arXiv subjects

Tim B. Miller

Publications and source records attributed to Tim B. Miller.

72 records · Page 4Linked to original sources

arXiv2023

Rest-Frame Near-Infrared Radial Light Profiles up to z=3 from JWST/NIRCam: Wavelength Dependence of the Sérsic Index

We examine the wavelength dependence of radial light profiles based on Sérsic index $n$ measurements of 1067 galaxies with M$_*\geq$ 10$^{9.5}$M$_\odot$ and in the redshift range $0.5 < z < 3$. The sample and rest-frame optical light profiles are drawn from CANDELS$+$3D-HST; rest-frame near-infrared light profiles are inferred from CEERS JWST/NIRCam imaging. $n$ shows only weak dependence on wavelength, regardless of redshift, galaxy mass and type: on average, star-forming galaxies have $n = 1-1.5$ and quiescent galaxies have $n = 3-4$ in the rest-frame optical and near-infrared. The strong correlation at all wavelengths between $n$ and star-formation activity implies a physical connection between the radial stellar mass profile and star-formation activity. The main caveat is that the current sample is too small to discern trends for the most massive galaxies (M$_* > 10^{11}M_\odot$).

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arXiv2023

Stellar Half-Mass Radii of $0.5<z<2.3$ Galaxies: Comparison with JWST/NIRCam Half-Light Radii

We use CEERS JWST/NIRCam imaging to measure rest-frame near-IR light profiles of $>$500 $M_\star>10^{10}~M_\odot$ galaxies in the redshift range $0.5 10^{11}~M_\odot$) quiescent galaxies between $z=0.5$ and $z=2.3$, again comparing $R_{\rm{opt}}$, \rmass~and \rmassd. We conclude that the main tenets of the size evolution narrative established over the past 20 years, based on rest-frame optical light profile analysis, still hold in the era of JWST/NIRCam observations in the rest-frame near-IR.

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arXiv2023

pysersic: A Python package for determining galaxy structural properties via Bayesian inference, accelerated with jax

A standard practice in extragalactic population studies is the fitting of parametric models to galaxy images. From such fits, key structural parameters of galaxies such as total flux and effective radius (size) can be extracted. One of the most popular parametric forms is that of the Sérsic profile, which is flexible enough to reasonably fit the light distribution of nearly all galaxies. Here we present pysersic, a Bayesian framework created to facilitate the inference of structural parameters from galaxy images. Pysersic is written in pure Python, and is built using the jax framework, allowing for just-in-time compilation, auto-differentiation and seamless execution on CPUs, GPUs or TPUs. Inference is performed with the numpyro package using gradient based methods, e.g., No U-Turn Sampling, for efficient and robust posterior estimation in only a few minutes on a modern laptop. Pysersic is designed to have a user-friendly interface, allowing users to fit single or multiple sources in a few lines of code, while also being flexible enough for integration into current and future analysis pipelines. In addition to sampling, pysersic can produce point estimates of the best model via optimization in several seconds, and approximate the posterior via stochastic variational inference. The use of the numpyro probabilistic language provides future extensibility to arbitrary models beyond the Sérsic.

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arXiv2023

Early JWST imaging reveals strong optical and NIR color gradients in galaxies at $z\sim2$ driven mostly by dust

Recent studies have shown that galaxies at cosmic noon are redder in the center and bluer in the outskirts, mirroring results in the local universe. These color gradients could be caused by either gradients in the stellar age or dust opacity; however, distinguishing between these two causes is impossible with rest-frame optical photometry alone. Here we investigate the underlying causes of the gradients from spatially-resolved rest-frame $U-V$ vs. $V-J$ color-color diagrams, measured from early observations with the James Webb Space Telescope. We use $1\, μm - 4\, μm$ NIRCam photometry from the CEERS survey of a sample of 54 galaxies with $M_* / M_\odot>10$ at redshifts $1.7<z<2.3$ selected from the 3D-HST catalog. We model the light profiles in the F115W, F200W and F356W NIRCam bands using \texttt{imcascade}, a Bayesian implementation of the Multi-Gaussian expansion (MGE) technique which flexibly represents galaxy profiles using a series of Gaussians. We construct resolved rest-frame $U-V$ and $V-J$ color profiles. The majority of star-forming galaxies have negative gradients (i.e. redder in the center, bluer in the outskirts) in both $U-V$ and $V-J$ colors consistent with radially decreasing dust attenuation. A smaller population (roughly 15\%) of star-forming galaxies have positive $U-V$ but negative $V-J$ gradients implying centrally concentrated star-formation. For quiescent galaxies we find a diversity of UVJ color profiles, with roughly one-third showing star-formation in their center. This study showcases the potential of JWST to study the resolved stellar populations of galaxies at cosmic noon.

