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--- courses:ast403:extragalactic-distance-scale [2026/02/12 08:59] – shuvo
+++ courses:ast403:extragalactic-distance-scale [2026/02/12 09:05] (current) – shuvo
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 ===== Extragalactic Distance Scales =====
-Extragalactic distance determination relies on a **distance ladder** where absolute distances to nearby objects are used to calibrate relative indicators that reach further into the Universe. This calibration is essential to determine the **Hubble constant** ($H_0$), as redshift-based distances are only accurate if $H_0$ is known and peculiar velocities (local gravitational motions) are negligible.
+Extragalactic distance determination relies on a distance ladder where absolute distances to nearby objects are used to calibrate relative indicators that reach further into the Universe. This calibration is essential to determine the Hubble constant ($H_0$), as redshift-based distances are only accurate if $H_0$ is known and peculiar velocities (local gravitational motions) are negligible.
-**Primary Distance Indicators:**
+  * **Primary Distance Indicators:**
 Primary indicators are used to establish the first rungs of the ladder, often focusing on the Large Magellanic Cloud (LMC).
-**Geometric Method (SN 1987A):** One of the most precise methods involves the ring around supernova SN 1987A. By comparing the time delay between the illumination of the nearest and farthest parts of the ring with its angular diameter (~1.7"), astronomers derive a physical diameter and a distance of $D_{SN1987A} \approx 51.8 \text{ kpc}$.
+//Geometric Method (SN 1987A):// One of the most precise methods involves the ring around supernova SN 1987A. By comparing the time delay between the illumination of the nearest and farthest parts of the ring with its angular diameter (~1.7"), astronomers derive a physical diameter and a distance of $D_{SN1987A} \approx 51.8 \text{ kpc}$.
-**Cepheid Variables:** These young stars follow a well-defined period–luminosity (PL) relation, where their intrinsic luminosity ($L$) is related to their pulsation period ($P$): $P \propto L^{7/12}$. Calibrated in the LMC, Cepheids are visible with the Hubble Space Telescope (HST) out to the Virgo Cluster (~16 Mpc).
+//Cepheid Variables:// These young stars follow a well-defined period–luminosity (PL) relation, where their intrinsic luminosity ($L$) is related to their pulsation period ($P$): $P \propto L^{7/12}$. Calibrated in the LMC, Cepheids are visible with the Hubble Space Telescope (HST) out to the Virgo Cluster (~16 Mpc).
-**RR Lyrae Stars:** These Population II stars are found in globular clusters and the Galactic bulge. Their absolute visual magnitudes are nearly constant ($M_V \approx 0.6$), though more precise estimates account for metallicity:
+//RR Lyrae Stars:// These Population II stars are found in globular clusters and the Galactic bulge. Their absolute visual magnitudes are nearly constant ($M_V \approx 0.6$), though more precise estimates account for metallicity:
-**Secondary Distance Indicators**
+  * **Secondary Distance Indicators:**
-To reach distances where the **Hubble flow** dominates, secondary indicators are calibrated against Cepheid distances.
+To reach distances where the Hubble flow dominates, secondary indicators are calibrated against Cepheid distances.
-**Type Ia Supernovae (SN Ia):** Considered "standardizable candles," their maximum luminosity correlates with the shape (width) of their light curve. By applying a "stretch-factor" correction, their scatter in absolute magnitude is reduced to ~0.15 mag, allowing distance estimates at very high redshifts where they reveal the Universe's accelerated expansion.
+//Type Ia Supernovae (SN Ia):// Considered "standardizable candles," their maximum luminosity correlates with the shape (width) of their light curve. By applying a "stretch-factor" correction, their scatter in absolute magnitude is reduced to ~0.15 mag, allowing distance estimates at very high redshifts where they reveal the Universe's accelerated expansion.
-**Tully–Fisher Relation:** Used for spiral galaxies, it correlates total luminosity with maximum rotation velocity ($V_{max}$). It is often measured via the 21-cm H I line width.
+//Tully–Fisher Relation:// Used for spiral galaxies, it correlates total luminosity with maximum rotation velocity ($V_{max}$). It is often measured via the 21-cm H I line width.
-**Fundamental Plane and $D_n–\sigma$ Relation:** These relate the size, surface brightness, and velocity dispersion ($\sigma$) of elliptical galaxies. The $D_n–\sigma$ relation is particularly effective, relating the diameter within which a specific surface brightness is reached to $\sigma$: $D_n \propto \sigma^{1.4}$.
+//Fundamental Plane and $D_n–\sigma$ Relation:// These relate the size, surface brightness, and velocity dispersion ($\sigma$) of elliptical galaxies. The $D_n–\sigma$ relation is particularly effective, relating the diameter within which a specific surface brightness is reached to $\sigma$: $D_n \propto \sigma^{1.4}$.
-**Surface Brightness Fluctuations (SBF):** This method uses the Poisson noise in a galaxy's image; the relative fluctuations in surface brightness ($\sqrt{N}/N$) decrease as distance increases and more stars are included in each pixel.
+//Surface Brightness Fluctuations (SBF):// This method uses the Poisson noise in a galaxy's image; the relative fluctuations in surface brightness ($\sqrt{N}/N$) decrease as distance increases and more stars are included in each pixel.
-**Planetary Nebulae (PN):** The luminosity function of PN in a galaxy has a universal upper limit, providing a standard candle for galaxies of known type.
+//Planetary Nebulae (PN):// The luminosity function of PN in a galaxy has a universal upper limit, providing a standard candle for galaxies of known type.
+  * **Direct Cosmological Methods:**
-**Direct Cosmological Methods:**
 These methods can bypass the distance ladder to measure $H_0$ directly on cosmic scales.
-**Sunyaev–Zeldovich (SZ) Effect:** By combining the spectral distortion of the CMB (caused by hot gas in galaxy clusters) with the cluster's X-ray surface brightness ($I_X$), the **angular-diameter distance** ($D_A$) can be determined:
+//Sunyaev–Zeldovich (SZ) Effect:// By combining the spectral distortion of the CMB (caused by hot gas in galaxy clusters) with the cluster's X-ray surface brightness ($I_X$), the angular-diameter distance ($D_A$) can be determined:
-    $$D_A \propto \left( \frac{\Delta I_{\nu}}{I_{\nu}} \right)^2 \frac{1}{I_X}$$.
-**Gravitational Lens Time Delays:** Variations in the luminosity of a multiply-imaged quasar appear at different times in each image due to different path lengths and gravitational potentials. This time delay ($\Delta t$) is inversely proportional to $H_0$.
+//Gravitational Lens Time Delays:// Variations in the luminosity of a multiply-imaged quasar appear at different times in each image due to different path lengths and gravitational potentials. This time delay ($\Delta t$) is inversely proportional to $H_0$.
-**Baryonic Acoustic Oscillations (BAO):** These provide a standard rular based on the sound horizon at recombination, visible as a feature in the galaxy correlation function at separations of $\sim 100 \text{ Mpc}$.
+//Baryonic Acoustic Oscillations (BAO):// These provide a standard rular based on the sound horizon at recombination, visible as a feature in the galaxy correlation function at separations of $\sim 100 \text{ Mpc}$.