Star Formation History Research Articles

On the maximum age resolution achievable through stellar population synthesis models

ABSTRACT As the reconstruction of the star formation histories (SFHs) of galaxies from spectroscopic data becomes increasingly popular, we explore the best age resolution achievable with stellar population synthesis (SPS) models, relying on different constraints: broad-band colours, absorption indices, a combination of the two, and the full spectrum. We perform idealized experiments on SPS models and show that the minimum resolvable relative duration of a star formation episode (time difference between 10 per cent and 90 per cent of the stellar mass formed divided by the median age) is never better than 0.4, even when using spectra with signal-to-noise ratio (SNR) larger than 100 per Å. Typically, the best relative age resolution ranges between 0.4 and 0.7 over most of the age–metallicity plane, corresponding to minimum bin sizes for SFH sampling between 0.15 and 0.25 dex. This resolution makes the spectroscopic exploration of distant galaxies mandatory in order to reconstruct the early phases of galaxies’ SFHs. We show that spectroscopy with ${\rm SNR} \gtrsim 20\,$ Å−1 is essential for good age resolution. Remarkably, using the full spectrum does not prove significantly more effective than relying on absorption indices, especially at SNR ≲ 20 Å−1. We discuss the physical origins of the age resolution trends as a function of age and metallicity, and identify the presence of maxima in age resolution (i.e. minima in measurable relative time duration) at the characteristic ages that correspond to quick time variations in spectral absorption features. We connect these maxima to bumps commonly observed in reconstructed SFHs.

Formaldehyde observations of the Perseus Molecular Cloud

ABSTRACT Large-scale observations of the Perseus Molecular Cloud (MC) with Nanshan 26-m telescope are presented using the 6 cm ortho-H2CO (110–111) transition. As a probe of dense gas at low temperatures, the H2CO absorption extends over the main parts of the Perseus MC. A comparison of the H2CO absorption line with the 12CO and 13CO (J = 1–0) emissions shows that the H2CO and CO are similarly distributed over the Perseus but that H2CO correlates better with 13CO. Comparison with the Herschel-derived H2 column density shows that both the 13CO and H2CO column densities vary linearly with the H2 column density. The main parameters of H2CO absorption line data show a log-normal distribution, which suggests that the strong non-thermal line-broadening results from large-scale supersonic turbulence related to the star formation. Formaldehyde absorption serves well as a tracer of star formation activity and also the H2CO-to-13CO and H2CO-to-H2 abundances systematically trace the star formation activity in the six subregions of Perseus MC. The H2CO abundances anticorrelate with the number of prestellar and protostellar cores and the IRIS 12 μm flux in the six subregions and reveal the star formation history in the Perseus MC.

A Comparison of Star-formation Histories Derived from UniverseMachine and LEGA-C at 0.6 < z < 1

In this work, we compare star formation histories of massive (10.5 <log(M*/M⊙)< 12) galaxies in the UniverseMachine model to those measured from the Large Early Galaxy Astrophysics Census (LEGA-C) at 0.6 < z < 1. Following the LEGA-C study, we investigate how 50% (t 50) and 90% (t 90) formation timescales depend on total stellar mass. We find good agreement between the observed and model timescales for the star-forming population Δ t SF ≲ 1 Gyr across the full mass range. In contrast, the observed age-mass correlation is weaker for the quiescent population compared to UniverseMachine models (Δt Q ≲ 2 Gyr), especially at the high-mass end. This indicates continued star formation or additional processes in the most massive quiescent galaxies, a behavior not accounted for in the UniverseMachine model.

