A cross-correlation study of the Fermi-LAT γ-ray diffuse extragalactic signal

Journal Article · Mon Sep 12 00:00:00 EDT 2011 · Monthly Notices of the Royal Astronomical Society

DOI:https://doi.org/10.1111/j.1365-2966.2011.19200.x· OSTI ID:1357203

Xia, Jun -Qing ^[1]; Cuoco, Alessandro ^[2]; Branchini, Enzo ^[3]; Fornasa, Mattia ^[4]; Viel, Matteo ^[5]

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1. Scuola Internazionale di Studi Avanzati, Trieste (Italy)
2. Stockholm Univ., Stockholm (Sweden)
3. Univ. degli Studi 'Roma Tre', Roma (Italy); Osservatorio Astronomico di Brera, Milano (Italy); Sezione di Toma Tre, Roma (Italy)
4. Istituto de Astrofisica de Andalucia (CSIC), Granada (Spain)
5. INAF Osservatorio Astronomico di Trieste, Trieste (Italy); Sezione de Trieste, Trieste (Italy)

In this work, starting from 21 months of data from the Fermi Large Area Telescope (LAT), we derive maps of the residual isotropic γ-ray emission, a relevant fraction of which is expected to be contributed by the extragalactic diffuse γ-ray background (EGB). We search for the auto-correlation signals in the above γ-ray maps and for the cross-correlation signal with the angular distribution of different classes of objects that trace the large-scale structure of the Universe. We compute the angular two-point auto-correlation function of the residual Fermi-LAT maps at energies E > 1 GeV, E > 3 GeV and E > 30 GeV well above the Galactic plane and find no significant correlation signal. This is, indeed, what is expected if the EGB were contributed by BL Lacertae (BLLacs), Flat Spectrum Radio Quasars (FSRQs) or star-forming galaxies, since, in this case, the predicted signal is very weak. Then, we search for the Integrated Sachs–Wolfe (ISW) signature by cross-correlating the Fermi-LAT maps with the 7-year Wilkinson Microwave Anisotropy Probe (WMAP7) cosmic microwave background map. We find a cross-correlation consistent with zero, even though the expected signal is larger than that of the EGB auto-correlation. Lastly, in an attempt to constrain the nature of the γ-ray background, we cross-correlate the Fermi-LAT maps with the angular distributions of objects that may contribute to the EGB: quasi-stellar objects (QSOs) in the Sloan Digital Sky Survey Data Release 6 (SDSS-DR6) catalogue, NRAO VLA Sky Survey (NVSS) galaxies, Two Micron All Sky Survey (2MASS) galaxies and Luminous Red Galaxies (LRGs) in the SDSS catalogue. The cross-correlation is always consistent with zero, in agreement with theoretical expectations, but we find (with low statistical significance) some interesting features that may indicate that some specific classes of objects contribute to the EGB. A χ² analysis confirms that the correlation properties of the 21-month data do not provide strong constraints of the EGB origin. However, the results suggest that the situation will significantly improve with the 5- and 10-yr Fermi-LAT data. In future, the EGB analysis will then allow placing significant constraints on the nature of the EGB and might provide, in addition, a detection of the ISW signal.

• Research Organization: SLAC National Accelerator Lab., Menlo Park, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC02-76SF00515
• OSTI ID: 1357203
• Journal Information: Monthly Notices of the Royal Astronomical Society, Vol. 416, Issue 3; ISSN 0035-8711
• Publisher: Royal Astronomical Society
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
cosmology: observations
cosmology: theory
large-scale structure of Universe
gamma-rays: diffuse background