terms

作者Liang Xie发表在未分类

I put down some terms that I recall often during my research here. some cool theory I gathered from paper reading can be found here.

Disclaimar: those content are mostly from either copy-paste of introduction in literature or chatgpt-generated unfiltered web content. I do not guarantee that they are true.

theory:

BPT diagram: to differentiate ionizing sources in the universe, by comparing the flux ratios of emission line pairs log([O III] λ5007/Hβ) and log([O I] λ6300/Hα), log([N II] λ6583/Hα), and log([S II] λλ6716,31/Hα) etc.

EoR: During its first billion years, the Universe underwent a major phase transition for its main baryonic content, the so called Epoch of Reionization (EoR). The first stars and black holes produced an intense ultraviolet (UV) Corresponding radiation that gradually ionized the hydrogen atoms in the intergalactic medium (IGM), creating ionized bub- bles growing for approximately 1 Gyr, until they fully percolated at z ≤ 6. The cosmological sources responsible for this disrup-
tive event have been sought for long time, with conflicting opinions in favor of the two principal suspects, i.e. the star-forming galaxies (e.g., Finkelstein et al. 2019) and the active galactic nuclei; The beginning of reionization is less well constrained and depends sensitively on the nature of the ionizing sources. If rare objects such as quasars provided the bulk of the ionizing photons, reionization likely did not get well underway until z~10 (e.g., Madau & Haardt 2015). On the other hand, if young, massive stars dominated the ionizing photon budget, reionization may have started much sooner, although the constraints on the electron scattering optical depth to the cosmic microwave background (CMB) measured by Planck Collaboration et al. (2016b) imply that the halfway point came at z~8.

The Epoch of Reionization (EoR) represents a mile-stone in the evolution of our Universe. It represents the last major phase transformation of its gas from a cold (i.e. a few tens of K) and neutral to a fully ionized, hot (i.e. ≈ 104 K) state. This transformation was likely associated with the formation of the first stars, black holes and galaxies in our Universe. Understanding the reionization process is therefore intimately linked to understanding the formation of the first structures in our Universe – which represents one of the most basic and fundamental questions in astrophysics。

the escape fraction of ionizing photons: The fraction of ionizing photons which escape their host galaxy and so are able to ionize hydrogen in the inter-galactic medium (IGM) is a critical parameter in studies of the reionization era and early galaxy formation. However, the fraction of ionizing photons which escape their host galaxy into the IGM, is limited by intervening absorption in the inter-stellar medium (ISM). The value of this escape fraction (fesc) is therefore a critical parameter in studies of reionization, and remains relatively uncertain.

Lyman continuum photons: first off, they are photons. hydrogen is ionized by absorbing those photons. These are high-energy photons in the ultraviolet (UV) range of the electromagnetic spectrum. They are capable of ionizing hydrogen atoms by removing their electrons. These photons play a crucial role in the process of reionization, during which the early universe transitioned from a neutral state to an ionized state.


Lyman-alpha forest: In astronomical observations of distant quasars or active galactic nuclei, the Lyman-alpha forest appears as a series of absorption lines in the spectra. These lines are caused by the absorption of Lyman-alpha photons by intervening neutral hydrogen clouds. The Lyman-alpha forest provides valuable information about the distribution and properties of intergalactic gas and helps in understanding the large-scale structure of the universe.


Lyman-break galaxy (LBG): A Lyman-break galaxy is a type of high-redshift galaxy that is detected through a significant drop in its observed flux at wavelengths shorter than the Lyman limit (912 Angstroms). This drop occurs due to the absorption of Lyman continuum photons by neutral hydrogen in the galaxy itself or along the line of sight. Lyman-break galaxies are often associated with star-forming galaxies in the early universe.

Lyman-alpha emitters (LAE): LAEs are galaxies that emit a strong Lyman-alpha spectral line, which corresponds to a transition of a hydrogen electron from the n=2 to n=1 energy level. These galaxies are often associated with regions of active star formation, where massive stars ionize the surrounding gas, causing it to emit Lyman-alpha radiation. LAEs are valuable tools for studying the early universe and cosmic reionization, as their light can be detected from extremely high redshifts.


Gunn-Peterson trough: The Gunn-Peterson trough is an absorption feature observed in the spectra of distant quasars. It is caused by the almost complete absorption of photons with energies just above the Lyman limit by neutral hydrogen in the intergalactic medium. The presence of the Gunn-Peterson trough indicates a largely neutral state of the intergalactic medium and helps in studying the epoch of reionization in the early universe.The Lyman-𝛼 Forest and Gunn-Peterson troughs are two extremes on the scale of absorption features that are left by neutral hydrogen in intergalactic space.

observation:

Binning:In spectroscopy, the data is often represented as a two-dimensional array, commonly known as a data cube or a spectral cube. This representation, binning, is used because spectroscopic observations are not only taken at a single point or wavelength but are obtained as a function of both spatial position and wavelength/frequency.

Grating: In spectroscopy, a grating is an optical component that disperses light into its individual wavelengths. It consists of a large number of parallel grooves or lines ruled onto a reflective or transparent surface. When light passes through or reflects off the grating, it is split into its constituent colors (wavelengths), creating a spectrum.

Grism: A grism is a combination of a grating and a prism. It functions similarly to a grating but also includes a prism element. The grism disperses light like a grating, but the prism component helps correct for the angular dispersion, resulting in a more uniform dispersion across the spectrum.

Slit: In astronomical spectroscopy, a slit is a narrow, elongated opening in the instrument that limits the amount of light entering the spectrograph. The purpose of the slit is to improve the spectral resolution of the instrument by allowing only a thin section of light from a specific part of the observed object to pass through. This helps to reduce the effects of light from neighboring regions and provides a clearer and more detailed spectrum.

Echelle: An echelle spectrometer (or simply, echelle) is a high-resolution spectrograph used in astronomy to obtain detailed spectra of celestial objects. It uses a special type of diffraction grating known as an “echelle grating.” The echelle grating has a very high groove density, which allows it to disperse light at very high resolution. Echelle spectrographs are particularly well-suited for studying objects with many spectral lines close together, such as stars with complex atmospheres or binary stars. They are capable of producing spectra with narrow, well-defined lines that reveal fine details about the composition and physical properties of astronomical objects.

Flux Conversion: https://hea-www.harvard.edu/~pgreen/figs/Conversions.pdf, http://physics.uwyo.edu/~chip/Classes/ASTR4610/Lec_Distances.pdf

PypeIt terms:

Calibration frames:

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