The dual role of outflows in quenching satellites of low-mass hosts: NGC 3109

ABSTRACT While dwarf galaxies observed in the field are overwhelmingly star forming, dwarf galaxies in environments as dense or denser than the Milky Way are overwhelmingly quenched. In this paper, we explore quenching in the lower density environment of the Small-Magellanic-Cloud-mass galaxy NGC 3109 (M$_* \sim 10^8 \, \text{M}_\odot$), which hosts two known dwarf satellite galaxies (Antlia and Antlia B), both of which are ${\rm H}\, \rm{\small I}$ deficient compared to similar galaxies in the field and have recently stopped forming stars. Using a new semi-analytic model in concert with the measured star formation histories and gas masses of the two dwarf satellite galaxies, we show that they could not have been quenched solely by direct ram pressure stripping of their interstellar media, as is common in denser environments. Instead, we find that separation of the satellites from pristine gas inflows, coupled with stellar-feedback-driven outflows from the satellites (jointly referred to as the starvation quenching model), can quench the satellites on time-scales consistent with their likely infall times into NGC 3109’s halo. It is currently believed that starvation is caused by ‘weak’ ram pressure that prevents low-density, weakly bound gas from being accreted on to the dwarf satellite, but cannot directly remove the denser interstellar medium. This suggests that star-formation-driven outflows serve two purposes in quenching satellites in low-mass environments: outflows from the host form a low-density circumgalactic medium that cannot directly strip the interstellar media from its satellites, but is sufficient to remove loosely bound gaseous outflows from the dwarf satellites driven by their own star formation.

The JWST Resolved Stellar Populations Early Release Science Program. IV. The Star Formation History of the Local Group Galaxy WLM

We present the first star formation history (SFH) and age–metallicity relation (AMR) derived from resolved stellar populations imaged with the JWST NIRCam instrument. The target is the Local Group star-forming galaxy WLM at 970 kpc. The depth of the color–magnitude diagram (CMD) reaches below the oldest main sequence turnoff with a signal-to-noise ratio = 10 at M F090W = + 4.6 mag. This is the deepest CMD for any galaxy that is not a satellite of the Milky Way. We use Hubble Space Telescope (HST) optical imaging that overlaps with the NIRCam observations to directly evaluate the SFHs derived based on data from the two great observatories. The JWST and HST-based SFHs are in excellent agreement. We use the metallicity distribution function measured from stellar spectra to confirm the trends in the AMRs based on the JWST data. Together, these results confirm the efficacy of recovering an SFH and AMR with the NIRCam F090W−F150W filter combination, and validate the sensitivity and accuracy of stellar evolution libraries in the near-infrared relative to the optical for SFH recovery work. From the JWST data, WLM shows an early onset to star formation, followed by an extended pause post-reionization before star formation reignites, which is qualitatively similar to what has been observed in the isolated galaxies Leo A and Aquarius. Quantitatively, 15% of the stellar mass formed in the first Gyr, while only 10% formed over the next ∼5 Gyr. The stellar mass then rapidly doubled in ∼2.5 Gyr, followed by constant star formation over the last ∼5 Gyr.

Outshining by Recent Star Formation Prevents the Accurate Measurement of High-z Galaxy Stellar Masses

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.

A Census of Star Formation Histories of Massive Galaxies at 0.6 < z < 1 from Spectrophotometric Modeling Using Bagpipes and Prospector

We present individual star formation histories (SFHs) of ∼3000 massive galaxies (log(M */M ⊙) > 10.5) from the Large Early Galaxy Astrophysics Census spectroscopic survey at a lookback time of ∼7 billion yr and quantify the population trends leveraging 20 hr deep-integrated spectra of these ∼1800 star-forming and ∼1200 quiescent galaxies at 0.6 < z < 1.0. Essentially all galaxies at this epoch contain stars of age <3 Gyr, in contrast with older massive galaxies today, facilitating better recovery of previous generations of star formation at cosmic noon and earlier. We conduct spectrophotometric analysis using parametric and nonparametric Bayesian stellar population synthesis modeling tools—Bagpipes and Prospector—to constrain the median SFHs of this mass complete sample and characterize population trends. A consistent picture arises for the late-time stellar mass growth when quantified as t 50 and t 90, corresponding to the age of the Universe when galaxies formed 50% and 90% of their total stellar mass, although the two methods disagree at the earliest formation times (e.g., t 10). Our results reveal trends in both stellar mass and stellar velocity dispersion as in the local Universe—low-mass galaxies with shallower potential wells grow their stellar masses later in cosmic history compared to high-mass galaxies. Unlike local quiescent galaxies, the median duration of late-time star formation (τ SF,late = t 90–t 50) does not consistently depend on the stellar mass. This census sets a benchmark for future deep spectrophotometric studies of the more distant Universe.

The Star Formation History of the Young Open Cluster NGC 2244: Study of the Binary Fraction

A star formation history (SFH) is the stellar record of the various processes that shaped a galaxy and star clusters since its birth to its current physical state. Studying the young open cluster NGC 2244, and in particular its massive stellar population, can improve our understanding of its formation mechanism, stellar evolution, feedback, and provide a basis for a better understanding of the formation of extragalactic star clusters. In this contribution, we present the first steps towards the derivation of the SFH of NGC 2244. We discuss the first results concerning the membership determination and the observed binary fractions of O- and B-type stars among NGC 2244. Finally, an overview of the forthcoming steps is also presented.

