Are the most metal-poor galaxies young?

Local low-metallicity galaxies with signatures of possible accretion activity are ideal laboratories in which to search for the lowest-mass black holes and study their impact on the host galaxy. Here we present the first JWST NIRSpec IFS observations of SDSS J120122.30+021108.3, a nearby (z = 0.00354) extremely metal-poor dwarf galaxy with no optical signatures of accretion activity but identified by the Wide-field Infrared Survey Explorer to have red mid-infrared (MIR) colors consistent with active galactic nuclei (AGNs). We identify over 100 lines between ∼1.7 and 5.2 μm, an unresolved nuclear continuum source with an extremely steep spectral slope consistent with hot dust from an AGN (F ν ≈ ν −1.5), and a plethora of H i, He i, and H2 lines, with no lines from heavier elements, CO or ice absorption features, or polycyclic aromatic hydrocarbons (PAHs). While the bright central unresolved source (≲5 pc) is suggestive of an AGN, there are no He ii lines or coronal lines identified in the spectrum, and, importantly, there is no evidence that the radiation field is harder in the nuclear source compared with surrounding regions. The emission-line spectrum can be explained by a young (<5 Myr) nuclear star cluster with stellar mass ∼3 × 104 M ⊙, but the unresolved continuum source is more typical for a deeply embedded AGN powered by a black hole of minimum mass ∼1450 M ⊙. These observations reveal that either a metal-poor stellar population can heat the dust to extremely high temperatures, a result that would have significant impact on the reliability of MIR color selection in AGN surveys and our understanding of the properties of the interstellar medium and young stars in metal-poor galaxies, or an accreting intermediate-mass black hole is deeply embedded and hidden even at near-infrared wavelengths.

Context. To understand the structure and evolution of massive stars, systematic surveys of the Local Group galaxies have been undertaken, to find these objects in environments of different chemical abundances. We focus on the metal-poor irregular galaxy IC 1613 to analyze the stellar and wind structure of its low-metallicity massive stars. We ultimately aim to study the metallicity-dependent driving mechanism of the winds of blue massive stars and use metal-poor massive stars of the Local Volume as a proxy for the stars in the early Universe.Aims. In a previous paper we produced a list of OB associations in IC 1613. Their properties are not only a powerful aid towards finding the most interesting candidate massive stars, but also reveal the structure and recent star formation history of the galaxy. We characterize these OB associations and study their connection with the galactic global properties.Methods. The reddening-free Q parameter is a powerful tool in the photometric analysis of young populations of massive stars, since it exhibits a smaller degree of degeneracy with OB spectral types than the B −V color. The color–magnitude diagram (Q vs. V ) of the OB associations in IC 1613 is studied to determine their age and mass, and confirm the population of young massive stars.Results. We identified more than 10 stars with M ≥ 50 M ⊙ . Spectral classification available for some of them confirm their massive nature, yet we find the common discrepancy with the spectroscopically derived masses. There is a general increasing trend of the mass of the most massive member with the number of members of each association, but not with the stellar density. The average diameter of the associations of this catalog is 40 pc, half the historically considered typical size of OB associations. Size increases with the association population. The distribution of the groups strongly correlates with that of neutral and ionized hydrogen. We find the largest dispersion of association ages in the bubble region of the galaxy where hydrogen is abundant, implying that recent star formation has proceeded over a longer period of time than in the rest of the galaxy, and is still ongoing. Very young associations are found at the west of the galaxy far from the bubble region, traditionally considered the sole locus of star formation, but still rich in neutral hydrogen. Conclusions. The contrast in the stellar properties derived from photometry and spectroscopy (when the latter is available) shows that the Q pseudo–color is very useful for estimating the parameters of OB stars when only photometric observations exist. This work helped define an extensive pool of candidate OB stars for subsequent spectroscopic analyses designed to study the structure and winds of metal-poor massive stars.

