Cumulative Number Density Research Articles

Textural Study of Vesicles in Tagish Lake (C2-ung) Meteorite Fusion Crust: Constraints on Vesicle Formation during Their Entry into the Earth’s Atmosphere

Vesicles are characteristic structures within the outer layer of many stony meteorites’ fusion crusts. Although these features are well-developed in hydrated carbonaceous chondrites and some micrometeorites, their formation mechanism remains poorly understood. This study provides new insights into the understanding of physical vesiculation processes by presenting the results of vesicle size distribution (VSD)—i.e., a quantitative method for vesicle analysis—applied to the study of the Tagish Lake (C2-ung) meteorite fusion crust. Tagish Lake was chosen because it shows a scoriaceous texture and a significant number of vesicles (about 24,000 vesicles/mm2), thus allowing statistical analysis. Vesicles range from being spherical to irregular-shaped and from a few μm to ~70 μm (equivalent diameter) in size. Vesicle size distribution and cumulative number density analyses show a high nucleation event and a fractal distribution of the vesicle population, respectively. We suggest these features are due to disequilibrium degassing processes, which simultaneously produce continuous/accelerating vesicle nucleation and growth. Finally, possible analogies between the scoriaceous Tagish Lake fusion crust and the space-weathered “frothy layer” on the surface of Ryugu’s grains could be found in terms of vesicularity.

Chasing nomadic worlds: A new class of deep space missions

Nomadic worlds, i.e., objects not gravitationally bound to any star(s), are of great interest to planetary science and astrobiology. They have garnered attention recently due to constraints derived from microlensing surveys and the recent discovery of interstellar planetesimals. In this paper, we roughly estimate the prevalence of nomadic worlds with radii of 100km≲R≲104km. The cumulative number density n>>R appears to follow a heuristic power law given by n>∝R−3. Therefore, smaller objects are probably much more numerous than larger rocky nomadic planets, and statistically more likely to have members relatively close to the inner Solar system. Our results suggest that tens to hundreds of planet-sized nomadic worlds might populate the spherical volume centered on Earth and circumscribed by Proxima Centauri, and may thus comprise closer interstellar targets than any planets bound to stars. For the first time, we systematically analyze the feasibility of exploring these unbounded objects via deep space missions. We investigate what near-future propulsion systems could allow us to reach nomadic worlds of radius >R in a 50-year flight timescale. Objects with R∼100 km are within the purview of multiple propulsion methods such as electric sails, laser electric propulsion, and solar sails. In contrast, nomadic worlds with R≳1000 km are accessible by laser sails (and perhaps nuclear fusion), thereby underscoring their vast potential for deep space exploration.

The ALMA Spectroscopic Survey in the HUDF: A Search for [C ii] Emitters at 6 ≤ z ≤ 8

Abstract The Atacama Large Millimeter/submillimeter Array (ALMA) Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) Band 6 scan (212–272 GHz) covers potential [C ii] emission in galaxies at 6 ≤ z ≤ 8 throughout a 2.9 arcmin2 area. By selecting on known Lyα emitters (LAEs) and photometric dropout galaxies in the field, we perform targeted searches down to a 5σ [C ii] luminosity depth L [C II] ∼ 2.0 × 108 L ⊙, corresponding roughly to star formation rates (SFRs) of 10–20 M ⊙ yr−1 when applying a locally calibrated conversion for star-forming galaxies, yielding zero detections. While the majority of galaxies in this sample are characterized by lower SFRs, the resulting upper limits on [C ii] luminosity in these sources are consistent with the current literature sample of targeted ALMA observations of z = 6–7 LAEs and Lyman-break galaxies (LBGs), as well as the locally calibrated relations between L [C ii] and SFR—with the exception of a single [C ii]-deficient, UV-luminous LBG. We also perform a blind search for [C ii]-bright galaxies that may have been missed by optical selections, resulting in an upper limit on the cumulative number density of [C ii] sources with L [C II] > 2.0 × 108 L ⊙ (5σ) to be less than 1.8 × 10−4 Mpc−3 (90% confidence level). At this luminosity depth and volume coverage, we present an observed evolution of the [C ii] luminosity function from z = 6–8 to z ∼ 0 by comparing the ASPECS measurement to literature results at lower redshift.