Stochastic Modeling of Star Formation Histories. III. Constraints from Physically Motivated Gaussian Processes

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 Hα and UV-based SFR indicators, Hδ 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.

Constraining the top-light initial mass function in the extended ultraviolet disk of M 83

Context. The universality or non-universality of the initial mass function (IMF) has significant implications for determining star formation rates and star formation histories from photometric properties of stellar populations. Aims. We reexamine whether the IMF is deficient in high-mass stars (top-light) in the low-density environment of the outer disk of M 83 and constrain the shape of the IMF therein. Methods. Using archival Galaxy Evolution Explorer (GALEX) far ultraviolet (FUV) and near ultraviolet (NUV) data and new deep OmegaCAM narrowband Hα imaging, we constructed a catalog of FUV-selected objects in the outer disk of M 83. We counted Hα-bright clusters and clusters that are blue in FUV−NUV in the catalog, measured the maximum flux ratio FHα/fλFUV among the clusters, and measured the total flux ratio ΣFHα/ΣfλFUV over the catalog. We then compared these measurements to predictions from stellar population synthesis models made with a standard Salpeter IMF, truncated IMFs, and steep IMFs. We also investigated the effect of varying the assumed internal extinction on our results. Results. We are not able to reproduce our observations with models using the standard Salpeter IMF or the truncated IMFs. It is only when assuming an average internal extinction of 0.10 &lt; AV &lt; 0.15 in the outer disk stellar clusters that models with steep IMFs (α &gt; 3.1) simultaneously reproduce the observed cluster counts, the maximum observed FHα/fλFUV, and the observed ΣFHα/ΣfλFUV. Conclusions. Our results support a non-universal IMF that is deficient in high-mass stars in low-density environments.

Chemodynamical Nature of the Anticenter Stream and Monoceros Ring

In the epoch of deep photometric surveys, a large number of substructures—e.g., overdensities and streams—have been identified. With the help of astrometry and spectroscopy, the community has revealed a complex picture of our Milky Way (MW) after investigating their origins. The off-plane substructures the Anticenter Stream (ACS) and Monoceros Ring (MNC), once considered as dissolving dwarf galaxies, were later found to share similar kinematics and metallicity with the Galactic outer thin disk. In this work, we aim to chemically tag ACS and MNC with high-accuracy abundances from the APOGEE survey. By extrapolating chemical abundance trends in the outer thin-disk region (10 < R GC < 18 kpc, 0 < ∣Z GC∣ < 3 kpc), we found that ACS and MNC stars show consistent chemical abundances as the extrapolating values for 12 elements, including C, N, O, Mg, Al, Si, K, Ca, Cr, Mn, Co, and Ni. The similar chemical patterns indicate that ACS and MNC have a similar star formation history as the MW outer thin disk, while we also excluded their dwarf galaxy association, as they are distinctive in multiple chemical spaces. The ages of ACS and MNC stars are consistent with the time of the first Sagittarius dSph passage, indicating their possible connection.

Dark Sage: Next-generation semi-analytic galaxy evolution with multidimensional structure and minimal free parameters

Abstract After more than five years of development, we present a new version of Dark Sage, a semi-analytic model (SAM) of galaxy formation that breaks the mould for models of its kind. Included among the major changes is an overhauled treatment of stellar feedback that is derived from energy conservation, operates on local scales, affects gas gradually over time rather than instantaneously, and predicts a mass loading factor for every galaxy. Building on the model’s resolved angular momentum structure of galaxies, we now consider the heating of stellar discs, delivering predictions for disc structure both radially and vertically. We add a further dimension to stellar discs by tracking the distribution of stellar ages in each annulus. Each annulus–age bin has its own velocity dispersion and metallicity evolved in the model. This allows Dark Sage to make structural predictions for galaxies that previously only hydrodynamic simulations could. We present the model as run on the merger trees of the highest-resolution gravity-only simulation of the MillenniumTNG suite. Despite its additional complexity relative to other SAMs, Dark Sage only has three free parameters, the least of any SAM, which we calibrate exclusively against the cosmic star formation history and the $z = 0$ stellar and H i mass functions using a particle-swarm optimisation method. The Dark Sage codebase, written in C and python, is publicly available at https://github.com/arhstevens/DarkSage.

