Abstract
We investigate whether the considerable diversity in the satellite populations of nearby Milky Way (MW)-mass galaxies is connected with the diversity in their host’s merger histories. ...Analyzing eight nearby galaxies with extensive observations of their satellite populations and stellar halos, we characterize each galaxy’s merger history using the metric of its most dominant merger,
M
⋆,Dom
, defined as the greater of either its total accreted stellar mass or most massive current satellite. We find an unexpectedly tight relationship between these galaxies’ number of
M
V
< − 9 satellites within 150 kpc (
N
Sat
) and
M
⋆,Dom
. This relationship remains even after accounting for differences in galaxy mass. Using the star formation and orbital histories of satellites around the MW and M81, we demonstrate that both likely evolved along the
M
⋆,Dom
–
N
Sat
relation during their current dominant mergers with the Large Magellanic Cloud and M82, respectively. We investigate the presence of this relation in galaxy formation models, including using the Feedback In Realistic Environments (FIRE) simulations to directly compare to the observations. We find no relation between
M
⋆,Dom
and
N
Sat
in FIRE, and a universally large scatter in
N
Sat
with
M
⋆,Dom
across simulations—in direct contrast with the tightness of the empirical relation. This acute difference in the observed and predicted scaling relation between two fundamental galaxy properties signals that current simulations do not sufficiently reproduce diverse merger histories and their effects on satellite populations. Explaining the emergence of this relation is therefore essential for obtaining a complete understanding of galaxy formation.
Aims.
We study the stellar (i.e., rest-optical) and dust-obscured star-forming (i.e., rest-mid-infrared) morphologies (i.e., sizes and Sérsic indices) of star-forming galaxies (SFGs) at 0.1 <
z
< ...2.5.
Methods.
We combined
Hubble
Space Telescope (HST) images from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) with JWST images from the Cosmic Evolution Early Release Science (CEERS) survey to measure the stellar and dust-obscured star formation distributions of 69 SFGs. Rest-mid-infrared (rest-MIR) morphologies were determined using a Markov chain Monte Carlo (MCMC) approach applied to the sharpest Mid-InfraRed Instrument (MIRI) images (i.e., shortest wavelength) dominated by dust emission (
S
ν
dust
/
S
ν
total
> 75%), as inferred for each galaxy from our optical-to-far-infrared spectral energy distribution fits with
CIGALE
. Rest-MIR Sérsic indices were only measured for the brightest MIRI sources, that is, with a signal-to-noise (S/N) greater than 75 (35 galaxies). At a lower S/N, simulations do indeed show that simultaneous measurements of both the size and Sérsic index become less reliable. We extended our study to fainter sources (i.e.,
S
/
N
> 10; 69 galaxies) by restricting our structural analysis to their rest-MIR sizes (
Re
MIR
) and by fixing their Sérsic index to a value of one.
Results.
Our MIRI-selected sample corresponds to a mass-complete sample (> 80%) of SFGs down to stellar masses 10
9.5
, 10
9.5
, and 10
10
M
⊙
at
z
∼ 0.3, 1, and 2, respectively. The rest-MIR Sérsic index of bright galaxies (
S
/
N
> 75) has a median value of 0.7
−0.3
+0.8
(the range corresponds to the 16th and 84th percentiles), which is in good agreement with their median rest-optical Sérsic indices. The Sérsic indices as well as the distribution of the axis ratio of these galaxies suggest that they have a disk-like morphology in the rest-MIR. Galaxies above the main sequence (MS) of star formation (i.e., starbursts) have rest-MIR sizes that are, on average, a factor ∼2 smaller than their rest-optical sizes (
Re
Opt.
). The median rest-optical to rest-MIR size ratio of MS galaxies increases with their stellar mass, from 1.1
−0.2
+0.4
at ∼10
9.8
M
⊙
to 1.6
−0.3
+1.0
at ∼10
11
M
⊙
. This mass-dependent trend resembles the one found in the literature between the rest-optical and rest-near-infrared sizes of SFGs, suggesting that it is primarily due to radial color gradients affecting rest-optical sizes and that the sizes of the stellar and star-forming components of SFGs are, on average, consistent at all masses. There is, however, a small population of SFGs (∼15%) with a compact star-forming component embedded in a larger stellar structure, with Re
Opt.
c
> 1.8 × Re
MIR
. This population could be the missing link between galaxies with an extended stellar component and those with a compact stellar component, the so-called blue nuggets.
