[Paper Review] The interstellar environment in the outer Galaxy as seen in gamma rays by Fermi

L. Tibaldo, I. A. Grenier+1 more|arXiv (Cornell University)|Dec 2, 2010

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This study uses Fermi LAT gamma-ray data to map cosmic ray (CR) densities and molecular gas calibration (X_CO) across the outer Galaxy, revealing a flatter CR emissivity gradient than predicted by standard propagation models. It finds a factor-of-two increase in X_CO from the Gould Belt to the local spur, with no further variation to the Perseus arm, and suggests large CR halo heights (>10 kpc) or flat CR source distributions are needed to reconcile observations with theory.

ABSTRACT

Gamma-ray emission produced by interactions between cosmic rays (CRs) and interstellar gas traces the product of their densities throughout the Milky Way. The outer Galaxy is a privileged target of investigation to separate interstellar structures seen along the line of sight. Recent observations by the Fermi Large Area Telescope (LAT) shed light on open questions of the EGRET era about the distribution of CR densities and the census of the interstellar medium. The gradient of gamma-ray emissivities measured in the outer Galaxy is significantly flatter than predictions from widely used CR propagation models given the rapid decline of putative CR sources beyond the solar circle. Large propagation volumes, with halo heights up to 20 kpc, or a flat CR source distribution are required to match the data. Other viable possibilities include non-uniform CR diffusion properties or more gas than accounted for by the radio/mm-wave data. Gamma-ray data constrain the evolution of the Xco=N(H2)/W(CO) ratio within a few kpc from the Sun. There is a significant increase by a factor 2 from nearby clouds in the Gould Belt to the local spur. No further significant variations are measured from the local spur to the Perseus spiral arm. At the level of statistical accuracy provided by the LAT data, the most important source of uncertainty, often overlooked so far, is due to the optical depth correction applied to derive the column densities of H I. Reliable determinations of the amount of atomic gas in the plane are key to better probe the properties of CRs in the Galaxy.

Motivation & Objective

• To measure cosmic ray (CR) densities in the outer Galaxy using gamma-ray emissivity from Fermi LAT.
• To calibrate the X_CO ratio (N(H₂)/W_CO) across different Galactic regions using gamma-ray and radio/mm-wave data.
• To resolve the cosmic-ray gradient problem by comparing observed gamma-ray emissivities with predictions from CR propagation models.
• To assess the impact of H I optical depth corrections on gas column density measurements and their effect on CR emissivity uncertainties.
• To evaluate the role of dark gas and non-uniform diffusion in explaining discrepancies between observations and models.

Proposed method

• Used Fermi Large Area Telescope (LAT) gamma-ray data to extract gamma-ray emissivity per H atom (q_HI) and per CO line intensity (q_CO) in multiple regions of the second and third Galactic quadrants.
• Applied likelihood-based component separation to disentangle gamma-ray emission from CR interactions with atomic and molecular gas.
• Calculated X_CO as q_CO / (2 × q_HI) to derive the molecular gas mass conversion factor across Galactocentric distances.
• Employed the GALPROP code to model CR propagation, varying halo height and source distribution profiles to match observed emissivity gradients.
• Explored uncertainties from H I spin temperature and optical depth corrections by testing a range of T_S values and their impact on N(H I) and q_HI.
• Compared results with predictions from Strong et al. (2000) and Nakanishi & Sofue (2003) to assess validity of X_CO and CR source models.

Experimental results

Research questions

• RQ1 What is the radial gradient of cosmic ray density in the outer Galaxy as traced by gamma-ray emissivity?
• RQ2 How does the X_CO ratio vary with Galactocentric distance, and what does this imply for molecular gas mass estimates?
• RQ3 Why is the observed gamma-ray emissivity gradient flatter than predicted by standard CR propagation models?
• RQ4 To what extent do uncertainties in H I optical depth correction affect the reliability of CR emissivity measurements?
• RQ5 What CR propagation parameters (e.g., halo height, source distribution) are required to reconcile observations with simulations?

Key findings

• The gamma-ray emissivity per H atom (q_HI) shows no significant gradient beyond R ≈ 11 kpc, indicating a remarkably flat cosmic ray density distribution in the outer Galaxy.
• A factor-of-two increase in the X_CO ratio is observed from the Gould Belt (8.5–8.8 kpc) to the local spur (8.8–10 kpc), with no further significant variation to the Perseus spiral arm (R ≈ 14 kpc).
• The observed X_CO values are systematically lower than predictions from Nakanishi & Sofue (2003) based on virial masses, and do not support the order-of-magnitude increase proposed by Strong et al. (2000).
• Standard CR propagation models with a declining source profile beyond the solar circle fail to reproduce the flat emissivity gradient, requiring either halo heights of 10–20 kpc or a flat source distribution beyond R ≈ 10 kpc.
• Uncertainties in H I optical depth correction dominate the error budget for q_HI, highlighting its critical role in gas mass and CR density determinations.
• Alternative explanations such as non-uniform diffusion or unaccounted dark gas in the outer disk may also contribute to the observed discrepancy with models.