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Dritan Kodra

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Richard P Martin

Stanford University

Ucla

Antonis K . Petrides

Open University of Cyprus

Silvia Barbantani

Università Cattolica del Sacro Cuore (Catholic University of the Sacred Heart)

Onur Güneş Ayas

Yildiz Technical University

University of Cambridge

George Sampatakakis

University of Patras

Luciana Gabriela Soares Santoprete

Centre National de la Recherche Scientifique / French National Centre for Scientific Research

Alessandro Barchiesi

New York University

Titika Dimitroulia

National and Kapodistrian University of Athens

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Papers by Dritan Kodra

Is Current Practice Adhering to Guidelines Proposed for Metabolite Identification in LC-MS Untargeted Metabolomics? A Meta-Analysis of the Literature

Journal of Proteome Research

Metabolite identification remains a bottleneck and a still unregulated area in untargeted LC-MS m... more Metabolite identification remains a bottleneck and a still unregulated area in untargeted LC-MS metabolomics. The metabolomics research community and, in particular, the metabolomics standards initiative (MSI) proposed minimum reporting standards for metabolomics including those for reporting metabolite identification as long ago as 2007. Initially, four levels were proposed ranging from level 1 (unambiguously identified analyte) to level 4 (unidentified analyte). This scheme was expanded in 2014, by independent research groups, to give five levels of confidence. Both schemes provided guidance to the researcher and described the logical steps that had to be made to reach a confident reporting level. These guidelines have been presented and discussed extensively, becoming well-known to authors, editors, and reviewers for academic publications. Despite continuous promotion within the metabolomics community, the application of such guidelines is questionable. The scope of this meta-analysis was to systematically review the current LC-MS-based literature and effectively determine the proportion of papers following the proposed guidelines. Also, within the scope of this meta-analysis was the measurement of the actual identification levels reported in the literature, that is to find how many of the published papers really reached full metabolite identification (level 1) and how many papers did not reach this level.

Cosmological perturbations with non-ideal fluids

Major Mergers in CANDELS up to z=3: Calibrating the Close-Pair Method Using Semi-Analytic Models and Baryonic Mass Ratio Estimates

The Galaxy Morphology-Density Relation at High Redshift with Candels

One of the biggest open questions regarding the evolution of the galaxy population over time, is ... more One of the biggest open questions regarding the evolution of the galaxy population over time, is how their properties (such as their morphologies) are affected by their local environment, e.g. the density of matter in the region where they are found. In the local universe, studies have shown that elliptical galaxies are found predominantly in the central regions of galaxy clusters where densities are higher, while disk galaxies reside in regions of lower densities such as the edges of clusters and the low-density "field". We investigate if this pattern continues to exist at earlier times by using data from the CANDELS collaboration at redshifts up to $z \sim 3$. For this, we make use of photometric redshift probability distributions (photo-z PDFs) for the galaxies observed by CANDELS. This required the development of new statistical methods to improve the quality of the PDFs measured by the CANDELS team, described in the thesis. We have used 3D-HST grism redshifts as well ...

FoodOmicsGR_RI: A Consortium for Comprehensive Molecular Characterisation of Food Products

Metabolites

The national infrastructure FoodOmicsGR_RI coordinates research efforts from eight Greek Universi... more The national infrastructure FoodOmicsGR_RI coordinates research efforts from eight Greek Universities and Research Centers in a network aiming to support research and development (R&D) in the agri-food sector. The goals of FoodOmicsGR_RI are the comprehensive in-depth characterization of foods using cutting-edge omics technologies and the support of dietary/nutrition studies. The network combines strong omics expertise with expert field/application scientists (food/nutrition sciences, plant protection/plant growth, animal husbandry, apiculture and 10 other fields). Human resources involve more than 60 staff scientists and more than 30 recruits. State-of-the-art technologies and instrumentation is available for the comprehensive mapping of the food composition and available genetic resources, the assessment of the distinct value of foods, and the effect of nutritional intervention on the metabolic profile of biological samples of consumers and animal models. The consortium has the kn...

