Astrophysical Black holes: An Explanation for the Galaxy Quenching

This paper proposes that astrophysical black holes (ABHs) without event horizons provide a more viable explanation for long-term galaxy quenching than classical black holes, due to their ability to generate significantly stronger accretion disk winds and more efficient feedback mechanisms.

The Gist

The Big Mystery: Why Do Galaxies Stop Having Babies?

Imagine a galaxy as a giant, bustling city. The "stars" are the people, and the "gas" is the food supply. For a long time, astronomers have been puzzled by a strange phenomenon: Galaxy Quenching.

This is when a galaxy suddenly stops making new stars. It's like a city that suddenly stops having babies, even though it still has plenty of food (gas) sitting in the suburbs (the halo). The gas is there, but it just won't turn into stars.

For decades, the leading theory has been that Supermassive Black Holes (SMBHs) at the center of these galaxies act like a "stop sign." When they eat gas, they shoot out massive jets of energy (feedback) that heat up the gas, preventing it from cooling down and forming stars.

But there's a problem: Recent observations (especially from the James Webb Space Telescope) show galaxies stopping their star formation way too early in the universe's history. The standard black hole model struggles to explain how they could stop the process so quickly and efficiently.

The New Idea: The "Event Horizon" Might Be a Myth

The authors of this paper, Jay Verma Trivedi, Pankaj Joshi, and colleagues, propose a radical new idea. They suggest that the objects at the center of these galaxies might not be the black holes we think they are.

The Standard Black Hole (The "One-Way Door"):
Think of a traditional black hole like a trash can with a lid that only opens inward. Once you throw something in (matter), it falls past a point of no return called the Event Horizon. Once it crosses that line, it's gone forever. It can't come back out, and it can't push anything else out.

• The Limit: Because of this lid, the "accretion disk" (the swirling pile of food before it falls in) stops at a safe distance. It can't get too close to the center. This limits how much energy it can release.

The "Astrophysical Black Hole" (ABH) (The "Open Kitchen"):
The authors propose these objects are Ultra-Compact Objects without Event Horizons. Let's call them ABHs.

• The Difference: Imagine a kitchen with no back door. The food (matter) can be pushed all the way to the very center, right up against the "stove" (the singularity).
• The Result: Because the food can get closer to the center, the heat and pressure become unimaginable. It's like squeezing a sponge until it's dry; the energy release is massive.

How This Explains the "Quenching"

The paper argues that these ABHs are much better at stopping star formation than traditional black holes. Here is the analogy:

1. The Wind Analogy:
   • Traditional Black Hole: Imagine a fan blowing air. Because the fan blades are stuck at a certain distance from the motor (the Event Horizon), the wind is strong, but it has a limit. If the motor is small, the wind might not be strong enough to blow away the clouds of gas in a young galaxy.
   • The ABH: Now imagine a fan where the blades can get right up against the motor. The air gets compressed and heated to extreme levels before it even leaves the fan. This creates a supersonic jet of wind.
   • The Outcome: This super-wind blows the gas out of the galaxy (or heats it so much it can't cool down) much faster and more effectively. This explains why galaxies stop making stars so quickly in the early universe.

Two Types of "Star Quenchers"

The paper suggests two ways this happens:

1. The Long-Term Stopper (The Supermassive ABH)

• Who: The giant object at the center of a big galaxy.
• How: It slowly builds up a "naked singularity" (a point of infinite density with no lid). As it eats, it releases a constant, powerful stream of high-energy particles. Over millions of years, this acts like a permanent "No Star Formation" sign, keeping the galaxy "quenched" forever.

2. The Short-Term Stopper (The Stellar-Mass ABH)

• Who: A massive star that is dying.
• How: When a huge star runs out of fuel, it collapses. Usually, it becomes a black hole with a lid. But the authors suggest that for a brief moment before the lid forms, it becomes a temporary ABH.
• The Burst: During this split second, it releases a massive burst of energy—like a supernova on steroids. This sudden explosion can blow away the gas in a small galaxy, stopping star formation for a while. This explains the "mini-quenched" galaxies found by JWST.

