Galactic Types and Cosmic Activities

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ver wondered about quasars, Blazars, AGNs, or Different types of Galaxies? Well, you've come to the right place. Here in this post, we'll explore them, and we'll tell you their key features and properties. Hello and welcome to our Blog Called CosmicWisdom, where we talk about the Cosmos and its valuable wisdom in simple and accessible language. Let's begin.

Introduction:

Galaxies are not a rare word in our daily life, and most people might have heard this word. A galaxy is much like a nation of celestial objects and structures. In this nation, Billions of Stars, planets, nebulae, Black holes, Neutron Stars, White dwarfs, brown dwarfs, and many other countless types of celestial objects bind together by gravity, thanks to dark matter. Galaxies might have different types, and in our cosmos, every galaxy has its own shape, size, appearance, and much more.

According to modern studies, Dark matter helps with gravity so that Stars and galaxies can evolve together. It is done when a large gas cloud collapses due to its own gravity, but keep in mind that galaxies form wherever there is a large amount of Dark matter. You can think of it just as water brings the garbage together sometimes and forms a messy zone in the lake, pond, or ocean.

Before we dive into their types and explore them, let's briefly know about a typical galaxy's common features:

a) Nucleus =>

It's the central bright or diffused region, which mostly hosts the Supermassive Black hole, regulates the Star formation and AGN Activities. Stars form in the highest densities in this region. As we move away from this zone, Stellar density gradually decreases. Around the outskirts of Galaxies, Stars might be thousands of light-years apart from each other.

All the Stars orbit this central Region of the galaxy, not the Supermassive Black hole itself, although it lies in the center. The Nucleus can host AGN if it's active. We'll also explore its different kinds of activities further in this post.

b) Central bulge =>

Many galaxies, especially the Spirals and lenticulars, show a bright central region around their nuclei, which may be shiny, dense, and rich with nebulae, stars, and Stellar activities. Some bulges show a bar-like structure whose formation mechanism is a research topic, and several theories have been proposed, but no significant results. When a galaxy hosts a crystal clear Bar, we called Strong Barred galaxy, or its opposite, will be called Weak Barred galaxy.

c) Interstellar medium (ISM) =>

Whenever you see a galaxy or our own Milky Way's arms, you'll notice some faint, diffused dust or fog-like thing. It contains Stellar winds, Gases like Hydrogen. Stars form from this Gas fog; its local properties are diverse from region to region. If it contains a molecular hydrogen cloudy zone, it has the potential to form stars.

When stars are being formed, they eject their gas and dust into the interstellar medium; hence the ISM enriches the stars, and Stars nourish with different gases when they die or eject their material.

Even though all Galaxies have a similar ISM composition, it varies slightly from Galaxy to Galaxy; that's why our Milky Way and Andromeda galaxies have K and M-type (low mass) stars abundant. Whereas the Evil Eye or Phantom Galaxies possess a higher abundance of O or B (massive) Stars.

Alright, you now know the basic things required for this post, and we can proceed to the core point of this post. Let's explore different types of Galaxies.

Morphological basis

The widely known galactic classification system is called the Hubble sequence, which accounts for the morphology of a galaxy, meaning it mainly looks for a galaxy's shape and appearance without going much deeper. In this classification system, Galaxies have some shapes like spiral, barred Spiral, lenticular, elliptical, and Irregular. It doesn't account for size, activity, and other properties, and it is easier to memorize. Let's see them closely.

Irregular Galaxy (Irr):

As its name suggests, it doesn't have any defined shape. In the Classification, we use words like Irr to denote its type. As we said, Size has nothing to do with a galaxy's shape, unlike planets, which are spherical when massive and large, but Galaxies have their own problems. As you know, they are not solid or have defined boundaries. All galaxies behave like Gases or fogs where stars act as atoms. It means their shape is constantly changing, but it takes thousands of years due to their vastness.

