Gamma-ray Large Area Space Telescope

1. Gamma-ray Large Area Space Telescope Luminous Infrared Galaxies as Sources for the LAT* GLAST-for-Lunch S. W. Digel Hansen Experimental Physics Laboratory, Stanford Univ. * From Torres, Reimer, Domingo-Santamaria, & SD, 2004 ApJ, 607, L99

2. PowerPoint as a didactic crutch http://www.edwardtufte.com/tufte/powerpoint

3. Gamma-ray emission from the interstellar medium • High-energy g-rays are produced in cosmic-ray interactions with interstellar gas and photons • Cosmic-ray production is associated with regions of massive star formation (SNRs, colliding OB stellar winds) • This represents approximately 90% of the high-energy g-ray luminosity of the Milky Way (~106 solar) • EGRET (Compton Observatory, 1991-2000) view of the sky: ~60% of EGRET γ-rays were diffuse emission from the Milky Way (~30% isotropic emission, and ~10% from detected point sources) EGRET >100 MeV, Phase 1-5

4. Diffuse emission from external galaxies • The well-known extragalactic sources detected by EGRET are blazars, with the gamma-ray emission associated with jets from accreting supermassive black holes • Rare & distant, but very luminous (and beamed emission) • Only one other external galaxy has been detected in the light of its diffuse emission – LMC • The problem is distance: Milky Way at 1 Mpc would have a flux of about 2.5 x 10-8 cm-2 s-1 (>100 MeV), well below EGRET’s detection limit EGRET IRAS LMC (1.9 ± 0.4) x 10-7 cm-2 s-1 30 Doradus: extensive massive SFR and molecular clouds

5. e– e+ GLAST Large Area Telescope Gamma-ray Large Area Space Telescope • LAT is a pair conversion telescope with solid state (Si strip) technology • Within its first few weeks, the LAT will double the number of celestial g-rays ever detected • 5-year design life, goal of 10 years Spectrum Astro 1.8 m Tracker ACD 3000 kg Calorimeter

6. Estimation of g-ray fluxes for external galaxies Neglecting inverse Compton & electron Bremsstrahlung • g-ray number luminosity • Corresponding integrated flux • For IACTs (Völk et al. 1996): Mass of proton γ-ray emissivity Mass of ISM Approximate flux limit for LAT Cosmic-ray density enhancement factor (relative to local) Galaxies that stand any chance of being detected by IACTs will not be so distant for g-g attenuation by the EBL to be an issue in detectability Flux limit for new generation IACTs is ~1 × 10-13 cm-2 s-1(~ 1 × 10-12 cm-2 s-1 for >100 GeV)

7. SNR in Arp 220 VLBI (Smith et al. 2002) Luminous Infrared Galaxies • Discovery: IRAS (1983) • Most abundant galaxies in the local universe at high bolometric luminosity • Prototype ULIG is Arp 220 (72 Mpc), although many were not known optically ~100 pc Nuclei of merging gas-rich galaxies Sanders & Mirabel (1996)

8. Upper limits from EGRET • 2s upper limits for the nearest Luminous Infrared Galaxies (>1011 L☼ in IR) • Data from entire EGRET mission were used (LIGs are steady sources) • Typical flux limit ~5 × 10-8 cm-2 s-1

9. Detectability of LIGs with LAT • Properties that favor gamma-ray detectability also favor starburst galaxies • Large M, with high average gas density • Recent HCN survey of Gao & Solomon (2003) of IR and CO-bright galaxies, and nearby spirals • Allows estimate of SFR (from HCN luminosity) and minimum required k for detection by LAT and IACTs (from HCN + CO intensities and distance) • Several nearby starburst galaxies and a number of Luminous Infrared Galaxies are well above the detectability line MW Minimum k required for detectability with LAT

10. Summary • Luminous infrared galaxies and starburst galaxies should be a new class of g-ray source detected by the LAT and new generation IACTs • Detectability – generation of enough g-ray flux – requires large quantities of interstellar gas with a large fraction at high density (i.e., supporting ongoing massive star formation) Reference (with much more detail): Torres et al. 2004, ApJL, 607, L99

11. Backup slide

12. Distributions of required CR density enhancements