Large-scale Structures in the CANDELS Fields: The Role of the Environment in Star Formation Activity

The Astrophysical Journal

We present a robust method, weighted von Mises kernel density estimation, along with boundary cor... more We present a robust method, weighted von Mises kernel density estimation, along with boundary correction to reconstruct the underlying number density field of galaxies. We apply this method to galaxies brighter than HST/F160w ≤ 26 AB mag at the redshift range of 0.4 ≤ z ≤ 5 in the five CANDELS fields (GOODS-N, GOODS-S, EGS, UDS, and COSMOS). We then use these measurements to explore the environmental dependence of the star formation activity of galaxies. We find strong evidence of environmental quenching for massive galaxies (M 10 11 M) out to z ∼ 3.5 such that an over-dense environment hosts 20% more massive quiescent galaxies compared to an under-dense region. We also find that environmental quenching efficiency grows with stellar mass and reaches ∼ 60% for massive galaxies at z ∼ 0.5. The environmental quenching is also more efficient in comparison to the stellar mass quenching for low mass galaxies (M 10 10 M) at low and intermediate redshifts (z 1.2). Our findings concur thoroughly with the "over-consumption" quenching model where the termination of cool gas accretion (cosmological starvation) happens in an over-dense environment and the galaxy starts to consume its remaining gas reservoir in depletion time. The depletion time depends on the stellar mass and could explain the evolution of environmental quenching efficiency with the stellar mass.

Can intrinsic alignments of elongated low-mass galaxies be used to map the cosmic web at high redshift?

Monthly Notices of the Royal Astronomical Society

Hubble Space Telescope observations show that low-mass (M* = 109 − 1010M⊙) galaxies at high redsh... more Hubble Space Telescope observations show that low-mass (M* = 109 − 1010M⊙) galaxies at high redshift (z = 1.0 − 2.5) tend to be elongated (prolate) rather than disky (oblate) or spheroidal. This is explained in zoom-in cosmological hydrodynamical simulations by the fact that these galaxies are forming in cosmic web filaments where accretion happens preferentially along the direction of elongation. We ask whether the elongated morphology of these galaxies allows them to be used as effective tracers of cosmic web filaments at high redshift via their intrinsic alignments. Using mock lightcones and spectroscopically-confirmed galaxy pairs from the CANDELS survey, we test two types of alignments: (1) between the galaxy major axis and the direction to nearby galaxies of any mass, and (2) between the major axes of nearby pairs of low-mass, likely prolate, galaxies. The mock lightcones predict strong signals in 3D real space, 3D redshift space, and 2D projected redshift space for both types...

The CANDELS/SHARDS Multiwavelength Catalog in GOODS-N: Photometry, Photometric Redshifts, Stellar Masses, Emission-line Fluxes, and Star Formation Rates

The Astrophysical Journal Supplement Series

We present a WFC3 F160W (H-band) selected catalog in the CANDELS/GOODS-N field containing photome... more We present a WFC3 F160W (H-band) selected catalog in the CANDELS/GOODS-N field containing photometry from the ultraviolet (UV) to the far-infrared (IR), photometric redshifts and stellar parameters derived from the analysis of the multi-wavelength data. The catalog contains 35,445 sources over the 171 arcmin 2 of the CANDELS F160W mosaic. The 5σ detection limits (within an aperture of radius 0. ′′ 17) of the mosaic range between H = 27.8, 28.2 and 28.7 in the wide, intermediate and deep regions, that span approximately 50%, 15% and 35% of the total area. The multi-wavelength photometry includes broad-band data from UV (U band from KPNO and LBC), optical (HST/ACS F435W, F606W, F775W, F814W, and F850LP), near-to-mid IR (HST/WFC3 F105W, F125W, F140W and F160W, Subaru/MOIRCS Ks, CFHT/Megacam K, and Spitzer/IRAC 3.6, 4.5, 5.8, 8.0 µm) and far IR (Spitzer/MIPS 24µm, HERSCHEL/PACS 100 and 160µm, SPIRE 250, 350 and 500µm) observations. In addition, the catalog also includes, optical medium-band data (R∼ 50) in 25 consecutive bands, λ = 500 to 950 nm, from the SHARDS survey and WFC3 IR spectroscopic observations with the G102 and G141 grisms (R∼ 210 and 130). The use of higher spectral resolution data to estimate photometric redshifts provides very high, and nearly uniform, precision from z = 0 − 2.5. The comparison to 1,485 good quality spectroscopic redshifts up to z ∼ 3 yields ∆z/(1+z spec)=0.0032 and an outlier fraction of η =4.3%. In addition to the multi-band photometry, we release added-value catalogs with emission line fluxes, stellar masses, dust attenuations, UV-and IR-based star formation rates and rest-frame colors.