Constraining cosmology with big data statistics of cosmological graphs

ABSTRACT By utilizing large-scale graph analytic tools implemented in the modern big data platform, apache spark, we investigate the topological structure of gravitational clustering in five different universes produced by cosmological N-body simulations with varying parameters: (1) a WMAP 5-yr compatible ΛCDM cosmology, (2) two different dark energy equation of state variants, and (3) two different cosmic matter density variants. For the big data calculations, we use a custom build of standalone Spark/Hadoop cluster at Korea Institute for Advanced Study and Dataproc Compute Engine in Google Cloud Platform with sample sizes ranging from 7 to 200 million. We find that among the many possible graph-topological measures, three simple ones: (1) the average of number of neighbours (the so-called average vertex degree) α, (2) closed-to-connected triple fraction (the so-called transitivity) $\tau _\Delta$, and (3) the cumulative number density ns ≥ 5 of subgraphs with connected component size s ≥ 5, can effectively discriminate among the five model universes. Since these graph-topological measures are directly related with the usual n-points correlation functions of the cosmic density field, graph-topological statistics powered by big data computational infrastructure opens a new, intuitive, and computationally efficient window into the dark Universe.

High-redshift JWST predictions from IllustrisTNG: dust modelling and galaxy luminosity functions

ABSTRACT The James Webb Space Telescope (JWST) promises to revolutionize our understanding of the early Universe, and contrasting its upcoming observations with predictions of the Λ cold dark matter model requires detailed theoretical forecasts. Here, we exploit the large dynamic range of the IllustrisTNG simulation suite, TNG50, TNG100, and TNG300, to derive multiband galaxy luminosity functions from z = 2 to z = 10. We put particular emphasis on the exploration of different dust attenuation models to determine galaxy luminosity functions for the rest-frame ultraviolet (UV), and apparent wide NIRCam bands. Our most detailed dust model is based on continuum Monte Carlo radiative transfer calculations employing observationally calibrated dust properties. This calibration results in constraints on the redshift evolution of the dust attenuation normalization and dust-to-metal ratios yielding a stronger redshift evolution of the attenuation normalization compared to most previous theoretical studies. Overall we find good agreement between the rest-frame UV luminosity functions and observational data for all redshifts, also beyond the regimes used for the dust model calibrations. Furthermore, we also recover the observed high-redshift (z = 4–6) UV luminosity versus stellar mass relation, the H α versus star formation rate relation, and the H α luminosity function at z = 2. The bright end (MUV > −19.5) cumulative galaxy number densities are consistent with observational data. For the F200W NIRCam band, we predict that JWST will detect ∼80 (∼200) galaxies with a signal-to-noise ratio of 10 (5) within the NIRCam field of view, $2.2\times 2.2 \, {\rm arcmin}^{2}$, for a total exposure time of $10^5\, {\rm s}$ in the redshift range z = 8 ± 0.5. These numbers drop to ∼10 (∼40) for an exposure time of $10^4\, {\rm s}$.

Effect of interstellar objects on metallicity of low-mass first stars formed in a cosmological model

We investigate metal pollution onto the surface of low-mass population III stars (Pop. III survivors) via interstellar objects floating in the Galactic interstellar medium. Only recently, Tanikawa et al. analytically estimated how much metal should collide to an orbiting Pop. III survivor encouraged by the recent discovery of 'Oumuamua and suggested that ISOs are the most dominant contributor of metal enrichment of Pop. III survivors. When we consider a distribution of interstellar objects in the Galactic disc, Pop. III survivors' orbits are significant properties to estimate the accretion rate of them though Tanikawa et al. assumed one modelled orbit. To take more realistic orbits into calculating the accretion rate, we use a high-resolution cosmological $N$-body simulation that resolves dark matter minihaloes. Pop. III survivors located at solar neighbourhood have a number of chances of ISO($> 100$ m) collisions, typically $5\times10^6$ times in the last $5$ Gyr, which is one order of magnitude greater than estimated in the previous study. When we assume a power-law parameter $\alpha$ of the ISO cumulative number density with size greater than $D$ as $n \propto D^{-\alpha}$, $0.80 \, M_{\odot}$ stars should be typically polluted [Fe/H]$\sim -2$ for the case of $\alpha=2.0$. Even in the cases of $0.70$ and $0.75 \, M_{\odot}$ stars, the typical surface metallicity are around [Fe/H]$=-6 \sim -5$. From the presence of stars with their [Fe/H], we can constrain on the lower limit of the power $\alpha$, as $\alpha \gtrsim 2.0$, which is consistent with $\alpha$ of km-size asteroids and comets in the solar system. Furthermore, we provide six candidates as the ISO-polluted Pop. III stars in the case of $\alpha \sim 2.5$. Metal-poor stars so far discovered are possible to be metal-free Pop. III stars on birth.

