Halo Growth Research Articles

Suppression of beam halo in an RF linac using a hollow electron beam

AbstractHigh-intensity electron linacs have severe space-charge effects that lead to the production of beam halo which degrade the beam quality. For a given charge per bunch, hollow beams have a weaker nonlinear space-charge force. In this paper, we have investigated the possibility of using hollow beam to control halo growth in linacs. We simulate the dynamics of such a beam in a 17 MeV radio frequency linac using ASTRA beam dynamics code and show that it experiences a smaller emittance growth as well as reduced beam halo. The results suggest that using a hollow beam, high charge per bunch could be propagated and accelerated in a radio frequency linac.

A one-dimensional hydrodynamic model for accretion, cooling, and heating of gas in dark matter haloes from z = 6 to z = 0

We study an idealized 1D model for the evolution of hot gas in dark matter halos for redshifts $z=[0,6]$. We introduce a numerical setup incorporating cosmological accretion of gas, along with the growth of the halo, based on the Van den Bosch model for the average growth of halos as a function of cosmic time. We evolve one-dimensional Lagrangian shells with radiative cooling of the gas and heating due to feedback from the gas cooling and moving in toward the center. A simple Bondi accretion model on to a central black hole is used to include feedback heating. The setup captures some of the key characteristics of spherically symmetric accretion onto the halos: formation of virial shocks slightly outside $r_{200}$ and long-term thermal balance in the form of cooling and heating cycles. The gas density outside our initial halos at $z=6$ is constrained by requiring that the baryon fraction within the virial radius for non-radiative evolution be equal to the universal value at almost all times. The total mass in the cold phase (taken to be $\sim 10^4$ K) within $40$ kpc is tracked as a function of the halo mass and redshift. We compare the evolution of the cold gas mass to the observed stellar-mass versus halo mass relations, following which, we can constrain the feedback energy required for different halo masses and redshifts. We also compare and match the hot gas density and temperature profiles for our most massive halo to those of clusters observed upto redshift $2$. Our model is thus an improvement over the semi-analytic models in which isothermal condition and $\rho \propto r^{-2}$ are assumed.

The Galaxy–Halo Connection in Low-mass Halos

Abstract Properties of galaxies vary systematically with the mass of their parent dark matter halos. This basic galaxy–halo connection shows a fair amount of scatter whose origin is not fully understood. Here, we study how differences in the halo assembly history affect central galaxies in low-mass (M halo < 1012 M ⊙) halos at z = 2–6 with the help of the MassiveFIRE suite of cosmological simulations. In contrast to previous works that tie galaxy properties to halo concentration and halo-formation redshift, we focus on halo growth rate as a measure of assembly history. We find that, at fixed halo mass, faster-growing halos tend to have lower stellar masses and higher star formation rates (SFRs) per unit stellar mass but similar overall SFRs. We provide a simple explanation for these findings with the help of an analytic model that captures approximately the behavior of our hydrodynamical simulations. Specifically, among halos of a given current mass, quickly growing halos have lower stellar masses (and thus higher specific SFRs) because they were less massive and had comparably lower cold-gas masses and SFRs in the past than slowly growing halos. By combining these findings with estimates for the scatter of the halo growth rate, we show that variations in growth rate at fixed halo mass may largely explain the scatter of the stellar mass–halo mass relation. In contrast, halo growth variations likely play only a minor role in the scatter of the star-forming sequence in low-mass galaxies.

Climbing halo merger trees with TreeFrog

AbstractWe present TreeFrog, a massively parallel halo merger tree builder that is capable comparing different halo catalogues and producing halo merger trees. The code is written in c++11, use the MPI and OpenMP API’s for parallelisation, and includes python tools to read/manipulate the data products produced. The code correlates binding energy sorted particle ID lists between halo catalogues, determining optimal descendant/progenitor matches using multiple snapshots, a merit function that maximises the number of shared particles using pseudo-radial moments, and a scheme for correcting halo merger tree pathologies. Focusing on VELOCIraptor catalogues for this work, we demonstrate how searching multiple snapshots spanning a dynamical time significantly reduces the number of stranded halos, those lacking a descendant or a progenitor, critically correcting poorly resolved halos. We present a new merit function that improves the distinction between primary and secondary progenitors, reducing tree pathologies. We find FOF accretion rates and merger rates show similar mass ratio dependence. The model merger rates from Poole, et al. [2017, 472, 3659] agree with the measured net growth of halos through mergers.