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arXiv2022

Rest-frame near-infrared sizes of galaxies at cosmic noon: objects in JWST's mirror are smaller than they appeared

Galaxy sizes and their evolution over cosmic time have been studied for decades and serve as key tests of galaxy formation models. However, at $z\gtrsim1$ these studies have been limited by a lack of deep, high-resolution rest-frame infrared imaging that accurately traces galaxy stellar mass distributions. Here, we leverage the new capabilities of the James Webb Space Telescope to measure the 4.4$μ$m sizes of ${\sim}1000$ galaxies with $\log{\rm{M}_*/\rm{M}_\odot}\ge9$ and $1.0\le z \le 2.5$ from public CEERS imaging in the EGS deep field. We compare the sizes of galaxies measured from NIRCam imaging at 4.4$μ$m ($λ_{\mathrm{rest}}\sim1.6μ$m) with sizes measured at $1.5μ$m ($λ_{\mathrm{rest}}\sim5500$A). We find that, on average, galaxy half-light radii are $\sim8$% smaller at 4.4$μ$m than 1.5$μ$m in this sample. This size difference is markedly stronger at higher stellar masses and redder rest-frame $V-J$ colors: galaxies with ${\rm M}_* \sim 10^{11}\,{\rm M}_\odot$ have 4.4$μ$m sizes that are $\sim 25$% smaller than their 1.5$μ$m sizes. Our results indicate that galaxy mass profiles are significantly more compact than their rest-frame optical light profiles at cosmic noon, and demonstrate that spatial variations in age and attenuation are important, particularly for massive galaxies. The trend that we find here impacts our understanding of the size growth and evolution of galaxies, and suggests that previous studies based on rest-frame optical light may not have captured the mass-weighted structural evolution of galaxies. This paper represents a first step towards a new understanding of the morphologies of early massive galaxies enabled by JWST's infrared window into the distant universe.

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arXiv2022

JWST reveals a population of ultra-red, flattened disk galaxies at 2<z<6 previously missed by HST

With just a month of data, JWST is already transforming our view of the Universe, revealing and resolving starlight in unprecedented populations of galaxies. Although ``HST-dark" galaxies have previously been detected at long wavelengths, these observations generally suffer from a lack of spatial resolution which limits our ability to characterize their sizes and morphologies. Here we report on a first view of starlight from a subset of the HST-dark population that are bright with JWST/NIRCam (4.4$μ$m<24.5mag) and very faint or even invisible with HST ($<$1.6$μ$m). In this Letter we focus on a dramatic and unanticipated population of physically extended galaxies ($\gtrsim$0.17''). These 12 galaxies have photometric redshifts $2<z<6$, high stellar masses $M_{\star}\gtrsim 10^{10}~M_{\odot}$, and significant dust-attenuated star formation. Surprisingly, the galaxies have elongated projected axis ratios at 4.4$μ$m, suggesting that the population is disk-dominated or prolate. Most of the galaxies appear red at all radii, suggesting significant dust attenuation throughout. We refer to these red, disky, HST-dark galaxies as Ultra-red Flattened Objects (UFOs). With $r_e$(F444W)$\sim1-2$~kpc, the galaxies are similar in size to compact massive galaxies at $z\sim2$ and the cores of massive galaxies and S0s at $z\sim0$. The stellar masses, sizes, and morphologies of the sample suggest that some could be progenitors of lenticular or fast-rotating galaxies in the local Universe. The existence of this population suggests that our previous censuses of the universe may have missed massive, dusty edge-on disks, in addition to dust-obscured starbursts.

