Differences

This shows you the differences between two versions of the page.

--- courses:ast403:lyman-break-technique [2026/03/26 06:28] – [The "Dropout" Observation Method] shuvo
+++ courses:ast403:lyman-break-technique [2026/03/26 07:49] (current) – [Advantages and Limitations] shuvo
@@ Line 24: / Line 24: @@
 If a galaxy is at a redshift of $z = 3$, the Lyman break is shifted from $912 \mathring{A}$ to roughly $3648 \mathring{A}$, moving it out of the extreme UV and into the visible part of the spectrum.
+[{{ :courses:ast403:highz_galaxy.jpeg?600 | Fig 1: The shift of Lyman$\alpha$ from UV to IR at $z=11$. }}]
 ===== The "Dropout" Observation Method =====
 Astronomers do not usually have the time to take detailed spectra of every single point of light in the sky to see where this break occurs. Instead, they use a highly efficient shortcut called broadband photometry.
@@ Line 31: / Line 32: @@
 They take images of the same patch of sky using multiple color filters—for example, Ultraviolet (U), Blue (B), Visible/Green (V), Red (R), and Infrared (I).
-**Finding a $z \approx 3$ Galaxy:** Because the Lyman break is shifted to about $3600 \text{ \AA}$, all light bluer than this is absorbed. When astronomers look at their images, the galaxy will be completely invisible in the U filter (which captures light around $3000 \mathring{A}$ - $4000 \mathring{A}$) but will suddenly appear brightly in the B, V, and R filters. Because the galaxy "drops out" of the U-band image, it is called a U-dropout.\\
-**Finding a $z \approx 4$ Galaxy:** At this distance, the break shifts further into the visible spectrum (around $4500 \mathring{A}$). The galaxy will now be invisible in both the U and B filters, but visible in the V filter and beyond. This is a B-dropout.
+[{{ :courses:ast403:dropout.jpg?600 | Fig 2: Dropout technique to find high-redshift galaxies.}}]
+**Finding a $z \approx 3$ Galaxy:** Because the Lyman break is shifted to about $3600 \mathring{A}$, all light bluer than this is absorbed. When astronomers look at their images, the galaxy will be completely invisible in the U filter (which captures light around $3000 \mathring{A}$ - $4000 \mathring{A}$) but will suddenly appear brightly in the B, V, and R filters. Because the galaxy "drops out" of the U-band image, it is called a U-dropout.\\
+**Finding a $z \approx 4$ Galaxy:** At this distance, the break shifts further into the visible spectrum (around $4500 \mathring{A}$). The galaxy will now be invisible in both the U and B filters, but visible in the V filter and beyond. This is a B-dropout.\\
 **Finding a $z \approx 5$ Galaxy:** The break shifts further, making the galaxy a V-dropout.
 Galaxies found using this method are collectively referred to as **Lyman-Break Galaxies (LBGs)**.
 ===== Advantages and Limitations =====
 **Advantages:**
-**Efficiency:** It allows astronomers to survey thousands of galaxies in a single image. By simply comparing brightness across a few filters, they can isolate a reliable list of high-redshift candidates without needing time-consuming spectroscopy for every object.
+**Efficiency:** It allows astronomers to survey thousands of galaxies in a single image. By simply comparing brightness across a few filters, they can isolate a reliable list of high-redshift candidates without needing time-consuming spectroscopy for every object.\\
+[{{ :courses:ast403:dropout_photo.png?600 | Fig 3: Dropout technique with HST filters showing photometric data.}}]
 **Targeting:** It helps optimize expensive telescope time. Once astronomers identify a list of dropouts, they can point massive spectrographs (like those on the Keck or VLT telescopes) exactly at those targets to confirm their exact redshift.
 **Limitations:**
-**Interlopers:** The technique is not foolproof. A "dropout" can sometimes be faked by a highly reddened, dusty galaxy at a lower redshift, or by certain types of cool, low-mass stars (like brown dwarfs) within our own Milky Way, which also have sharp drops in their bluer spectra.
+**Interlopers:** The technique is not foolproof. A "dropout" can sometimes be faked by a highly reddened, dusty galaxy at a lower redshift, or by certain types of cool, low-mass stars (like brown dwarfs) within our own Milky Way, which also have sharp drops in their bluer spectra.\\
 **Selection Bias:** It primarily detects incredibly bright, actively star-forming galaxies (since they produce the UV light necessary for a strong break). Quiescent (dead) galaxies or highly dust-obscured galaxies at the same redshift might be missed entirely.
+[{{ :courses:ast403:interloopers.jpg?600 | Fig 4: Color-color diagram for LBGs at z ~ 3 (light grey shaded zone). Black dots are the complete sample of galaxies in the SDSS photometric catalog. Red filled dots represent the sample of LBGs spectroscopically confirmed to be}}]