Late Bloomer Galaxies: Growing Up in Cosmic Autumn

Abstract Late bloomers (LBs) are massive ( ) galaxies at z < 1 that formed the majority of their stars within ∼2 Gyr of the epoch of observation. Our improved methodology for deriving star formation histories (SFHs) of galaxies at redshifts 0.45 < z < 0.75 from the Carnegie-Spitzer-IMACS Survey includes confidence intervals that robustly distinguish LBs from “old” galaxies. We use simulated SFHs to test for “false positives” and contamination from old galaxies to demonstrate that the late-bloomer population is not an artifact of our template modeling technique. We show that LBs account for ∼20% of z ∼ 0.6 galaxies with masses of the modern Milky Way, with a moderate dependence on mass. We take advantage of a 1% overlap of our sample with HST (CANDELS) imaging to construct a “gold standard” catalog of 74 galaxies with high-confidence SFHs, SEDs, basic data, and HST images to facilitate comparison with future studies by others. This small subset suggests that galaxies with both old and young SFHs cover the full range of morphology and environment (excluding rich groups or clusters), albeit with a mild but suggestive correlation with the local environment. We begin the investigation of whether LBs of sufficient mass and frequency are produced in current-generation ΛCDM-based semianalytic models of galaxy formation. In terms of halo growth, we find a late-assembling halo fraction within a factor of two of our late bloomer fraction. However, sufficiently delaying star formation in such halos may be a challenge for the baryon component of such models.

Lagrangian Cosmological Perturbation Theory at Shell Crossing.

We consider the growth of primordial dark matter halos seeded by three crossed initial sine waves of various amplitudes. Using a Lagrangian treatment of cosmological gravitational dynamics, we examine the convergence properties of a high-order perturbative expansion in the vicinity of shell crossing by comparing the analytical results with state-of-the-art high resolution Vlasov-Poisson simulations. Based on a quantitative exploration of parameter space, we study explicitly for the first time the convergence speed of the perturbative series and find, in agreement with intuition, that it slows down when going from quasi-one-dimensional initial conditions (one sine wave dominating) to quasitriaxial symmetry (three sine waves with same amplitude). In most cases, the system structure at collapse time is, as expected, very similar to what is obtained with simple one-dimensional dynamics, except in the quasitriaxial regime, where the phase-space sheet presents a velocity spike. In all cases, the perturbative series exhibits a generic convergence behavior as fast as an exponential of a power law of the order of the expansion, allowing one to numerically extrapolate it to infinite order. The results of such an extrapolation agree remarkably well with the simulations, even at shell crossing.

How black holes stop their host galaxy from growing without AGN feedback

Super-massive black holes (SMBHs) with $M_{\bullet} \sim 10^9 $ M$_{\odot}$ at $z>6$ likely originate from massive seed black holes (BHs). We investigate the consequences of seeding SMBHs with direct collapse BHs (DCBHs) ($M_{\bullet}=10^{4-6}\, \mathrm{M}_\odot$) on proto-galactic disc growth. We show that even in the absence of direct feedback effects, the growth of seed BHs reduces the development of gravitational instabilities in host galaxy discs, suppressing star formation and confining stars to a narrow ring in the disc and leading to galaxies at $z \sim 6$ which lie above the local BH-stellar mass relation. The relative magnitude of cosmic and BH accretion rates governs the evolution of the BH-stellar mass relation. For typical DCBH formation epochs, $z_{\rm{i}} \sim 10$, we find star formation is inhibited in haloes growing at the average rate predicted by $\Lambda$CDM which host BHs capable of reaching $M_{\bullet}\sim 10^9 \, \mathrm{M}_{\odot}$ by $z\gtrsim6$. Slower growing BHs cause a delay in the onset of star formation; a $M_{\bullet} \sim 10^6 $ M$_{\odot}$ seed growing at $0.25$ times the Eddington limit will delay star formation by $\sim100$ Myr. This delay is reduced by a factor of $\sim10$ if the halo growth rate is increased by $\sim 0.6\, \sigma$. Our results suggest that SMBHs seeded by DCBHs and their host galaxies form in separate progenitor haloes. In the absence of subsequent mergers, higher than average cosmic accretion or earlier seed formation ($z_{\rm i} \sim 20$) are required to place the evolving BH on the local BH-stellar mass relation by $z=6$.

