The Astrophysical Journal, 784:138 (6pp), 2014 April 1 doi:10.1088/0004-637X/784/2/138 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. AN EMPIRICAL DETERMINATION OF THE INTERGALACTIC BACKGROUND LIGHT USING NEAR-INFRARED DEEP GALAXY SURVEY DATA OUT TO 5 μm AND THE GAMMA-RAY OPACITY OF THE UNIVERSE Sean T. Scully1, Matthew A. Malkan2, and Floyd W. Stecker2,3 1 Department of Physics and Astronomy, James Madison University, Harrisonburg, VA 22807, USA 2 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA; [email protected] 3 Astrophysics Science Division, NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA Received 2014 January 21; accepted 2014 February 24; published 2014 March 14 ABSTRACT We extend our previous model-independent determination of the intergalactic background light, based purely on galaxy survey data, out to a wavelength of 5 μm. Our approach enables us to constrain the range of photon densities, based on the uncertainties from observationally determined luminosity densities and colors. We further determine a 68% confidence upper and lower limit on the opacity of the universe to γ -rays up to energies of 1.6/(1 + z)TeV. A comparison of our lower limit redshift-dependent opacity curves to the opacity limits derived from the results of both ground-based air Cerenkov telescope and Fermi-LAT observations of PKS 1424+240 allows us to place a new upper limit on the redshift of this source, independent of IBL modeling. Key words: BL Lacertae objects: individual (PKS 1424+240) – diffuse radiation – gamma rays: general Online-only material: color figure <z< 1. INTRODUCTION even out to redshifts 2 4, have become sufficiently complete to eliminate the necessity of using model-based ap- Past work on estimating the spectral and redshift charac- proaches. The high-redshift galaxies are sufficiently sampled teristics of the intergalactic photon density (IBL; with the to enable us to interpolate between observationally determined z = 0 IBL usually referred to as the EBL) has depended luminosity densities obtained for specific wavebands. We use on various assumptions as to the evolution of stellar pop- observationally determined galaxy colors and their associated ulations and dust absorption in galaxies. There have also errors to fill in the gaps in wavelength bands where there are been attempts to probe the extragalactic background light less data. (EBL) using studies of blazar γ -ray spectra (Ackermann Here we apply the technique of SMS to determine the IBL et al. 2012; Abramowski et al. 2013), an approach originally to a wavelength of ∼5 μm, thus extending our previous range suggested by Stecker et al. (1992). of γ -ray opacities of the universe to energies up to 1.6 TeV. We previously have pursued an independent, fully empiri- This range is not only relevant for the sources with the highest cal approach to calculating the IBL using deep galaxy survey redshifts observed by the Fermi-LAT Telescope, but also for data (Stecker et al. 2012—hereafter SMS; see also Helgason & results in the sub-TeV and TeV energy range made using ground- Kashlinsky 2012). In this paper, we extend the previous SMS based air Cerenkov telescopes. Our technique to determine the results on the γ -ray opacity of the universe to TeV energies expected γ -ray opacity of the universe is thus complementary to accessible to ground-based air Cerenkov telescopes, here in- the technique of using γ -ray observations directly, because the cluding galaxy survey data extending beyond the photometric intrinsic (unabsorbed) spectra of the γ -ray sources are uncertain. I band to the M band (i.e., 0.8 μmto∼5 μm). We stress that To further show the utility of our approach, we apply our the SMS approach, being completely observationally based, lower limit opacities to the results of both ground-based air is model independent, relying only on published luminosity Cerenkov telescope observations and Fermi-LAT observations densities. Such an approach has a particular advantage over of the BL Lac object PKS 1424+240, which allows us to place model-based methods, as it enables a determination of both the a true upper limit on the redshift of this source, independent of IBL and its observational uncertainties without making any IBL models. Previous observations of the ultraviolet absorption assumptions about how the galaxy luminosity functions (LFs) from the intergalactic medium of this object place a lower limit evolve, as some approaches require (e.g. Malkan & Stecker on its redshift of z .6035 (Furniss et al. 2013). 