ABSTRACT
Understanding the variability of galaxy star formation histories (SFHs) across a range of time-scales provides insight into the underlying physical processes that regulate star formation ...within galaxies. We compile the SFHs of galaxies at z = 0 from an extensive set of models, ranging from cosmological hydrodynamical simulations (Illustris, IllustrisTNG, Mufasa, Simba, EAGLE), zoom simulations (FIRE-2, g14, and Marvel/Justice League), semi-analytic models (Santa Cruz SAM) and empirical models (UniverseMachine), and quantify the variability of these SFHs on different time-scales using the power spectral density (PSD) formalism. We find that the PSDs are well described by broken power laws, and variability on long time-scales (≳1 Gyr) accounts for most of the power in galaxy SFHs. Most hydrodynamical models show increased variability on shorter time-scales (≲300 Myr) with decreasing stellar mass. Quenching can induce ∼0.4−1 dex of additional power on time-scales >1 Gyr. The dark matter accretion histories of galaxies have remarkably self-similar PSDs and are coherent with the in situ star formation on time-scales >3 Gyr. There is considerable diversity among the different models in their (i) power due to star formation rate variability at a given time-scale, (ii) amount of correlation with adjacent time-scales (PSD slope), (iii) evolution of median PSDs with stellar mass, and (iv) presence and locations of breaks in the PSDs. The PSD framework is a useful space to study the SFHs of galaxies since model predictions vary widely. Observational constraints in this space will help constrain the relative strengths of the physical processes responsible for this variability.
Abstract
We study the spatially resolved stellar populations of 444 galaxies at 0.3 <
z
< 6.0 in two clusters (WHL 0137–08 and MACS 0647+70) and a blank field, combining imaging data from the Hubble ...Space Telescope and JWST to perform spatially resolved spectral energy distribution (SED) modeling using
piXedfit
. The high spatial resolution of the imaging data combined with magnification from gravitational lensing in the cluster fields allows us to resolve a large fraction of our galaxies (109) to subkiloparsec scales. At redshifts around cosmic noon and higher (2.5 ≲
z
≲ 6.0), we find mass-doubling times to be independent of radius, inferred from flat specific star formation rate (sSFR) radial profiles and similarities between the half-mass and half-SFR radii. At lower redshifts (1.5 ≲
z
≲ 2.5), a significant fraction of our star-forming galaxies shows evidence for nuclear starbursts, inferred from a centrally elevated sSFR and a much smaller half-SFR radius compared to the half-mass radius. At later epochs, we find more galaxies suppress star formation in their centers but are still actively forming stars in the disk. Overall, these trends point toward a picture of inside-out galaxy growth consistent with theoretical models and simulations. We also observe a tight relationship between the central mass surface density and global stellar mass with ∼0.38 dex scatter. Our analysis demonstrates the potential of spatially resolved SED analysis with JWST data. Future analysis with larger samples will be able to further explore the assembly of galaxy mass and the growth of their structures.
The star formation histories (SFHs) of galaxies contain imprints of the physical processes responsible for regulating star formation during galaxy growth and quenching. We improve the Dense Basis SFH ...reconstruction method of Iyer & Gawiser, introducing a nonparametric description of the SFH based on the lookback times at which a galaxy assembles certain quantiles of its stellar mass. The method uses Gaussian processes to create smooth SFHs independent of any functional form, with a flexible number of parameters that is adjusted to extract the maximum amount of information from the SEDs being fit. Applying the method to reconstruct the SFHs of 48,791 galaxies with H < 25 at 0.5 < z < 3.0 across the five Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey fields, we study the evolution of galaxies over time. We quantify the fraction of galaxies that show multiple major episodes of star formation, finding that the median time between two peaks of star formation is , where tuniv is the age of the universe at a given redshift and remains roughly constant with stellar mass. Correlating SFHs with morphology allows us to compare the timescales on which the SFHs decline for different morphological classifications, ranging from for galaxies with spiral arms to for spheroids at 0.5 < z < 1.0 with 1010 < M* < 1010.5M . The Gaussian process-based SFH description provides a general approach to reconstruct smooth, flexible, nonparametric SFH posteriors for galaxies that can be incorporated into Bayesian SED fitting codes to minimize the bias in estimating physical parameters due to SFH parameterization.