The SFR–M * Correlation Extends to Low Mass at High Redshift

The Astrophysical Journal

To achieve a fuller understanding of galaxy evolution, SED fitting can be used to recover quantit... more To achieve a fuller understanding of galaxy evolution, SED fitting can be used to recover quantities beyond stellar masses (M *) and star formation rates (SFRs). We use star formation histories (SFHs) reconstructed via the Dense Basis method of Iyer & Gawiser for a sample of 17,873 galaxies at 0.5<z<6 in the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey GOODS-S field to study the nature and evolution of the SFR-M * correlation. The reconstructed SFHs represent trajectories in SFR-M * space, enabling us to study galaxies at epochs earlier than observed by propagating them backward in time along these trajectories. We study the SFR-M * correlation at z=1, 2, 3, 4, 5, 6 using both direct fits to galaxies observed at those epochs and SFR-M * trajectories of galaxies observed at lower redshifts. The SFR-M * correlations obtained using the two approaches are found to be consistent with each other through a K-S test. Validation tests using SFHs from semi-analytic models and cosmological hydrodynamical simulations confirm the sensitivity of the method to changes in the slope, normalization, and shape of the SFR-M * correlation. This technique allows us to further probe the low-mass regime of the correlation at high z by ∼1 dex and over an effective volume of ∼10×larger than possible with just direct fits. We find that the SFR-M * correlation is consistent with being linear down to M * ∼10 6 M e at z>4. The evolution of the correlation is well described by = -() t log SFR 0.80 0.029 0.017 0.010 univ *--() M t log 6.487 0.282 0.039 0.008 univ , where t univ is the age of the universe in Gyr.

Major merging history in CANDELS. I. Evolution of the incidence of massive galaxy–galaxy pairs from z = 3 to z ∼ 0

Monthly Notices of the Royal Astronomical Society

The rate of major galaxy-galaxy merging is theoretically predicted to steadily increase with reds... more The rate of major galaxy-galaxy merging is theoretically predicted to steadily increase with redshift during the peak epoch of massive galaxy development (1≤z≤3). We use close-pair statistics to objectively study the incidence of massive galaxies (stellar M 1 >2×10 10 M) hosting major companions (1≤M 1 /M 2 ≤4; i.e., <4:1) at six epochs spanning 0<z<3. We select companions from a nearly complete, mass-limited (≥ 5×10 9 M) sample of 23,696 galaxies in the five CANDELS fields and the SDSS. Using 5 − 50 kpc projected separation and close redshift proximity criteria, we find that the major companion fraction f mc (z) based on stellar mass-ratio (MR) selection increases from 6% (z∼0) to 16% (z∼0.8), then turns over at z∼1 and decreases to 7% (z∼3). Instead, if we use a major F160W flux ratio (FR) selection, we find that f mc (z) increases steadily until z = 3 owing to increasing contamination from minor (MR>4:1) companions at z > 1. We show that these evolutionary trends are statistically robust to changes in companion proximity. We find disagreements between published results are resolved when selection criteria are closely matched. If we compute merger rates using constant fraction-to-rate conversion factors (C merg,pair =0.6 and T obs,pair =0.65Gyr), we find that MR rates disagree with theoretical predictions at z>1.5. Instead, if we use an evolving T obs,pair (z)∝(1 + z) −2 from Snyder et al., our MR-based rates agree with theory at 0<z<3. Our analysis underscores the need for detailed calibration of C merg,pair and T obs,pair as a function of redshift, mass and companion selection criteria to better constrain the empirical major merger history.

Galaxy Inclination and the IRX–β Relation: Effects on UV Star Formation Rate Measurements at Intermediate to High Redshifts

