Jack Singal

PHAT: PHoto-z Accuracy Testing

Context. Photometric redshifts (photo-zs) have become an essential tool in extragalactic astronomy. Many current and upcoming observing programmes require great accuracy of photo-zs to reach their scientific goals. Aims. Here we introduce PHAT, the PHoto-z Accuracy Testing programme, an international initiative to test and compare different methods of photo-z estimation. Methods. Two different test environments are set up, one (PHAT0) based on simulations to test the basic functionality of the different photo-z codes, and another one (PHAT1) based on data from the GOODS survey including 18-band photometry and similar to 2000 spectroscopic redshifts. Results. The accuracy of the different methods is expressed and ranked by the global photo-z bias, scatter, and outlier rates. While most methods agree very well on PHAT0 there are differences in the handling of the Lyman-alpha forest for higher redshifts. Furthermore, different methods produce photo-z scatters that can differ by up to a factor of two even in this idealised case. A larger spread in accuracy is found for PHAT1. Few methods benefit from the addition of mid-IR photometry. The accuracy of the other methods is unaffected or suffers when IRAC data are included. Remaining biases and systematic effects can be explained by shortcomings in the different template sets (especially in the mid-IR) and the use of priors on the one hand and an insufficient training set on the other hand. Some strategies to overcome these problems are identified by comparing the methods in detail. Scatters of 4-8% in Delta z/(1 + z) were obtained, consistent with other studies. However, somewhat larger outlier rates (\textgreater 7.5% with Delta z/(1 + z) \textgreater 0.15; \textgreater 4.5% after cleaning) are found for all codes that can only partly be explained by AGN or issues in the photometry or the spec-z catalogue. Some outliers were probably missed in comparisons of photo-zs to other, less complete spectroscopic surveys in the past. There is a general trend that empirical codes produce smaller biases than template-based codes. Conclusions. The systematic, quantitative comparison of different photo-z codes presented here is a snapshot of the current state-of-the-art of photo-z estimation and sets a standard for the assessment of photo-z accuracy in the future. The rather large outlier rates reported here for PHAT1 on real data should be investigated further since they are most probably also present (and possibly hidden) in many other studies. The test data sets are publicly available and can be used to compare new, upcoming methods to established ones and help in guiding future photo-z method development.

ARCADE 2 Measurement of the Absolute Sky Brightness at 3-90 GHz

The ARCADE 2 instrument has measured the absolute temperature of the sky at frequencies 3, 8, 10, 30, and 90 GHz, using an open-aperture cryogenic instrument observing at balloon altitudes with no emissive windows between the beam-forming optics and the sky. An external blackbody calibrator provides an in situ reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approximating the cosmic microwave background (CMB) temperature. A linear model is used to compare the output of each radiometer to a set of thermometers on the instrument. Small corrections are made for the residual emission from the flight train, balloon, atmosphere, and foreground Galactic emission. The ARCADE 2 data alone show an excess radio rise of 54 ± 6 mK at 3.3 GHz in addition to a CMB temperature of 2.731 ± 0.004 K. Combining the ARCADE 2 data with data from the literature shows an excess power-law spectrum of T = 24.1 ± 2. 1( K) (ν/ν0) −2.599±0.036 from 22 MHz to 10 GHz (ν0 = 310 MHz) in addition to a CMB temperature of 2.725 ± 0.001 K.The ARCADE 2 instrument has measured the absolute temperature of the sky at frequencies 3, 8, 10, 30, and 90 GHz, using an open-aperture cryogenic instrument observing at balloon altitudes with no emissive windows between the beam-forming optics and the sky. An external blackbody calibrator provides an in situ reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approximating the CMB temperature. A linear model is used to compare the output of each radiometer to a set of thermometers on the instrument. Small corrections are made for the residual emission from the flight train, balloon, atmosphere, and foreground Galactic emission. The ARCADE 2 data alone show an extragalactic rise of 50 ± 7 mK at 3.3 GHz in addition to a CMB temperature of 2.730 ± .004 K. Combining the ARCADE 2 data with data from the literature shows a background power law spectrum of T = 1.26 ± 0.09 [K] (�/�0) −2.60±0.04 from 22 MHz to 10 GHz (�0 = 1 GHz) in addition to a CMB temperature of 2.725 ± .001 K. Subject headings: cosmology: Cosmic Microwave Background — cosmology: Observations