An Irr Galaxy could be as small as just a few thousand light-years; on the other hand, they may be extended to millions of light-years if many galaxies are interacting or colliding with each other.

Sometimes they may look like Diffused Nebulae or large clouds, but scale always matters; typically, a nebula can't have the properties of a galaxy, that's only a way to distinguish between a Vast nebula and an Irr Galaxy.

The real-life examples of Irr galaxies are: LMC (25 kly), SMC (13 kly), IC 10 (4800 ly), Antennae (635 kly). Their Approximate sizes are given in Brackets.

Elliptical Galaxy (E):

Like its name, these galaxies look like a globular cluster of faint stars, so don't be confused. These galaxies are smooth, visually featureless except for a glowing central nucleus. They usually possess neither gas nor dust, at least on visual and observational levels, yet there are many exceptions, and some do feature those structures. 

According to scientists, elliptical galaxies have older star populations and show a high abundance of low-mass aging stars; they mainly show interstellar activities when they collide with other galaxies. Elliptical galaxies are classified from E0 to E7, where 0 is almost spherical, and 7 is oval-shaped.

When Stars orbit the central region in Random circular paths, unlike planet-like flat trajectories, the galaxy doesn't possess a disk; instead, it looks like a spheroid cluster of countless stars.

The interstellar medium of these galaxies is relatively quiescent. Stars usually have lower masses than Spiral galaxies. The size of elliptical galaxies can vary from a few thousand light-years to millions of light-years. For example, the satellite galaxies of Andromeda are very small, whereas we have gigantic galaxies like Hercules A, M87, or IC 1101.

Many ellipticals have active nuclei and host AGNs, which we will explore right after this section.

Lenticular Galaxy (S0):

These galaxies look like flat disks but have no spiral arms; some studies suggest they are older types of galaxies where there is no significant difference in ISM, and it is distributed uniformly. Flat disks form when Stars orbit the central bulge in planet-like flat paths.

It is thought that lenticular galaxies are bridges between Elliptical galaxies and Spiral galaxies and can be categorized as ES. Lenticulars are denoted by S0. These galaxies evolve passively, meaning no significant changes or visual features.

Lenticulars are unique in many ways; their nucleus region matches more with elliptical or E galaxies, whereas their disk part is similar to spiral galaxies. These galaxies have mainly dust instead of Gas, which means they seem to have used most of their interstellar matter in star formation, hence we observe dust.

Some lenticulars showing similarities with Elliptical are categorized as ES, and Barred Lenticulars are denoted by SB0, and here's their further description:

The real-life examples are: Spindle Galaxy (76 kly), NGC 1460 (43 kly), and NGC 1387 (45 kly). Approximate sizes are given in brackets.

Spiral Galaxy (S):

Flat disk-like galaxies with spiral arms; these arms suggest different rotational and compositional traits in the ISM, hence we see spiral arms. Almost two-thirds of known spiral galaxies host a bar in their bulges, and we call them Spiral Barred galaxies. Our own Milky Way galaxy has a bar, and it's classified as SBc.

In their classification, including both Barless and Barred spirals, Sa and SBa have diffused arms which almost blend with ISM, while Sc and SBc have the clearest spiral arms, and Sb and SBb share in between features.

 

M77 (90 kly), Pinwheel (252 kly), and Triangulum (61 kly) are some famous Spiral galaxies, whereas Andromeda (152 kly), NGC 1300 (130 kly) and our Milky Way (87 kly) are Famous Spiral barred galaxies.

Here, our first section is complete, and we can now proceed to the next section, where we will learn about their functional types and roles in their local cosmos.

Functions and features

When we talk about features and functions, we basically look for which activities are happening, what kinds of unusual or common features they have, and several such things. As you know, astronomy is basically a science of light, where light or photons are analyzed in spectrographs and spectrometers, and we estimate their source, nature, and effect. In this post, we aren't looking for stellar light or emissions; we are focusing on Galactic emissions or AGN's emissions.

