1,720,963 research outputs found
Dynamical Properties and Assembly of Galaxies in the Epoch of Reionization
Full text linkIn a quest for understanding the nature of the furthest galaxies in the Universe, a large amount of
effort has been expended to build large telescopes as well as to perform sophisticated cosmological
simulations. Despite these achievements, we still know little about the structural and dynamical
properties of star-forming galaxies belonging to the epoch when the age of the Universe was less
than a Giga year i.e. the Epoch of Reionization (EoR). Indeed, galaxy dynamics in such a distant
universe is an uncharted field and we do not have solid answers to several fundamental questions
about the structure of EoR galaxies:
1- What are the dynamical and morphological properties of EoR galaxies?
2- Do we expect galaxies to form their disk structure as early as the EoR?
3- Are there any signatures of galaxy-galaxy mergers in such early epochs?
4- Are EoR galaxies rotationally supported or dispersion-dominated systems?
5- What is the energy source powering the gas velocity dispersion in EoR galaxies?
Atacama Large Millimeter/submillimeter Array (ALMA), being the most powerful millimeter/
sub-millimeter interferometer on Earth, has been playing a revolutionary role in the field of high-z
galaxy dynamics by providing spatially-resolved emission-line observations. However, as we aim
at studying the first galaxies that appeared in the Universe, the limiting angular resolutions and
signal-to-noise ratio of the observations significantly limit such studies. These limitations can be
overcome either by performing deeper observations or targeting lensed galaxies. While waiting for
such high-quality data to be available for EoR galaxies, we can address the problem theoretically.
This Thesis focuses on the study of structural and dynamical properties of EoR galaxies by utilizing
analytical modelings (when possible) as well as state-of-the-art hydrodynamical simulations
of galaxies. Since our aim is to provide a solid comparison and/or prediction for the upcoming
observations, we develop a framework in which a common cross-talk among observations and pure
theoretical works becomes possible. The Thesis is structured as follows:
1- In Chapter 1, we give the reader the necessary theoretical and observational background
information to follow the whole Thesis. A brief review of the conventional galaxy formation in the context of the standard model of cosmology is followed by explaining the current status
of observations of distant galaxies with particular attention to the EoR.
2- In Chapter 2, we introduce the ISM physics including far-infrared (FIR) line emissions, then
we introduce semi-analytical models of galaxies that we have developed to get the first insights
on the properties of [C II] emission line coming from high-z galaxies either as the kinematics
or star formation tracer. We end this Chapter by explaining the modeling features of a suite
of hydrodynamical simulations used in the rest of the Thesis.
3- In Chapter 3, we explore different kinematical features of EoR galaxies and their connection
with the assembly process as imprinted in the FIR line emission profiles. This is achieved
by tracing the evolution of a simulated galaxy from z = 7 to z = 6 through the FIR
[C II] emission.
4- In Chapter 4, we study the structure of spatially resolved, line-of-sight velocity dispersion
in EoR galaxies traced by [C II] line emission in the redshift range of 6 < z < 8. We also
quantify the contribution of the different energy sources powering such velocity dispersions
in EoR galaxies.
5- In Chapter 5, we address one of the recent puzzling issues in high-z galaxy dynamics studies,
which is related to the observations of surprisingly cold galactic disks at high redshift universe.
This problem is addressed by studying the dynamical properties of a large sample of simulated
EoR galaxies in the redshift range of 6 ≤ z < 9 probed by [C II] and nebular Hα emission.
6- Finally in Chapter 6, we present the conclusions and the future prospects
Blue monsters : why are JWST super-early, massive galaxies so blue?