Deviations from hydrostatic equilibrium in the circumgalactic medium: spinning hot haloes and accelerating flows

Hydrostatic equilibrium (HSE), where the thermal pressure gradient balances the force of gravity, is tested across a range of simulated EAGLE haloes from Milky Way L* haloes (M200|$\approx$| 1012M⊙) to cluster scales. Clusters (M200|$\approx$|1014M⊙) reproduce previous results with thermal pressure responsible for |$\sim$|90 per cent of the support against gravity, but this fraction drops for group-sized haloes (M200|$\approx$| 1013M⊙) and is even lower (40–70 per cent) for L* haloes between 0.1 and 0.3R200. Energy from feedback grows relative to the binding energy of a halo towards lower mass resulting in greater deviations from HSE. Tangential motions comprise the largest deviation from HSE in L* haloes indicating that the hot circumgalactic medium (CGM) has significant subcentrifugal rotation and angular momentum spin parameters 2−3 × higher than the dark matter spin parameters. Thermal feedback can buoyantly rise to the outer CGM of M200 ≲ 1012M⊙ haloes, both moving baryons beyond R200 and feeding uncorrelated tangential motions. The resulting hot halo density and rotation profiles show promising agreement with X-ray observations of the inner Milky Way halo, and we discuss future observational prospects to detect spinning hot haloes around other galaxies. Acceleration and radial streaming motions also comprise significant deviations from HSE, especially net outward accelerations seen in L* and group haloes indicating active feedback. Black hole feedback acts in a preventative manner during the later growth of group haloes, applying significant accelerations via shocks that do not feed tangential motions. We argue that HSE is a poor assumption for the CGM, especially in the inner regions, and rotating baryonic hot haloes are a critical consideration for analytic models of the CGM.

Approximations to galaxy star formation rate histories: properties and uses of two examples

Galaxies evolve via a complex interaction of numerous different physical processes, scales and components. In spite of this, overall trends often appear. Simplified models for galaxy histories can be used to search for and capture such emergent trends, and thus to interpret and compare results of galaxy formation models to each other and to nature. Here, two approximations are applied to galaxy integrated star formation rate histories, drawn from a semi-analytic model grafted onto a dark matter simulation. Both a lognormal functional form and principal component analysis (PCA) approximate the integrated star formation rate histories fairly well. Machine learning, based upon simplified galaxy halo histories, is somewhat successful at recovering both fits. The fits to the histories give fixed time star formation rates which have notable scatter from their true fixed time rates at final time, especially for quiescent and "green valley" galaxies, and more so for the PCA fit. For classifying galaxies into subfamilies sharing similar integrated histories, both approximations are better than using final stellar mass or specific star formation rate. Several subsamples from the simulation illustrate how these simple parameterizations can provide points of contact for comparisons between different galaxy formation samples, or more generally, models. As a side result, the halo masses of simulated galaxies with early peak star formation rate (according to the lognormal fit) lie on one of two tracks. The small fraction of galaxies with a lower halo mass at peak star formation rate appear to stall in their halo growth, even though they are central in their host halos.

Possible Imprints of Cold-mode Accretion on the Present-day Properties of Disk Galaxies

Abstract Recent theoretical studies suggest that a significant part of the primordial gas accretes onto forming galaxies as narrow filaments of cold gas without building a shock and experiencing heating. Using a simple model of disk galaxy evolution that combines the growth of dark matter halos predicted by cosmological simulations with a hypothetical form of cold-mode accretion, we investigate how this cold-accretion mode affects the formation process of disk galaxies. It is found that the shock-heating and cold-accretion models produce compatible results for low-mass galaxies owing to the short cooling timescale in such galaxies. However, cold accretion significantly alters the evolution of disk galaxies more massive than the Milky Way and puts observable fingerprints on their present properties. For a galaxy with a virial mass , the scale length of the stellar disk is larger by 41% in the cold-accretion model than in the shock-heating model, with the former model reproducing the steep rise in the size–mass relation observed at the high-mass end. Furthermore, the stellar component of massive galaxies becomes significantly redder (0.66 in u − r at ), and the observed color–mass relation in nearby galaxies is qualitatively reproduced. These results suggest that large disk galaxies with red optical colors may be the product of cold-mode accretion. The essential role of cold accretion is to promote disk formation in the intermediate-evolution phase ( ) by providing the primordial gas having large angular momentum and to terminate late-epoch accretion, quenching star formation and making massive galaxies red.