You can easily memorize with this chart below, where we've distilled the major galactic types, and remember that many of these classes overlap. For example, an outburst galaxy can show Quiet or loud emission as well as Seyfert signatures; these are basically properties rather than rigid classes. Also, if an AGN was active millions of years ago but now appears Quiet, then we may classify it as an Inactive galaxy.

Inactive Galactic Nuclei:

Supermassive Black hole of a Quiet Galaxy

It is a well-known fact that most of the galaxies, except for dwarf and Irregular ones, have Supermassive Black holes (SMBH) in their central part of Nucleus. This part appears bright and huge, but no significant activities.

We look for its emissions in all parts of Electromagnetic radiation, then we conclude whether it is inactive or active. In the quiet phase of an SMBH, it consumes local gas and dust, bears an accretion disk and jets, but this doesn't affect at least the Galaxy's nucleus levels.

However, Modern Theories and studies suggest that Active or Calm is not a permanent trait; it's a cycle. Which means an SMBH looking quiet could become active in the future.

Our own Milky Way galaxy has two bubbles (Fermi bubbles) of Gamma rays and X-rays extending 30,000 light-years. Studies suggest these are the emissions from our SMBH Sagittarius A*. It's not like common active galaxies' emissions, but in miniature form.

Active Galactic Nuclei:

A Variable AGN with its Galaxy

It's the opposite of the previous category. These Galaxies' SMBH consume Gas and dust in enormous amounts, form torus-shaped thick accretion disks, and several violent forces occur and show their effects. They also release plasma Jets with tremendous energies, which are enough to bring massive changes to the whole Galactic or Intergalactic ranges.

Why SMBH is important?

Earlier, we talked that some massive stars like Wolf-Rayets, O and B types, including supernovae, help or regulate stellar formation and overall bring variations in the ISM. Let's see it briefly, then you'll have a crystal-clear idea why SMBHs are the kings of their Galaxies.

ISM is just a mixture of interstellar dust and gas, like Hydrogen, whose atoms are constantly moving and colliding. When ionizing radiations like Gamma rays, X-rays, or Ultraviolet pass through here, the electrons or other subatomic particles absorb their energies with photons and move at higher speeds and some also tend to make new bonds.

This way, their habits are also changed, and several new compounds form. For example, a common Nebula consists mainly of atomic hydrogen, which means Hydrogen atoms are freely floating. If any trigger, like a supernova shockwave, radiation, or an SMBH jet, or even some other similar events, occurs, those hydrogen atoms form Hydrogen Molecules, meaning two H atoms are now bound together, and we call this region a molecular cloud. These are perfect to form new stars within this zone.

Also, notice that Molecular clouds won't form stars automatically, because their molecules must overcome their repulsion or they should collapse under their gravity, therefore they'll keep wandering and colliding with each other just like they were in atomic form.

If no trigger occurs, then it won't collapse because only shockwave, jets, or radiation can alter the chemistry of the molecular cloud and forces them to collapse and join to form stars. This also works as a flip-flop switch; if a molecular cloud zone is already forming stars, jets, or a shockwave arrives, then it may stop forming stars or change their evolution rates.

Now trigger scale matters, how long a Star or Supernova can emit ionizing radiation, send its message "Hey nebulae! Create stars or stop it." Roughly, a star can do it just 10-20 light-years, whereas a supernova does it 50-100 light-years, and an SMBH Jet can send the message in almost the entire Galaxy or its galactic neighborhood. If we combine the effects of local and large-scale triggers, Stellar formation stops somewhere or starts, but doesn't start/stop together.

 We can call OB and WR Stars a local government, Supernovae as Senators and SMBH as the President in a galaxy. All these helps in the usage of Interstellar material to start/stop stellar formation. While non-AGN galaxies like ours do the same on miniature levels.

Alright, you now know about Why Supermassive Black holes are so necessary. Now, let's see their classification criteria and conditions so that you won't be confused when we move further.