The UNCOVER Survey: A First-look HST+JWST Catalog of Galaxy Redshifts and Stellar Population Properties Spanning 0.2 ≲ z ≲ 15

The recent UNCOVER survey with the James Webb Space Telescope (JWST) exploits the nearby cluster A2744 to create the deepest view of our Universe to date by leveraging strong gravitational lensing. In this work, we perform photometric fitting of more than 50,000 robustly detected sources out to z ∼ 15. We show the redshift evolution of stellar ages, star formation rates, and rest-frame colors across the full range of 0.2 ≲ z ≲ 15. The galaxy properties are inferred using the Prospector Bayesian inference framework using informative Prospector-β priors on the masses and star formation histories to produce joint redshift and stellar populations posteriors. Additionally, lensing magnification is performed on the fly to ensure consistency with the scale-dependent priors. We show that this approach produces excellent photometric redshifts with σ NMAD ∼ 0.03, of a similar quality to the established photometric redshift code EAzY. In line with the open-source scientific objective of this Treasury survey, we publicly release the stellar population catalog with this paper, derived from our photometric catalog adapting aperture sizes based on source profiles. This release (the catalog and all related documentation are accessible via the UNCOVER survey web page: https://jwst-uncover.github.io/DR2.html#SPSCatalogs with a copy deposited to Zenodo at doi:10.5281/zenodo.8401181) includes posterior moments, maximum likelihood spectra, star formation histories, and full posterior distributions, offering a rich data set to explore the processes governing galaxy formation and evolution over a parameter space now accessible by JWST.

What Holes in the Gas Distribution of Nearly Face-on Galaxies Can Tell Us about the Host Disk Parameters: The Case of the NGC 628 Southeast Superbubble

Here we explore the impact of all major factors, such as the nonhomogeneous gas distribution, galactic rotation, and gravity, on the observational appearance of superbubbles in nearly face-on spiral galaxies. The results of our 3D numerical simulations are compared to the observed gas column density distribution in the largest southeast superbubble in the late-type spiral galaxy NGC 628. We make use of the star formation history inside the bubble derived from the resolved stellar population seen in Hubble Space Telescope images to obtain its energy and demonstrate that the results of numerical simulations are in good agreement with the observed gas surface density distribution. We also show that the observed gas column density distribution constrains the gaseous disk scale height and the midplane gas density if the energy input rate can be obtained from observations. This implies that observations of large holes in the interstellar gas distribution and their stellar populations have the potential power to solve the midplane gas density–gaseous disk scale height degeneracy problem in nearly face-on galaxies. The possible role of superbubbles in driving the secondary star formation in galaxies is also briefly discussed.

Pure Spectroscopic Constraints on UV Luminosity Functions and Cosmic Star Formation History from 25 Galaxies at z spec = 8.61–13.20 Confirmed with JWST/NIRSpec

We present pure spectroscopic constraints on the UV luminosity functions and cosmic star formation rate (SFR) densities from 25 galaxies at z spec = 8.61–13.20. By reducing the JWST/NIRSpec spectra taken in multiple programs of Early Release Observation, Early Release Science, General Observer, and Director’s Discretionary Time observations with our analysis technique, we independently confirm 16 galaxies at z spec = 8.61–11.40, including new redshift determinations, and a bright interloper at z spec = 4.91 that was claimed as a photometric candidate at z ∼ 16. In conjunction with nine galaxies at redshifts up to z spec = 13.20 in the literature, we make a sample of 25 spectroscopically confirmed galaxies in total and carefully derive the best estimates and lower limits of the UV luminosity functions. These UV luminosity function constraints are consistent with the previous photometric estimates within the uncertainties and indicate mild redshift evolution toward z ∼ 12, showing tensions with some theoretical models of rapid evolution. With these spectroscopic constraints, we obtain firm lower limits of the cosmic SFR densities and spectroscopically confirm a high SFR density at z ∼ 12 beyond the constant star formation efficiency models, which supports earlier claims from the photometric studies. While there are no spectroscopically confirmed galaxies with very large stellar masses violating the ΛCDM model due to the removal of the bright interloper, we confirm star-forming galaxies at z spec = 11–13 with stellar masses much higher than model predictions. Our results indicate possibilities of high star formation efficiency (>5%), a hidden active galactic nucleus, a top-heavy initial mass function (possibly with Population III), and large scatter/variance. Having these successful and unsuccessful spectroscopy results, we suggest observational strategies for efficiently removing low-redshift interlopers for future JWST programs.

