title: A low level of extragalactic background light as revealed by gamma-rays from blazars.
abstract: The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the 'first stars', which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light. An alternative approach is to study the absorption features imprinted on the gamma-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons. Here we report the discovery of gamma-ray emission from the blazars H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources-in particular from the first stars formed. This result also indicates that intergalactic space is more transparent to gamma-rays than previously thought.
authors.affiliation: Max-Planck-Institut für Kernphysik, PO Box 103980, D 69029 Heidelberg, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Aharonian
authors.forename: A G
authors.initials: AG
authors.lastname: Akhperjanian
authors.forename: A R
authors.initials: AR
authors.lastname: Bazer-Bachi
authors.forename: M
authors.initials: M
authors.lastname: Beilicke
authors.forename: W
authors.initials: W
authors.lastname: Benbow
authors.forename: D
authors.initials: D
authors.lastname: Berge
authors.forename: K
authors.initials: K
authors.lastname: Bernlöhr
authors.forename: C
authors.initials: C
authors.lastname: Boisson
authors.forename: O
authors.initials: O
authors.lastname: Bolz
authors.forename: V
authors.initials: V
authors.lastname: Borrel
authors.forename: I
authors.initials: I
authors.lastname: Braun
authors.forename: F
authors.initials: F
authors.lastname: Breitling
authors.forename: A M
authors.initials: AM
authors.lastname: Brown
authors.forename: P M
authors.initials: PM
authors.lastname: Chadwick
authors.forename: L-M
authors.initials: LM
authors.lastname: Chounet
authors.forename: R
authors.initials: R
authors.lastname: Cornils
authors.forename: L
authors.initials: L
authors.lastname: Costamante
authors.forename: B
authors.initials: B
authors.lastname: Degrange
authors.forename: H J
authors.initials: HJ
authors.lastname: Dickinson
authors.forename: A
authors.initials: A
authors.lastname: Djannati-Ataï
authors.forename: L O'C
authors.initials: LO
authors.lastname: Drury
authors.forename: G
authors.initials: G
authors.lastname: Dubus
authors.forename: D
authors.initials: D
authors.lastname: Emmanoulopoulos
authors.forename: P
authors.initials: P
authors.lastname: Espigat
authors.forename: F
authors.initials: F
authors.lastname: Feinstein
authors.forename: G
authors.initials: G
authors.lastname: Fontaine
authors.forename: Y
authors.initials: Y
authors.lastname: Fuchs
authors.forename: S
authors.initials: S
authors.lastname: Funk
authors.forename: Y A
authors.initials: YA
authors.lastname: Gallant
authors.forename: B
authors.initials: B
authors.lastname: Giebels
authors.forename: S
authors.initials: S
authors.lastname: Gillessen
authors.forename: J F
authors.initials: JF
authors.lastname: Glicenstein
authors.forename: P
authors.initials: P
authors.lastname: Goret
authors.forename: C
authors.initials: C
authors.lastname: Hadjichristidis
authors.forename: D
authors.initials: D
authors.lastname: Hauser
authors.forename: M
authors.initials: M
authors.lastname: Hauser
authors.forename: G
authors.initials: G
authors.lastname: Heinzelmann
authors.forename: G
authors.initials: G
authors.lastname: Henri
authors.forename: G
authors.initials: G
authors.lastname: Hermann
authors.forename: J A
authors.initials: JA
authors.lastname: Hinton
authors.forename: W
authors.initials: W
authors.lastname: Hofmann
authors.forename: M
authors.initials: M
authors.lastname: Holleran
authors.forename: D
authors.initials: D
authors.lastname: Horns
authors.forename: A
authors.initials: A
authors.lastname: Jacholkowska
authors.forename: O C
authors.initials: OC
authors.lastname: de Jager
authors.forename: B