The Effect of Dark Matter–Dark Radiation Interactions on Halo Abundance: A Press–Schechter Approach

Abstract We study halo mass functions with the Press–Schechter formalism for interacting dark matter models, where matter power spectra are damped due to dark acoustic oscillations in the early universe. After adopting a smooth window function, we calibrate the analytical model with numerical simulations from the “effective theory of structure formation” (ETHOS) project and fix the model parameters in the high-mass regime, M h ≳ 3 × 1010 M ⊙. We also perform high-resolution cosmological simulations with halo masses down to M h ∼ 108 M ⊙ to cover a wide mass range for comparison. Although the model is calibrated with ETHOS1 and cold dark matter simulations for high halo masses at redshift z = 0, it successfully reproduces simulations for two other ETHOS models in the low-mass regime at low and high redshifts. As an application, we compare the cumulative number density of halos to that of observed galaxies at z = 6, and find that the interacting dark matter models with a kinetic decoupling temperature below 0.5 keV is disfavored. We also perform the abundance-matching analysis and derive the stellar–halo mass relation for these models at z = 4. Suppression in halo abundance leads to less massive halos that host observed galaxies in the stellar mass range M * ≃ 10 5 – 10 7 M ⊙ .

Origin of 1I/’Oumuamua. II. An Ejected Exo-Oort Cloud Object?

Abstract 1I/’Oumuamua is the first detected interstellar interloper. We test the hypothesis that it is representative of a background population of exo-Oort cloud objects ejected under the effect of post-main sequence mass loss and stellar encounters. We do this by comparing the cumulative number density of interstellar objects inferred from the detection of 1I/’Oumuamua to that expected from these two clearing processes. We consider the 0.08–8 M ⊙ mass range, take into account the dependencies with stellar mass, Galactocentric distance, and evolutionary state, and consider a wide range of size distributions for the ejected objects. Our conclusion is that 1I/’Oumuamua is likely not representative of this background population, even though there are large uncertainties in the masses and size distributions of the exo-Oort Clouds. We discuss whether the number density of free-floating, planetary-mass objects derived from gravitational microlensing surveys could be used as a discriminating measurement regarding 1I/’Oumuamua’s origin (given their potential common origin). We conclude that this is challenged by the mass limitation of the surveys and the resulting uncertainty of the mass distribution of the free floaters. The detection of interlopers may be one of the few observational constraints of the small end of this population, with the caveat that, as we conclude here and in Moro-Martín (2018), in the case of 1I/’Oumuamua, it might not be appropriate to assume this object is representative of an isotropic background population, which makes the derivation of a number density very challenging.

Metal pollution of low-mass Population III stars through accretion of interstellar objects like ‘Oumuamua