Using the optical spectrum obtained with the Kitt Peak Ohio State Multi-Object Spectrograph mounted on the Apache Point Observatory 3.5 m Telescope and the Sloan Digital Sky Survey spectrum, we study the properties of one of the most metal-poor dwarf star-forming galaxies (SFGs) in the local Universe, J1046+4047. The galaxy, with a redshift z = 0.04874, was selected from the Data Release 16 of the SDSS. Its properties are among the most extreme for SFGs in several ways. The oxygen abundance 12 + log(O/H) = 7.082 ± 0.016 in J1046+4047 is among the lowest ever observed. With an absolute magnitude Mg = −16.51 mag, a low stellar mass M⋆ = 1.8 × 106 M⊙, and a very low mass-to-light ratio M⋆/Lg ∼ 0.0029 (in solar units), J1046+4047 has a very high specific star formation rate sSFR ∼ 430 Gyr−1, indicating very active ongoing star formation. Another striking feature of J1046+4047 is that it possesses a ratio O32 = I([O iii] λ5007)/I([O ii] λ3727) ∼ 57. Using this extremely high O32, we have confirmed and improved the strong-line calibration for the determination of oxygen abundances in the most metal-deficient galaxies, in the range 12 + log(O/H) ≲ 7.65. This improved method is applicable for all galaxies with O32 ≤ 60. We find the H α emission line in J1046+4047 to be enhanced by some non-recombination processes and thus cannot be used for the determination of interstellar extinction.

Context. Local galaxies follow scaling relations between their mass and stellar population properties, such as age and metallicity. These relations encode fundamental information about the past evolutionary history of galaxies. Aims. We want to revise stellar population scaling relations of local galaxies by leveraging the largest spectroscopic dataset provided by Data Release Seven of the Sloan Digital Sky Survey (SDSS DR7) at 0.005 < z < 0.22, using improved stellar population synthesis (SPS) models and novel SDSS aperture bias corrections. Methods. We applied statistical weights to account for selection biases and implemented corrections to the SDSS fibre aperture limitations. Within a Bayesian framework, we estimated the stellar masses, mean stellar ages, and mean stellar metallicities by comparing the spectral indices and photometry with composite stellar population models. We adopted state-of-the-art ingredients and updated prescriptions to better capture the complexity of galaxies star formation and chemical enrichment histories. We also tested different models and priors. Results. We estimated light-weighted mean stellar ages for 354 977 galaxies ( S/N ≥ 10) and metallicities for 89 852 galaxies ( S/N ≥ 20), and studied their dependence on stellar mass. Our key findings include: i) a revised bimodal distribution in the mass-age plane, with a young sequence (dominant at low masses) and an old sequence (dominant at high masses), partly overlapping in mass and transitioning at M tr = 10 10.80 M ⊙ ; ii) a mass-metallicity relation (MZR) shifted ∼0.2 dex higher than in previous studies. Our aperture corrections produce mass-dependent reductions in masses, ages, and metallicities, enhancing the young sequence and steepening the MZR at low masses; iii) using H α -based star formation rate classification, we found that while star-forming+young and quiescent+old correspondences generally hold, some exceptions exist for many galaxies. Quiescent galaxies show flatter, less scattered MZR than star-forming ones, converging at high masses; and, finally, iv) different SPS modelling assumptions significantly impact results, with star formation and chemical enrichment histories having the strongest effects. Conclusions. These revised relations provide new benchmarks for galaxy evolution studies and simulations. Systematic effects of 0.1 − 0.2 dex can arise from uncorrected aperture biases and different SPS modelling choices. Consistent assumptions should be applied when comparing observations and models.

Time is one of the most fundamental and common of all conceptions; it is also one of the most illusive. The qualifying words, “from the astronomer's standpoint” have been added in order to warn off all optimistic philosophers who might otherwise be led to expect that what is to follow will contain learned remarks upon abstruse aspects of the subject of time, such as these—What is time? Has it an absolute quality, or is it purely relative in the sense in which the theory of relativity regards both it and space? Is time directional? How does the mind of man become conscious of the passing of time? Ought we to regard entropy as “time's arrow” in the physical world, as has been suggested by Sir A. S. Eddington? Are time past and time future equally real in the sense in which the passing moment which at a certain critical instant we label the present, is real? These are problems which very few people can discuss to the advantage of others.

We present Hubble Space Telescope (HST) and Spitzer Space Telescope images and photometry of the extremely metal-poor (Z 0.03 Z☉) blue dwarf galaxy CGCG 269–049. The HST images reveal a large population of red giant and asymptotic giant branch stars, ruling out the possibility that the galaxy has recently formed. From the magnitude of the tip of the red giant branch, we measure a distance to CGCG 269–049 of only 4.9 ± 0.4 Mpc. The spectral energy distribution of the galaxy between ~3.6-70 μm is also best fitted by emission from predominantly ~10 Gyr old stars, with a component of thermal dust emission having a temperature of 52 ± 10 K. The HST and Spitzer photometry indicate that more than 60% of the CGCG 269–049 stellar mass consists of stars ~1-10 Gyr old, similar to other local blue dwarf galaxies. Our HST Hα image shows no evidence of a supernova-driven outflow that could be removing metals from the galaxy, nor do we find evidence that such outflows occurred in the past. Taken together with the CGCG 269–049 large ratio of neutral hydrogen mass to stellar mass (~10), these results are consistent with recent simulations in which the metal deficiency of local dwarf galaxies results mainly from inefficient star formation, rather than youth or the escape of supernova ejecta.