authors.initials: B
authors.lastname: Khélifi
authors.forename: S
authors.initials: S
authors.lastname: Klages
authors.forename: Nu
authors.initials: N
authors.lastname: Komin
authors.forename: A
authors.initials: A
authors.lastname: Konopelko
authors.forename: I J
authors.initials: IJ
authors.lastname: Latham
authors.forename: R
authors.initials: R
authors.lastname: Le Gallou
authors.forename: A
authors.initials: A
authors.lastname: Lemière
authors.forename: M
authors.initials: M
authors.lastname: Lemoine-Goumard
authors.forename: N
authors.initials: N
authors.lastname: Leroy
authors.forename: T
authors.initials: T
authors.lastname: Lohse
authors.forename: J M
authors.initials: JM
authors.lastname: Martin
authors.forename: O
authors.initials: O
authors.lastname: Martineau-Huynh
authors.forename: A
authors.initials: A
authors.lastname: Marcowith
authors.forename: C
authors.initials: C
authors.lastname: Masterson
authors.forename: T J L
authors.initials: TJ
authors.lastname: McComb
authors.forename: M
authors.initials: M
authors.lastname: de Naurois
authors.forename: S J
authors.initials: SJ
authors.lastname: Nolan
authors.forename: A
authors.initials: A
authors.lastname: Noutsos
authors.forename: K J
authors.initials: KJ
authors.lastname: Orford
authors.forename: J L
authors.initials: JL
authors.lastname: Osborne
authors.forename: M
authors.initials: M
authors.lastname: Ouchrif
authors.forename: M
authors.initials: M
authors.lastname: Panter
authors.forename: G
authors.initials: G
authors.lastname: Pelletier
authors.forename: S
authors.initials: S
authors.lastname: Pita
authors.forename: G
authors.initials: G
authors.lastname: Pühlhofer
authors.forename: M
authors.initials: M
authors.lastname: Punch
authors.forename: B C
authors.initials: BC
authors.lastname: Raubenheimer
authors.forename: M
authors.initials: M
authors.lastname: Raue
authors.forename: J
authors.initials: J
authors.lastname: Raux
authors.forename: S M
authors.initials: SM
authors.lastname: Rayner
authors.forename: A
authors.initials: A
authors.lastname: Reimer
authors.forename: O
authors.initials: O
authors.lastname: Reimer
authors.forename: J
authors.initials: J
authors.lastname: Ripken
authors.forename: L
authors.initials: L
authors.lastname: Rob
authors.forename: L
authors.initials: L
authors.lastname: Rolland
authors.forename: G
authors.initials: G
authors.lastname: Rowell
authors.forename: V
authors.initials: V
authors.lastname: Sahakian
authors.forename: L
authors.initials: L
authors.lastname: Saugé
authors.forename: S
authors.initials: S
authors.lastname: Schlenker
authors.forename: R
authors.initials: R
authors.lastname: Schlickeiser
authors.forename: C
authors.initials: C
authors.lastname: Schuster
authors.forename: U
authors.initials: U
authors.lastname: Schwanke
authors.forename: M
authors.initials: M
authors.lastname: Siewert
authors.forename: H
authors.initials: H
authors.lastname: Sol
authors.forename: D
authors.initials: D
authors.lastname: Spangler
authors.forename: R
authors.initials: R
authors.lastname: Steenkamp
authors.forename: C
authors.initials: C
authors.lastname: Stegmann
authors.forename: J-P
authors.initials: JP
authors.lastname: Tavernet
authors.forename: R
authors.initials: R
authors.lastname: Terrier
authors.forename: C G
authors.initials: CG
authors.lastname: Théoret
authors.forename: M
authors.initials: M
authors.lastname: Tluczykont
authors.forename: C
authors.initials: C
authors.lastname: van Eldik
authors.forename: G
authors.initials: G
authors.lastname: Vasileiadis
authors.forename: C
authors.initials: C
authors.lastname: Venter
authors.forename: P
authors.initials: P
authors.lastname: Vincent
authors.forename: H J
authors.initials: HJ
authors.lastname: Völk
authors.forename: S J
authors.initials: SJ
authors.lastname: Wagner