Abstract We calculate the accretion mass of interstellar objects (ISOs) like ‘Oumuamua on to low-mass Population III stars (Pop. III survivors), and estimate the surface pollution of Pop. III survivors. The ISO number density estimated from the discovery of ‘Oumuamua is so high (∼0.2 au−3) that Pop. III survivors have chances of colliding with ISOs ≳ 105 times per 1 Gyr. ‘Oumuamua itself would be sublimated near Pop. III survivors, since its size is small; ∼100 m. However, ISOs with size ≳3 km would reach the Pop. III survivor surfaces. Supposing an ISO cumulative number density with size larger than D is n∝D−α, Pop. III survivors can accrete ISO mass ≳10−16 M⊙, or ISO iron mass ≳10−17 M⊙, if α < 4. This iron mass is larger than the accretion mass of interstellar medium (ISM) by several orders of magnitude. Taking into account material mixing in the convection zone of Pop. III survivors, we find that their surface pollution is typically [Fe/H] ≲ −8 in most cases; however, the surface pollution of Pop. III survivors with 0.8 M⊙ can be [Fe/H] ≳ −6 because of the very shallow convective layer. If we consider Pop. III survivors located at the Galactocentric distance of 8 kpc, the dependence of the metal pollution is as follows. If α > 4, Pop. III survivors have no chance at colliding with ISOs with D ≳ 3 km, and remain metal-free. If 3 < α < 4, Pop. III survivors would be most polluted by ISOs up to [Fe/H] ∼ −7. If α < 3 up to D ∼ 10 km, Pop. III survivors could hide in metal-poor stars so far discovered. Pop. III survivors would be more polluted with decreasing Galactocentric distance. Although the metal pollution depends on α and the Galactocentric distance, we first show the importance of ISOs for the metal pollution of Pop. III survivors.

The Mass Growth and Stellar Ages of Galaxies: Observations versus Simulations

Abstract Using observed stellar mass functions out to z = 5, we measure the main progenitor stellar mass growth of descendant galaxies with masses of at using an evolving cumulative number density selection. From these mass growth histories, we are able to measure the time at which half the total stellar mass of the descendant galaxy was assembled, t a , which in order of decreasing mass corresponds to redshifts of , and 0.51. We compare this to the median light-weighted stellar age t * ( and 0.37) of a sample of low-redshift SDSS galaxies (from the literature) and find the timescales are consistent with more massive galaxies forming a higher fraction of their stars ex situ compared to lower-mass descendants. We find that both t * and t a strongly correlate with mass, which is in contrast to what is found in the EAGLE hydrodynamical simulation that shows a flat relationship between t a and M *. However, the semi-analytic model (SAM) of Henriques et al. is consistent with the observations in both t a and t * with M *, showing that the most recent SAMs are better able to decouple the evolution of the baryons from the dark matter in lower-mass galaxies.

The Rest-frame Optical (900 nm) Galaxy Luminosity Function at z ∼ 4–7: Abundance Matching Points to Limited Evolution in the MSTAR/MHALO Ratio at z ≥ 4

Abstract We present the first determination of the galaxy luminosity function (LF) at z ∼ 4, 5, 6, and 7, in the rest-frame optical at (z′ band). The rest-frame optical light traces the content in low-mass evolved stars (∼stellar mass—M *), minimizing potential measurement biases for M *. Moreover, it is less affected by nebular line emission contamination and dust attenuation, is independent of stellar population models, and can be probed up to z ∼ 8 through Spitzer/IRAC. Our analysis leverages the unique full-depth Spitzer/IRAC 3.6–8.0 μm data over the CANDELS/GOODS-N, CANDELS/GOODS-S, and COSMOS/UltraVISTA fields. We find that, at absolute magnitudes where is fainter than mag, linearly correlates with . At brighter , presents a turnover, suggesting that the stellar mass-to-light ratio could be characterized by a very broad range of values at high stellar masses. Median-stacking analyses recover an roughly independent on for mag, but exponentially increasing at brighter magnitudes. We find that the evolution of the LF marginally prefers a pure luminosity evolution over a pure density evolution, with the characteristic luminosity decreasing by a factor of between z ∼ 4 and z ∼ 7. Direct application of the recovered generates stellar mass functions consistent with average measurements from the literature. Measurements of the stellar-to-halo mass ratio at fixed cumulative number density show that it is roughly constant with redshift for . This is also supported by the fact that the evolution of the LF at can be accounted for by a rigid displacement in luminosity, corresponding to the evolution of the halo mass from abundance matching.

