1/ Let's talk about gravitational lensing!

Ever wondered why distant galaxies look distorted?

How can lensing help us study galaxies?

What's the mystery behind this fascinating yet incredibly creepy smiley face in space?

A

2/ Just like optical lensing that we encounter in daily life - GRAVITY can cause lensing and create images too!!

The crux lies in general relativity: massive objects warp spacetime.

Light rays, following shortest paths, get bent due to space and time themselves being "curved".

3/ Because light has been deflected from its original path before reaching us, we observe the source as being in a different position than it actually is - we see images!

This video from @almaobs provides a great visual of the lensing mechanism!

4/ Some terminology:

Source: background object emitting the light we eventually observe (star, galaxy, quasar, etc)

Lens: massive foreground object that deflects the path of light rays and create images of the source (star, galaxy, cluster of galaxies, etc)

5/ Under certain conditions (like a lens that's dense enough), we get "Strong Lensing": characterised by the production of:
MULTIPLE images,
Beautiful giant arcs,
and/or Einstein Rings!!!

That's right - a single source can result in MANY images under the right conditions!

6/ Common image configurations in strong lensing include:

Multiple images (top left)

Giant arcs (top right)

Einstein cross of 4 images (bottom left)

Einstein Ring (bottom right)

- often a combination of many of the above!

7/ The geometry and alignment of the source and lens relative to our line-of-sight can also affect the types of images we see!

This Wikipedia gif shows how images (white) change depending on the lens position (blue) relative to our line of sight (into the screen in this case)

8/ Usually for strong lensing, the lens is a galaxy OR a cluster of galaxies - these are massive enough to result in multiple images

There are also phenomena like weak lensing or microlensing, but we're going to focus on strong lensing today and how we use it to study galaxies.

9/ How does strong lensing help us understand galaxies???

It was originally viewed as a cool result of general relativity, but has become a powerful astrophysical tool.

A MAJOR application is in measuring the TOTAL mass of galaxies & galaxy clusters (including dark matter)!!!

10/ Bottom line: we can measure the mass of stars and gas in a galaxy (or cluster), BUT measuring the mass of dark matter in them is REALLY HARD, since we cannot directly detect it! The only way we can "detect" dark matter is its gravity...

Enter lensing!

11/ KEY IDEA: lensing depends on the TOTAL MASS of the lens! It doesn't discriminate between ordinary (stars/gas) & dark matter - ALL mass in a lens will affect what we observe.

This is very powerful! Now we have a method that depends on the TOTAL mass, not just ordinary matter.

12/ We use a technique called "lens modelling": we fit models to the lens system to know its TOTAL mass (or total surface mass density) based on the multiple images we see.

Simple example: a MORE MASSIVE lens produces a LARGER Einstein Ring (or giant arc with larger radius)!

13/ Eventually we need to more than pen & paper calculations - "lensing codes" (computer programs) allow us to accurately measure total masses, sizes, and other properties of the lens galaxy or cluster.

Creating these lensing codes is an area of research unto itself!

14/ A lens model is "better" when it more accurately reproduces the observed multiple images - it's how we fine tune our calculation of total mass in the lens galaxy or cluster!

Left: a galaxy cluster
Right: its mass distribution shown in blue (calculated using lens models)

15/ But wait, there's more

Measuring masses is not the only application of strong lensing . . .

It also helps us observe REALLY DISTANT GALAXIES!!!

16/ Strong lensing can also MAGNIFY images - acting as a cosmic telescope!!!

A far away galaxy might be impossible to observe by ordinary telescopes . . .

but if there's a foreground galaxy cluster; BOOM - now we can observe the distant galaxy because it'll get magnified!!!

17/ In the past decade, this has led to a revolution in studying the first galaxies that formed in the early Universe - thanks to lensing magnification!

The following image from Salmon et al. 2018 shows a magnified view of a galaxy that existed more than 13 BILLION YEARS AGO!!!

18/ Once we have lens models for the foreground cluster, we can even use them to modify the images and reconstruct what the distant source galaxy ACTUALLY looks like!!!

This example from Yang et al. 2020 shows an observed image of a distant galaxy vs the reconstructed source.

19/ Gravitational lensing has countless applications in many areas of modern astronomy - galaxies, quasars, dark matter, exoplanets, and more!

I wanted to share a couple of them that I find especially cool.

What do you find most fascinating about lensing? Comment below.