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arXiv2022

Tidal Distortions in NGC1052-DF2 and NGC1052-DF4: Independent Evidence for a Lack of Dark Matter

Two ultra diffuse galaxies in the same group, NGC1052-DF2 and NGC1052-DF4, have been found to have little or no dark matter and to host unusually luminous globular cluster populations. Such low mass diffuse objects in a group environment are easily disrupted and are expected to show evidence of tidal distortions. In this work we present deep new imaging of the NGC1052 group, obtained with the Dragonfly Telephoto Array, to test this hypothesis. We find that both galaxies show strong position angle twists and are significantly more elongated at their outskirts than in their interiors. The group's central massive elliptical NGC1052 is the most likely source of these tidal disturbances. The observed distortions imply that the galaxies have a low total mass or are very close to NGC1052. Considering constraints on the galaxies' relative distances, we infer that the dark matter halo masses of these galaxies cannot be much greater than their stellar masses. Calculating pericenters from the distortions, we find that the galaxies are on highly elliptical orbits, with a ratio of pericenter to present-day radius Rperi/R0~0.1 if the galaxies are dark matter-free and Rperi/R0~0.01 if they have a normal dark halo. Our findings provide strong evidence, independent of kinematic constraints, that both galaxies are dark matter deficient. Furthermore, the similarity of the tidal features in NGC1052-DF2 and NGC1052-DF4 strongly suggests that they arose at comparable distances from NGC1052. In Appendix A, we describe sbcontrast, a robust method to determine the surface brightness limit of images.

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arXiv2022

Color gradients and half-mass radii of galaxies out to $z=2$ in the CANDELS/3D-HST fields: further evidence for important differences in the evolution of mass-weighted and light-weighted sizes

Recent studies have indicated that the ratio between half-mass and half-light radii, $r_{\rm mass} / r_{\rm light}$, varies significantly as a function of stellar mass and redshift, complicating the interpretation of the ubiquitous $r_{\rm light}- M_*$ relation. To investigate, in this study we construct the light and color profiles of $\sim 3000$ galaxies at $1 10.25$ using $\texttt{imcascade}$, a Bayesian implementation of the Multi-Gaussian expansion (MGE) technique. $\texttt{imcascade}$ flexibly represents galaxy profiles using a series of Gaussians, free of any a-priori parameterization. We find that both star-forming and quiescent galaxies have on average negative color gradients. For star forming galaxies, we find steeper gradients that evolve with redshift and correlate with dust content. Using the color gradients as a proxy for gradients in the $M/L$ ratio we measure half mass radii for our sample of galaxies. There is significant scatter in individual $r_{\rm mass} / r_{\rm light}$ ratios, which is correlated with variation in the color gradients. We find that the median $r_{\rm mass} / r_{\rm light}$ ratio evolves from 0.75 at $z=2$ to 0.5 at $z=1$, consistent with previous results. We characterize the $r_{\rm mass}- M_*$ relation and we find that it has a shallower slope and shows less redshift evolution than the $r_{\rm light} - M_*$ relation. This applies both to star-forming and quiescent galaxies. We discuss some of the implications of using $r_{\rm mass}$ instead of $r_{\rm light}$, including an investigation of the size-inclination bias and a comparison to numerical simulations.

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arXiv2021

A Method To Characterize the Wide-Angle Point Spread Function of Astronomical Images

Uncertainty in the wide-angle Point Spread Function (PSF) at large angles (tens of arcseconds and beyond) is one of the dominant sources of error in a number of important quantities in observational astronomy. Examples include the stellar mass and shape of galactic halos and the maximum extent of starlight in the disks of nearby galaxies. However, modeling the wide-angle PSF has long been a challenge in astronomical imaging. In this paper, we present a self-consistent method to model the wide-angle PSF in images. Scattered light from multiple bright stars is fitted simultaneously with a background model to characterize the extended wing of the PSF using a Bayesian framework operating on pixel-by-pixel level. The method is demonstrated using our software elderflower and is applied to data from the Dragonfly Telephoto Array to model its PSF out to 20-25 arcminutes. We compare the wide-angle PSF of Dragonfly to that of a number of other telescopes, including the SDSS PSF, and show that on scales of arcminutes the scattered light in the Dragonfly PSF is markedly lower than that of other wide-field imaging telescopes. The energy in the wings of the Dragonfly point-spread function is sufficiently low that optical cleanliness plays an important role in defining the PSF. This component of the PSF can be modelled accurately, highlighting the power of our self-contained approach.