Brightest group galaxies – II: the relative contribution of BGGs to the total baryon content of groups at z < 1.3

We performed a detailed study of the evolution of the star formation rate (SFR) and stellar mass of the brightest group galaxies (BGGs) and their relative contribution to the total baryon budget within $R_{200}$ ($f^{BGG}_{b,200}$). The sample comprises 407 BGGs selected from X-ray galaxy groups ($M_{200}=10^{12.8}-10^{14} \;M_{\odot}$) out to $z\sim1.3$ identified in the COSMOS, XMM-LSS, and AEGIS fields. We find that BGGs constitute two distinct populations of quiescent and star-forming galaxies and their mean SFR is $\sim2$ dex higher than the median SFR at $ z<1.3 $. Both the mean and the median SFRs decline with time by $>2$ dex. The mean (median) of stellar mass of BGGs has grown by $0.3$ dex since $z=1.3$ to the present day. We show that up to $\sim45\% $ of the stellar mass growth in a star-forming BGG can be due to its star-formation activity. With respect to $f^{BGG}_{b,200}$, we find it to increase with decreasing redshift by $\sim0.35$ dex while decreasing with halo mass in a redshift dependent manner. We show that the slope of the relation between $f^{BGG}_{b,200}$ and halo mass increases negatively with decreasing redshift. This trend is driven by an insufficient star-formation in BGGs, compared to the halo growth rate. We separately show the BGGs with the 20\% highest $f^{BGG}_{b,200}$ are generally non-star-forming galaxies and grow in mass by processes not related to star formation (e.g., dry mergers and tidal striping). We present the $ M_\star-M_h $ and $ M_\star/M_h-M_h $ relations and compare them with semi-analytic model predictions and a number of results from the literature. We quantify the intrinsic scatter in stellar mass of BGGs at fixed halo mass ($\sigma_{log M_{\star}}$) and find that $\sigma_{log M_{\star}}$ increases from 0.3 dex at $ z\sim0.2 $ to 0.5 dex at $ z\sim1.0 $ due to the bimodal distribution of stellar mass.

Isolation, Screening and Methionine Production by Bacillus species isolated from Poultry Waste

The current study was focused on isolating and identifying methionine producing bacteria from poultry waste. The bacteria were isolated from poultry waste and the ones which specifically produced methionine on solid agar medium, using a methionine requiring auxotroph (Escherichia coli) were kept for further analysis. Methionine assay was also carried out after fermentation to quantify methionine produced by each isolate. A total of 10 bacteria were isolated of which 2 were confirmed as active methionine producers after halo growth was observed on the solid agar medium. These 2 isolates which produced higher amount of methionine were ultimately selected for further investigation. The methionine producing isolates which were designated PW1 and PW3 were identified as Bacillus sp. respectively after biochemical tests were carried out. The test carried out include; gram staining, motility test, catalase test, citrate test, hemolysis test, indole test, penicillin sensitivity, oxidase test and crystal colony formation. Where PW3 (Bacillus sp.) produced greater amount of methionine (0.54 mg/l), while PW1 (Bacillus sp.) produced 0.20 mg/l after the completion of the methionine assay. Keywords: Keywords: Methionine production, Bacillus species, poultry waste.

The build up of the correlation between halo spin and the large-scale structure

Both simulations and observations have confirmed that the spin of haloes/galaxies is correlated with the large scale structure (LSS) with a mass dependence such that the spin of low-mass haloes/galaxies tend to be parallel with the LSS, while that of massive haloes/galaxies tend to be perpendicular with the LSS. It is still unclear how this mass dependence is built up over time. We use N-body simulations to trace the evolution of the halo spin-LSS correlation and find that at early times the spin of all halo progenitors is parallel with the LSS. As time goes on, mass collapsing around massive halo is more isotropic, especially the recent mass accretion along the slowest collapsing direction is significant and it brings the halo spin to be perpendicular with the LSS. Adopting the $fractional$ $anisotropy$ (FA) parameter to describe the degree of anisotropy of the large-scale environment, we find that the spin-LSS correlation is a strong function of the environment such that a higher FA (more anisotropic environment) leads to an aligned signal, and a lower anisotropy leads to a misaligned signal. In general, our results show that the spin-LSS correlation is a combined consequence of mass flow and halo growth within the cosmic web. Our predicted environmental dependence between spin and large-scale structure can be further tested using galaxy surveys.

Antagonism of Trichoderma spp. strains against pea (Pisum sativum L.) Fusarium wilt caused by Fusarium oxysporum f. sp. pisi.