INTERPRETATION OF THE ARCADE 2 ABSOLUTE SKY BRIGHTNESS MEASUREMENT

We use absolutely calibrated data between 3 and 90 GHz from the 2006 balloon flight of the ARCADE 2 instrument, along with previous measurements at other frequencies, to constrain models of extragalactic emission. Such emission is a combination of the cosmic microwave background (CMB) monopole, Galactic foreground emission, the integrated contribution of radio emission from external galaxies, any spectral distortions present in the CMB, and any other extragalactic source. After removal of estimates of foreground emission from our own Galaxy, and an estimated contribution of external galaxies, we present fits to a combination of the flat-spectrum CMB and potential spectral distortions in the CMB. We find 2σ upper limits to CMB spectral distortions of μ< 6 × 10 −4 and |Yff| < 1 × 10 −4 . We also find a significant detection of a residual signal beyond that, which can be explained by the CMB plus the integrated radio emission from galaxies estimated from existing surveys. This residual signal may be due to an underestimated galactic foreground contribution, an unaccounted for contribution of a background of radio sources, or some combination of both. The residual signal is consistent with emission in the form of a power law with amplitude 18.4 ± 2.1 K at 0.31 GHz and a spectral index of −2.57 ± 0.05.

ARCADE 2 OBSERVATIONS OF GALACTIC RADIO EMISSION

We use absolutely calibrated data from the ARCADE 2 flight in 2006 July to model Galactic emission at frequencies 3, 8, and 10 GHz. The spatial structure in the data is consistent with a superposition of free–free and synchrotron emission. Emission with spatial morphology traced by the Haslam 408 MHz survey has spectral index βsynch =− 2.5 ± 0.1, with free–free emission contributing 0.10 ± 0.01 of the total Galactic plane emission in the lowest ARCADE 2 band at 3.15 GHz. We estimate the total Galactic emission toward the polar caps using either a simple plane-parallel model with csc |b| dependence or a model of high-latitude radio emission traced by the COBE/FIRAS map of Cii emission. Both methods are consistent with a single power law over the frequency range 22 MHz to 10 GHz, with total Galactic emission toward the north polar cap TGal = 10.12 ± 0.90 K and spectral index β =− 2.55 ± 0.03 at reference frequency 0.31 GHz. Emission associated with the plane-parallel structure accounts for only 30% of the observed high-latitude sky temperature, with the residual in either a Galactic halo or an isotropic extragalactic background. The well-calibrated ARCADE 2 maps provide a new test for spinning dust emission, based on the integrated intensity of emission from the Galactic plane instead of cross-correlations with the thermal dust spatial morphology. The Galactic plane intensity measured by ARCADE 2 is fainter than predicted by models without spinning dust and is consistent with spinning dust contributing 0.4 ± 0.1 of the Galactic plane emission at 23 GHz.

Sources of the Radio Background Considered

We investigate different scenarios for the origin of the extragalactic radio background. The surface brightness of the background, as reported by the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission 2 (ARCADE 2) collaboration, is several times higher than that which would result from currently observed radio sources. We consider contributions to the background from diffuse synchrotron emission from clusters and the intergalactic medium, previously unrecognized flux from low-surface-brightness regions of radio sources and faint point sources below the flux limit of existing surveys. By examining radio source counts available in the literature, we conclude that most of the radio background is produced by radio point sources that dominate at sub-μJy fluxes. We show that a truly diffuse background produced by electrons far from galaxies is ruled out because such energetic electrons would overproduce the observed X-ray/γ -ray background through inverse Compton scattering of the other photon fields. Unrecognized flux from low-surface-brightness regions of extended radio sources, or moderate flux sources missed entirely by radio source count surveys, cannot explain the bulk of the observed background but may contribute as much as 10 per cent. We consider both radio supernovae and radio-quiet quasars as candidate sources for the background and show that both fail to produce it at the observed level because of an insufficient number of objects and total flux, although radio-quiet quasars contribute at the level of at least a few per cent. We conclude that if the radio background is at the level reported, a majority of the total surface brightness would have to be produced by ordinary star-forming galaxies above redshift 1 characterized by an evolving radio–far-infrared correlation, which changes towards the radio loud with redshift.

Flux and Photon Spectral Index Distributions of Fermi-LAT Blazars And Contribution To The Extragalactic Gamma-ray Background

We present a determination of the distributions of the photon spectral index and gamma-ray flux—the so-called log N-log S relation—for the 352 blazars detected with a greater than approximately 7σ detection threshold and located above ±20° Galactic latitude by the Large Area Telescope of the Fermi Gamma-ray Space Telescope in its first year catalog. Because the flux detection threshold depends on the photon index, the observed raw distributions do not provide the true log N-log S counts or the true distribution of the photon index. We use the non-parametric methods developed by Efron and Petrosian to reconstruct the intrinsic distributions from the observed ones which account for the data truncations introduced by observational bias and includes the effects of the possible correlation between the two variables. We demonstrate the robustness of our procedures using a simulated data set of blazars and then apply these to the real data and find that for the population as a whole the intrinsic flux distribution can be represented by a broken power law with high and low indices of –2.37 ± 0.13 and –1.70 ± 0.26, respectively, and the intrinsic photon index distribution can be represented by a Gaussian with mean of 2.41 ± 0.13 and width of 0.25 ± 0.03. We also find the intrinsic distributions for the sub-populations of BL Lac and flat spectrum radio quasar type blazars separately. We then calculate the contribution of Fermi blazars to the diffuse extragalactic gamma-ray background radiation. Under the assumption that the flux distribution of blazars continues to arbitrarily low fluxes, we calculate the best-fit contribution of all blazars to the total extragalactic gamma-ray output to be 60%, with a large uncertainty.

