Did you know that our galaxy—the Milky Way—is full of invisible clouds of gas? Even though we can't see them with our eyes, scientists have a clever way to detect and study this gas using radio waves from space. One special kind of gas in the Milky Way is neutral hydrogen (HI), which sends out a very small radio wave.
This is where the MeerKAT Absorption Line Survey (MALS) becomes important. The third release of the MeerKAT Absorption Line Survey (MALS) contains data from around 20,000 radio sources. This means that a large amount of information is available for researchers to study, helping to investigate various cosmic phenomena, such as galaxy formation and other aspects of the universe. Like distant galaxies and quasars, these sources send radio waves that pass through the Milky Way’s gas before reaching Earth. By studying how the gas affects these signals, scientists can learn a lot about the structure and motion of our galaxy.
So, what can we discover with this data?
1. How Gas Moves in the Galaxy?
The gas in the Milky Way isn’t still—it moves! Using the MALS data, we can measure the speed and direction of this motion. This helps us create a map of how the gas flows throughout the galaxy.
2. Understanding Galactic Rotation
Our galaxy spins like a giant pinwheel. By studying the motion of hydrogen gas, we can see how fast different parts of the Milky Way are rotating. This even gives clues about the invisible dark matter that holds our galaxy together.
3. Spotting Turbulence in Space
Sometimes, gas in space moves in a very messy or swirling way, similar to how wind moves in the sky. This is called turbulence. By studying the 21-cm signal, we can look closely at these small movements and understand the “rough weather” happening in space.
4. Finding Peculiar Motions
Not all gas follows smooth, expected paths. Some gas might be pulled into the galaxy, pushed out by stars, or moved by shock waves. These unusual movements are called peculiar motions, and the MALS survey helps us detect them.
Why does it matter?
Studying these signals helps us understand how galaxies form, how stars are born, and how the Milky Way evolved over time.
What Can You Study with MALS HI 21-cm Data?
The HI 21-cm line data helps us understand the behavior of neutral hydrogen in our galaxy. Here’s what we can study in more detail:
1. HI Velocity Structure:
-
Velocity Profiles:
By looking at how the hydrogen's signals shift in frequency (called the Doppler shift), we can figure out how fast the gas is moving toward or away from us. This tells us how the gas moves in different parts of the galaxy, like its movement within different spiral arms or specific clouds. -
Cloud Identification:
When we look at the data, we can break the signals into smaller pieces, each representing a different cloud of hydrogen. This helps us identify individual gas clouds, and we can learn how fast they are moving and where they are located in the galaxy. -
High-Velocity Clouds (HVCs):
Some hydrogen clouds move much faster than the regular gas in the galaxy. These are called high-velocity clouds, and their speeds do not follow the normal rotation of the galaxy. Studying these clouds helps us understand gas that’s moving outside the regular disk of the galaxy, such as gas that may be falling into the galaxy or gas pushed away by powerful forces.
2. Galactic Rotation:
-
Rotation Curve:
We can use the movement of hydrogen gas in different parts of the galaxy to map out how fast the entire galaxy is rotating. By studying how the speed changes with distance from the center, we can figure out the galaxy’s rotation pattern. -
Spiral Arm Dynamics:
The gas in the spiral arms of the galaxy moves in specific ways. By looking at the velocities of the gas in these areas, we can learn about how the arms are formed and how they behave. -
Differential Rotation:
The speed at which different parts of the galaxy rotate can vary. The inner parts of the galaxy may rotate faster than the outer parts. By studying this difference, we can learn more about the galaxy’s mass and structure.
3. Turbulence:
-
Velocity Dispersion:
The hydrogen gas in the galaxy doesn’t always move smoothly. Instead, it has irregular motions or turbulence. By measuring how spread out the velocities of the gas are, we can understand how chaotic the gas movements are and how turbulent energy is spread across the galaxy. -
Power Spectra:
We can study the strength of turbulent movements at different scales. This means looking at how large or small the turbulence is, like comparing big gusts of wind to small breezes. This helps us understand the turbulence’s behavior in space. -
Turbulent Driving Mechanisms:
Turbulence can be caused by different things, like explosions from dying stars (supernovae), strong winds from young stars, or forces inside the galaxy. By studying where the turbulence is strongest, we can figure out what causes it. Turbulence refers to random, chaotic motions of HI gas, driven by energy inputs like supernova explosions, stellar winds, or gravitational instabilities.
4. Peculiar Motions:
-
Non-Circular Motions:
Not all gas in the galaxy moves in perfect circles. Some gas moves in strange ways, deviating from the regular circular motion of the galaxy. This could be caused by things like the galaxy's bar or spiral arms interacting with the gas. -
Outflows/Inflows:
Gas can flow in and out of the galaxy, moving either toward the center (inflows) or away from it (outflows). These motions are often linked to processes that push or pull gas, like exploding stars or gas falling into the galaxy. -
Local Anomalies:
Sometimes, specific regions in the galaxy show unusual movements of gas. This can happen because of nearby events like stars forming, gas being pushed by explosions, or interactions with other smaller galaxies. By studying these, we can learn more about how the galaxy evolves.
- hi_velocity_distribution.png:
Histogram of HI velocities.
- hi_velocity_map.png:
Galactic map of HI velocities.
- rotation_curve.png:
Galactic rotation curve.
- turbulent_dispersion.png:
Histogram of turbulent velocity dispersions.
- turbulence_map.png:
Galactic map of turbulent dispersions.
- peculiar_motions.png: Galactic map of peculiar motions (velocities deviating >20 km/s from expected rotation).