The average structural evolution of massive galaxies can be reliably estimated using cumulative galaxy number densities

Abstract Galaxy evolution can be studied observationally by linking progenitor and descendant galaxies through an evolving cumulative number density (CND) selection. This procedure can reproduce the expected evolution of the median stellar mass from abundance matching. However, models predict an increasing scatter in main progenitor masses at higher redshifts, which makes galaxy selection at the median mass unrepresentative. Consequently, there is no guarantee that the evolution of other galaxy properties deduced from this selection is reliable. Despite this concern, we show that this procedure approximately reproduces the evolution of the average stellar density profile of main progenitors of M ≈ 1011.5M⊙ galaxies, when applied to the EAGLE hydrodynamical simulation. At z ≳ 3.5, the aperture masses disagree by about a factor 2, but this discrepancy disappears when we include the expected scatter in cumulative number densities. The evolution of the average density profile in EAGLE broadly agrees with observations from UltraVISTA and CANDELS, suggesting an inside-out growth history for these massive galaxies over 0 ≲ z ≲ 5. However, for z ≲ 2, the inside-out growth trend is stronger in EAGLE. We conclude that CND matching gives reasonably accurate results when applied to the evolution of the mean density profile of massive galaxies.

The Mass, Color, and Structural Evolution of Today’s Massive Galaxies Since z ∼ 5

Abstract In this paper, we use stacking analysis to trace the mass growth, color evolution, and structural evolution of present-day massive galaxies ( ) out to z = 5. We utilize the exceptional depth and area of the latest UltraVISTA data release, combined with the depth and unparalleled seeing of CANDELS to gather a large, mass-selected sample of galaxies in the NIR (rest-frame optical to UV). Progenitors of present-day massive galaxies are identified via an evolving cumulative number density selection, which accounts for the effects of merging to correct for the systematic biases introduced using a fixed cumulative number density selection, and find progenitors grow in stellar mass by since z = 5. Using stacking, we analyze the structural parameters of the progenitors and find that most of the stellar mass content in the central regions was in place by , and while galaxies continue to assemble mass at all radii, the outskirts experience the largest fractional increase in stellar mass. However, we find evidence of significant stellar mass build-up at beyond probing an era of significant mass assembly in the interiors of present-day massive galaxies. We also compare mass assembly from progenitors in this study to the EAGLE simulation and find qualitatively similar assembly with z at . We identify as a distinct epoch in the evolution of massive galaxies where progenitors transitioned from growing in mass and size primarily through in situ star formation in disks to a period of efficient growth in r e consistent with the minor merger scenario.

An improved probabilistic approach for linking progenitor and descendant galaxy populations using comoving number density

Galaxy populations at different cosmic epochs are often linked together by comoving cumulative number density in observational studies. Many theoretical works, however, have shown that the number densities of tracked galaxy populations evolve in bulk and spread out over time. We present a number density method for linking progenitor and descendant galaxy populations which takes both of these effects into account. We define probability distribution functions that capture the evolution and dispersion of galaxy populations in comoving number density space, and use these functions to assign galaxies at one redshift $z_f$ probabilities of being progenitors or descendants of a galaxy population at another redshift $z_0$. These probabilities are then used as weights for calculating distributions of physical properties such as stellar mass, star formation rate, or velocity dispersion within the progenitor/descendant population. We demonstrate that this probabilistic method provides more accurate predictions for the evolution of physical properties then either the assumption of a constant number density or the assumption of an evolving number density in a bin of fixed width by comparing the predictions against galaxy populations directly tracked through a cosmological simulation. We find that the constant number density method performs most poorly at recovering galaxy properties, the evolving number method density slightly better, and the probabilistic number density method best of all. The improvement is present for predictions of both stellar mass as well as inferred quantities such as star formation rate and velocity dispersion which were not included in the number density fits. We demonstrate that this method can also be applied robustly and easily to observational data, and provide a code package for doing so.