The antagonistic effectiveness of native strains of Trichoderma spp. on Fusarium oxysporum f. sp. pisi. in vitro, greenhouse and field conditions, were evaluated. In laboratory conditions, the antagonistic capacity of 12 strains of Trichoderma spp., strains C2, C7, C12 and C21, exhibited a better behavior measured by the following variables: inhibition halo and mycelial growth. In greenhouse conditions, the four strains, which showed the best in vitro antagonistic behavior were evaluated using a DIA experimental design with factorial arrangement for three factors which corresponded to strain, concentration and dose. The results of this evaluation, showed that strains C12 and C21 at doses of 20 mL, and at concentrations of 108 and 106 conidia.mL-1, respectively. The best antagonistic response was determined by variables as follows: plant height, fresh root weight and incidence. Under field conditions, the evaluations were carried out in the municipalities of Ipiales, Pupiales and Gualmatán, in the Department of Nariño, Colombia. In each location, a BCA experimental design was used with four treatments and five replicates, treatments were as follows: C12 strains at 108 concentration, C21 at 106 concentration, chemical control and absolute control. In Gualmatan location, C12 and C21 strains, showed no antagonistic capacity, whereas in Ipiales and Pupiales locations, strain C12, presented a lower incidence of F. oxysporum than the control, but with no effect on yields. In Pupiales location, C21 strain surpassed in performance to the control treatment, even though the two treatments had similar incidence.

Assembly Pathways and the Growth of Massive Early-Type Galaxies

Based on data from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, I present results on the assembly pathways, dark matter content and halo growth of massive early-type galaxies. Using galaxy starlight information we find that such galaxies had an early dissipative phase followed by a second phase of halo growth from largely minor mergers (and in rare cases major mergers). Thus our result fits in well with the two-phase scenario of galaxy formation. We also used globular cluster radial velocities to measure the enclosed mass within 5 effective radii. The resulting dark matter fractions reveal a few galaxies with very low dark matter fractions that are not captured in the latest cosmological models. Multiple solutions are possible, but none yet is convincing. Translating dark matter fractions into epochs of halo assembly, we show that low mass galaxies tend to grow via gas-rich accretion, while high mass galaxies grow via gas-poor mergers.

ZOMG – I. How the cosmic web inhibits halo growth and generates assembly bias

The clustering of dark matter haloes with fixed mass depends on their formation history, an effect known as assembly bias. We use zoom N -body simulations to investigate the origin of this phenomenon. For each halo at redshift z=0, we determine the time in which the physical volume containing its final mass becomes stable. We consider five examples for which this happens at z~1.5 and two that do not stabilize by z=0. The zoom simulations show that early-collapsing haloes do not grow in mass at z=0 while late-forming ones show a net inflow. The reason is that 'accreting' haloes are located at the nodes of a network of thin filaments feeding them. Conversely, each 'stalled' halo lies within a prominent filament that is thicker than the halo size. Infalling material from the surroundings becomes part of the filament while matter within it recedes from the halo. We conclude that assembly bias originates from quenching halo growth due to tidal forces following the formation of non-linear structures in the cosmic web, as previously conjectured in the literature. Also the internal dynamics of the haloes change: the velocity anisotropy profile is biased towards radial (tangential) orbits in accreting (stalled) haloes. Our findings reveal the cause of the yet unexplained dependence of halo clustering on the anisotropy. Finally, we extend the excursion-set theory to account for these effects. A simple criterion based on the ellipticity of the linear tidal field combined with the spherical collapse model provides excellent predictions for both classes of haloes.

Log-normal Star Formation Histories in Simulated and Observed Galaxies

Abstract Gladders et al. have recently suggested that the star formation histories (SFHs) of individual galaxies are characterized by a log-normal function in time, implying a slow decline rather than rapid quenching. We test their conjecture on theoretical SFHs from the cosmological simulation Illustris and on observationally inferred SFHs. While the log-normal form necessarily ignores short-lived features such as starbursts, it fits the overall shape of the majority of SFHs very well. In particular, 85% of the cumulative SFHs are fitted to within a maximum error of 5% of the total stellar mass formed, and 99% to within 10%. The log-normal performs systematically better than the commonly used delayed-τ model, and is superseded only by functions with more than three free parameters. Poor fits are mostly found in galaxies that were rapidly quenched after becoming satellites. We explore the log-normal parameter space of normalization, peak time, and full width at half maximum, and find that the simulated and observed samples occupy similar regions, though Illustris predicts wider, later-forming SFHs on average. The ensemble of log-normal fits correctly reproduces complex metrics such as the evolution of Illustris galaxies across the star formation main sequence, but overpredicts their quenching timescales. SFHs in Illustris are a diverse population not determined by any one physical property of galaxies, but follow a tight relation, where . We show that such a relation can be explained qualitatively (though not quantitatively) by a close connection between the growth of dark matter halos and their galaxies.