The Astrophysical Journal

At intermediate and high redshifts, measurements of galaxy star-formation rates are usually based... more At intermediate and high redshifts, measurements of galaxy star-formation rates are usually based on rest-frame ultraviolet (UV) data. A correction for dust attenuation, A UV , is needed for these measurements. This correction is typically inferred from UV spectral slopes (β) using an equation known as "Meurer's Relation." In this paper, we study this relation at a redshift of 1.5 using images and photometric measurements in the rest-frame UV (HST) through mid-infrared (Spitzer). It is shown that massive star-forming galaxies (above 10 10 M) have dust corrections that are dependent on their inclination to the line-of-sight. Edge-on galaxies have higher A UV and infrared excess (IRX=L(IR)/L(UV)) than face-on galaxies at a given β. Interestingly, dust corrections for low-mass star-forming galaxies do not depend on inclination. This is likely because more massive galaxies have more disk-like shapes/kinematics, while low-mass galaxies are more prolate and have more disturbed kinematics. To account for an inclination-dependent dust correction, a modified Meurer's Relation is derived: A UV = 4.43 + 1.99β − 1.73(b/a − 0.67), where b/a is the galaxy axis ratio. This inclinationdependence of A UV can be explained by a two-component model of the dust distribution inside galaxies. In such a model, the dust attenuation of edge-on galaxies has a higher contribution from a mixture component (dust uniformly mixed with stars in the diffuse interstellar medium), and a lower contribution from a birth cloud component (near-spherical dust shells surrounding young stars in H II regions) than that of face-on galaxies. The difference is caused by the larger path-lengths through disks at higher inclinations.

Large-scale Structures in the CANDELS Fields: The Role of the Environment in Star Formation Activity

The Astrophysical Journal, Feb 6, 2020

We present a robust method, weighted von Mises kernel density estimation, along with boundary cor... more We present a robust method, weighted von Mises kernel density estimation, along with boundary correction to reconstruct the underlying number density field of galaxies. We apply this method to galaxies brighter than HST/F160w ≤ 26 AB mag at the redshift range of 0.4 ≤ z ≤ 5 in the five CANDELS fields (GOODS-N, GOODS-S, EGS, UDS, and COSMOS). We then use these measurements to explore the environmental dependence of the star formation activity of galaxies. We find strong evidence of environmental quenching for massive galaxies (M 10 11 M) out to z ∼ 3.5 such that an over-dense environment hosts 20% more massive quiescent galaxies compared to an under-dense region. We also find that environmental quenching efficiency grows with stellar mass and reaches ∼ 60% for massive galaxies at z ∼ 0.5. The environmental quenching is also more efficient in comparison to the stellar mass quenching for low mass galaxies (M 10 10 M) at low and intermediate redshifts (z 1.2). Our findings concur thoroughly with the "over-consumption" quenching model where the termination of cool gas accretion (cosmological starvation) happens in an over-dense environment and the galaxy starts to consume its remaining gas reservoir in depletion time. The depletion time depends on the stellar mass and could explain the evolution of environmental quenching efficiency with the stellar mass.

Is Current Practice Adhering to Guidelines Proposed for Metabolite Identification in LC-MS Untargeted Metabolomics? A Meta-Analysis of the Literature

Journal of Proteome Research

Metabolite identification remains a bottleneck and a still unregulated area in untargeted LC-MS m... more Metabolite identification remains a bottleneck and a still unregulated area in untargeted LC-MS metabolomics. The metabolomics research community and, in particular, the metabolomics standards initiative (MSI) proposed minimum reporting standards for metabolomics including those for reporting metabolite identification as long ago as 2007. Initially, four levels were proposed ranging from level 1 (unambiguously identified analyte) to level 4 (unidentified analyte). This scheme was expanded in 2014, by independent research groups, to give five levels of confidence. Both schemes provided guidance to the researcher and described the logical steps that had to be made to reach a confident reporting level. These guidelines have been presented and discussed extensively, becoming well-known to authors, editors, and reviewers for academic publications. Despite continuous promotion within the metabolomics community, the application of such guidelines is questionable. The scope of this meta-analysis was to systematically review the current LC-MS-based literature and effectively determine the proportion of papers following the proposed guidelines. Also, within the scope of this meta-analysis was the measurement of the actual identification levels reported in the literature, that is to find how many of the published papers really reached full metabolite identification (level 1) and how many papers did not reach this level.