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arXiv2021

Bayesian fitting of multi-Gaussian expansion models to galaxy images

Fitting parameterized models to images of galaxies has become the standard for measuring galaxy morphology. This forward modelling technique allows one to account for the PSF to effectively study semi-resolved galaxies. However, using a specific parameterization for a galaxy's surface brightness profile can bias measurements if it is not an accurate representation. Furthermore, it can be difficult to assess systematic errors in parameterized profiles. To overcome these issues we employ the Multi-Gaussian expansion (MGE) method of representing a galaxy's profile together with a Bayesian framework for fitting images. MGE flexibly represents a galaxy's profile using a series of Gaussians. We introduce a novel Bayesian inference approach which uses pre-rendered Gaussian components, which greatly speeds up computation time and makes it feasible to run the fitting code on large samples of galaxies. We demonstrate our method with a series of validation tests. By injecting galaxies, with properties similar to those observed at $z\sim1.5$, into deep HST observations we show that it can accurately recover total fluxes and effective radii of realistic galaxies. Additionally we use degraded images of local galaxies to show that our method can recover realistic galaxy surface brightness and color profiles. Our implementation is available in an open source python package $\texttt{imcascade}$, which contains all methods needed for the preparation of images, fitting and analysis of results.

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arXiv2021

The Dragonfly Wide Field Survey. II. Accurate Total Luminosities and Colors of Nearby Massive Galaxies and Implications for the Galaxy Stellar Mass Function

Stellar mass estimates of massive galaxies are susceptible to systematic errors in their photometry, due to their extended light profiles. In this study we use data from the Dragonfly Wide Field Survey (DWFS) to accurately measure the total luminosities and colors of nearby massive galaxies. The low surface brightness limits of the survey ($μ_g \approx $ 31 mag arcsec $^{-2}$ on a one arcmin scale) allows us to implement a method, based on integrating the 1-D surface brightness profile, that is minimally dependent on any parameterization. We construct a sample of 1188 massive galaxies with $\log M_*/M_\odot > 10.75$ based on the Galaxy Mass and Assembly (GAMA) survey and measure their total luminosities and $g-r$ colors. We then compare our measurements to various established methods applied to imaging from the Sloan Digital Sky Survey (SDSS), focusing on those favored by the GAMA survey. In general, we find that galaxies are brighter in the $r$ band by an average of ${\sim}0.05$ mag and bluer in $g-r$ colors by $\sim 0.06$ mag compared to the GAMA measurements. These two differences have opposite effects on the stellar mass estimates. The total luminosities are larger by $5\%$ but the mass-to-light ratios are lower by $\sim 10\%$. The combined effect is that the stellar mass estimate of massive galaxies decreases by $7\%$. This, in turn, implies a small change in number density of massive galaxies: $\leq 30\%$ at $\log M_*/M_\odot \geq 11$.

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arXiv2020

The Dragonfly Wide Field Survey. I. Telescope, Survey Design and Data Characterization

We present a description of the Dragonfly Wide Field Survey (DWFS), a deep photometric survey of a wide area of sky. The DWFS covers 330 $\mathrm{deg}^2$ in the equatorial GAMA fields and the Stripe 82 fields in the SDSS $g$ and $r$ bands. It is carried out with the 48-lens Dragonfly Telephoto Array, a telescope that is optimized for the detection of low surface brightness emission. The main goal of the survey is to study the dwarf galaxy population beyond the Local Group. In this paper, we describe the survey design and show early results. We reach $1σ$ depths of $μ_g\approx 31$ mag arcsec$^{-2}$ on arcminute scales and show that Milky Way satellites such as Sextans, Bootes, and Ursa Major should be detectable out to $D\gtrsim 10$ Mpc. We also provide an overview of the elements and operation of the 48-lens Dragonfly telescope and a detailed description of its data reduction pipeline. The pipeline is fully automated, with individual frames subjected to a rigorous series of quality tests. The sky subtraction is performed in two stages, ensuring that emission features with spatial scales up to $\sim 0.^{\circ}9 \times 0.^{\circ}6$ are preserved. The DWFS provides unparalleled sensitivity to low surface brightness features on arcminute scales.