A large difference in the progenitor masses of active and passive galaxies in the EAGLE simulation

Abstract Cumulative number density matching of galaxies is a method to observationally connect descendent galaxies to their typical main progenitors at higher redshifts and thereby to assess the evolution of galaxy properties. The accuracy of this method is limited due to galaxy merging and scatter in the stellar mass growth history of individual galaxies. Behroozi et al. have introduced a refinement of the method, based on abundance matching of observed galaxies to the Bolshoi dark matter-only simulation. The EAGLE cosmological hydrosimulation is well suited to test this method, because it reproduces the observed evolution of the galaxy stellar mass function and the passive fraction. We find agreement with the Behroozi et al. method for the complete sample of main progenitors of z = 0 galaxies, but we also find a strong dependence on the current star formation rate. Passive galaxies with a stellar mass up to 1010.75 M⊙ have a completely different median mass history than active galaxies of the same mass. This difference persists if we only select central galaxies. This means that the cumulative number density method should be applied separately to active and passive galaxies. Even then, the typical main progenitor of a z = 0 galaxy already spans two orders of magnitude in stellar mass at z = 2.

CONNECTING THE DOTS: TRACKING GALAXY EVOLUTION USING CONSTANT CUMULATIVE NUMBER DENSITY AT 3 ≤ z ≤ 7

ABSTRACT Using the cosmological smoothed particle hydrodynamical code GADGET-3, we make a realistic assessment of the technique of using constant cumulative number density as a tracer of galaxy evolution at high redshift. We find that over a redshift range of 3 ≤ z ≤ 7 one can on average track the growth of the stellar mass of a population of galaxies selected from the same cumulative number density bin to within ∼0.20 dex. Over the stellar mass range that we probe ( 10 10.4 ≤ M s / M ⊙ ≤ 10 10.8 ?> at z = 3 and 10 8.5 ≤ M s / M ⊙ ≤ 10 9.6 ?> at z = 7), one can reduce this bias by selecting galaxies based on an evolving cumulative number density. We find that this cumulative number density evolution exhibits a trend toward higher values which can be quantified by simple linear formulations of −0.10Δz for descendants and 0.12Δz for progenitors. Utilizing such an evolving cumulative number density increases the accuracy of descendant/progenitor tracking by a factor of ∼2. This result is in excellent agreement, within 0.10 dex, with abundance matching results over the same redshift range. However, we find that our more physically realistic cosmological hydrodynamic simulations produce a much larger scatter in descendant/progenitor stellar masses than previous studies, particularly when tracking progenitors. This large scatter makes the application of either the constant cumulative number density or evolving cumulative number density technique limited to average stellar masses of populations only, as the diverse mass assembly histories caused by stochastic physical processes such as gas accretion or mergers lead to an even larger scatter in other physical properties such as metallicity and star formation rate.

FROM DIVERSITY TO DICHOTOMY, AND QUENCHING: MILKY-WAY-LIKE AND MASSIVE-GALAXY PROGENITORS AT 0.5 <z< 3.0

Using the HST/WFC3 and ACS multi-band imaging data taken in CANDELS and 3D-HST, we study the general properties and the diversity of the progenitors of the Milky Way (MWs) and local massive galaxy (MGs) at 0.5 < z < 3.0, based on a constant cumulative number density analysis. After careful data reduction and stacking analysis, we conduct a radially resolved pixel SED fitting to obtain the radial distributions of the stellar mass and rest-frame colors. The stellar mass of MWs increases in self-similar way, irrespective of the radial distance, while that of MGs grows in inside-out way where they obtain ~ 75% of the total mass at outer (> 2.5 kpc) radius since z ~ 2. Although the radial mass profiles evolve in distinct ways, the formation and quenching of the central dense region (or bulge) ahead of the outer disk formation are found to be common for both systems. The sudden reddening of bulge at z ~ 1.6 and z ~ 2.4 for MWs and MGs, respectively, suggests the formation of bulge and would give a clue to the different gas accretion histories and quenching. A new approach to evaluate the morphological diversity is conducted by using the average surface density profile and its dispersion. The variety of the radial mass profiles for MGs peaks at higher redshift (z > 2.8), and then rapidly converges to more uniform shape at z < 1.5, while that for MWs remains in the outer region over the redshift. Compared with the observed star formation rates and color profiles, the evolution of variety is consistently explained by the star formation activities.