Radio emission:

Of course, AGNs are not a radar station or a telecom company's tower, but it works similarly in terms of radiation. There are two types of AGN Radio emission: Loud and Quiet. Note that these are not types of AGN but features. We are looking at their Radio emission's quality and the overall properties.

Radio wavelengths are especially studied frequencies between 1.4 and 5 GHz. Radio Loud (RL) sources are often rare and emit higher Radio luminosities, usually at the 5 GHz band, and these produce larger Jets than the hosting galaxies. It's mainly found in elliptical galaxies. Whereas Radio Quiet (RQ) AGNs are common and found in 80-90% of Radio sources, many of them are hosted by Spiral galaxies. Some of these are called Seyferts as well, but they are slightly different. Also, their Jets or emissions are relatively weak.

Radio-emitting Structures:

Most of AGNs show structures like jets, lobes, fumes, or nebula-like structures, which can emit all kinds of radiation, but as we said earlier, we look for its Radio maps between 1.4-5 GHz. Along with RL-RQ variations, the shape and quality of these structures are studied or classified by a class system.

In 1974, two astronomers named Bernard Fanaroff and Julia Riley studied about 57 radio sources and created the Fanaroff-Riley system, commonly known as FR-I and FR-II groups. Let's see these classes quickly.

FR-I Jets that lose brightness as they move away from the AGN

FR-I class is recognized by its radio-emitting lobes, which lose brightness as it goes far away from the source; this is also called Edge-darkened lobes. On the other hand, the FR-II is just the opposite of the previous class, whose lobes get shiny in Radio wavelengths as they move farther from the source. This is called Edge-brightened lobes.

FR-II Jets that gets luminous when they move away from the AGN

Also, if those lobes extend about 2.2 million light-years or 0.7 megaparsecs, we call it "Giant Radio Galaxy". The Alcyoneus, Porphyrion, Hercules A and Centaurus A are striking examples of Edge-brightened galaxies.

Why Radio wavelength?

Note that FR-I and FR-II classes are exclusive to radio-emitting structures that come from the Supermassive Black Hole of the hosting galaxy. In a nutshell, whether its spiral, elliptical, lenticular, quasar, blazar, Seyfert, or any kind of AGN galaxy, it must show Radio lobes from 1.4 to 5 GHz frequencies: only then those two classes matter.

Let's see if AGN shines in all kinds of radiation types, then why is Radio emission too important? It has many benefits, for example:

1) Radio waves between 1.4-5 GHz can pass through easily in a gas and dust medium, whereas other radiations might be absorbed or blocked, overall obscuring the galaxy or structure.

2) Around scales of millions of light-years, other radiations like visible or X-rays might fade or become invisible, but Radio waves still shine.

3) The FR-I and FR-II-like emissions might appear in other radiation bands, but they wouldn't be sharp or clear. The Radio maps are too obvious, and it helps the scientists better than any other kind of radiation.

4) The Radio interferometers of the 1970s were sensitive to such radio emissions and structures, which were hard to impossible to detect with other Visible, UV, or X-ray surveys.

Due to all those reasons, Radio wavelengths are a better aid to study the Galactic and AGN activities. Now you know the classification terms, and we can swiftly move to the AGN types.

Low Ionization Nuclear Emission Region (LINER):

This is our first group of AGN-like galaxies. You can consider them a bridge between powerful AGNs and Inactive galaxies because their emissions are not as powerful as common AGNs, but they do emit radiation and show some structures, often rare though. The shape of the galaxy doesn't matter here, but the activity of a supermassive black hole does.

You remember, there were talks of Fermi bubbles in our Milky Way galaxy, which were the bubbles of gamma rays and X-rays extending 30,000 light-years. Yeah, it's probable that our Supermassive Black Hole Sagittarius A* had some LINER-like activities about 8 billion years ago. This was weak compared to Regular AGN, but enough to bring a message of Stellar formation on a galactic level.

