LeMMINGs VII: 5 GHz, 50 mas e-MERLIN observations of a statistically complete sample of nearby AGN

This paper presents high-resolution 5 GHz e-MERLIN observations of a statistically complete sample of nearby galaxies, revealing that compact radio cores and jets are the primary manifestation of black hole activity in the local Universe, with up to 30% of galaxies hosting such radio-active nuclei.

The Gist

Imagine the universe as a vast, crowded city. In the center of almost every building (galaxy) in this city, there is a massive, invisible engine: a Supermassive Black Hole. Most of the time, these engines are "idling" or running on very low power. They aren't screaming with energy like the famous, bright quasars we see in movies; instead, they are whispering, barely making a sound.

This paper is like a team of detectives using a super-powerful, high-tech microscope to listen to these whispers across the entire city.

The Mission: LeMMINGs VII

The team, known as LeMMINGs (Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies), decided to take a census of 280 nearby galaxies. Their goal? To find out how many of these black holes are actually "awake" and active, even if they are very quiet.

Previously, they had taken a look at these galaxies using a radio frequency of 1.5 GHz (think of this as looking at the city with a standard pair of binoculars). They found some activity, but the view was a bit blurry. Sometimes, they couldn't tell if the noise they heard was coming from the black hole in the center or just from a busy construction site (star formation) nearby.

In this new study, they switched to 5 GHz.

• The Analogy: If 1.5 GHz was like looking at a city from a hill with binoculars, 5 GHz is like using a high-powered telescope from a drone hovering right above the buildings. It offers four times better resolution.
• The Result: They could zoom in so closely that they could ignore the "construction noise" (star formation) and focus purely on the engine room (the black hole).

What They Found

Out of the 280 galaxies they studied, they found 68 with clear signs of a "whispering" black hole. That's about 24% of the sample.

Here is the breakdown of their discoveries:

1. The "Active" Neighborhoods (LINERs and Seyferts):
   These are galaxies that already look a bit "active" in visible light. The team found that almost all of these have a compact, bright radio core.

   • Analogy: These are like houses with the lights on and the music playing. It's no surprise to find a party going on inside.

2. The "Quiet" Neighborhoods (H II and Absorption Line Galaxies):
   These galaxies look completely normal and "dead" in visible light. You wouldn't guess they had a black hole.

   • The Surprise: Even here, they found radio signals in about 8% of these galaxies.
   • The Twist: In some of these "quiet" galaxies, the radio signal might not be a black hole at all, but rather a massive explosion of new stars (a starburst) or a tidal disruption event (where a black hole ate a star). It's like hearing a noise in a quiet house and realizing it's either a sleeping baby (a quiet black hole) or a dog barking (star formation).

3. The Shape of the Noise:

   • Compact Cores: Most of the signals they found were tiny, tight dots (less than 10 light-years across). This is the "smoking gun" of a black hole.
   • Jets: About 22% of the active galaxies showed long, thin streams of energy shooting out (jets).
   • The 5 GHz Advantage: In the older, blurrier 1.5 GHz images, 38% of the galaxies looked like they had jets. But with the sharper 5 GHz "lens," many of those jets disappeared! Why? Because the jets were actually faint, spread-out clouds of gas that the high-resolution camera simply "filtered out," leaving only the true, compact black hole core. This proves that high-resolution is key to not getting fooled.

Why Does This Matter?

For a long time, astronomers thought black holes were either "on" (bright and loud) or "off" (invisible). This paper suggests that the reality is much more common: Most black holes are in a "low-power mode."

• The "Low-Luminosity" Reality: The study suggests that about 30% of all nearby galaxies host a black hole that is actively eating, but just very slowly. These are called Low-Luminosity AGN (LLAGN).
• The Hosts: These quiet black holes seem to prefer living in "early-type" galaxies (smooth, round, or oval-shaped galaxies) rather than the spiral, pinwheel-shaped ones.

The Takeaway

Imagine trying to hear a pin drop in a noisy stadium. If you use a regular microphone (low resolution), you hear a lot of background noise and can't be sure what you're hearing. But if you use a laser-focused microphone (5 GHz e-MERLIN), you can isolate that single pin drop.

This paper tells us that the universe is full of these "pin drops." By using the sharpest radio eyes we have, we've learned that black holes are everywhere, but they are often very quiet, very compact, and hiding in plain sight. To find them all, we need to keep looking with higher resolution and sensitivity, because the faintest whispers are often the most common.

Technical Summary

Here is a detailed technical summary of the paper "LeMMINGs VII: 5 GHz, 50 mas e-MERLIN observations of a statistically complete sample of nearby AGN".

1. Problem Statement

The primary goal of this research is to conduct an unbiased, statistically complete census of nuclear radio activity in the local Universe to understand the prevalence and nature of Low-Luminosity Active Galactic Nuclei (LLAGN). While supermassive black holes (SMBHs) are believed to reside in most galaxies, many are in a "low accretion" state, making them difficult to detect.

• Limitations of Previous Surveys: Previous radio surveys of the Palomar sample (e.g., using the VLA or MERLIN at 1.5 GHz) often suffered from limited resolution (arcsecond scale) and sensitivity. This led to confusion between genuine nuclear emission (AGN) and other processes like circumnuclear star formation (SF) or supernova remnants.
• The Need for Higher Resolution: To distinguish compact AGN cores (often <10 pc) from extended structures, higher angular resolution and better spatial filtering are required. Additionally, multi-frequency data is needed to calculate spectral indices, a key diagnostic for distinguishing thermal (SF) from non-thermal (AGN) emission.