Abstract
We report the discovery of a low-mass
z
= 5.200 ± 0.002 galaxy that is in the process of ceasing its star formation. The galaxy, MACS0417-z5BBG, is multiply imaged with magnification factors ...∼40 by the galaxy cluster MACS J0417.5-1154, observed as part of the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). Using observations of MACS0417-z5BBG with a JWST/NIRSpec Prism spectrum and NIRCam imaging, we investigate the mechanism responsible for the cessation of star formation of the galaxy and speculate about possibilities for its future. Using spectrophotometric fitting, we find a remarkably low stellar mass of
M
*
=
4.3
±
0.8
0.9
×
10
7
M
⊙
, less than 1% of the characteristic stellar mass at
z
∼ 5. We measure a delensed rest-UV half-light radius in the source plane of
30
±
5
7
pc and measure a star formation rate from H
α
of
0.14
±
0.12
0.17
M
⊙
yr
−1
. We find that under the assumption of a double power-law star formation history, MACS0417-z5BBG has seen a recent rise in star formation, peaking ∼10–30 Myr ago and declining precipitously since then. Together, these measurements reveal a low-mass, extremely compact galaxy which is in the process of ceasing star formation. We investigate the possibilities of mechanisms that have led to the cessation of star formation in MACS0417-z5BBG, considering stellar and active galactic nuclei (AGN) feedback and environmental processes. We can likely rule out most environmental processes but leave open the possibility of a low-mass AGN that does not leave a broad-line imprint on the spectrum or that MACS0417-z5BBG could be a star-forming galaxy in the lull of a bursty star formation history.
Abstract
Galaxy formation and evolution involve a variety of effectively stochastic processes that operate over different timescales. The extended regulator model provides an analytic framework for ...the resulting variability (or “burstiness”) in galaxy-wide star formation due to these processes. It does this by relating the variability in Fourier space to the effective timescales of stochastic gas inflow, equilibrium, and dynamical processes influencing giant molecular clouds' creation and destruction using the power spectral density (PSD) formalism. We use the connection between the PSD and autocovariance function for general stochastic processes to reformulate this model as an autocovariance function, which we use to model variability in galaxy star formation histories (SFHs) using physically motivated Gaussian processes in log star formation rate (SFR) space. Using stellar population synthesis models, we then explore how changes in model stochasticity can affect spectral signatures across galaxy populations with properties similar to the Milky Way and present-day dwarfs, as well as at higher redshifts. We find that, even at fixed scatter, perturbations to the stochasticity model (changing timescales vs. overall variability) leave unique spectral signatures across both idealized and more realistic galaxy populations. Distributions of spectral features including H
α
and UV-based SFR indicators, H
δ
and Ca H and K absorption-line strengths,
D
n
(4000), and broadband colors provide testable predictions for galaxy populations from present and upcoming surveys with the Hubble Space Telescope, James Webb Space Telescope, and Nancy Grace Roman Space Telescope. The Gaussian process SFH framework provides a fast, flexible implementation of physical covariance models for the next generation of spectral energy distribution modeling tools. Code to reproduce our results can be found at
https://github.com/kartheikiyer/GP-SFH
.
Abstract
Using data from JWST, we analyse the compact sources (“sparkles”) located around a remarkable
z
spec
= 1.378 galaxy (the ‘Sparkler) that is strongly gravitationally lensed by the
z
= 0.39 ...galaxy cluster SMACS J0723.3-7327. Several of these compact sources can be cross-identified in multiple images, making it clear that they are associated with the host galaxy. Combining data from JWSTs Near-Infrared Camera (NIRCam) with archival data from the Hubble Space Telescope (HST), we perform 0.4–4.4
μ
m photometry on these objects, finding several of them to be very red and consistent with the colors of quenched, old stellar systems. Morphological fits confirm that these red sources are spatially unresolved even in the strongly magnified JWST/NIRCam images, while the JWST/NIRISS spectra show O
iii
λ
5007 emission in the body of the Sparkler but no indication of star formation in the red compact sparkles. The most natural interpretation of these compact red companions to the Sparkler is that they are evolved globular clusters seen at
z
= 1.378. Applying
Dense Basis
spectral energy distribution fitting to the sample, we infer formation redshifts of
z
form
∼ 7–11 for these globular cluster candidates, corresponding to ages of ∼3.9–4.1 Gyr at the epoch of observation and a formation time just ∼0.5 Gyr after the Big Bang. If confirmed with additional spectroscopy, these red, compact sparkles represent the first evolved globular clusters found at high redshift, which could be among the earliest observed objects to have quenched their star formation in the universe, and may open a new window into understanding globular cluster formation. Data and code to reproduce our results will be made available at
http://canucs-jwst.com/sparkler.html
.
ABSTRACT
We present a new method for inferring galaxy star formation histories (SFH) using machine learning methods coupled with two cosmological hydrodynamic simulations. We train convolutional ...neural networks to learn the relationship between synthetic galaxy spectra and high-resolution SFHs from the eagle and Illustris models. To evaluate our SFH reconstruction we use Symmetric Mean Absolute Percentage Error (SMAPE), which acts as a true percentage error in the low error regime. On dust-attenuated spectra we achieve high test accuracy (median SMAPE = 10.5 per cent). Including the effects of simulated observational noise increases the error (12.5 per cent), however this is alleviated by including multiple realizations of the noise, which increases the training set size and reduces overfitting (10.9 per cent). We also make estimates for the observational and modelling errors. To further evaluate the generalization properties we apply models trained on one simulation to spectra from the other, which leads to only a small increase in the error (median SMAPE $\sim 15{\,{\rm {per\, cent}}}$). We apply each trained model to SDSS DR7 spectra, and find smoother histories than in the $\textsf{vespa}$ catalogue. This new approach complements the results of existing spectral energy distribution fitting techniques, providing SFHs directly motivated by the results of the latest cosmological simulations.