During those 8 billion years, its radiation-emitting structures have faded too much, but at the time it was newly formed, it was a huge structure of plasma and interstellar material, just like we see in films.

Some Present day LINERs are Sombrero galaxy (94 kly diameter), M94 or Cat's eye Galaxy (81 kly) and NGC 5005 or Caldwell 29 (125 kly).

Radio Wavelength Visible Light

Here's an example of a LINER galaxy, you won't be able to see anything special with your eyes, it looks like a normal galaxy. Therefore, you need divine vision to see the Radio emission. That's why we've added a slider that unlocks the divine vision to see the radio emission and activities related to it.

Radio Galaxy:

Please keep in mind that every type of AGN or similar features we're talking about behaves like a gigantic star, which can emit the photons (particles of light) with wavelengths across the Electromagnetic spectrum, Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, and Gamma ray. But it's not the stellar light, it actually comes from when the SMBH is accreting matter in tremendous amounts and forms a torus-like accretion disk, and approx 40% of matter can't fall into the black hole, instead it keeps swirling around the Black hole. Loses energy in forms of radiation and jets.

Since we talked about radiation here, it could mean any one or all of those 7 types of radiation. In the case of Radio galaxies, The Radio bands are dominant, and that's why they are called Radio AGNs and the hosting galaxy is known as a radio galaxy.

Radio galaxies belong to the radio-loud class, which may have jets matching FR-I or FR-II or no jets at all. These are like Giant Radar stations that signal their presence in our cosmos. 

The famous examples of Radio galaxies are M87, Alcyoneus and Porphyrion galaxies. The M87 is quite popular among amateur astronomers and the scientific communities. This elliptical galaxy is extended across 132 kly, and the rest of the details you know, you can Google it, otherwise.

The Alcyonues galaxy was referred to as the largest known galaxy because of its Jets extending 17 million light-years, even larger than the IC 1101 elliptical galaxy, which is about 6 million light-years in expanse.

Let's be clear that jets, fumes, and ejections are not used to measure the Galaxy's dimensions. The actual Alcyoneus galaxy is just 242 kly in size.

The Porphyrion galaxy also hosts one of the largest discovered Jet systems, which are extended in 23 million light-years. Imagine it's 25 million, not 2.5, almost all the way from the Milky Way, Andromeda, and Triangulum Galaxies could be fitted in its radio-emitting structures, only created by a Powerful SMBH.

Seyfert Galaxy:

These are related to Radio Quiet (RQ) groups, which are mainly hosted by Spiral galaxies. These were first discovered by Edward A. Faith and Vesto M. Slipher in 1908 during the study of the NGC 1068 galaxy. In 1943, Carl Keenan Seyfert studied them deeply, and this class is named after him.

 If you look at them with your optical telescope, you might ignore them by saying Aww! It's just another spiral galaxy. If you use the Radio or other wavelength-based telescopes or instruments, then the real secret resides there. These are actually light versions of Quasars. The only difference is that quasars outshine their host galaxies and appear from billions of light-years away. Seyferts, on the other hand, their nucleus is brighter than the combined brightness of all its stars in the galaxy.

Seyfert AGNs have less massive SMBHs, not as weak as ours, though.

The Seyferts have two classes based on emissions. Type I Seyferts, NGC 6831 as an example, are known for bright UV and X-rays, which heavily ionize their local ISM gases. Type II Seyferts reveal their bright cores when you use Infrared to see them. NGC 3081 is an example of this class. However, in some cases, a single Seyfert galaxy can show both Type I and II-like regions, which denotes how the ISM responds to the Radiation from the AGN.

Some other examples of Seyfert Galaxies are: M77, Circinus galaxy, Cygnus A, and Centaurus A.