In the local Universe, 10% of massive elliptical galaxies are observed to exhibit a peculiar property: a substantial excess of ultraviolet emission (UVX) over what is expected from their old, red stellar populations. Several origins for the UVX have been proposed, including a population of hot young stars, or a population of old, blue horizontal branch or extended horizontal branch (BHB or EHB) stars that have undergone substantial mass loss from their outer atmospheres. We explore the radial distribution of ultraviolet excess (UVX) in a selection of 49 nearby E/S0-type galaxies by measuring the extended photometry in the UV-midIR with GALEX, SDSS and WISE. We compare UV/optical and UV/mid-IR colors with the Flexible Stellar Population Synthesis with EHB models (Conroy & Gunn 2010). We find that combined WISE mid-IR and GALEX UV colors are more effective in distinguishing models than optical colors, and that the UV/mid-IR combination is sensitive to EHB fraction. There are strong color gradients with the outer radii bluer than the inner half-light radii by ~1 magnitude. This color difference is easily accounted for with a BHB fraction increase of 0.25 with radius. We estimated the average ages for the inner and outer radii are 7.0+/-0.3 Gyr, and 6.2+/-0.2 Gyr, respectively, with the implication that the outer regions are likely to have formed ~1 Gyr after the inner regions. Additionally, we find that metallicity gradients are likely not a significant factor in the color difference. The separation of color between the inner and outer regions, which agrees with a specific stellar population difference (e.g., higher EHB populations), and the ~0.5-2 Gyr age difference suggests multi-stage formation. Our results are best explained by inside-out formation: rapid star formation within the core at early epochs (>4 Gyr ago) and at least one later stage starburst event coinciding with z~1

In order to know the formation epoch of the oldest elliptical galaxies as a function of mass and observed redshift, a statistical analysis of 333 extremely red objects (EROs) classified as old galaxies (OGs) at 0.8 {<=} z {<=} 2.3 is carried out. Once we get M{sub V} and (B - V) at rest for each galaxy, we calculate the average variation of this intrinsic color with redshift and derive the average age through a synthesis model (the code for the calculation of the age has been made publicly available). The average gradient of the (B - V) color at rest of EROs/OGs is 0.07-0.10 Gyr{sup -1} for a fixed luminosity. The stars in these extremely red elliptical galaxies were formed when the universe was {approx} 2 Gyr old on average. We have not found a significant enough dependence on the observed redshift and stellar mass. This fits a scenario in which the stellar formation of the objects that we denominate as EROs-OGs is more intense at higher redshifts, at which the stellar populations of the most massive galaxies form earlier than or at the same time as less massive galaxies.

From several searches of the area common to the Sloan Digital Sky Survey and\nthe United Kingdom Infrared Telescope Infrared Deep Sky Survey, we have\nselected 22 luminous galaxies between $z \\sim$ 0.4 and $z \\sim$ 0.9 that have\ncolors and sizes similar to those of the compact quiescent galaxies at $z&gt;2$.\nBy exploring structural parameters and stellar populations, we found that most\nof these galaxies actually formed most of their stars at $z&lt;2$ and are\ngenerally less compact than those found at $z &gt; 2$. Several of these young\nobjects are disk-like or possibly prolate. This lines up with several previous\nstudies which found that massive quiescent galaxies at high redshifts often\nhave disk-like morphologies. If these galaxies were to be confirmed to be\ndisk-like, their formation mechanism must be able to account for both\ncompactness and disks. On the other hand, if these galaxies were to be\nconfirmed to be prolate, the fact that prolate galaxies do not exist in the\nlocal universe would indicate that galaxy formation mechanisms have evolved\nover cosmic time. We also found five galaxies forming over 80% of their stellar\nmasses at $z&gt;2$. Three of these galaxies appear to have been modified to have\nspheroid-like morphologies, in agreement with the scenario of "inside-out"\nbuildup of massive galaxies. The remaining galaxies, SDSS\\,J014355.21+133451.4\nand SDSS\\,J115836.93+021535.1, have truly old stellar populations and disk-like\nmorphologies. These two objects would be good candidates for nearly unmodified\ncompact quiescent galaxies from high redshifts that are worth future study.\n