Dark-ages Reionization &amp; Galaxy Formation Simulation VIII. Suppressed growth of dark matter halos during the Epoch of Reionization

We investigate how the hydrostatic suppression of baryonic accretion affects the growth rate of dark matter halos during the Epoch of Reionization. By comparing halo properties in a simplistic hydrodynamic simulation in which gas only cools adiabatically, with its collisionless equivalent, we find that halo growth is slowed as hydrostatic forces prevent gas from collapsing. In our simulations, at the high redshifts relevant for reionization (between ${\sim}6$ and ${\sim}11$), halos that host dwarf galaxies ($\lesssim 10^{9} \mathrm{M_\odot}$) can be reduced by up to a factor of 2 in mass due to the hydrostatic pressure of baryons. Consequently, the inclusion of baryonic effects reduces the amplitude of the low mass tail of the halo mass function by factors of 2 to 4. In addition, we find that the fraction of baryons in dark matter halos hosting dwarf galaxies at high redshift never exceeds ${\sim}90\%$ of the cosmic baryon fraction. When implementing baryonic processes, including cooling, star formation, supernova feedback and reionization, the suppression effects become more significant with further reductions of ${\sim}30\%$ to 60\%. Although convergence tests suggest that the suppression may become weaker in higher resolution simulations, this suppressed growth will be important for semi-analytic models of galaxy formation, in which the halo mass inherited from an underlying N-body simulation directly determines galaxy properties. Based on the adiabatic simulation, we provide tables to account for these effects in N-body simulations, and present a modification of the halo mass function along with explanatory analytic calculations.

Substructure of fuzzy dark matter haloes

We derive the halo mass function (HMF) for fuzzy dark matter (FDM) by solving the excursion set problem explicitly with a mass-dependent barrier function, which has not been done before. We find that compared to the naive approach of the Sheth–Tormen HMF for FDM, our approach has a higher cutoff mass and the cutoff mass changes less strongly with redshifts. Using merger trees constructed with a modified version of the Lacey & Cole formalism that accounts for suppressed small-scale power and the scale-dependent growth of FDM haloes and the semi-analytic galacticus code, we study the statistics of halo substructure including the effects from dynamical friction and tidal stripping. We find that if the dark matter is a mixture of cold dark matter (CDM) and FDM, there will be a suppression on the halo substructure on small scales which may be able to solve the missing satellites problem faced by the pure CDM model. The suppression becomes stronger with increasing FDM fraction or decreasing FDM mass. Thus, it may be used to constrain the FDM model.

How Do Gas Hydrates Spread on a Substrate?

Growth of gas hydrates as fast-growing polycrystalline crusts at interfaces between water and guest phases is well documented, but the mechanisms of hydrate growth on solid substrates are much less known. We report here on cyclopentane (CP) hydrate spreading on glass (fused silica) under CP. As seen for methane hydrate by Beltrán and Servio (Cryst. Growth Des. 2010, 10, 4339−4347), CP hydrate grows on glass as a “halo” radiating from the contact line of a “primary” drop. Complementary optical microscopies at micron resolution here allow identification of the mechanisms of halo growth and melting. We conclude that forms of water on the substrate control halo spreading, namely, a precursor film near the contact line and a breath figure (dew) condensed from the CP (halo spreading at ≤2 μm s–1 at T ≈ 0 °C or subcooling ∼7 °C), and “leap-frogging” (at ∼10 μm s–1) over “secondary” drops left behind by melting a previous halo. Halo thickening, about 5 nm s–1, is attributed to water condensation, either incorporation of water dissolved in CP (like ablimation) or settling of water “fog” from the CP. Halos spread slower on untreated, compared to hydrophilic, glass, an effect attributed to the quantity of water present on the substrate; a similar trend is noted when the CP phase is not pre-equilibrated with water prior to the experiment. No hydrate halo was detected on hydrophobized (silane-treated) glass, where the breath figure is absent.