[Paper Review] Upper limits on the VHE $γ$-ray flux from the ULIRG Arp 220 and other galaxies with VERITAS

Henrike Fleischhack|arXiv (Cornell University)|Aug 24, 2015

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This study presents the most sensitive upper limits to date on very-high-energy (VHE) gamma-ray emission from the ultra-luminous infrared galaxy Arp 220 and three other galaxies using VERITAS observations. Despite no significant detection after over 30 hours of integration, the upper limits constrain theoretical models of cosmic-ray acceleration and VHE gamma-ray production in starburst and ULIRG environments, excluding the most optimistic predictions at 99% confidence level.

ABSTRACT

The cores of ultra-luminous infrared galaxies (ULIRGs) are very dense environments, with a high rate of star formation and supernova explosions. They are thought to be sites of cosmic-ray acceleration, and are predicted to emit $γ$-rays in the GeV to TeV range. So far, no ULIRG has been detected in $γ$-rays. Arp 220, the closest ULIRG to Earth, has been well studied, and detailed models of $γ$-ray production in this galaxy are available. They predict a rather hard $γ$-ray spectrum up to several TeV. Due to its large rate of star formation, high gas density, and its close proximity to Earth, Arp 220 is thought to be a very good candidate for observations in very-high-energy (VHE; 100 GeV - 100 TeV) $γ$-rays. Arp 220 was observed by the VERITAS telescopes for more than 30 hours with no significant excess over the cosmic-ray background. The upper limits on the VHE $γ$-ray flux of Arp 220 derived from these observations are the most sensitive limits presented so far and are starting to constrain theoretical models. We also present upper limits for the VHE flux from the ULIRG IRAS 17204-0014, the starburst galaxy IC 342, and the active galaxy 3C321.

Motivation & Objective

• To search for VHE gamma-ray emission from Arp 220, a nearby ULIRG with high star formation and dense interstellar medium, which is predicted to emit VHE gamma rays.
• To test theoretical models of VHE gamma-ray production via cosmic-ray interactions in dense star-forming environments.
• To improve constraints on VHE flux predictions for starburst and ULIRG galaxies using the most sensitive observational limits to date.
• To assess the consistency of VHE-FIR luminosity correlations in star-forming galaxies, especially for non-detections.
• To evaluate the sensitivity of current IACTs like VERITAS to detect VHE emission from high-luminosity, high-density galaxies.

Proposed method

• VERITAS performed deep observations totaling over 30 hours on Arp 220 and three other galaxies: IRAS 17204-0014, IC 342, and 3C321.
• The analysis used standard gamma-ray source extraction and background modeling techniques to search for excess events above cosmic-ray background.
• Upper limits on integral flux were calculated assuming a power-law spectrum with indices of -2.5 for starburst galaxies and -3.0 for the active galaxy 3C321.
• Systematic uncertainties from the VERITAS energy scale (20%) were propagated, affecting flux limits by up to 40% depending on spectral index.
• Theoretical flux predictions were compared to upper limits using a scaling relation based on M82's measured VHE flux and FIR luminosity.
• Model comparisons were made using published predictions from multiple theoretical frameworks, including those assuming calorimetric conditions and EBL absorption.

Experimental results

Research questions

• RQ1 What are the most stringent upper limits on VHE gamma-ray flux from Arp 220 and other star-forming galaxies using VERITAS?
• RQ2 To what extent do the VERITAS upper limits constrain existing theoretical models of VHE gamma-ray emission in ULIRGs?
• RQ3 Can the VHE-FIR luminosity correlation be validated or challenged by non-detections in high-luminosity star-forming galaxies?
• RQ4 How do systematic uncertainties in energy scale and spectral index assumptions affect the interpretation of non-detections?
• RQ5 What future observational capabilities are required to detect or rule out VHE emission from Arp 220?

Key findings

• VERITAS observed Arp 220 for over 30 hours with no significant gamma-ray excess, setting a 99% confidence level upper limit of ≤ 1.90 × 10⁻¹⁰ ph cm⁻² s⁻¹ for E > 500 GeV.
• The upper limit for Arp 220 is the most sensitive to date and excludes the most optimistic theoretical models of VHE emission at 99% confidence level.
• The upper limit for IC 342 is ≤ 1.6 × 10⁻¹⁰ ph cm⁻² s⁻¹ (E > 500 GeV), consistent with a rough prediction based on the M82 scaling relation.
• For IRAS 17204-0014, the upper limit is ≤ 1.0 × 10⁻¹⁰ ph cm⁻² s⁻¹ (E > 600 GeV), also consistent with the M82-based flux estimate.
• The upper limit for the active galaxy 3C321 is ≤ 1.90 × 10⁻¹⁰ ph cm⁻¹ s⁻¹ (E > 500 GeV), derived assuming a spectral index of -3.0.
• The results are consistent with the lower end of theoretical predictions for Arp 220, indicating that current IACTs are approaching the sensitivity needed to detect or rule out emission.