2. Methodology

The study utilizes the Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies (LeMMINGs) survey, focusing on a statistically complete sample of 280 nearby galaxies ($<120$ Mpc) selected from the Palomar bright spectroscopic sample.

• Observations:
  • Frequency: 5 GHz (C-band).
  • Instrument: e-MERLIN array (6 antennas: Mk2, Kn, De, Pi, Da, Cm; Lovell telescope excluded).
  • Resolution: $\sim50$ mas (milliarcseconds), corresponding to $\sim5$ pc at the median distance of the sample (20 Mpc).
  • Strategy: "Snap-shot" imaging mode (8 hours total, visiting fields once per hour) to fill the $uv$-plane and create circular synthesized beams.
• Data Reduction:
  • Calibrated using the e-MERLIN CASA Pipeline (eMCP) and WSClean.
  • Self-Calibration: Performed on 70/280 fields using bright in-beam sources to improve sensitivity and phase stability.
  • Imaging: Produced full-resolution images (50 mas) and tapered images (140 mas) to match the resolution of the previously published 1.5 GHz LeMMINGs data for direct comparison.
• Source Identification:
  • Used updated Gaia DR3 optical positions where available, cross-referenced with NED/Simbad.
  • Searched for radio emission within a 1.5 arcsec radius of the optical nucleus.
  • Detection threshold set at **$5\sigma$** (median sensitivity of$0.33$mJy beam$^{-1}$) to minimize false positives, compared to the$3\sigma$ threshold used in the 1.5 GHz release.

3. Key Contributions

• Deepest High-Resolution Survey: This represents the deepest, statistically complete radio-band survey of the local Universe at sub-arcsecond resolution, probing linear scales of $\sim5$ pc.
• Multi-Frequency Diagnostics: By combining 5 GHz data with existing 1.5 GHz data, the authors provide spectral index measurements ($\alpha$) for a large sample, crucial for identifying flat-spectrum AGN cores versus steep-spectrum jet or SF emission.
• Refined Positional Accuracy: The study highlights the importance of using Gaia positions for nuclear alignment, identifying cases where previous catalog positions were offset by foreground stars or misidentifications.
• Morphological Classification: A systematic classification of radio morphologies (Compact Core, One-sided Jet, Triple, Complex) specifically for the 5 GHz dataset, allowing for a cleaner separation of nuclear vs. extended emission.

4. Key Results

• Detection Rate:
  • Nuclear radio emission was detected in 68/280 sources (24%) at 5 GHz.
  • This is lower than the 1.5 GHz detection rate (38%), primarily because the higher resolution and $5\sigma$ threshold resolved out extended, low-surface-brightness structures (e.g., star-forming regions) that were visible at 1.5 GHz.
• Source Properties:
  • Luminosity: Core luminosities range from $10^{35}$to$10^{41.9}$erg s$^{-1}$, with a median of$7.1 \times 10^{36}$erg s$^{-1}$.
  • Brightness Temperature: Brightest nuclei (LINERs and Seyferts) show $T_B \ge 10^6$ K, confirming non-thermal AGN activity.
  • Morphology: 78% of detections are compact ($<10$ pc, "Core" morphology). Only 22% show extended, jet-like features (up to 380 pc).
• Host Galaxy Dependence:
  • "Active" Galaxies (LINERs/Seyferts): High detection rate (47%). LINERs dominate the high-luminosity tail.
  • "Inactive" Galaxies (H II and Absorption Line Galaxies): Low detection rate (8%). Most detections here are likely LLAGN, though some may be star formation or supernova remnants.
  • Morphology: Early-type galaxies (Ellipticals/Lenticulars) have a significantly higher detection fraction (45%) compared to late-type spirals (17%).
• Spectral Indices:
  • Detected sources generally show flat ($\alpha \approx 0$) or slightly inverted spectra, consistent with compact AGN cores.
  • Sources undetected at 5 GHz but detected at 1.5 GHz often have steep spectra, suggesting they are resolved out or dominated by optically thin synchrotron emission (jets/SF).
• Specific Cases:
  • NGC 3690 (Arp 299-B): Detected as a tidal disruption event.
  • NGC 5194: Only detected in tapered images, highlighting sensitivity limits.
  • NGC 2655 & NGC 4589: Show complex "triple" structures resolved at 5 GHz, with high X-ray absorption suggesting obscured AGN.

5. Significance and Conclusions

• LLAGN Dominance: The study concludes that LLAGN are the primary manifestation of black hole activity in the local Universe. Approximately 30% of local galaxies host a radio-active nucleus when combining 1.5 GHz and 5 GHz data.
• Resolution is Critical: High-resolution imaging is essential to disentangle genuine nuclear accretion from circumnuclear star formation. Lower-resolution surveys overestimate the prevalence of "active" nuclei in late-type galaxies by including SF emission.
• Future Requirements: To detect the faintest LLAGN and resolve jets in all nearby galaxies, future surveys require:
  • Sensitivity improvements (targeting $\le 33$ $\mu$Jy beam$^{-1}$, a 4x increase in observing time).
  • Better $uv$-coverage (potentially including the Lovell telescope) to image large-scale jets without resolution loss.
• Physical Drivers: Radio emission drivers diverge by host type: Early-type galaxies are dominated by compact LLAGN cores, while Late-type galaxies show lower detection rates where non-AGN processes (SF) often dominate the nuclear radio emission.

This paper establishes a new benchmark for local AGN studies, demonstrating that a significant fraction of "inactive" galaxies actually harbor weakly accreting SMBHs, provided they are observed with sufficient resolution and sensitivity.