The residual entropy scaling of viscosity was applied to pure refrigerants, including natural refrigerants, hydrofluoroolefins, hydrochlorofluoroolefins, perfluorocarbons, hydrofluorocarbons, ...chlorofluorocarbons, and hydrochlorofluorocarbons and their mixtures. Experimental temperature, pressure, and viscosity data of 39 pure refrigerants, including more than 15,000 experimental data values from more than 400 literature sources, were used to build a univariate correlation function between the reduced residual viscosity and the dimensionless residual entropy. The correlation function contains only four fitted parameters and a fluid-specific scaling factor. Approximately, 80.0% of the experimental data are predicted within 5.0% when the fluid-specific fitted parameters are used. About 80.0% of the experimental data collapse onto one single curve within 7.9% when the global fitted parameters and the fluid-specific scaling factor were adopted for the correlation function. The correlation function is able to predict mixture viscosity without any additional empirical parameters. Approximately, 80.0% of the experimental data of 27 binary or multi-component mixtures composed by the investigated pure components, encompassing 2890 experimental values from more than 20 literature sources, agree with the correlation function within 7.9%, which is as good as the comparison for pure fluids. The commonly used extended corresponding states model has as many as four more parameters for each pair of components and has been optimized for some of the binaries; therefore, it generally yields better agreement than the proposed correlation function for binary mixtures but similar performance for multi-component mixtures.
Abstract
We present a new approach to measuring the thickness of a partially face-on stellar disk, using dust geometry. In a moderately-inclined disk galaxy, the fraction of reddened stars is ...expected to be 50% everywhere, assuming that dust lies in a thin midplane. In a thickened disk, however, a wide range of radii project onto the line of sight. Assuming stellar density declines with radius, this geometrical projection leads to differences in the numbers of stars on the near and far sides of the thin dust layer. The fraction of reddened stars will thus differ from the 50% prediction, with a deviation that becomes larger for puffier disks. We map the fraction of reddened red giant branch (RGB) stars across M31, which shows prominent dust lanes on only one side of the major axis. The fraction of reddened stars varies systematically from 20% to 80%, which requires that these stars have an exponential scale height
h
z
that is 0.14 ± 0.015 times the exponential scale length (
h
r
≈ 5.5 kpc). M31's RGB stars must therefore have
h
z
= 770 ± 80 pc, which is far thicker than the Milky Way’s thin disk, but comparable to its thick disk. The lack of a significant thin disk in M31 is unexpected, but consistent with its interaction history and high disk velocity dispersion. We suggest that asymmetric reddening be used as a generic criterion for identifying “thick disk”-dominated systems, and discuss prospects for future 3D tomographic mapping of the gas and stars in M31.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted) We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 147 stellar targets along the ...line of sight to the newly discovered "ultrafaint" stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru 1). Based on simultaneous estimates of line of sight velocity and stellar-atmospheric parameters, we identify 8 and 7 stars as probable members of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to resolve an internal velocity dispersion of ... km s super(-1) about a mean of ... km s super(-1)(solar rest frame), and to estimate a mean metallicity of Fe/H = ... These results place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies, suggesting a dominant dark matter component with dynamical mass ... M sub(middot in circle) enclosed within the central ~160 pc, and dynamical mass-to-light ratio ... M sub(middot in circle)/Lv .middot in circle. For Gru 1 we estimate a mean velocity of ... km s super(-1) and a mean metallicity of Fe/H = ... but our sample does not resolve Gru 1's velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic phase space suggest that their orbits are among the most energetic within a distance of <, ~ 300 kpc. Moreover, their proximity to each other in this space arises naturally if both objects are trailing the Large Magellanic Cloud.
ABSTRACT Although there has been much progress in understanding how galaxies evolve, we still do not understand how and when they stop forming stars and become quiescent. We address this by applying ...our galaxy spectral energy distribution models, which incorporate physically motivated star formation histories (SFHs) from cosmological simulations, to a sample of quiescent galaxies at . A total of 845 quiescent galaxies with multi-band photometry spanning rest-frame ultraviolet through near-infrared wavelengths are selected from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) data set. We compute median SFHs of these galaxies in bins of stellar mass and redshift. At all redshifts and stellar masses, the median SFHs rise, reach a peak, and then decline to reach quiescence. At high redshift, we find that the rise and decline are fast, as expected, because the universe is young. At low redshift, the duration of these phases depends strongly on stellar mass. Low-mass galaxies ( ) grow on average slowly, take a long time to reach their peak of star formation ( Gyr), and then the declining phase is fast ( Gyr). Conversely, high-mass galaxies ( ) grow on average fast ( Gyr), and, after reaching their peak, decrease the star formation slowly ( ). These findings are consistent with galaxy stellar mass being a driving factor in determining how evolved galaxies are, with high-mass galaxies being the most evolved at any time (i.e., downsizing). The different durations we observe in the declining phases also suggest that low- and high-mass galaxies experience different quenching mechanisms, which operate on different timescales.
