The galactic center had been very difficult to study for many years and for very valid reasons! Take, for instance, the fact that it is very difficult to observe by optical telescopes due to being obscured by the gas and dust in the optical wavelength. What this means is that the gas and dust hinder the radiation in the visible range of the spectrum and hence can’t be seen in the literal sense.
But that doesn’t mean that all the radiation is obstructed. So instead, we make use of infrared, radio, and x-ray radiation to map and image it. This is the radiation that slips through the clouds of dust and gas and is able to reach us. This is how we are able to understand and image the Galactic Center (GC). And we can all agree that the gas and dust are hiding some very complex and interesting structures.
Onto the list of some of the things found in the GC. First, we have a large number of supernovae remnants, gas nebulae, globular clusters, X-ray binaries, etc. We also have found an abundance of HI gas in this region. This gas cloud extends to about 1 kpc from the center. Then from around 2 kpc, we detected ~ 3 x 10^7 solar masses of atomic Hydrogen!
No doubt there is so much extinction in the optical wavelength!
The Baade's Window
There are some regions in the GC where the extinction is considerably lower and we are able to peer into these regions using our optical telescopes. One such very significant region is called the Baade’s Window.
Coincidentally, this relatively clearer space (Baade’s Window) lies in our line of sight and gives a clear view into the most important part of the GC - it's core. This region is called the Sagittarius A system and constitutes the innermost 8 pc of the GC. It is a radio source and houses more complex structures, one of which is the powerhouse of our galaxy.
Components of Sagittarius A
The Circumnuclear ring (CNR):
The ring in orange (Figure 1), labelled as a circumnuclear disk is a torus shaped (imagine a donut) structure composed of molecular gas and orbiting the central black hole. The yellow line shows the inclination of the galactic plane relative to our line of sight. Its main physical features are as follows:
Radii: between 2 pc and 8pc.
Inclination to the galactic plane: 20 degrees
Rotational velocity: ~110 km (68 miles)/s
Now the question is- How was this ring formed?
If you closely observe Figure 2, you can see that even though the outer boundary of the CNR is not very defined, its inner boundary is pretty sharp. So, it seems that it is a result of some energetic event that happened within the last 10^5 years. The reason for this time frame is because within 10^5 years the turbulent motion would otherwise have erased it completely. So, for it to have existed by now, it has to have formed within that time.
Sagittarius A East:
Next, we come to the shell-like structure in blue which is called Sagittarius A East or simply Sgr A East. It is a non-thermal source meaning there is no radiation emission due to heat. But the most intriguing fact about this structure is that it is a very special type of Supernova (SN) remnant. Its age can be anywhere between 100 to 5000 years but what researchers found unusual was its spectrum. Even though it resembled the Type Ia SN a lot, the amounts of elements suggested by the spectrum were very different. It was hence given a very special category of SN called Type Iax.
A leading theory suggests that this special type of SN is caused by thermonuclear reactions that travel through their progenitors at a slower pace than the progenitors of Type Ia. This slower pace directly affects the explosion making it weaker and hence the differences in the elements produced by it.
Sagittarius A West:
The red spiral-like structure in Figure 3 and the cyan (blue) structure in Figure 1 is known as Sgr A West or simply the ‘Minispiral’. This one is a thermal source and consists of HII. The three arms of this spiral-like structure are rotating in a counterclockwise direction. Have you noticed the emphasis on the word spiral-like? It does look like a spiral structure with three arms. So why isn’t it just called a spiral structure? The answer is in its 3D image. In reality, it's just three streams of gas and dust clouds orbiting the blackhole and probably inflowing into it. As expected, the velocities of infall lie around the 1000 km/s range.
The circle in gray in Figure 1 shows the region where the central nuclear star cluster resides with the mass of ~ 1 x 10^6 Solar masses concentrated in just 1 pc! And at the center is the black hole called Sgr A* also read as Sagittarius A star. It is a compact and strong radio source and was observed to occupy a region lesser than 3 AU. The star S2 is orbiting Sgr A* at a speed of ~5000 km/s at its closest approach. The gas cloud labeled G2 is believed to be a star remnant because it was seen to pass through Sgr A* region without falling into the event horizon. This can only be possible if the gravitational field of this object was larger than normal gas clouds, suggesting it is a star remnant.
Glossary
HI gas: (HydrogenI) This signifies that the Hydrogen gas is in zero (1-1) excitation state. In short, the Hydrogen gas is in its ground state.
pc/kpc: (parsec/kiloparsec) This is a unit of measurement of distance used in astronomy. 1pc = 3.26 lightyears.
Solar Mass: This is a unit of measurement of masses of stars in astronomy. It is used in comparison with the mass of the Sun. So 1 Solar Mass would mean the star has the same mass as the Sun. 2 Solar Masses would mean the star is 2x the mass of the Sun and so on.
Extinction: In astronomy, extinction is used to denote the dimming of light due to being absorbed and scattered due to things such as dust, gas, atmosphere etc.
Progenitors: The stars that have now exploded into into supernovae are called the progenitor stars or simply progenitors.
HII: (HydrogenII) is Hydrogen gas in its first (2-1) excitation state.
AU: (Astronomical Unit) is the average distance between Earth and Sun. When parsec is too big a number to describe the distance in astronomy, we use AU.
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