Cosmological perturbations with non-ideal fluids

Major Mergers in CANDELS up to z=3: Calibrating the Close-Pair Method Using Semi-Analytic Models and Baryonic Mass Ratio Estimates

The Galaxy Morphology-Density Relation at High Redshift with Candels

One of the biggest open questions regarding the evolution of the galaxy population over time, is ... more One of the biggest open questions regarding the evolution of the galaxy population over time, is how their properties (such as their morphologies) are affected by their local environment, e.g. the density of matter in the region where they are found. In the local universe, studies have shown that elliptical galaxies are found predominantly in the central regions of galaxy clusters where densities are higher, while disk galaxies reside in regions of lower densities such as the edges of clusters and the low-density "field". We investigate if this pattern continues to exist at earlier times by using data from the CANDELS collaboration at redshifts up to $z \sim 3$. For this, we make use of photometric redshift probability distributions (photo-z PDFs) for the galaxies observed by CANDELS. This required the development of new statistical methods to improve the quality of the PDFs measured by the CANDELS team, described in the thesis. We have used 3D-HST grism redshifts as well ...

FoodOmicsGR_RI: A Consortium for Comprehensive Molecular Characterisation of Food Products

Metabolites

The national infrastructure FoodOmicsGR_RI coordinates research efforts from eight Greek Universi... more The national infrastructure FoodOmicsGR_RI coordinates research efforts from eight Greek Universities and Research Centers in a network aiming to support research and development (R&D) in the agri-food sector. The goals of FoodOmicsGR_RI are the comprehensive in-depth characterization of foods using cutting-edge omics technologies and the support of dietary/nutrition studies. The network combines strong omics expertise with expert field/application scientists (food/nutrition sciences, plant protection/plant growth, animal husbandry, apiculture and 10 other fields). Human resources involve more than 60 staff scientists and more than 30 recruits. State-of-the-art technologies and instrumentation is available for the comprehensive mapping of the food composition and available genetic resources, the assessment of the distinct value of foods, and the effect of nutritional intervention on the metabolic profile of biological samples of consumers and animal models. The consortium has the kn...

Large-scale Structures in the CANDELS Fields: The Role of the Environment in Star Formation Activity

The Astrophysical Journal

We present a robust method, weighted von Mises kernel density estimation, along with boundary cor... more We present a robust method, weighted von Mises kernel density estimation, along with boundary correction to reconstruct the underlying number density field of galaxies. We apply this method to galaxies brighter than HST/F160w ≤ 26 AB mag at the redshift range of 0.4 ≤ z ≤ 5 in the five CANDELS fields (GOODS-N, GOODS-S, EGS, UDS, and COSMOS). We then use these measurements to explore the environmental dependence of the star formation activity of galaxies. We find strong evidence of environmental quenching for massive galaxies (M 10 11 M) out to z ∼ 3.5 such that an over-dense environment hosts 20% more massive quiescent galaxies compared to an under-dense region. We also find that environmental quenching efficiency grows with stellar mass and reaches ∼ 60% for massive galaxies at z ∼ 0.5. The environmental quenching is also more efficient in comparison to the stellar mass quenching for low mass galaxies (M 10 10 M) at low and intermediate redshifts (z 1.2). Our findings concur thoroughly with the "over-consumption" quenching model where the termination of cool gas accretion (cosmological starvation) happens in an over-dense environment and the galaxy starts to consume its remaining gas reservoir in depletion time. The depletion time depends on the stellar mass and could explain the evolution of environmental quenching efficiency with the stellar mass.

Can intrinsic alignments of elongated low-mass galaxies be used to map the cosmic web at high redshift?

Monthly Notices of the Royal Astronomical Society

Hubble Space Telescope observations show that low-mass (M* = 109 − 1010M⊙) galaxies at high redsh... more Hubble Space Telescope observations show that low-mass (M* = 109 − 1010M⊙) galaxies at high redshift (z = 1.0 − 2.5) tend to be elongated (prolate) rather than disky (oblate) or spheroidal. This is explained in zoom-in cosmological hydrodynamical simulations by the fact that these galaxies are forming in cosmic web filaments where accretion happens preferentially along the direction of elongation. We ask whether the elongated morphology of these galaxies allows them to be used as effective tracers of cosmic web filaments at high redshift via their intrinsic alignments. Using mock lightcones and spectroscopically-confirmed galaxy pairs from the CANDELS survey, we test two types of alignments: (1) between the galaxy major axis and the direction to nearby galaxies of any mass, and (2) between the major axes of nearby pairs of low-mass, likely prolate, galaxies. The mock lightcones predict strong signals in 3D real space, 3D redshift space, and 2D projected redshift space for both types...