SDSS-IV MaNGA: how do star formation histories affect gas-phase abundances?

ABSTRACT Gas-phase abundances in galaxies are the products of those galaxies’ evolutionary histories. The star formation history (SFH) of a region might therefore be expected to influence that region’s present day gaseous abundances. Here, we employ data from the MaNGA survey to explore how local gas metallicities relate to SFHs of galaxy regions. We combine MaNGA emission line measurements with SFH classifications from absorption line spectra to compare gas-phase abundances in star-forming regions with those in regions classified as starburst, post-starburst, and green valley. We find that starburst regions contain gas that is more pristine than in normal star-forming regions, in terms of O/H and N/O; we further find that post-starburst regions (which have experienced stochastic SFHs) behave very similarly to ordinary star-forming regions (which have experienced far smoother SFHs) in O/H–N/O space. We argue from this that gas is diluted significantly by pristine infall but is then re-enriched rapidly after a starburst event, making gas-phase abundances insensitive to the precise form of the SFH at late times. We also find that green valley regions possess slightly elevated N/O abundances at a given O/H; this is potentially due to a reduced star formation efficiency in such regions, but it could also point to late-time rejuvenation of green valley regions in our sample.

Bursty star formation and galaxy–galaxy interactions in low-mass galaxies 1 Gyr after the Big Bang

ABSTRACT We use CANUCS JWST/NIRCam imaging of galaxies behind the gravitationally-lensing cluster MACS J0417.5-1154 to investigate star formation burstiness in low-mass (M⋆ ∼ 108 M⊙) galaxies at z ∼ 4.7–6.5. Our sample of 123 galaxies is selected using the Lyman break selection and photometric emission-line excess methods. Sixty per cent of the 123 galaxies in this sample have Hα-to-UV flux ratios that deviate significantly from the range of Hα-to-UV ratio values consistent with smooth and steady star formation histories. This large fraction indicates that the majority of low-mass galaxies is experiencing bursty star formation histories at high redshift. We also searched for interacting galaxies in our sample and found that they are remarkably common ($\sim 40~{{\ \rm per\ cent}}$ of the sample). Compared to non-interacting galaxies, interacting galaxies are more likely to have very low Hα-to-UV ratios, suggesting that galaxy–galaxy interactions enhance star formation burstiness and enable faster quenching (with time-scales of ≲100 Myr) that follows the rapid rise of star formation activity. Given the high frequency of galaxy–galaxy interactions and the rapid SFR fluctuations they appear to cause, we conclude that galaxy–galaxy interactions could be a leading cause of bursty star formation in low-mass, high-z galaxies. They could thus play a significant role in the evolution of the galaxy population at early cosmological times.

EPOCHS IX. When cosmic dawn breaks: evidence for evolved stellar populations in 7 &amp;lt; z &amp;lt; 12 galaxies from PEARLS GTO and public NIRCam imaging

ABSTRACT The presence of evolved stars in high-redshift galaxies can place valuable indirect constraints on the onset of star formation in the Universe. Thus, we use PEARLS GTO (Prime Extragalactic Areas for Reionization and Lensing Science Guaranteed Time Observations) and public NIRCam (Near Infrared Camera) photometric data to search for Balmer-break candidate galaxies at 7 &amp;lt; z &amp;lt; 12. We find that our Balmer-break candidates at z ∼ 10.5 tend to be older (115 Myr), have lower inferred [O iii] + Hβ equivalent widths (120 Å), have lower specific star formation rates (6 Gyr−1) and redder UV slopes (β = −1.8) than our control sample of galaxies. However, these trends all become less strong at z ∼ 8, where the F444W filter now probes the strong rest-frame optical emission lines, thus providing additional constraints on the current star formation activity of these galaxies. Indeed, the bursty nature of epoch of reionization galaxies can lead to a disconnect between their current spectral energy distribution (SED) profiles and their more extended star formation histories. We discuss how strong emission lines, the cumulative effect of weak emission lines, dusty continua, and active galactic nuclei can all contribute to the photometric excess seen in the rest-frame optical, thus mimicking the signature of a Balmer break. Additional medium-band imaging will thus be essential to more robustly identify Balmer-break galaxies. However, the Balmer break alone cannot serve as a definitive proxy for the stellar age of galaxies, being complexly dependent on the star formation history. Ultimately, deep Near Infrared Spectrograph (NIRSpec) continuum spectroscopy and Mid-Infrared Instrument (MIRI) imaging will provide the strongest indirect constraints on the formation era of the first galaxies in the Universe, thereby revealing when cosmic dawn breaks.