Why Should We Care? (The "Detective" Work)

The paper isn't just math; it offers ways to test this idea:

• The "Shadows": If we look at the "shadow" of these objects (like the famous image of the black hole in M87 or Sagittarius A*), an ABH might look slightly different because the light behaves differently near a "naked" singularity compared to a black hole with a lid.
• The Accelerating Winds: The paper points out that some gas winds in galaxies are speeding up over time. A traditional black hole struggles to explain this acceleration. An ABH, however, gets more efficient at shooting out winds as it collapses, perfectly explaining why the winds are speeding up.
• The "Parker Wind": They suggest that if we see specific magnetic patterns in these winds (like a specific type of electric current), it could be the "smoking gun" that proves these objects don't have event horizons.

The Bottom Line

The universe is full of ultra-dense objects. For a long time, we assumed they all had "event horizons" (invisible walls). This paper suggests that maybe some of them don't.

If they don't have these walls, they are super-efficient energy machines. They can blow away gas and stop star formation much better than standard black holes. This could solve the mystery of why the universe stopped making stars so early in its history.

In short: The universe might not be full of "black holes" with lids. It might be full of "Astrophysical Black Holes" that are open, chaotic, and incredibly powerful, acting as the ultimate cosmic traffic cops that shut down star factories.

Technical Summary

1. Problem Statement

The paper addresses the persistent astrophysical puzzle of galaxy quenching: the cessation of star formation in galaxies despite the presence of abundant hot gas in their halos (the "cooling problem").

• The Limitation of Standard Models: Conventional models attribute quenching to feedback from Active Galactic Nuclei (AGN) powered by Supermassive Black Holes (SMBHs). However, these models face challenges:
  • AGN are often found in gas-rich galaxies, suggesting they do not always effectively deplete the gas reservoir.
  • Standard SMBHs possess an Event Horizon (EH). Matter accretion is truncated at the Innermost Stable Circular Orbit (ISCO), located at $3R_s$ (Schwarzschild radius) for non-rotating black holes. This limits the efficiency of energy extraction and particle generation.
  • Recent JWST observations have revealed massive, quiescent galaxies at very high redshifts ($z > 3$) and even a "mini" quenched galaxy at $z=7.3$ (when the universe was only ~700 Myr old). Standard feedback mechanisms struggle to explain such rapid and sustained quenching in the early universe.
• The Hypothesis: The authors propose that the central compact objects in galaxies may not be traditional black holes with event horizons, but rather Astrophysical Black Holes (ABHs)—ultra-compact objects (potentially naked singularities) without event horizons.

2. Methodology

The authors employ a combination of theoretical general relativity, accretion disk physics, and hydrodynamic analysis to compare standard Black Holes (BHs) with horizonless ABHs.

• Theoretical Framework:
  • They utilize JMN (Joshi-Malafarina-Narayan) models (JMN-1 and JMN-2), which describe the gravitational collapse of anisotropic fluids and perfect fluids, respectively. These models result in globally naked singularities (ultra-compact objects without trapped surfaces/EHs).
  • They derive the spacetime metrics for these collapsing clouds to analyze the behavior of accretion disks.
• Accretion Disk Analysis:
  • They calculate the radiative flux and luminosity ($L_\infty$) for accretion disks in general spherically symmetric metrics.
  • Key Distinction: In standard BHs, the disk is truncated at the ISCO. In ABHs, the absence of an EH allows the disk to extend all the way to the central ultra-dense region (approaching the singularity).
• Wind Generation Modeling:
  • They apply Newtonian formalism (with radiation pressure) to derive the escape condition for particles ejected from the disk.
  • They compare the normalized disk luminosity ($\Gamma_d$) required to drive winds in BHs vs. ABHs.
  • They reference recent General Relativistic Magnetohydrodynamic (GRMHD) simulations of naked singularities to validate jet/outflow generation.
• Feedback Efficiency:
  • They adapt the Silk-Rees model to estimate the mass required for a central object to expel all gas from a host galaxy, comparing the radiative and mechanical efficiencies of BHs and ABHs in both "Quasar" (high accretion) and "Radio" (low accretion) modes.

