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--- courses:ast403:the-local-group [2026/03/08 22:37] – created shuvo
+++ courses:ast403:the-local-group [2026/03/09 09:58] (current) – [Satellite Systems and Interactions] shuvo
@@ Line 1: / Line 1: @@
 ====== The Local Group ======
-The **Local Group** is a gravitationally bound collection of approximately 35 to 40 known galaxies, including our own Milky Way, situated within a sphere of roughly **1 megaparsec (Mpc)** in radius. It represents a locally overdense region of the Universe where mutual gravitational attraction has overcome the general Hubble expansion.
+The Local Group is a gravitationally bound collection of approximately 35 to 40 known galaxies, including our own Milky Way, situated within a sphere of roughly 1 megaparsec (Mpc) in radius. It represents a locally overdense region of the Universe where mutual gravitational attraction has overcome the general Hubble expansion.
-### **Phenomenology and Membership**
+==== Phenomenology and Membership ====
-The group’s population is dominated by three large spiral galaxies, which together produce about **90% of its total luminosity**:
+The group’s population is dominated by three large spiral galaxies, which together produce about 90% of its total luminosity:
-*   **Andromeda (M31):** The most luminous member, an Sb-type spiral located approximately 770 kpc away. It is about 50% more luminous than the Milky Way and possesses a larger disk scale length of 6–7 kpc.
-*   **The Milky Way:** An Sbc or Sc-type spiral and the second most luminous member.
+**Andromeda (M31):** The most luminous member, an Sb-type spiral located approximately 770 kpc away. It is about 50% more luminous than the Milky Way and possesses a larger disk scale length of 6–7 kpc.
-*   **M33 (Triangulum):** An Sc or Scd spiral that is much less luminous, contributing only about 20% of the Milky Way’s light.
+**The Milky Way:** An Sbc or Sc-type spiral and the second most luminous member.
+**M33 (Triangulum):** An Sc or Scd spiral that is much less luminous, contributing only about 20% of the Milky Way’s light.
 The remaining members are primarily **dwarf galaxies**, which are far more numerous but significantly fainter. These include the **Large and Small Magellanic Clouds** (LMC and SMC), which are irregular satellite galaxies orbiting the Milky Way at distances of ~50 kpc and ~60 kpc, respectively. Other members include various dwarf ellipticals (dE), such as M31's satellite **M32**, and numerous extremely diffuse dwarf spheroidal (dSph) galaxies like **Fornax**, **Sculptor**, and **Leo I**.
-### **Dynamics and Mass Distribution**
+[{{ :courses:ast403:localgroup.jpg?600 | Fig 1: Schematic view of the Local Group of galaxies. Milky Way is at the center.}}]
+==== Dynamics and Mass Distribution ====
 The Local Group exhibits complex internal and global motions:
-*   **Closing Velocity:** The Milky Way and Andromeda are currently approaching each other at approximately **120 km s⁻¹**.
-*   **Global Motion:** The entire Local Group moves at a peculiar velocity of approximately **600 km s⁻¹** relative to the cosmic microwave background (CMB) rest-frame.
-*   **Mass and Dark Matter:** The total mass of the Local Group is estimated to be between **$3 \times 10^{12} M_{\odot}$ and $4.8 \times 10^{12} M_{\odot}$**. Because visible stars and gas account for only about 5% of this mass, the group is heavily dominated by **dark matter**, resulting in a high mass-to-light ratio of $M/L \gtrsim 80 M_{\odot}/L_{\odot}$.
-### **Satellite Systems and Interactions**
+**Closing Velocity:** The Milky Way and Andromeda are currently approaching each other at approximately 120 km s⁻¹.
+**Global Motion:** The entire Local Group moves at a peculiar velocity of approximately 600 km s⁻¹ relative to the cosmic microwave background (CMB) rest-frame.
+**Mass and Dark Matter:** The total mass of the Local Group is estimated to be between $3 \times 10^{12} M_{\odot}$ and $4.8 \times 10^{12} M_{\odot}$. Because visible stars and gas account for only about 5% of this mass, the group is heavily dominated by dark matter, resulting in a high mass-to-light ratio of $M/L \gtrsim 80 M_{\odot}/L_{\odot}$.
+==== Satellite Systems and Interactions ====
 Many of the smaller members are organized into satellite systems around the two dominant spirals. The Milky Way’s 11 known satellites, for instance, form a highly flattened, planar distribution nearly perpendicular to the Galactic plane. These systems often show signs of tidal interaction:
-*   **Magellanic Stream:** A long band of neutral hydrogen stripped from the Magellanic Clouds by the Milky Way's tidal forces roughly 200 million years ago.
-*   **Sagittarius Dwarf Spheroidal:** This is the closest galaxy to Earth (24 kpc) and is currently being incorporated into the Milky Way, with its stars and globular clusters being stripped away by tidal forces.
-### **Future Evolution**
+**Magellanic Stream:** A long band of neutral hydrogen stripped from the Magellanic Clouds by the Milky Way's tidal forces roughly 200 million years ago.
-The Local Group is a dynamic system whose components are destined to merge. The Milky Way and Andromeda are predicted to undergo a **direct collision and merger** in approximately **3 to 4 billion years**, likely forming a single, larger elliptical or giant S0 galaxy. Additionally, smaller satellites like the LMC are expected to merge with the Milky Way even sooner, likely within the next **3 gigayears**, due to the effects of **dynamical friction**. Over time, the Local Group will likely become a "fossil group" dominated by one massive central galaxy.
+**Sagittarius Dwarf Spheroidal:** This is the closest galaxy to Earth (24 kpc) and is currently being incorporated into the Milky Way, with its stars and globular clusters being stripped away by tidal forces.
+[{{ :courses:ast403:milkyway_stream.jpg?600 | Fig 2: HI in the Magellanic Clouds and the Magellanic Stream.}}]
+==== Future Evolution ====
+The Local Group is a dynamic system whose components are destined to merge. The Milky Way and Andromeda are predicted to undergo a direct collision and merger in approximately 3 to 4 billion years, likely forming a single, larger elliptical or giant S0 galaxy. Additionally, smaller satellites like the LMC are expected to merge with the Milky Way even sooner, likely within the next 3 gigayears, due to the effects of dynamical friction. Over time, the Local Group will likely become a "fossil group" dominated by one massive central galaxy.