The CANDELS/SHARDS Multiwavelength Catalog in GOODS-N: Photometry, Photometric Redshifts, Stellar Masses, Emission-line Fluxes, and Star Formation Rates

The Astrophysical Journal Supplement Series

We present a WFC3 F160W (H-band) selected catalog in the CANDELS/GOODS-N field containing photome... more We present a WFC3 F160W (H-band) selected catalog in the CANDELS/GOODS-N field containing photometry from the ultraviolet (UV) to the far-infrared (IR), photometric redshifts and stellar parameters derived from the analysis of the multi-wavelength data. The catalog contains 35,445 sources over the 171 arcmin 2 of the CANDELS F160W mosaic. The 5σ detection limits (within an aperture of radius 0. ′′ 17) of the mosaic range between H = 27.8, 28.2 and 28.7 in the wide, intermediate and deep regions, that span approximately 50%, 15% and 35% of the total area. The multi-wavelength photometry includes broad-band data from UV (U band from KPNO and LBC), optical (HST/ACS F435W, F606W, F775W, F814W, and F850LP), near-to-mid IR (HST/WFC3 F105W, F125W, F140W and F160W, Subaru/MOIRCS Ks, CFHT/Megacam K, and Spitzer/IRAC 3.6, 4.5, 5.8, 8.0 µm) and far IR (Spitzer/MIPS 24µm, HERSCHEL/PACS 100 and 160µm, SPIRE 250, 350 and 500µm) observations. In addition, the catalog also includes, optical medium-band data (R∼ 50) in 25 consecutive bands, λ = 500 to 950 nm, from the SHARDS survey and WFC3 IR spectroscopic observations with the G102 and G141 grisms (R∼ 210 and 130). The use of higher spectral resolution data to estimate photometric redshifts provides very high, and nearly uniform, precision from z = 0 − 2.5. The comparison to 1,485 good quality spectroscopic redshifts up to z ∼ 3 yields ∆z/(1+z spec)=0.0032 and an outlier fraction of η =4.3%. In addition to the multi-band photometry, we release added-value catalogs with emission line fluxes, stellar masses, dust attenuations, UV-and IR-based star formation rates and rest-frame colors.

The SFR–M * Correlation Extends to Low Mass at High Redshift

The Astrophysical Journal

To achieve a fuller understanding of galaxy evolution, SED fitting can be used to recover quantit... more To achieve a fuller understanding of galaxy evolution, SED fitting can be used to recover quantities beyond stellar masses (M *) and star formation rates (SFRs). We use star formation histories (SFHs) reconstructed via the Dense Basis method of Iyer & Gawiser for a sample of 17,873 galaxies at 0.5<z<6 in the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey GOODS-S field to study the nature and evolution of the SFR-M * correlation. The reconstructed SFHs represent trajectories in SFR-M * space, enabling us to study galaxies at epochs earlier than observed by propagating them backward in time along these trajectories. We study the SFR-M * correlation at z=1, 2, 3, 4, 5, 6 using both direct fits to galaxies observed at those epochs and SFR-M * trajectories of galaxies observed at lower redshifts. The SFR-M * correlations obtained using the two approaches are found to be consistent with each other through a K-S test. Validation tests using SFHs from semi-analytic models and cosmological hydrodynamical simulations confirm the sensitivity of the method to changes in the slope, normalization, and shape of the SFR-M * correlation. This technique allows us to further probe the low-mass regime of the correlation at high z by ∼1 dex and over an effective volume of ∼10×larger than possible with just direct fits. We find that the SFR-M * correlation is consistent with being linear down to M * ∼10 6 M e at z>4. The evolution of the correlation is well described by = -() t log SFR 0.80 0.029 0.017 0.010 univ *--() M t log 6.487 0.282 0.039 0.008 univ , where t univ is the age of the universe in Gyr.

Major merging history in CANDELS. I. Evolution of the incidence of massive galaxy–galaxy pairs from z = 3 to z ∼ 0