Full text linkThe recent James Webb Space Telescope tentative discovery of a population of super-early (redshift z > 10), relatively massive (stellar mass M* = 108-109M⊙) and evolved (metallicity Z ≈ 0.1 Z⊙) galaxies, which nevertheless show blue (β ≃ -2.6) spectra, and very small dust attenuation (AV ≲ 0.02), challenges our interpretation of these systems. To solve the puzzle, we propose two solutions in which dust is either (a) ejected by radiation pressure, or (b) segregated with respect to UV-emitting regions. We clarify the conditions for which the two scenarios apply, and show that they can be discriminated by ALMA observations, such as the recent non-detection of the 88μm dust continuum in GHZ2 (z ≃ 12) favouring dust ejection
Interpreting ALMA non-detections of JWST super-early galaxies
Full text linkRecent attempts to detect [O iii] 88μm emission from super-early (z > 10) galaxy candidates observed by JWST have been unsuccessful. Non-detections can be either due to wrong photometric redshifts or to the faintness of the line in such early systems. By using zoom-in simulations, we show that if redshifts of these galaxies are confirmed, they are faint and mostly fall below the local metal-poor [OIII]−SFR relation as a result of their low ionization parameter, Uion ≲ 10−3. Such low Uion values are found in galaxies that are in an early assembly stage, and whose stars are still embedded in high-density natal clouds. However, the most luminous galaxy in our sample (log[L[OIII]/L⊙]=8.4, Uion ≈ 0.1) could be detected by ALMA in only 2.8 h.Recent attempts to detect [O iii] 88, μm emission from super-early (z > 10) galaxy candidates observed by JWST have been unsuccessful. Non-detections can be either due to wrong photometric redshifts or to the faintness of the line in such early systems. By using zoom-in simulations, we show that if redshifts of these galaxies are confirmed, they are faint and mostly fall below the local metal-poor [O, small III}]-SFR relation as a result of their low ionization parameter, Uion 10-3. Such low Uion values are found in galaxies that are in an early assembly stage, and whose stars are still embedded in high-density natal clouds. However, the most luminous galaxy in our sample (log [L[O, small III]L⊙] = 8.4, Uion ≈ 0.1) could be detected by ALMA in only 2.8 h
The evolution of dust attenuation in z ≈ 2–12 galaxies observed by JWST
No full textA sizable fraction of the heavy elements synthesized by stars in galaxies condenses into submicrometre-sized solid-state particles known as dust grains. Dust produces a wavelength-dependent attenuation, A(lambda), of the galaxy emission, thereby substantially altering its observed properties. Locally, A(lambda) is in general the sum of a power law and a ultraviolet feature ('bump') produced by small, carbon-based grains. However, scant information exists regarding its evolution across cosmic time. Here, leveraging data from 173 galaxies observed by the James Webb Space Telescope in the redshift range z = 2-12, we report the detection of the ultraviolet bump in a z approximate to 7.55 galaxy (when the Universe was only similar to 700 Myr old) and show that the power-law slope and the bump strength decrease towards high redshifts. We propose that the flat A(lambda) shape at early epochs is produced by large grains newly formed in supernova ejecta, which act as the main dust factories at such early epochs. Importantly, these grains have undergone minimal reprocessing in the interstellar medium due to the limited available cosmic time. This discovery offers crucial insights into the redshift-dependent evolution of dust attenuation properties, shedding light on the role of supernovae-driven dust production, grain size distribution at early cosmic times and the processes driving dust evolution at later epochs
≈ 7 and its connection with the interstellar medium properties
Full text linkWe exploit moderately resolved [O III], [C II] and dust continuum ALMA observations to derive the gas density (n), the gas-phase metallicity (Z) and the deviation from the Kennicutt-Schmidt (KS) relation (κs) on ≈sub−kpc
scales in the interstellar medium (ISM) of five bright Lyman Break Galaxies at the Epoch of Reionization (z ≈ 7). To do so, we use GLAM, a state-of-art, physically motivated Bayesian model that links the [C II]and [O III] surface brightness (Σ[CII], Σ[OIII]) and the SFR surface density (ΣSFR) to n, κs, and Z. All five sources are characterized by a central starbursting region, where the Σgas vs ΣSFR align ≈10 × above the KS relation (κs ≈ 10). This translates into gas depletion times in the range tdep ≈ 80 − 250 Myr. The inner starbursting centers are characterized by higher gas density (log (n/cm−3) ≈ 2.5 − 3.0) and higher metallicity (log (Z/Z⊙) ≈ −0.5) than the galaxy outskirts. We derive marginally negative radial metallicity gradients (∇log Z ≈ −0.03 ± 0.07 dex/kpc), and a dust temperature (Td ≈ 32 − 38 K) that anticorrelates with the gas depletion time
Deep into the structure of the first galaxies: SERRA views
Full text linkWe study the formation and evolution of a sample of Lyman Break Galaxies in
the Epoch of Reionization by using high-resolution (),
cosmological zoom-in simulations part of the SERRA suite. In SERRA, we follow
the interstellar medium (ISM) thermo-chemical non-equilibrium evolution, and
perform on-the-fly radiative transfer of the interstellar radiation field
(ISRF). The simulation outputs are post-processed to compute the emission of
far infrared lines ([CII], [NII], and [OIII]). At , the most massive
galaxy, `Freesia', has an age , stellar mass
, and a star formation rate
, due to a recent burst.
Freesia has two stellar components (A and B) separated by ; other 11 galaxies are found within . The
mean ISRF in the Habing band is and is spatially uniform; in
contrast, the ionisation parameter is , and
has a patchy distribution peaked at the location of star-forming sites. The
resulting ionising escape fraction from Freesia is .
While [CII] emission is extended (radius 1.54 kpc), [OIII] is concentrated in
Freesia-A (0.85 kpc), where the ratio . As many high- galaxies, Freesia lies below the local [CII]-SFR
relation. We show that this is the general consequence of a starburst phase
(pushing the galaxy above the Kennicutt-Schmidt relation) which
disrupts/photodissociates the emitting molecular clouds around star-forming
sites. Metallicity has a sub-dominant impact on the amplitude of [CII]-SFR
deviations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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