1998, 2001; Kneiske et al. 2002; Stecker et al. 2006; Our results give the γ -ray opacity as a function of energy Franceschini et al. 2008; Finke et al. 2010; Kneiske & Dole and redshift to within an observationally determined 68% 2010) or by using semi-analytic models that make assumptions confidence band. Therefore, a direct comparison with γ -ray concerning galaxy evolution, stellar population synthesis mod- spectral data will allow an independent determination of the els, star formation rates, and dust attenuation (e.g., Gilmore et al. previously suggested effects on such spectra. These effects 2009; Somerville et al. 2012). Dom´ınguez et al. (2011)usedthe include secondary γ -ray production generated by (1) cosmic- redshift evolution of the K-band LFs of Cirasuolo et al. (2010), ray interactions along the line of sight to the source (Essey together with model templates based on AEGIS data to model et al. 2010; Essey & Kusenko 2012) and (2) line-of-sight the IBL. A recent detailed review of the problem has been given photon–axion oscillations during propagation (e.g., De Angelis by Dwek & Krennrich (2013). et al. 2007). Both of these processes can produce an apparent Direct integration of the galaxy survey data in the wave- reduction of the pair-production opacity effect as derived from length range from the far-ultraviolet (FUV) to ∼5 μm has now γ -ray spectra alone. Thus our results can be critical in analyzing become possible. Observational data in this wavelength range, the implications of present and future γ -ray spectral data. 1 The Astrophysical Journal, 784:138 (6pp), 2014 April 1 Scully, Malkan, & Stecker 2. DETERMINATION OF THE IBL FROM GALAXY Table 1 PHOTON EMISSIVITIES Average Colors Waveband Relative νLν Std. Deviation Min. Value Max. Value Galaxy LFs, Φν (L), can be determined by properly sampling galaxies in a survey for a given wavelength and accounting J (1.2 μm) 1a –– – for biases. As in SMS, in this work, we chose to use only H (1.6 μm) 1.19 0.08 1.09 1.37 those references that give values for the integrated LF, i.e., K (2.2 μm) 0.97 0.12 0.83 1.23 L (3.5 μm) 0.70 0.15 0.43 0.99 the luminosity density (LD), ρL (z). This is because there is ν μ generally a lack of knowledge about the covariance of the errors M (4.8 m) 0.50 0.16 0.3 0.84 in the Schechter function parameters used to determine the dominant statistical errors in their analyses. Sufficient numbers Notes. The bias in the colors toward bluer averages occurs because there are not many galaxies in the very red categories, so the mean color stays on the blue of galaxies need to be observed up and down the LF for a reliable side of (Max + Min)/2. constraint on their total LD, i.e., the integral of the LF. It is not aAll differences are relative to the J band. even guaranteed that a Schechter function will always provide the best analytic fit to the LF data. The co-moving radiation field. They obtained well defined complete LFs in the local energy density, uν (z), is derived from the equivalent co-moving redshift bin of z 0.2. They then derived galaxy luminosity specific emissivity Eν (z) = ρL (z). ν densities and generated a best-fit SED (their Figure 13). We take 2.1. Luminosity Densities these data to be representative of low-redshift galaxies (z 0.5). To estimate the red and NIR SEDs of galaxies at higher In this work, we use the same set of galaxy survey data and redshifts, we have used the template SEDs derived by Kriek resulting luminosity densities that covered rest-frame wave- et al. (2010, 2011).4 Those authors utilized extensive multi- lengths from the FUV to the I band as in SMS; however, we band photometry obtained for a sample of 3500 K-band selected now extend our calculations to the J and K bands. We have ex- galaxies, at redshifts between z = 0.5 and z = 2.0, close to a cluded the H band due to a paucity of observational data but mass-limited sample of galaxies. For redshifts between z = 0.5 we have checked that our approach is consistent with what data and z = 1.0, the galaxy stellar masses are uniformly distributed 9.7 11.3 exists in this band. We further use the colors derived in the next between the range of 10 and 10 M. At redshifts from subsection to extend the calculation into the L and M bands (out 1 to 2, the mass range covered by these SEDs shrinks to to 4.8 μm). 1010.3–1011.3. Kriek et al. (2010) grouped their galaxy SEDs into The co-moving radiation energy density uν (z) is the time 30 average templates spanning the full range from the bluest to integral of the co-moving specific emissivity Eν (z), the reddest galaxies.