Tracing galaxy populations through cosmic time: a critical test of methods for connecting the same galaxies between different redshifts at z &lt; 3

Connecting galaxies with their descendants (or progenitors) at different redshifts can yield strong constraints on galaxy evolution. Observational studies have historically selected samples of galaxies using a physical quantity, such as stellar mass, either above a constant limit or at a constant cumulative number density. Investigation into the efficacy of these selection methods has not been fully explored. Using a set of four semi-analytical models based on the output of the Millennium Simulation, we find that selecting galaxies at a constant number density (in the range $-4.3 < \log\ n\ [\mathrm{Mpc}^{-3}\ h^{3}] < -3.0$) is superior to a constant stellar mass selected sample, although it still has significant limitations. Recovery of the average stellar mass, stellar mass density and average star-formation rate is highly dependent on the choice of number density but can all be recovered to within $<50\%$ at the commonly employed choice of $\log\ n\ [\mathrm{Mpc}^{-3}\ h^{3}] = -4.0$, corresponding to $\log M_\odot / h \sim 11.2$ at $z=0$, but this increases at lower mass limits. We show that there is a large scatter between the location of a given galaxy in a rank ordering based on stellar mass between different redshifts. We find that the inferred velocity dispersion may be a better tracer of galaxy properties, although further investigation is warranted into simulating this property. Finally, we find that over large redshift ranges selection at a constant number density is more effective in tracing the progenitors of modern galaxies than vice-versa.

OBSERVATIONS OF ENVIRONMENTAL QUENCHING IN GROUPS IN THE 11 GYR SINCEz= 2.5: DIFFERENT QUENCHING FOR CENTRAL AND SATELLITE GALAXIES

We present direct observational evidence for star formation quenching in galaxy groups in the redshift range 0<z<2.5. We utilize a large sample of nearly 6000 groups, selected by fixed cumulative number density from three photometric catalogs, to follow the evolving quiescent fractions of central and satellite galaxies over roughly 11 Gyr. At z~0, central galaxies in our sample range in stellar mass from Milky Way/M31 analogs (M=6.5x10^10 M\solar) to nearby massive ellipticals (M=1.5x10^11 M\solar). Satellite galaxies in the same groups reach masses as low as twice that of the Large Magellanic Cloud (M=6.5x10^9 M\solar). Using statistical background subtraction, we measure the average rest-frame colors of galaxies in our groups and calculate the evolving quiescent fractions of centrals and satellites over seven redshift bins. Our analysis shows clear evidence for star formation quenching in group halos, with a different quenching onset for centrals and their satellite galaxies. Using halo mass estimates for our central galaxies, we find that star formation shuts off in centrals when typical halo masses reach between 10^12 and 10^13 M\solar, consistent with predictions from the halo quenching model. In contrast, satellite galaxies in the same groups most likely undergo quenching by environmental processes, whose onset is delayed with respect to their central galaxy. Although star formation is suppressed in all galaxies over time, the processes that govern quenching are different for centrals and satellites. While mass plays an important role in determining the star formation activity of central galaxies, quenching in satellite galaxies is dominated by the environment in which they reside.

Martensite transformation in bulk and polycrystalline austenite

Martensite is a heterogeneous, nucleation-controlled, and autocatalytic transformation with strong industrial relevance due to its ability to impart the properties of engineering steels during production, manufacturing, and use. In this work, a formal model of the spread of martensite nucleation permitted obtaining the cumulative number density of embryos in a bulk, polycrystalline Fe-31 wt%Ni-0.02 wt%C alloy based on standard metallography measurements. The result shows that the density of embryos available to initiate the reaction in a bulk; polycrystalline Fe-31 wt%Ni-0.02 wt%C alloy is significantly smaller than the density of pre-existent martensite embryos available in a similar alloy constituted of isolated small particles or the particulate. In a polycrystal, the present authors have previously proposed that embryos that do initiate the transformation originate from a volume of influence of the grain boundaries. Therefore, in addition to the usual concept of embryo potency, a new concept, namely, embryo association must be introduced. In a polycrystal, fewer but better localized, i.e. embryos associated with the grain boundaries, which are located within the volume of influence of the grain boundaries, initiate martensite transformation larger number of embryos which may exist within the grains. According to this view, this difference in the embryo association in the polycrystal and in the particulate is the main factor contributing to the difference in the martensite density of embryos available to propagate the transformation observed in this work.