🔭 astrophysics

Extragalactic microlensing through Ultra Diffuse Galaxies

This paper investigates the feasibility of detecting extragalactic microlensing events through local Ultra Diffuse Galaxies like NGC1052-DF2, concluding that while current rates are low for specific targets, future surveys like Euclid and LSST could enable a statistically significant sample to independently constrain the initial mass function and stellar multiplicity in these dark matter-deficient systems.

Original authors: Sung Kei Li, Thomas Broadhurst, Jose M. Diego, Jeremy Lim, Jose M. Palencia, James Nianias

Published 2026-04-14

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CC BY 4.0

Original authors: Sung Kei Li, Thomas Broadhurst, Jose M. Diego, Jeremy Lim, Jose M. Palencia, James Nianias

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, slightly foggy room. Usually, when we look at distant stars in faraway galaxies, they are too dim to see, like trying to spot a single firefly in a dark forest from a mile away.

However, astronomers have a trick up their sleeve called gravitational microlensing. Think of this as nature's own magnifying glass. When a massive object (like a star) passes directly between us and a distant background star, its gravity bends and focuses the light, making the background star suddenly appear much brighter—like a spotlight turning on.

This paper proposes a new, exciting way to use this trick, but with a twist: instead of using massive galaxy clusters as the magnifying glass, the authors suggest using Ultra Diffuse Galaxies (UDGs).

The Cast of Characters

The Magnifying Glass (The Lens): These are the UDGs. Imagine a UDG as a "ghost galaxy." It's huge in size (as big as our Milky Way) but incredibly faint and spread out, containing very few stars. They are so dim and diffuse that they don't act like a giant lens that distorts the whole image; instead, they act like a sheet of tiny, scattered magnifying glasses floating in space.
The Target (The Source): These are the bright, active galaxies sitting far behind the UDGs.
The Detective (The Telescope): We have powerful tools like the James Webb Space Telescope (JWST) and the upcoming Vera Rubin Observatory (LSST) that can watch these galaxies over time.

The Big Idea: A Game of Cosmic Hide-and-Seek

The authors, led by Sung Kei Li, asked a simple question: Can we use these "ghost galaxies" to spot individual stars in the galaxies behind them?

They used a specific ghost galaxy called NGC1052-DF2 as a test case. They ran the numbers to see if it was theoretically possible.

The Good News:
Yes, it is possible! If a star in a distant galaxy lines up perfectly with a star inside the UDG, the UDG's star acts as a lens, boosting the distant star's brightness by a factor of 100,000 or more. It's like finding a needle in a haystack, but the needle suddenly glows like a neon sign.

The Bad News (The Reality Check):
While it's possible, it's incredibly rare.

The "Needle in a Haystack" Problem: Because UDGs are so sparse (few stars), the chances of a perfect alignment happening are tiny.
The Case of NGC1052-DF2: The authors calculated that for this specific galaxy, we might only see one event every 20 years with our best telescopes. It's like waiting for a specific lottery ticket to win, but the ticket is only sold once every two decades.
Why? The background galaxies behind this specific UDG are too far away or not bright enough to be seen even with the magnification.

The Real Hope: Finding the Right "Ghost"

Just because this ghost galaxy isn't a good target doesn't mean the idea is dead. The authors suggest that if we scan the entire sky with the LSST (a telescope that will take pictures of the whole sky every few nights), we will find thousands of other UDGs.

If we find UDGs that are sitting in front of nearby, active star-forming galaxies, the odds improve dramatically. The authors estimate that with a full-sky survey, we might catch 1 to 10 events per year.

Why Do We Care? (The "So What?")

Why go through all this trouble to catch a rare flash of light? Because these events are a treasure trove of information:

Counting the Invisible: UDGs are mysterious. Some seem to have almost no dark matter. By counting how often these microlensing events happen, we can weigh the stars inside the UDG more accurately. It's like trying to guess how many people are in a dark room by listening to how often their footsteps echo.
The "Family" Test: The paper suggests that if we catch enough of these events, we might see "double" flashes. This would tell us if stars in these ghost galaxies come in pairs (binary stars) or are mostly single. This helps us understand how stars are born in these strange, low-density environments.
The Initial Mass Function (IMF): This is a fancy way of asking: "Are there more small stars or big stars in these galaxies?" The answer might be different in UDGs than in our own Milky Way, which would rewrite our textbooks on how stars form.

The Bottom Line

This paper is a proposal for a new game of cosmic hide-and-seek. While the specific test case (NGC1052-DF2) is a bit of a "dry spell," the authors are confident that with the upcoming LSST and JWST, we will find the right "ghost galaxies" to act as our magnifying glasses.

If we succeed, we won't just be seeing distant stars; we'll be using them to weigh the invisible stuff in the universe and understand the family dynamics of stars in the most elusive galaxies of all. It's a long shot, but in astronomy, the biggest discoveries often come from the rarest moments.

1. Problem Statement

While stellar microlensing is a well-established tool for studying the Milky Way, the Small and Large Magellanic Clouds, and even M31, its application at cosmological distances is limited. Currently, individual stars at high redshifts are only detectable when their light is magnified by strong gravitational lensing from massive foreground galaxy clusters.