Determination of the Intrinsic Luminosity Time Correlation in the X-Ray Afterglows of Gamma-Ray Bursts

Gamma-ray bursts (GRBs), which have been observed up to redshifts z 9.5, can be good probes of the early universe and have the potential to test cosmological models. Dainottis analysis of GRB Swift afterglow light curves with known redshifts and a definite X-ray plateau shows an anti-correlation between the rest-frame time when the plateau ends (the plateau end time) and the calculated luminosity at that time (or approximately an anti-correlation between plateau duration and luminosity). Here, we present an update of this correlation with a larger data sample of 101 GRBs with good light curves. Since some of this correlation could result from the redshift dependences of these intrinsic parameters, namely, their cosmological evolution, we use the Efron-Petrosian method to reveal the intrinsic nature of this correlation. We find that a substantial part of the correlation is intrinsic and describe how we recover it and how this can be used to constrain physical models of the plateau emission, the origin of which is still unknown. The present result could help to clarify the debated nature of the plateau emission.

The Radio and Optical Luminosity Evolution of Quasars. II. The SDSS Sample

We determine the radio and optical luminosity evolutions and the true distribution of the radio loudness parameter R, defined as the ratio of the radio to optical luminosity, for a set of more than 5000 quasars combining SDSS optical and FIRST radio data. We apply the method of Efron and Petrosian to access the intrinsic distribution parameters, taking into account the truncations and correlations inherent in the data. We find that the population exhibits strong positive evolution with redshift in both wavebands, with somewhat greater radio evolution than optical. With the luminosity evolutions accounted for, we determine the density evolutions and local radio and optical luminosity functions. The intrinsic distribution of the radio loudness parameter R is found to be quite different than the observed one, and is smooth with no evidence of a bi-modality in radio loudness. The results we find are in general agreement with the previous analysis of Singal et al. 2011 which used POSS-I optical and FIRST radio data.

On the Radio and Optical Luminosity Evolution of Quasars

We calculate simultaneously the radio and optical luminosity evolutions of quasars, and the distribution in radio loudness R defined as the ratio of radio and optical luminosities, using a flux limited data set containing 636 quasars with radio and optical fluxes from White et al. We first note that when dealing with multivariate data it is imperative to first determine the true correlations among the variables, not those introduced by the observational selection effects, before obtaining the individual distributions of the variables. We use the methods developed by Efron and Petrosian which are designed to obtain unbiased correlations, distributions, and evolution with redshift from a data set truncated due to observational biases. It is found that as expected the population of quasars exhibits strong positive correlation between the radio and optical luminosities and that this correlation deviates from a simple linear relation in a way indicating that more luminous quasars are more radio loud. We also find that there is a strong luminosity evolution with redshift in both wavebands, with significantly higher radio than optical evolution. We conclude that the luminosity evolution obtained by arbitrarily separating the sources into radio loud (R > 10) and radio quiet (R < 10) populations introduces significant biases that skew the result considerably. We also construct the local radio and optical luminosity functions and the density evolution. Finally, we consider the distribution of the radio loudness parameter R obtained from careful treatment of the selection effects and luminosity evolutions with that obtained from the raw data without such considerations. We find a significant difference between the two distributions and no clear sign of bi-modality in the true distribution. Our results indicate therefore, somewhat surprisingly, that there is no critical switch in the efficiency of the production of disk outflows/jets between very radio quiet and very radio loud quasars, but rather a smooth transition. Also, this efficiency seems higher for the high-redshift and more luminous sources in the considered sample.

THE ARCADE 2 INSTRUMENT

The second generation Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE 2) instrument is a balloon-borne experiment to measure the radiometric temperature of the cosmic microwave background and Galactic and extragalactic emission at six frequencies from 3 to 90 GHz. ARCADE 2 utilizes a double-nulled design where emission from the sky is compared to that from an external cryogenic full-aperture blackbody calibrator by cryogenic switching radiometers containing internal blackbody reference loads. In order to further minimize sources of systematic error, ARCADE 2 features a cold fully open aperture with all radiometrically active components maintained at near 2.7 K without windows or other warm objects, achieved through a novel thermal design. We discuss the design and performance of the ARCADE 2 instrument in its 2005 and 2006 flights.