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The Atacama Cosmology Telescope: Extragalactic Point Sources in the Southern Surveys at 150, 220 and 280 GHz observed between 2008-2010

Original authors: Cristian Vargas, Carlos H. López-Caraballo, Elia S. Battistelli, Rolando Dunner, Gerrit Farren, Megan Gralla, Kirsten R. Hall, Carlos Hervías-Caimapo, Matt Hilton, Adam D. Hincks, Kevin Huffenberger

Published 2026-06-03

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CC BY 4.0

Original authors: Cristian Vargas, Carlos H. López-Caraballo, Elia S. Battistelli, Rolando Dunner, Gerrit Farren, Megan Gralla, Kirsten R. Hall, Carlos Hervías-Caimapo, Matt Hilton, Adam D. Hincks, Kevin Huffenberger, Tobias Marriage, Tony Mroczkowski, Michael D. Niemack, Lyman Page, Bruce Partridge, Felipe Rojas, Francesca Rizzo, Cristóbal Sifón, Suzanne Staggs, Edward J. Wollack

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, noisy radio station. Most of the time, you hear a static hiss (the Cosmic Microwave Background, or the afterglow of the Big Bang). But sometimes, you hear a clear, loud song coming from a specific direction. Those "songs" are galaxies and active black holes.

This paper is like a detailed music catalog created by a team of astronomers using the Atacama Cosmology Telescope (ACT) in the high, dry mountains of Chile. Between 2008 and 2010, they pointed their telescope at a patch of the southern sky about 600 times the size of the full moon. They listened at three different "frequencies" (150, 220, and 280 GHz), which is like tuning a radio to different stations to hear different types of music.

Here is what they found, broken down simply:

1. The Two Types of "Musicians"

The astronomers found 483 distinct sources (galaxies or black holes) in their data. They realized these sources fall into two main groups, based on how their "volume" changes as they tune the radio dial:

The Synchrotron Singers (284 sources): These are like electric guitar players. They are mostly active black holes (AGNs) shooting out jets of particles. Their "music" gets quieter as you tune to higher frequencies. They are the "rock stars" of the radio universe.
The Dusty Choirs (183 sources): These are like a warm, humming choir. They are galaxies full of cold dust, heated by newborn stars. Their "music" gets louder as you tune to higher frequencies. These are the "dusty emitters."

2. The "Time-Travel" Aspect

The telescope looked at the same patch of sky in three different years (2008, 2009, and 2010). This allowed the team to check if the "musicians" were changing their tune.

The Rock Stars (Synchrotron): They found that these sources are very active. Their brightness changes by about 35% from year to year. It's like a rock star who sometimes plays a loud solo and other times plays quietly.
The Dusty Choirs: These sources are very stable. Their brightness didn't change at all between the years. They are the reliable, steady background singers.

3. The "New" Discoveries

The team cross-referenced their list with other famous catalogs (like lists of radio stars or infrared galaxies).

They confirmed that 88% of the "Rock Stars" were already known to other telescopes.
However, about 49% of the "Dusty Choirs" were new discoveries. These are dusty galaxies that were hiding from other telescopes but were loud and clear in the ACT's high-frequency data.

4. The "New" Frequency (280 GHz)

One of the biggest achievements of this paper is that they released new maps at 280 GHz, a frequency they hadn't fully used before. Think of this as adding a brand new instrument to the orchestra. This higher frequency was crucial for spotting the "Dusty Choirs" because they shine brightest at these high pitches.

5. Counting the Musicians

The team counted how many sources they found at different brightness levels (number counts).

For the "Rock Stars," their counts matched what other scientists predicted.
For the "Dusty Choirs," their counts mostly matched predictions, except at the very highest frequency (280 GHz). At this frequency, some computer models predicted there should be more dusty galaxies than they actually found. This suggests the models might be overestimating how many dusty galaxies exist in that specific range.

The Bottom Line

This paper is essentially a giant, multi-year, multi-frequency directory of extragalactic sources in the southern sky. It tells us:

We can separate "electric" galaxies from "dusty" galaxies by listening to them at different frequencies.
The "electric" ones change their brightness often, while the "dusty" ones stay steady.
We found nearly 100 new dusty galaxies that were previously hidden.
The new 280 GHz maps are a valuable tool for future telescopes to study the universe's history.

The authors are releasing this catalog and the new maps to the public, acting as a map for other astronomers to find these cosmic "musicians" and study them further.

Technical Summary: Extragalactic Point Sources in the Southern Surveys at 150, 220, and 280 GHz

Problem and Motivation
The Atacama Cosmology Telescope (ACT) was designed primarily to map the Cosmic Microwave Background (CMB). However, the millimeter-wavelength surveys conducted by ACT are contaminated by foreground emissions from extragalactic sources. To accurately extract cosmological information from the CMB, these foregrounds must be identified, characterized, and separated. While previous ACT works cataloged sources in the equatorial region and at 150 GHz in the south, a comprehensive multi-frequency, multi-epoch catalog for the southern surveys covering 150, 220, and 280 GHz was lacking. This paper addresses the need to characterize the spectral properties, variability, and number counts of these foreground populations—specifically distinguishing between non-thermal synchrotron emitters (typically Active Galactic Nuclei, AGNs) and thermal dust emitters (Dusty Star-Forming Galaxies, DSFGs)—to improve CMB foreground removal and advance the study of galaxy evolution.