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arXiv2020

Dynamical self-friction: how mass loss slows you down

We investigate dynamical self-friction, the process by which material that is stripped from a subhalo torques its remaining bound remnant, which causes it to lose orbital angular momentum. By running idealized simulations of a subhalo orbiting within an analytical host halo potential, we isolate the effect of self-friction from traditional dynamical friction due to the host halo. While at some points in a subhalo's orbit the torque of the stripped material can boost the orbital angular momentum of the remnant, the net effect over the long term is orbital decay regardless of the initial orbital parameters or subhalo mass. In order to quantify the strength of self-friction, we run a suite of simulations spanning typical host-to-subhalo mass ratios and orbital parameters. We find that the time-scale for self-friction, defined as the exponential decay time of the subhalo's orbital angular momentum, scales with mass ratio and orbital circularity similar to standard dynamical friction. The decay time due to self-friction is roughly an order of magnitude longer, suggesting that self-friction only contributes at the 10 percent level. However, along more radial orbits, self-friction can occasionally dominate over dynamical friction close to pericentric passage, where mass stripping is intense. This is also the epoch at which the self-friction torque undergoes large and rapid changes in both magnitude and direction, indicating that self-friction is an important process to consider when modeling pericentric passages of subhaloes and their associated satellite galaxies.

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arXiv2020

Investigating overdensities around z>6 galaxies through ALMA observations of [CII]

We present a search for companion [CII] emitters to known luminous sources at $6<$ z $<6.5$ in deep, archival ALMA observations. The observations are deep enough to detect sources with L$_{\rm [CII]} \sim 10^8$ at z $\sim6$. We identify four robust line detections from a blind search of five deep fields centered on ultra-luminous infrared galaxies and QSOs, over an order of magnitude more than expected based on current observations and predictions, suggesting that these objects may be highly biased tracers of mass in the early Universe. We find these companion lines to have comparable properties to other known galaxies at the same epoch. All companions lie less than 650 km s$^{-1}$ and between 20 -- 70 kpc (projected) from their central source, providing a constraint on their halo masses of the central galaxies ranging from 2.5$\times$10$^{12}$ M$_\odot$ to 4$\times$10$^{13}$ M$_\odot$. To place these discoveries in context, we employ a mock galaxy catalog to estimate the luminosity function for [CII] during reionization and compare to our observations. The simulations support this result by showing a similar level of elevated counts found around such luminous sources.

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arXiv2019

DASH: a library of dynamical subhalo evolution

The abundance and demographics of dark matter substructure is important for many areas in astrophysics and cosmological $N$-body simulations have been the primary tool used to investigate them. However, it has recently become clear that the simulations are subject to numerical artefacts, which hampers a proper treatment of the tidal evolution of subhaloes. Unfortunately, no analytical models that accurately describe subhalo evolution exist either. We therefore present a library of idealized, high resolution $N$-body simulations of the tidal evolution of individual subhaloes that can be used to calibrate semi-analytical models and to complement cosmological simulations. The simulations focus on minor mergers, i.e., the mass of the subhalo is much smaller than that of the host halo, such that the impact of dynamical friction is negligible. This setup allows the adoption of a fixed analytical potential for modelling the host halo. The dynamical evolution of subhaloes is followed with $N$-body computations. In the library, four parameters, two of which characterize the subhalo orbit with respect to the host halo, and the two concentrations of the host- and subhalo, are varied over the ranges encountered in cosmological simulations. We show several representative examples from the library that illustrate the evolution of the subhalo mass and velocity dispersion profiles. Additionally, we make publicly available a pre-trained non-parametric model of the subhalo mass evolution based on random forest regression. This model is able to interpolate the simulation data at the 0.1\,dex level and provides efficient access to the data for further use in modelling.

