The Atacama Cosmology Telescope: Extragalactic Sources at 148 GHz in the 2008 Survey

TL;DR

This study characterizes extragalactic sources at 148 GHz in the ACT 2008 southern survey, delivering a 157-source catalog and detailed flux-density validation. It employs a two-dimensional matched filter, end-to-end simulations, and Bayesian deboosting to obtain robust fluxes and spectral indices, revealing a radio-dominated population with mild spectral steepening between 5–148 GHz. The observed differential counts are consistent with a Toffolatti-like radio population scaled by ≈0.34, and the inferred residual Poisson power from synchrotron sources is $C^{\rm Sync} \approx (2.8 \pm 0.3) \times 10^{-6} \mu{K}^2$, aligning with concurrent analyses of high-ℓ CMB data. Collectively, the results refine foreground models for CMB/SZ studies and inform expectations for source populations in future ACT and related mm-wave surveys.

Abstract

We report on extragalactic sources detected in a 455 square-degree map of the southern sky made with data at a frequency of 148 GHz from the Atacama Cosmology Telescope 2008 observing season. We provide a catalog of 157 sources with flux densities spanning two orders of magnitude: from 15 to 1500 mJy. Comparison to other catalogs shows that 98% of the ACT detections correspond to sources detected at lower radio frequencies. Three of the sources appear to be associated with the brightest cluster galaxies of low redshift X-ray selected galaxy clusters. Estimates of the radio to mm-wave spectral indices and differential counts of the sources further bolster the hypothesis that they are nearly all radio sources, and that their emission is not dominated by re-emission from warm dust. In a bright (>50 mJy) 148 GHz-selected sample with complete cross-identifications from the Australia Telescope 20 GHz survey, we observe an average steepening of the spectra between 5, 20, and 148 GHz with median spectral indices of $α_{\rm 5-20} = -0.07 \pm 0.06$, $α_{\rm 20-148} = -0.39 \pm0.04$, and $α_{\rm 5-148} = -0.20 \pm 0.03$. When the measured spectral indices are taken into account, the 148 GHz differential source counts are consistent with previous measurements at 30 GHz in the context of a source count model dominated by radio sources. Extrapolating with an appropriately rescaled model for the radio source counts, the Poisson contribution to the spatial power spectrum from synchrotron-dominated sources with flux density less than 20 mJy is $C^{\rm Sync} = (2.8 \pm 0.3) \times 10^{-6} \micro\kelvin^2$.

Figures (10)

• Figure 1: Sensitivity map with detections. The subset of the ACT 2008 148 GHz dataset considered for this study lies between right ascension $00^{\mathrm h}12^{\rm m}$ and $07^{\mathrm h}08^{\rm m}$ and declination $-56\degree11\hbox{$^\prime$}$ and $-49\degree00\hbox{$^\prime$}$ (455 square-degrees). The gray-scale encodes the rms of the map in mJy. The deepest data correspond to an exposure time of 23.5 minutes per square-arcminute and a 1$\sigma$ sensitivity of 2.5 mJy. White circles mark the locations of ACT sources. The diameter of each circle is proportional to the log of the associated source flux density. Towards the edge of the map, the noise properties display local variation. For this reason, detections with flux density values below 50 mJy have been discarded in regions where the rms exceeds 4.6 mJy, corresponding to less than 7 minutes of integration. This, together with an exclusion of all detections below 5.25 $\sigma$, accounts for the relative dearth of detections in areas of shallow coverage (See Section \ref{['subsec:sourceDetection']}.).
• Figure 2: Model ACT auto-power spectra used for the matched filter. Left. One-dimensional spectra decomposed by component. The CMB spectrum is taken from WMAP5 nolta/etal:2009. The undetected sources and the Sunyaev-Zel'dovich effect from galaxy clusters are 1D templates fit to the high-$\ell$ 148 GHz spectrum in fowler/etal:2010. The 1D noise spectrum is obtained by radially-binning the average of 2D spectra from ACT 148 GHz jackknife maps. The spectra are dominated at low-$\ell$ by the CMB and atmospheric noise and at high-$\ell$ by white detector and photon shot noise. Convolution of all celestial components by the ACT beam results in attenuation of the corresponding spectra at high-$\ell$. Right. The model two-dimensional auto power spectrum. The spectrum includes noise, CMB, undetected sources and the Sunyaev-Zel'dovich effect from galaxy clusters. The latter three components were obtained from the 1D models. The ACT noise is isotropic except for extra noise in scan parallel (from residual 1/f) and scan perpendicular (from detector row correlations) directions. For each rising and setting scan direction, two orthogonal bands of excess power are centered on the origin rotated $60^\degree$ with respect to one another. To minimize contamination, it is important to deweight data in these diagonal modes through the two-dimensional matched filter.
• Figure 3: 148 GHz map. The submap shown above is a sample 64 square-degrees of the survey field. The data have been matched filtered such that the gray-scale is in units of flux density (mJy). The inset shows the flux density distribution across the data weighted by the max-normalized square-root of the number of data per pixel $\sqrt{N_{\rm obs}(\hbox{\boldmath$x$})}/\sqrt{N_{\rm obs, max}}$. Thus the distribution represents the data in the deepest part of the map (although it uses weighted data from all regions). The data distribution is shown as a grey histogram on which is plotted a dashed Gaussian distribution with standard deviation 2.5 mJy. The positive non-Gaussian tail may, in part, be attributed to sources and the negative tail to ringing from the filter about sources as well as SZ (e.g., ACT-CL J0438-5419 from menanteau/etal:2010). Several sources are apparent as white points surrounded by darker rings from the filter. The white contour marks the transition at the edge of the map where the rms exceeds 4.6 mJy, corresponding to less than 7 minutes of integration per arcminute. In this region we have excluded detections below 50 mJy due to contamination from local noise. The source above the contour at $\delta \approx -50\degree$ has a flux density of 150 mJy and is included in the catalog.
• Figure 4: Astrometric Accuracy. The small filled circles are the positional offsets of ACT sources with S/N $>$ 20 from counterparts in AT20G. The error bars show the rms in right ascension (3.5$"$) and declination (3.3$"$) and are centered on the mean of the distribution: $-0.3\hbox{$^{\prime\prime}$}\pm0.7\hbox{$^{\prime\prime}$}$ in right ascension and $0.0\hbox{$^{\prime\prime}$}\pm0.6\hbox{$^{\prime\prime}$}$ in declination.
• Figure 5: Source Profile Fitting. The radially binned and normalized brightness profile observed for ACT-S J021046$-$510100 is represented by the points with error bars. The solid line running through the points is the measured ACT beam from hincks/etal:prep convolved with a Gaussian with $\sigma=3"$ plus an additive background term. The Gaussian convolution is intended to represent spreading of the beam by random pointing error. The value of $\sigma=3"$ describes the best-fit to a Gaussian-convolved beam plus background term. Note that $3"$ is consistent with pointing uncertainty from Figure \ref{['fig:pointing']}. The dashed line shows the model with background and beam amplitude fit to the data, but no Gaussian convolution. Without the Gaussian convolution, the background term increases to fit the data at small angles and forces the model high at large angles. The data clearly prefer the Gaussian-convolved model. Similar fits to the twenty brightest sources in the ACT data suggest bias in the reported flux density level due to misestimation of the source profile is at the sub-percent level. The error bars shown on the radially binned points are correlated.