Monthly Notices of the Royal Astronomical Society

The rate of major galaxy-galaxy merging is theoretically predicted to steadily increase with reds... more The rate of major galaxy-galaxy merging is theoretically predicted to steadily increase with redshift during the peak epoch of massive galaxy development (1≤z≤3). We use close-pair statistics to objectively study the incidence of massive galaxies (stellar M 1 >2×10 10 M) hosting major companions (1≤M 1 /M 2 ≤4; i.e., <4:1) at six epochs spanning 0<z<3. We select companions from a nearly complete, mass-limited (≥ 5×10 9 M) sample of 23,696 galaxies in the five CANDELS fields and the SDSS. Using 5 − 50 kpc projected separation and close redshift proximity criteria, we find that the major companion fraction f mc (z) based on stellar mass-ratio (MR) selection increases from 6% (z∼0) to 16% (z∼0.8), then turns over at z∼1 and decreases to 7% (z∼3). Instead, if we use a major F160W flux ratio (FR) selection, we find that f mc (z) increases steadily until z = 3 owing to increasing contamination from minor (MR>4:1) companions at z > 1. We show that these evolutionary trends are statistically robust to changes in companion proximity. We find disagreements between published results are resolved when selection criteria are closely matched. If we compute merger rates using constant fraction-to-rate conversion factors (C merg,pair =0.6 and T obs,pair =0.65Gyr), we find that MR rates disagree with theoretical predictions at z>1.5. Instead, if we use an evolving T obs,pair (z)∝(1 + z) −2 from Snyder et al., our MR-based rates agree with theory at 0<z<3. Our analysis underscores the need for detailed calibration of C merg,pair and T obs,pair as a function of redshift, mass and companion selection criteria to better constrain the empirical major merger history.

Galaxy Inclination and the IRX–β Relation: Effects on UV Star Formation Rate Measurements at Intermediate to High Redshifts

The Astrophysical Journal

At intermediate and high redshifts, measurements of galaxy star-formation rates are usually based... more At intermediate and high redshifts, measurements of galaxy star-formation rates are usually based on rest-frame ultraviolet (UV) data. A correction for dust attenuation, A UV , is needed for these measurements. This correction is typically inferred from UV spectral slopes (β) using an equation known as "Meurer's Relation." In this paper, we study this relation at a redshift of 1.5 using images and photometric measurements in the rest-frame UV (HST) through mid-infrared (Spitzer). It is shown that massive star-forming galaxies (above 10 10 M) have dust corrections that are dependent on their inclination to the line-of-sight. Edge-on galaxies have higher A UV and infrared excess (IRX=L(IR)/L(UV)) than face-on galaxies at a given β. Interestingly, dust corrections for low-mass star-forming galaxies do not depend on inclination. This is likely because more massive galaxies have more disk-like shapes/kinematics, while low-mass galaxies are more prolate and have more disturbed kinematics. To account for an inclination-dependent dust correction, a modified Meurer's Relation is derived: A UV = 4.43 + 1.99β − 1.73(b/a − 0.67), where b/a is the galaxy axis ratio. This inclinationdependence of A UV can be explained by a two-component model of the dust distribution inside galaxies. In such a model, the dust attenuation of edge-on galaxies has a higher contribution from a mixture component (dust uniformly mixed with stars in the diffuse interstellar medium), and a lower contribution from a birth cloud component (near-spherical dust shells surrounding young stars in H II regions) than that of face-on galaxies. The difference is caused by the larger path-lengths through disks at higher inclinations.

Large-scale Structures in the CANDELS Fields: The Role of the Environment in Star Formation Activity

The Astrophysical Journal, Feb 6, 2020

We present a robust method, weighted von Mises kernel density estimation, along with boundary cor... more We present a robust method, weighted von Mises kernel density estimation, along with boundary correction to reconstruct the underlying number density field of galaxies. We apply this method to galaxies brighter than HST/F160w ≤ 26 AB mag at the redshift range of 0.4 ≤ z ≤ 5 in the five CANDELS fields (GOODS-N, GOODS-S, EGS, UDS, and COSMOS). We then use these measurements to explore the environmental dependence of the star formation activity of galaxies. We find strong evidence of environmental quenching for massive galaxies (M 10 11 M) out to z ∼ 3.5 such that an over-dense environment hosts 20% more massive quiescent galaxies compared to an under-dense region. We also find that environmental quenching efficiency grows with stellar mass and reaches ∼ 60% for massive galaxies at z ∼ 0.5. The environmental quenching is also more efficient in comparison to the stellar mass quenching for low mass galaxies (M 10 10 M) at low and intermediate redshifts (z 1.2). Our findings concur thoroughly with the "over-consumption" quenching model where the termination of cool gas accretion (cosmological starvation) happens in an over-dense environment and the galaxy starts to consume its remaining gas reservoir in depletion time. The depletion time depends on the stellar mass and could explain the evolution of environmental quenching efficiency with the stellar mass.