3. Key Contributions and Results

A. Long-Term Quenching Mechanism

• Extended Accretion Disk: Because ABHs lack an event horizon, the accretion disk can extend to the center. As the collapse proceeds, the ISCO moves closer to the center, eventually allowing the disk to reach the ultra-strong gravity regime.
• Radiative Efficiency:
  • Standard BH: Radiative efficiency is capped at $\sim 6\%$ (for Schwarzschild) because the disk stops at the ISCO.
  • ABH: Radiative efficiency increases as the disk extends inward, asymptotically approaching 100% once the singularity forms.
• Wind Generation:
  • The authors demonstrate that ABHs can generate significantly stronger winds than BHs, even at low accretion rates.
  • In the "Radio mode" (low accretion), standard BHs struggle to generate radiation-driven winds. In contrast, ABHs, due to their high radiative efficiency and small effective ISCO, can drive powerful winds even with minimal accretion.
  • Result: ABHs are more effective at expelling gas and heating the interstellar medium (ISM), leading to sustained, long-term quenching.

B. Short-Term Quenching (Stellar-Mass ABHs)

• The paper introduces Stellar-Mass Astrophysical Black Holes (StMABHs). These form during the collapse of massive stars before an event horizon fully envelops the singularity.
• Transient Phase: During the brief period before horizon formation, the system can emit bursts of ultra-high-energy particles and radiation.
• Impact: These transient bursts could cause localized, short-term quenching (or "starburst-quench" cycles) in early galaxies, explaining the rapid cessation of star formation observed in high-redshift mini-galaxies.

C. Observational Signatures

• Spectral Differences: ABHs exhibit high-energy emission tails in their spectral luminosity distribution, unlike the sharp cutoff seen in Schwarzschild BHs (Fig. 1).
• Wind Acceleration: The paper links the observed acceleration of outflows in quasars (e.g., SBS 1408+544) to the increasing radiative efficiency of ABHs. As the ABH evolves, its radiation pressure increases, accelerating winds over time—a phenomenon difficult to explain with standard BHs.
• JWST Compatibility: The existence of quenched galaxies at $z=7.3$ with low stellar mass suggests a feedback mechanism that is highly efficient early on. StMABHs provide a mechanism for this, as they do not require the long growth timescales of massive SMBHs.
• Magnetic Signatures: The authors suggest that specific magnetic field structures in Parker-type winds (carrying strong electric currents) could be signatures of the transition from a horizonless object to a black hole.

4. Significance and Implications

• Alternative to Cosmic Censorship: The paper challenges the strict adherence to the Cosmic Censorship Conjecture (which forbids naked singularities). It argues that if naked singularities (ABHs) exist, they offer a more physically viable explanation for galaxy evolution than standard black holes.
• Resolution of the Cooling Problem: By allowing accretion disks to access the ultra-strong gravity regime, ABHs provide a mechanism for continuous, high-efficiency energy injection that can permanently suppress star formation, solving the "cooling problem" where gas fails to cool and form stars.
• Early Universe Evolution: The model provides a plausible explanation for the rapid quenching observed by JWST in the early universe, where standard SMBH growth models might be too slow to account for the observed data.
• Future Directions: The authors propose that future observations (JWST, EHT) and GRMHD simulations should focus on distinguishing between BHs and ABHs via:
  • Spectral energy distributions.
  • Wind acceleration profiles.
  • Shadow imaging (though JMN-1 mimics BH shadows, other parameters differ).
  • Signatures of quantum gravity effects near the singularity.

Conclusion

The paper posits that Astrophysical Black Holes (ABHs)—ultra-compact objects without event horizons—are a superior candidate for explaining galaxy quenching compared to traditional black holes. Their ability to sustain accretion disks closer to the center results in near-100% radiative efficiency, enabling powerful, continuous feedback (winds and jets) that effectively quenches star formation in both the early universe and massive galaxies today. This framework bridges gaps in current AGN feedback models and aligns with recent high-redshift observational anomalies.