Methodology
The study utilizes data from the Millimeter Bolometric Array Camera (MBAC) on the ACT, covering observations from the 2008, 2009, and 2010 seasons. The analysis focuses on three frequency bands: 150 GHz (f150), 220 GHz (f220), and 280 GHz (f280).

Data Processing: The dataset comprises 8 maps (excluding the high-noise 2009 f280 map). Maps were calibrated against the CMB power spectrum (referenced to WMAP and Planck). To improve noise characteristics, co-added maps were generated for overlapping regions using inverse-variance weighting.
Source Detection: A matched filter technique was employed to optimize the signal-to-noise ratio (S/N) of point sources embedded in CMB and atmospheric noise. The filter utilized the effective beam profile, which accounts for the instrument's instantaneous beam, pointing uncertainty (modeled as a 5" Gaussian), and the spectral properties of the sources.
Flux Measurement and Validation: Flux densities were measured by integrating the filtered map over the beam solid angle. The pipeline included iterative source detection (thresholds of 50, 5.0, and 3.5 S/N) and masking to prevent artifacts. Extensive simulations (approx. $10^6$ detections) were conducted to determine the purity, completeness, and flux boosting effects (systematic overestimation of flux at low S/N) of the catalog.
Classification: Sources were classified based on their spectral indices ( $\alpha$ , where $S_\nu \propto \nu^\alpha$ ) derived from flux measurements across the three bands. A bimodal distribution in spectral indices allowed for the separation of synchrotron-dominated sources ( $\alpha \lesssim 1$ ) and dust-dominated sources ( $\alpha \gtrsim 1$ ).
Cross-Matching: The catalog was cross-matched with nine external catalogs spanning radio to X-ray frequencies (including SUMSS, PMN, AT20G, SPT-SZ, Planck, IRAS, WISE, AKARI, and 2RXS) to identify counterparts and validate classifications.

Key Contributions

New Catalog: The paper presents a multi-frequency, multi-epoch catalog of 483 extragalactic point sources found in a sky area of $\sim600$ square degrees.
280 GHz Data Release: The authors release new 280 GHz maps and weights from the 2008 and 2010 seasons, calibrated to the CMB.
Population Separation: The catalog divides sources into two distinct populations: 284 synchrotron sources and 183 dusty source candidates.
Variability Analysis: The multi-epoch nature of the data allows for a study of source variability on yearly timescales.
Number Counts: Differential number counts ($dN/dS$) are provided for both populations at 150, 220, and 280 GHz, corrected for completeness and flux boosting.

Results

Source Counts and Classification: The catalog contains 284 synchrotron sources and 183 dusty candidates. Cross-matching with external catalogs identified counterparts for 251 synchrotron sources (88%) and 92 dusty sources (51%). This suggests that 91 dusty candidates are new detections not present in existing catalogs.
Spectral Properties:
- Synchrotron sources exhibit flat or negative spectral indices, with a mean $\alpha_{150-220} \approx -0.65$ .
- Dusty sources show positive spectral indices, with a mean $\alpha_{150-220} \approx 3.39$ .
- The 280 GHz band is crucial for identifying dusty sources, as their flux increases with frequency, whereas synchrotron flux decreases.
Variability: Synchrotron sources exhibit significant year-to-year variability, with a mean fractional variability of $35 \pm 2\%$ . In contrast, no significant variability was detected for the dusty source population, consistent with expectations for thermal dust emission.
Number Counts:
- Synchrotron: The measured counts agree well with previous measurements and theoretical models (e.g., Lagache et al. 2020, Bonato et al. 2019) across all frequencies.
- Dusty: At 150 and 220 GHz, the counts generally agree with models, though some models (e.g., Béthermin et al. 2011) tend to overestimate the number of sources at 150 GHz. At 280 GHz, the results agree with equatorial survey data at higher fluxes ( $>30$ mJy) but show discrepancies at lower fluxes. Notably, some models overestimate the number of dusty sources at 280 GHz.
Astrometry: Comparison with the AT20G catalog indicates a mean positional offset of $0.63''$ in right ascension and $0.31''$ in declination, with standard deviations of $4.7''$ and $3.5''$ , respectively.

Significance and Claims
The authors state that this work provides a valuable tool for cross-correlation and cross-matching with future high-frequency surveys, specifically the Simons Observatory, CMB-S4, and the Fred Young Submillimeter Telescope (CCAT-Prime), all of which will operate at 280 GHz. The release of calibrated 280 GHz maps and the identification of hundreds of new bright DSFG candidates (selected via a different function than 220 GHz catalogs) offer excellent targets for follow-up observations with facilities like ALMA and APEX. The observational counts presented serve to constrain theoretical models of galaxy formation and evolution, particularly in the less observationally constrained 280 GHz channel. The study confirms the utility of multi-frequency ACT data in separating foreground populations to refine CMB analyses and advance the understanding of high-redshift star formation.