The authors propose a novel approach: utilizing Ultra Diffuse Galaxies (UDGs) as "semi-transparent, thin lens planes." UDGs are galaxies with sizes comparable to the Milky Way but masses comparable to dwarfs, characterized by extremely low surface brightness. The specific scientific challenges addressed are:

Can UDGs act as effective microlenses to detect individual stars in background galaxies without the need for strong lensing clusters?
What are the expected event rates for current and future surveys (ZTF, LSST, JWST)?
Can this method provide independent constraints on the Initial Mass Function (IMF) and stellar multiplicity in low-density environments, particularly for UDGs like NGC1052-DF2 where the dominance of stellar mass over dark matter is debated?

2. Methodology

The authors employ a combination of analytical modeling, observational data analysis, and statistical estimation:

Case Study: They use the UDG NGC1052-DF2 (located at $\sim$ 20 Mpc) as a primary case study. They utilize JWST F090W imaging and VLT/MUSE spectroscopic data to identify five spectroscopically confirmed background galaxies behind it.
Theoretical Framework:
- Maximum Magnification: They calculate the maximum magnification ( $\mu_{max}$ ) attainable by a source star of radius $r_s$ passing a microlens of mass $M$ , using the Einstein radius ( $R_E$ ). They assume a single-lens approximation, justified by the low surface mass density of UDGs ( $\Sigma_\star \approx 5 M_\odot/\text{pc}^2$ ).
- Timescales: They estimate the duration of detectable events ( $t_b$ ) based on the relative transverse velocity (dominated by Hubble flow, $\sim$ 400 km/s) and the impact parameter required to reach a minimum magnification ( $\mu_{min}$ ) defined by survey depth limits.
- Event Rate Calculation: They derive the microlensing optical depth ( $\tau$ ) for a thin lens plane and combine it with the luminosity function of background galaxies. The expected number of events ( $E$ ) is calculated by integrating the source luminosity function (derived from Star Formation Rate, SFR) against the probability of magnification exceeding the detection threshold.
Data Sources:
- IMF & Stellar Tracks: Assumed a mean stellar mass of $0.3 M_\odot$ (Kroupa/Chabrier IMF) and used MIST isochrones to determine the brightest possible stars for given radii.
- SFR Estimation: Converted H- $\beta$ luminosities of background galaxies to SFR (assuming an intrinsic H $\alpha$ /H $\beta$ ratio of 2.9), noting that dust extinction was not accounted for, making these upper limits.
Simulations: A Monte Carlo simulation was performed to test the validity of the single-lens approximation by checking for the probability of Einstein ring overlaps (caustic networks) given the UDG's surface density.

3. Key Contributions

Proof of Concept: The paper establishes theoretically that UDGs can serve as microlenses for extragalactic stars, a regime previously unexplored due to the assumption that UDGs are too diffuse to cause significant microlensing.
Survey Feasibility Analysis: It provides a quantitative assessment of the detectability of UDG microlensing events across different facilities (ZTF, LSST, JWST, Euclid).
New Probe for Stellar Populations: It proposes a unique method to constrain the Initial Mass Function (IMF) and stellar multiplicity in UDGs. Unlike cluster lenses where strong lensing induces caustic crossings, UDG microlensing events are rare and isolated; therefore, detected caustic crossings would likely indicate intrinsic binary/multiple star systems rather than lens geometry.
Dark Matter Constraints: By measuring the stellar surface mass density ( $\Sigma_\star$ ) via event rates, the method offers a pathway to independently verify the dark matter content (or lack thereof) in UDGs.

4. Results

Detectability:
- JWST: Can theoretically detect events in background galaxies up to $z \sim 0.7$ if they are sufficiently bright.
- LSST: Can detect events up to $z \sim 0.2$ .
- ZTF: Limited to very local sources ( $z \lesssim 0.05$ ); effectively cannot detect events behind NGC1052-DF2.
Event Rates for NGC1052-DF2:
- The event rate is extremely low due to the lack of low-redshift, high-SFR background galaxies.
- LSST: $\sim 2 \times 10^{-8}$ events/year (effectively zero).
- JWST: $\sim 5.6 \times 10^{-2}$ events/year (one event expected every $\sim$ 20 years of constant monitoring).
- Conclusion: NGC1052-DF2 is not a prime target for immediate detection.
All-Sky Potential:
- Scaling up to the known population of $\sim$ 7,000 UDGs (from DR9), the authors estimate a total event rate of $O(1-10)$ events per year for LSST's all-sky monitoring.
- Euclid is identified as the ideal instrument for identifying suitable UDG candidates with low-redshift, star-forming background galaxies for follow-up.
Monte Carlo Findings:
- The probability of Einstein rings overlapping (creating caustic networks) due to geometric proximity is negligible ( $<0.2\%$ ).
- Therefore, any observed caustic crossing events would almost certainly arise from intrinsic stellar multiplicity (binaries) in the UDG lens population.

5. Significance

This work opens a new window into the study of Ultra Diffuse Galaxies and low-mass stellar populations:

IMF and Multiplicity: It offers a method to measure the stellar multiplicity fraction and IMF in low-density, low-metallicity environments (UDGs) independently of globular clusters. This is crucial for understanding star formation physics in extreme environments.
Dark Matter Nature: By constraining the stellar mass density via microlensing optical depth, the method can test claims that some UDGs (like NGC1052-DF2) lack dark matter or contain alternative dark matter candidates (e.g., Primordial Black Holes).
Future Observations: The paper provides a roadmap for the next decade of astronomy, suggesting that the combination of Euclid (for target selection), LSST (for high-cadence monitoring to capture light curves), and JWST (for deep follow-up) will make UDG microlensing a viable observational field, potentially yielding the first detections within the next decade.