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arXiv2019

A New View of the Size-Mass Distribution of Galaxies: Using $r_{20}$ and $r_{80}$ instead of $r_{50}$

When investigating the sizes of galaxies it is standard practice to use the half-light radius, $r_{50}$. Here we explore the effects of the size definition on the distribution of galaxies in the size -- stellar mass plane. Specifically, we consider $r_{20}$ and $r_{80}$, the radii that contain 20% and 80% of a galaxy's total luminosity, as determined from a Sersic profile fit, for galaxies in the 3D-HST/CANDELS and COSMOS-DASH surveys. These radii are calculated from size catalogs based on a simple calculation assuming a Sersic profile. We find that the size-mass distributions for $r_{20}$ and $r_{80}$ are markedly different from each other and also from the canonical $r_{50}$ distribution. The most striking difference is in the relative sizes of star forming and quiescent galaxies at fixed stellar mass. Whereas quiescent galaxies are smaller than star forming galaxies in $r_{50}$, this difference nearly vanishes for $r_{80}$. By contrast, the distance between the two populations increases for $r_{20}$. Considering all galaxies in a given stellar mass and redshift bin we detect a significant bimodality in the distribution of $r_{20}$, with one peak corresponding to star forming galaxies and the other to quiescent galaxies. We suggest that different measures of the size are tracing different physical processes within galaxies; $r_{20}$ is closely related to processes controlling the star formation rate of galaxies and $r_{80}$ may be sensitive to accretion processes and the relation of galaxies with their halos.

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arXiv2017

Stacked star formation rate profiles of bursty galaxies exhibit 'coherent' star formation

In a recent work based on 3200 stacked H$α$ maps of galaxies at $z \sim 1$, Nelson et al.~find evidence for `coherent star formation': the stacked SFR profiles of galaxies above (below) the 'star formation main sequence' (MS) are above (below) that of galaxies on the MS at all radii. One might interpret this result as inconsistent with highly bursty star formation and evidence that galaxies evolve smoothly along the MS rather than crossing it many times. We analyze six simulated galaxies at $z\sim1$ from the Feedback in Realistic Environments (FIRE) project in a manner analogous to the observations to test whether the above interpretations are correct. The trends in stacked SFR profiles are qualitatively consistent with those observed. However, SFR profiles of individual galaxies are much more complex than the stacked profiles: the former can be flat or even peak at large radii because of the highly clustered nature of star formation in the simulations. Moreover, the SFR profiles of individual galaxies above (below) the MS are not systematically above (below) those of MS galaxies at all radii. We conclude that the time-averaged coherent star formation evident stacks of observed galaxies is consistent with highly bursty, clumpy star formation of individual galaxies and is not evidence that galaxies evolve smoothly along the MS.

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arXiv2015

The bias of the submillimetre galaxy population: SMGs are poor tracers of the most massive structures in the z ~ 2 Universe

It is often claimed that overdensities of (or even individual bright) submillimetre-selected galaxies (SMGs) trace the assembly of the most-massive dark matter structures in the Universe. We test this claim by performing a counts-in-cells analysis of mock SMG catalogues derived from the Bolshoi cosmological simulation to investigate how well SMG associations trace the underlying dark matter structure. We find that SMGs exhibit a relatively complex bias: some regions of high SMG overdensity are underdense in terms of dark matter mass, and some regions of high dark matter overdensity contain no SMGs. Because of their rarity, Poisson noise causes scatter in the SMG overdensity at fixed dark matter overdensity. Consequently, rich associations of less-luminous, more-abundant galaxies (i.e. Lyman-break galaxy analogues) trace the highest dark matter overdensities much better than SMGs. Even on average, SMG associations are relatively poor tracers of the most significant dark matter overdensities because of 'downsizing': at z < ~2.5, the most-massive galaxies that reside in the highest dark matter overdensities have already had their star formation quenched and are thus no longer SMGs. At a given redshift, of the 10 per cent most-massive overdensities, only ~25 per cent contain at least one SMG, and less than a few per cent contain more than one SMG.

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