Recent work has shown that Milky Way-mass galaxies display an incredible range of stellar halo properties, yet the origin of this diversity is unclear. The nearby galaxy M81-currently interacting ...with M82 and NGC 3077-sheds unique light on this problem. We present a Subaru Hyper Suprime-Cam survey of the resolved stellar populations around M81, revealing M81's stellar halo in never-before-seen detail. We resolve the halo to unprecedented V-band equivalent surface brightnesses of 33 mag arcsec −2 and produce the first-ever global stellar mass density map for a Milky Way-mass stellar halo outside of the Local Group. Using the minor axis, we confirm M81's halo as one of the lowest mass and metal poorest known (M 1.16 × 109M , Fe/H −1.2)-indicating a relatively quiet prior accretion history. Yet, our global halo census finds that tidally unbound material from M82 and NGC 3077 provides a substantial infusion of metal-rich material (M 5.4 × 108 M , Fe/H −0.9). We further show that, following the accretion of its massive satellite M82 (and the LMC-like NGC 3077), M81 will host one of the most massive and metal-rich stellar halos in the nearby universe. Thus, the saga of M81: following a passive history, M81's merger with M82 will completely transform its halo from a low-mass, anemic halo rivaling the Milky Way, to a metal-rich behemoth rivaled only by systems such as M31. This dramatic transformation indicates that the observed diversity in stellar halo properties is primarily driven by diversity in the largest mergers these galaxies have experienced.
Residual entropy scaling (RES) of thermal conductivity was applied to pure refrigerants, including natural and halogenated refrigerants, and their mixtures. The reference equations of state and the ...mixture models implemented in the REFPROP software package were adopted to calculate the residual entropy, and the critical enhancement of thermal conductivity was taken into account with the RES approach for the first time. Experimental data of 39 pure fluids with more than 38,000 data points and of 31 mixtures with more than 7600 points were collected and analyzed. More than 95.4% of the data (within two standard deviations of the mean) of pure fluids collapse into a global dimensionless residual thermal conductivity versus scaled dimensionless residual entropy curve within 11.1% and those of mixtures are within 8.3%. This smooth, monotonically increasing curve was correlated with a polynomial function containing only four fitted parameters and one fluid-specific scaling factor. Each pure fluid has its individual scaling factor, and a simple mole-fraction-weighted mixing rule was applied for mixtures. The correlation function provides a reliable thermal conductivity prediction of pure fluids and, without any additional parameters, of mixtures. The proposed model yields a similar level of statistical agreement with the experimental data as the extended corresponding states model, which is the current state-of-the-art and has as many as four more parameters for each pair of components.
Although giant clumps of stars are thought to be crucial to galaxy formation and evolution, the most basic demographics of clumps are still uncertain, mainly because the definition of clumps has not ...been thoroughly discussed. In this paper, we carry out a study of the basic demographics of clumps in star-forming galaxies at 0.5 < z < 3, using our proposed physical definition that UV-bright clumps are discrete star-forming regions that individually contribute more than 8% of the rest-frame UV light of their galaxies. Clumps defined this way are significantly brighter than the H II regions of nearby large spiral galaxies, either individually or blended, when physical spatial resolution and cosmological dimming are considered. The clump contribution in the intermediate-mass and massive galaxies is possibly linked to the molecular gas fraction of the galaxies. The clump contribution to the SFR of star-forming galaxies, generally around 4%-10%, also shows dependence on the galaxy M, but for a given galaxy M, its dependence on the redshift is mild.
ABSTRACT We present estimates of intrinsic scatter in the star formation rate (SFR)-stellar mass (M*) correlation in the redshift range and in the mass range M . We utilize photometry in the Hubble ...Ultradeep Field (HUDF12) and Ultraviolet Ultra Deep Field (UVUDF) campaigns and CANDELS/GOODS-S and estimate SFR, M* from broadband spectral energy distributions and the best-available redshifts. The maximum depth of the UDF photometry (F160W 29.9 AB, 5 depth) probes the SFR-M* correlation down to 107M , a factor of 10-100× lower in M* than previous studies, and comparable to dwarf galaxies in the local universe. We find the slope of the SFR-M* relationship to be near unity at all redshifts and the normalization to decrease with cosmic time. We find a moderate increase in intrinsic scatter with cosmic time from 0.2 to 0.4 dex across the epoch of peak cosmic star formation. None of our redshift bins show a statistically significant increase in intrinsic scatter at low mass. However, it remains possible that intrinsic scatter increases at low mass on timescales shorter than ∼100 Myr. Our results are consistent with a picture of gradual and self-similar assembly of galaxies across more than three orders of magnitude in stellar mass from as low as 107M .
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