AGN or Star? How We Tell the Difference Using Optical Light

Optically Selected AGN (1000-8000A) 

Each AGN has a supermassive black hole at its center. And this SMBH is surrounded by an accretion disk that spins rapidly around it. And the SMBH swallows up whatever matters in the spinning accretion disk. This disk is a hot, bright disk that glows in UV and optical light.

The accretion disk is the main source of optical and UV light, but there are other regions that produce optical and UV light.

1. Broad Line Regions (BLRs): This is the region near the SMBH. In the broad line region, gases move at thousands of kilometers per second and emit broad emission lines(BELs).
2. Narrow Line Regions (NLRs): Narrow line regions are at the outer edge of the disk where gases move at about hundreds of kilometers/second and emit narrow emission lines.

We use optical telescopes to find AGN. The optical spectrum of the AGN gives us information about what is happening near the black hole, such as the motion of the gas, measure the width of BEL, estimates the size of BLR, Chemical Composition (Metallicity), Accretion Rate, Black Hole Mass (MBH), Redshift (z) and structure and many other things like:

• Continuum light → from the hot accretion disk

• Broad emission lines → fast-moving gas near the black hole

• Narrow emission lines → slower gas farther out

• Absorption lines → from winds (e.g., in BAL quasars)

• BAL quasars (Broad Absorption Line) → strong outflows, up to 30,000 km/s!

Q. 1 How do we know which spot is an AGN and which is a star when the whole sky is full of stars at night?

Ans. : Although AGNs look like stars (just tiny points of light), they behave very differently. For this we use three types of tools: photometry, spectroscopy and redshift estimation. 

Feature	Star	AGN
Spectrum	Absorption lines	Broad emission lines
Color	Follows standard patterns	Unusual colors
Brightness changes	Mostly constant	Flickers or changes
Multi-wavelength	Optical only	Bright in X-ray, IR, radio
Redshift	Very small (nearby)	High (far galaxies)

Technique	What should be measured?	What is the use?
Photometry	Brightness, color	Identify AGNs, estimate photometric redshift
Spectroscopy	Emission lines, line widths, continuum	Confirm AGN, black hole mass, accretion rate
Redshift	Wavelength shift in spectral lines	Distance, age of AGN, evolutionary studies

• Photometry: When we look for AGNs in the sky, we take pictures of the sky using different optical filters (e.g. red, green, blue) - this is photometry. when we take photo of sky then we use different filters like u, g, r, i, z (used in SDSS). Each filter captures a slice of light: from ultraviolet to red.

AGNs can also be distinguished from stars simply by their colors. we use colours (i.e., how bright they are in different filters).

Generally, AGNs and stars have different colour patterns, so we can distinguish them on a colour-colour diagram.

But the problem comes when...........now... think about redshift............

When the AGN is very far from us, and the light coming from it travels a long distance to reach us and due to this the light coming to us gets stretched due to the expanding universe, that is, its wavelength increases, due to which we start seeing the AGN as red. This effect is called redshift.

That is why at high redshift, the emission lines and blue light emitted from the AGN can also be seen in red or IR and some stars also appear red to us, so how do we know which is the AGN and which is the star? Because at some redshift, the color of the redshifted AGN light becomes almost similar to the color of some stars in our galaxy.

So we cannot tell from just photometry which point is the AGN and which is the star. This is called the selection problem.

Just because we can't see an AGN doesn't mean there isn't one. Rather, the AGN we can't see could be behind stars and that's why we can't see it.

Astronomers have found solutions to this by: 

1. using More filters to break the color parity.

2. combining optical and X-ray data to identify them.

3. using spectroscopy to reveal emission lines that only occur in AGN.

• Spectroscopy: Splitting light into differnt wavelength like rainbow. Spectrum of Stars and AGN are different. we can also differenciate between Star and AGN using spectroscopy. now "Selection problem" solved by Spectroscopy. 

When Light come from AGN. What You See:

1. Broad emission lines → from fast gas (close to black hole).
2. Narrow emission lines → from slower gas (farther out).
3. These lines are unique to AGN — just like fingerprints or DNA.

So:

Photometry = suspect sketch (approx idea)

Spectroscopy = DNA test (confirmed ID) ✅

• Redshift Estimates: When we look at the sky at night and see a galaxy shining very far away, we ask ourselves or think

“How far is it?”

“And how long ago did the light from that galaxy or star start its journey towards Earth?”

This is where redshift comes in!

In simple terms, redshift: The higher the redshift, the farther away the object is – and the older the light.

But how do we measure redshift?

When we use spectroscopy, we spread the incoming light out into a full rainbow and see how much the emission or absorption lines have shifted and we can calculate the redshift from that too, one at a time. But when we have millions of galaxies and AGN to calculate their redshift, we can't do spectroscopy for all of them again and again. So instead of spectroscopy, we use photometric redshift which involves filters to estimate the redshift. We have 2 types of filters 1. Broadband filters (like SDSS u,g,r,i,z) and 2. Narrowband filters (like J-PAS, ALHAMBRA)

1. Broadband filters: These are wide colour bands (like a broad brush). Using them, we can estimate the redshift of the objects by judging their size/colour. 5 filters (like SDSS), 

 u (ultraviolet)

 g (green)

 r (red)

 i (infrared)

 z (deeper infrared)

    2. Narrow band filters: Here we use 40-60 narrowband filters. Each filter is like a thin piece of paint, just a few nanometers wide. These filters can detect even the smallest changes in light and they give low-resolution spectroscopy.

So now, even without a spectrograph, you can:

• Accurately measure redshift (z)
• Find distant AGNs and galaxies
• Know when they existed in the history of the universe

Q2. Why do some AGNs go undetected in optical surveys, even though they are bright?

Reason 1: Colour confusion with stars at some redshifts

At redshifts z ~ 2.6 to 3.5, AGNs have similar colours to stars.

In photometric surveys (such as SDSS), AGNs and stars look similar in u, g, r, i, z filters. Therefore, AGNs hide among stars, making them difficult to detect. Only ~60% of AGNs are detected in this range.

Reason 2: Low luminosity AGNs are faint

Faint AGNs are not very bright, and their host galaxy's luminosity is too much, so much so that the light coming from faint AGNs does not reach us. The light from faint AGNs is lost there and the light that does come through makes them look just like normal galaxies in the images. Without spectroscopy, it's hard to tell if there's an AGN inside.

Reason 3: Type 2 AGNs are covered in dust, meaning they're not clearly visible. Their central light is blocked by dust, so we don't see bright optical signals. And they're often missed in optical surveys.

But some Type 2 AGNs are found in optical surveys! Because scattered light or strong emission lines reach us directly.

The reflected or scattered light reaches us after hitting dust particles or electrons in the path of the light.

AGN Detection Depends On...

Factor	Why it matters
Redshift	Affects color; some AGNs look like stars at high z
Luminosity	Faint AGNs get lost in galaxy light
Obscuration	Dust blocks the light from view
Colour overlap	AGNs can blend into the star population

• Most AGN have black holes between 1 million and 10 billion times the mass of the Sun, and they feed at different rates. By measuring how wide a broad emission line is, and knowing how bright the AGN is, we can estimate the mass of the black hole at its center — all using just optical light.
• Assume the gas is captured by gravity.
• We assume that the gas is rotating under the gravitational pull of the black hole – just like planets orbiting the Sun. This is called the virial assumption.