1. HI Velocity Structure:
- What it shows: A histogram (bar-like graph) of the speeds of hydrogen gas clouds relative to the Earth, measured in kilometers per second (km/s). The x-axis is the velocity, and the y-axis is how many gas clouds have that velocity. More dots since it includes all HI gas data.
- There are big spikes; they show common speeds of gas clouds. For
example, a peak near 0 km/s means a lot of gas is moving with the Earth’s
motion (local gas).
- Velocities range from, say, -100 to +100 km/s, which means gas is moving
in different directions (toward or away from us).
- Velocities far from 0 (e.g., >90 km/s) could be special gas clouds, like high-velocity clouds (HVCs), not orbiting normally in the galaxy.
- Why it matters: This plot tells you how fast and in what directions the hydrogen gas is moving.
2. HI Velocity Map:
- What it shows: A map of the galaxy with dots showing where gas clouds are located. The x-axis is Galactic longitude (l, like left-to-right position in the Galaxy), and the y-axis is Galactic latitude (b, like up-down position). The color of each dot shows the velocity of the gas (in km/s, with a color bar explaining the scale). Maps the normal speed of gas to study the galaxy's overall motion.
- Patterns:
Do colors (velocities) change smoothly across the map? For example, blue
dots (negative velocities) on one side and red dots (positive velocities)
on the other might show gas orbiting the galaxy.
- Clusters:
Are there groups of dots with similar colors? This could show gas in a
spiral arm or a cloud.
- High
latitude: Dots far from b=0 (the Galactic plane) with weird
velocities might be gas above or below the disk.
Why it matters: This map shows where gas with different speeds is located in the galaxy.
Example: If dots near l=30°, b=20° are mostly blue (-50 km/s), it means gas in that part of the Galactic plane is moving toward us with a speed of 50 km/s.3. Peculiar motions:
- What it shows: A map of gas clouds moving in unexpected ways (not following the galaxy’s normal rotation). Dots show locations (l, b), and colors show their velocities (km/s). Only gas with velocities >20 km/s different from expected rotation is shown. Finds objects with strange movements, like due to gravity or galaxy effects. Fewer dots because it only includes objects with weird speeds.
- Locations:
Are dots mostly at high |b| (away from the Galactic plane)? This could
mean gas being pushed up/down (e.g., by outflows).
- Colors:
Do velocities show a pattern (e.g., mostly positive or negative)? This
might indicate systematic motions like galactic winds.
- Clusters:
Groups of dots could point to specific events, like a supernova pushing
gas.
- Why
it matters: This plot highlights gas doing something unusual, like
cars veering off the racetrack. It can reveal exciting galactic events.
- What it shows: This figure represents a galactic rotation curve, which plots the circular velocity (in km/s) of stars or gas in a galaxy as a function of their distance from the galactic center (in kiloparsecs, kpc).
- The x-axis shows the galactic radius, ranging from 5 to 17 kpc.
- The y-axis shows the circular velocity, ranging from 210 to 230 km/s.
- The green and red dots represent observed circular velocities of objects (likely stars or gas clouds) at various distances from the galactic center.
- A dashed red line at 220 km/s (Constant velocity) shows the expected speed for a “flat” rotation curve.
- What
to look for:
- Alignment:
Do the dots cluster around 220 km/s? If so, the Galaxy’s rotation is
steady, like a solid disk.
- Scatter: If dots are spread out, it might mean errors in calculating distances or gas not following simple rotation.
Why it matters: This plot tells you how the Galaxy spins. A flat curve means the Galaxy has hidden mass (dark matter), keeping the rotation speed constant. The data points hover around 220 km/s across the range of radii, showing a "flat" rotation curve. This means the circular velocity does not decrease with increasing distance from the galactic center, as would be expected if the galaxy's mass were concentrated mostly at the center (like in a Keplerian orbit, where velocity decreases as the reciprocal of the square root of r, distance from the center). The flat curve indicates that the galaxy's mass is not concentrated at the center but is distributed in a roughly spherical halo extending far beyond the visible disk. This halo likely has a density profile that decreases gradually with radius.
Conclusion:
- X-axis: Galactic longitude (l, in degrees), ranging from 0° to 360°, with the Galactic center at l=0.
- Y-axis: Galactic latitude (b, in degrees), ranging from -90° to +90°, with the Galactic plane at b=0.
- Dots: Each dot represents an HI gas component (from the 2,096 filtered sources in the MALS dataset), with its position in Galactic coordinates (l,b).
Yellow indicates high turbulence. Dark purple typically indicates low turbulence.
Turbulence is higher near the Galactic plane due to active star formation, supernovae, and spiral arm dynamics. These processes inject energy, stirring the ISM.
At high latitudes, the ISM is less disturbed, with fewer energy sources like supernovae. Turbulence should be lower.
The Galactic plane (b=0, l=0 to 360 in degrees) is more turbulent, reflecting active star-forming regions and energy inputs.
Turbulence is generally symmetric above (b>0) and below (b<0) the Galactic plane near b≈0.
6. HI Turbulent Dispersion Distribution/Turbulent velocity dispersions:
- What it shows: A histogram of how much gas clouds are “jiggling” (turbulent motion) in km/s.
- What to look for:
- Typical
values: Most dispersions should be 5–15 km/s for normal HI gas. Very
high values (>20 km/s) might mean energetic regions (e.g., near
exploding stars).
- Shape:
A single peak means most gas has similar turbulence. Multiple peaks could
mean different types of gas clouds.