cosmic-ray density and the well-measured high-latitude gamma-ray emission intensity. We obtain results that are consistent with many previous studies, but depend on an entirely independent set of systematic uncertainties. Moreover, our results have the precision to untangle the tension in previous world leading measurements, quantitatively improving our models of local gas and dust.

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Constraining Solar Modulation

A critical uncertainty in understanding cosmic-rays in our Galaxy is that the cosmic-rays we can observe in our solar system are not representative of the galactic cosmic-ray population. To make it to Earth, cosmic-rays must first fight against the magnetic field and charged wind produce by the Sun -- losing energy in a time-dependent, charge-dependent, and rigidity-dependent way. Fortunately, because the cosmic-ray population in interstellar space is constant in time, and the effects induced by the Sun oscillate wildly (even on daily timescales), we can separate these effects and constrain the impact of solar modulation on the observed cosmic-ray population. We use this to better constrain the cosmic-ray population in the interstellar medium, which is a key observable in searches for dark matter or other new physics.

Cosmic-Rays Don't Drive Winds in M82

We construct the most detailed models of cosmic-ray injection, propagation, and energy losses in the starburst galaxy M82, including the first two-dimensional model of cosmic-ray injection and propagation in the dense M82 nuclear core. We find that the high-gas density of of the starburst core quickly cools high-energy protons injected by supernovae in teh starburst region. By comparing these gamma-ray constraints with the morphology of observed radio emission, we place strong constraints on the steady state cosmic-ray density above and below the galactic plane. Because we find the cosmic-ray density (and its gradient) to be small, we can rule out models where cosmic-rays drive the galactic winds in galaxies like M82.

Evidence for Cosmic-Ray Escape from the Small Magellanic Cloud

The Small Magellanic Cloud serves as an excellent test case to understand cosmic-ray propagation in small galaxies. Situated only 100 thousand light years away from the Milky Way, its proximity allows us to utilize gamma-ray observations to measure not only its diffuse gamma-ray flux, but also its gamma-ray morphology. We produce a targeted study of the Small Magellanic Cloud, and find that the gamma-ray spectrum includes a peculiar cutoff at energies above 12 GeV, which corresponds to cosmic-ray energies near 200 GeV. These gamma-rays do not appear to be produced by isolated sources within the galaxy, leading us to conclude that high-energy cosmic-rays are produced in the Small Magellanic cloud, but then efficiently escape at high energies before encounting gas. This has implications for cosmic-ray feedback and the dynamics of small galaxies.

Full Publication List:

12. Weighing the Local Interstellar Medium using Gamma Rays and Dust
Axel Widmark, Michael Korsmeier, Tim Linden
Physical Review Letters 130 161002 (2023)

11. Constraining the Charge-Sign and Rigidity-Dependence of Solar Modulation
Ilias Cholis, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 10 051

10. Cosmic Rays and Magnetic Fields in the Core and Halo of the Starbust M82: Implications for Galactic Wind Physics
Benjamin Buckman, Tim Linden, Todd Thompson
Monthly Notices of the Royal Astronomical Society 494 2679

9. Evidence for Cosmic-Ray Escape in the Small Magellanic Cloud using Fermi Gamma-rays
Laura Lopez, Katie Auchettl, Tim Linden, Alberto Bolatto, Todd Thompson, Enrico Ramirez-Ruiz
The Astrophysical Journal 867 44

8. HAWC Observations Strongly Favor Pulsar Interpretations of the Cosmic-Ray Positron Excess
Dan Hooper, Ilias Cholis, Tim Linden, Ke Fang
Physical Review D 96 103013

7. Evidence for the Stochastic Acceleration of Secondary Antiprotons by Supernova Remnants
Ilias Cholis, Dan Hooper, Tim Linden
Physical Review D 95 123007

6. Improved Cosmic-Ray Injection Models and the Galactic Center Gamma-Ray Excess
Eric Carlson, Tim Linden, Stefano Profumo
Physical Review D 94 063504

5. A Predictive Analytic Model for the Solar Modulation of Cosmic Rays
Ilias Cholis, Dan Hooper, Tim Linden
Physical Review D 93 4 043016

4. Putting Things Back Where They Belong: Tracing Cosmic-Ray Injection with H2
Eric Carlson, Tim Linden, Stefano Profumo
Physical Review Letters 117 111101

3. The Circular Polarization of Pulsar Wind Nebulae and the Cosmic-Ray Positron Excess
Tim Linden
The Astrophysical Journal 799 200 (2015)

2. Is the Ultra-High Energy Cosmic-Ray Excess Correlated with IceCube Neutrinos?
Ke Fang, Toshihiro Fujii, Tim Linden, Angela Olinto
The Astrophysical Journal, 794 126

1. Probing the Pulsar Origin of the Positron Fraction with Atmospheric Cherenkov Telescopes
Tim Linden, Stefano Profumo
The Astrophysical Journal, 772 18

Tim Linden

Assistant Professor, Stockholm University