Quasar Galaxy:

Now, these are the most awesome structures nowadays. They are probably the most powerful energy sources in the universe. In functional groups, they may be divided as RQ Quasars or QSO (Quasi Stellar-like Objects) or Radio-loud Quasars. Some experts prefer to use the term Quasar regardless of radio emission and qualities, but others might prefer to reserve Quasar for the extreme sources. They are closely related to Seyferts, but with enormous luminosities, they are mainly hosted by elliptical galaxies; however, some exceptions are always present.

In the 1950s, they were first identified as Bright Radio sources with unknown physical properties. Later on, it turned out that these are the most luminous objects in the cosmos, powered by Ultramassive black holes.

Imagine that Gamma rays, which are too expensive to produce, and they only originate in the universe's most powerful events. The Supernovae usually emit them for less than 2 seconds; a specialized Neutron star can sustain them for some days. Quasar emits it like a star for millions of years, outshining the entire galaxy, meaning no shortage of Gamma rays.

Supernova and Neutron stars might produce it in a few sparks or brief sessions. It's a tremendous source of energy that numerous Stars, Neutron Stars, mergers and apocalyptic events are tiny if we compare them against a common Quasar.

Like all AGNs, Quasars emit all kinds of radiation in mind-boggling amounts, which is why they appear from billions of light-years and hide their hosting galaxies. If you look at them closely or get a chance to orbit a Quasar, you'll scream, "What the hell is it? I can't see but a white blazing light everywhere."

Most of the Real-life Quasar depictions are either taken from different radiation filtering tools or just the artist's impressions. 3C 48, 3C 273, Tonantzintla 618, Phoenix A* are some of the famous Quasars. They do emit jets and Radio lobes, which is why they are studied with the help of Radio wavelengths.

Blazars:

Blazars are actually quasars, but their jets are pointed toward our solar system, which is the only reason they are scarier and shinier than Regular Quasars. There are almost 2800 Blazars, some of them are BL Lacertae, 3C 273, Merkarian 421 and 501 and TXS 0506+056. No need to fear since they are too far away to harm us, and our Ozone and Heliopause mostly deal with their jet particles if they could even arrive here.

Galactic Activities:

In this section, you'll know about the types of galaxies that may not show the activities connected to their nuclei directly. We have made two distinct groups; of course, much like our post of Nebulae, this classification is prepared for you to understand them clearly. Let's begin.

Merging galaxies:

As its name suggests, these are two or more galaxies that merge to become one giant galaxy. As we said earlier, Galaxies are like fluid (gas) reservoirs where stars behave like atoms. If you prepare a water plastic water cushion and press or pinch it, you'll notice that some part of the water moves accordingly. Overall, the water changes its shape since fluid acquires the shape of the containers in which they are kept.

If a galaxy comes too close to another galaxy, its tidal effect will start working and deformation takes place. This process takes millions of years because Galaxies are too large and we measure them in light-years, so a slight deformation, even the fastest one, would take millions of years to observe.

As in the image above, you can see the left side's galaxy lost its shape of one of its spiral arms, because that arm feels a much stronger pull from another galaxy due to proximity. The distribution of gravitational pull is unequal since they both exert gravitational effects on each other.

When the object feels an uneven gravitational effect due to a source of gravity, we call it the tidal force. It's the same effect which can be seen in planets, tides in Earth's seas, stars and even galaxies.

Notice that a galaxy is not an individual entity; those celestial objects bind each other by mutual gravitational force, which is why if you destabilize one portion of the galaxy, the other portions will respond to it, like pinching a pizza dough.

The Whirlpool, Antennae and Stephan's quintet galaxies are some of the colliding or interacting galaxies.

Starburst galaxy:

These galaxies show massive interstellar activities like stellar formation or ceasing it. It's the aftereffect of AGN; many of its effects we've discussed in the AGN section. However, such a galaxy may not possess an active nucleus, since stars usually take millions of years to form.

Those colorful spiral arms are revealed in UV or IR spectral studies because protostars produce UV and IR in massive amounts. The famous Starburst galaxies are M82, Baby Boom, and the Large Magellanic Cloud.