Abstract
We demonstrate that the inference of galaxy stellar masses via spectral energy distribution (SED) fitting techniques for galaxies formed in the first billion years after the Big Bang carries ...fundamental uncertainties owing to the loss of star formation history (SFH) information from the very first episodes of star formation in the integrated spectra of galaxies. While this early star formation can contribute substantially to the total stellar mass of high-redshift systems, ongoing star formation at the time of detection outshines the residual light from earlier bursts, hampering the determination of accurate stellar masses. As a result, order-of-magnitude uncertainties in stellar masses can be expected. We demonstrate this potential problem via direct numerical simulation of galaxy formation in a cosmological context. In detail, we carry out two cosmological simulations with significantly different stellar feedback models, which span a significant range in SFH burstiness. We compute the mock SEDs for these model galaxies at
z
= 7 via calculations of 3D dust radiative transfer, and then backward fit these SEDs with
prospector
SED fitting software. The uncertainties in derived stellar masses that we find for
z
> 7 galaxies motivate the development of new techniques and/or priors for SFH to model star formation in the early Universe.
Abstract
We investigate the connection between the regulation of star formation and the cycling of baryons both within and in and out of galaxies. We use idealized numerical simulations of Milky ...Way–mass galaxies, in which we vary the galaxy morphology and stellar feedback strength. By following individual gas parcels through the disk, spiral arms, and massive star-forming clumps, we quantify how gas moves through the different phases of the interstellar medium (ISM) and forms stars. We show that the residence time of gas in the dense ISM phase (
τ
SF
), the nature of spiral arms, and the clump properties depend on both the galaxy morphology and stellar feedback. We quantify signatures of the baryon cycle within galaxies using the temporal and spatial power spectrum density (PSD) of the star formation rate (SFR). Stronger stellar feedback leads to more bursty star formation while the correlation timescale of the SFH is longer, because stronger feedback dissolves the dense ISM phase, leading to a more homogeneous ISM and a decrease in
τ
SF
. The bulge strength has a similar effect: the deep gravitational potential in a bulge-dominant galaxy imposes a strong shear force that breaks apart gas clumps in the ISM; this subsequently inhibits the fragmentation of gas and therefore the star formation in the disk, leading to a decrease in the spatial power on scales of ∼1 kpc. We conclude that measurements of the temporal and spatial PSD of the SFR can provide constraints on the baryon cycle and the star formation process.
Abstract
We present JWST NIRSpec spectroscopy for 11 galaxy candidates with photometric redshifts of
z
≃ 9 − 13 and
M
UV
∈ −21, −18 newly identified in NIRCam images in the Cosmic Evolution Early ...Release Science Survey. We confirm emission line redshifts for 7 galaxies at
z
= 7.762–8.998 using spectra at ∼1–5
μ
m either with the NIRSpec prism or its three medium-resolution (
R
∼ 1000) gratings. For
z
≃ 9 photometric candidates, we achieve a high confirmation rate of ≃90%, which validates the classical dropout selection from NIRCam photometry. No robust emission lines are identified in three galaxy candidates at
z
> 10, where the strong O
iii
and H
β
lines would be redshifted beyond the wavelength range observed by NIRSpec, and the Ly
α
continuum break is not detected with the sensitivity of the current data. Compared with Hubble Space Telescope-selected bright galaxies (
M
UV
≃ −22) that are similarly spectroscopically confirmed at
z
≃ 8 − 9, these NIRCam-selected galaxies are characterized by lower star formation rates (SFRs; SFR ≃ 4
M
⊙
yr
−1
) and lower stellar masses (≃10
8
M
⊙
), but with higher specific SFR (≃40 Gyr
−1
), higher O
iii
+H
β
equivalent widths (≃1100 Å), and elevated production efficiency of ionizing photons (
log
(
ξ
ion
/
Hz
erg
−
1
)
≃
25.8
) induced by young stellar populations (<10 Myr) accounting for ≃20% of the galaxy mass, highlighting the key contribution of faint galaxies to cosmic reionization. Taking advantage of the homogeneous selection and sensitivity, we also investigate metallicity and ISM conditions with empirical calibrations using the O
iii
5008
/H
β
ratio. We find that galaxies at
z
≃ 8 − 9 have higher SFRs and lower metallicities than galaxies at similar stellar masses at
z
≃ 2 − 6, which is generally consistent with the current galaxy formation and evolution models.
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