Spitzer-selected z &amp;gt; 1.3 protocluster candidates in the LSST Deep Drilling Fields

ABSTRACT We have identified 189 candidate z &amp;gt; 1.3 protoclusters and clusters in the LSST Deep Drilling Fields. This sample will enable the measurement of the metal enrichment and star formation history of clusters during their early assembly period through the direct measurement of the rate of supernovae identified through the LSST. The protocluster sample was selected from galaxy overdensities in a Spitzer/IRAC colour-selected sample using criteria that were optimized for protocluster purity using a realistic light-cone. Our tests reveal that $60\!-\!80~{{\ \rm per\ cent}}$ of the identified candidates are likely to be genuine protoclusters or clusters, which is corroborated by a ∼4σ stacked X-ray signal from these structures. We provide photometric redshift estimates for 47 candidates which exhibit strong peaks in the photo-z distribution of their candidate members. However, the lack of a photo-z peak does not mean a candidate is not genuine, since we find a stacked X-ray signal of similar significance from both the candidates that exhibit photo-z peaks and those that do not. Tests on the light-cone reveal that our pursuit of a pure sample of protoclusters results in that sample being highly incomplete ($\sim 4~{{\ \rm per\ cent}}$) and heavily biased towards larger, richer, more massive, and more centrally concentrated protoclusters than the total protocluster population. Most ($\sim 75~{{\ \rm per\ cent}}$) of the selected protoclusters are likely to have a maximum collapsed halo mass of between 1013 and 1014 M⊙, with only $\sim 25~{{\ \rm per\ cent}}$ likely to be collapsed clusters above 1014 M⊙. However, the aforementioned bias ensures our sample is $\sim 50~{{\ \rm per\ cent}}$ complete for structures that have already collapsed into clusters more massive than 1014 M⊙.

The SAMI-Fornax Dwarfs Survey – IV. Star formation histories of dwarf and early-type galaxies: insights from full spectral fitting

ABSTRACT We present a study on the star formation histories (SFHs) of galaxies covering the range 104 &amp;lt; M⋆/M⊙ &amp;lt; 1012, leveraging full spectral fitting algorithms. Our sample consists of 31 dwarf galaxies from the SAMI-Fornax Survey with stellar masses between 107–$10^{9.5} \, {\rm M}_{\odot }$, early-type galaxies from the ATLAS3D project with stellar masses between 1010–$10^{12} \, {\rm M}_{\odot }$, and dwarf galaxies that are satellites of Andromeda and the Milky Way, with 104 &amp;lt; M⋆/M⊙ &amp;lt; 108. We find that galaxies from 107–$10^{8} \, {\rm M}_{\odot }$ exhibit the smallest star formation rates (SFRs), while the SFR increase as we move down or up in mass. In this sense, we find that some $10^{5} \, {\rm M}_{\odot }$ galaxies have cumulative SFHs that are comparable to those of $10^{12} \, {\rm M}_{\odot }$ galaxies. Our study shows that the evolution of giant galaxies is primarily governed by their internal properties, with time-scales that do not depend on their environmental location. In contrast, dwarf galaxies below $10^{8} \, {\rm M}_{\odot }$ can be significantly affected in dense environments, such as the inner regions of a cluster, that severely quench the galaxies before the assembly of their 50 per cent present-day mass. We find that, only dwarfs with stellar masses between 107–$10^{9} \, {\rm M}_{\odot }$ actively form stars nowadays, while less massive galaxies seem to remain unaffected by the environment due to the expulsion of most of their gas at an early stage in their evolution. Our study highlights and corroborates a critical threshold around $10^{8}-10^{9} \, {\rm M}_{\odot }$ in galaxy evolution from previous studies, separating more massive galaxies minimally impacted by the environment from those less massive galaxies quenched by it.