Context. The radial variations of the stellar populations properties within passive galaxies at high redshift contain information about their assembly mechanisms, based on which galaxy formation and evolution scenarios may be distinguished. Aims. The aim of this work is to give constraints on massive galaxy formation scenarios through one of the first analyses of age and metallicity gradients of the stellar populations in a sample of passive galaxies at z &gt; 1.6 based on spectroscopic data from the Hubble Space Telescope. Methods. We combined G141 deep slitless spectroscopic data and F160W photometric data of the spectroscopically passive galaxies at 1.6 &lt; z &lt; 2.4 with H160 &lt; 22.0 in the field of view of the cluster JKCS 041. We extracted spectra from different zones of the galaxies, and we analysed them by fitting them with a library of synthetic templates of stellar population models to obtain estimates of the age and metallicity gradients. Results. We obtained reliable measurements of age and metallicity parameters in different spatial zones of four galaxies. We performed spatially resolved measurements in individual high-redshift galaxies without the need of peculiar situations (i.e. gravitational lensing) for the first time. All four galaxies exhibit negative metallicity gradients. Their amplitude, similar to that measured in galaxies in the local Universe, suggests that the stellar populations of passive galaxies from z ∼ 2 to z = 0 are not redistributed. Conclusions. Although the sample we analysed is small, the results we obtained suggest that the main mechanism that determines the spatial distribution of the stellar population properties within passive galaxies is constrained in the first 3 Gyr of the Universe. This is consistent with the revised monolithic scenario.

The delay time distribution (DTD) of type Ia supernovae (SNe Ia) from star formation is an important clue to reveal the still unknown progenitor system of SNe Ia. Here, we report on a measurement of the SN Ia DTD in a delay time range of $t_{\rm Ia} = $ 0.1-8.0Gyr by using faint variable objects detected in the Subaru/XMM-Newton Deep Survey (SXDS) down to $i'$$\sim$ 25.5. We selected 65 SN candidates showing significant spatial offset from the nuclei of the host galaxies having an old stellar population at $z$$\sim$ 0.4-1.2, out of more than 1000 SXDS variable objects. Although spectroscopic type classification is not available for these, we quantitatively demonstrated that more than $\sim$80% of these should be SNe Ia. The DTD was derived using stellar age estimates of the old galaxies based on 9 band photometries from optical to mid-infrared wavelength. Combined with the observed SN Ia rate in elliptical galaxies at the local universe, the DTD in $t_{\rm Ia}$$\sim$ 0.1-10Gyr is well described by a featureless power-law as $f_{\rm D}$($t_{\rm Ia}$) $\propto$$t_{\rm Ia}^\alpha$ with $\alpha$$\sim$$-$1. The derived DTD is in excellent agreement with a generic prediction of the double-degenerate scenario, giving strong support to this scenario. In the single-degenerate (SD) scenario, although predictions by simple analytic formulations have broad DTD shapes that are similar to the observation, DTD shapes calculated by more detailed binary population synthesis tend to have strong peaks at characteristic time scales, which do not fit the observation. This result thus indicates either that the SD channel is not the major contributor to SNe Ia in an old stellar population, or that an improvement of binary population synthesis theory is required. Various sources of systematic uncertainties were examined and tested, but our main conclusions were not affected significantly.

Clusters of galaxies represent the largest laboratories in the universe for testing the incredibly chaotic physics governing the collapse of baryons into the stars, galaxies, groups, and diffuse clouds that we see today. Within the cluster environment, there are a wide variety of physical processes that may be acting to transform galaxies. In this thesis, we combine extensive Keck spectroscopy with wide-field HST imaging to perform a detailed case study of two intermediate redshift galaxy clusters, Cl 0024+1654 (z=0.395) and MS 0451-03 (z=0.540). Leveraging a comprehensive multiwavelength data set that spans the X-ray to infrared, and with spectral-line measurements serving as the key to revealing both the recent star-formation histories and kinematics of infalling galaxies, we aim to shed light on the environmental processes that could be acting to transform galaxies in clusters. We adopt a strategy to make maximal use of our HST-based morphologies by splitting our sample of cluster galaxies according to morphological type, characterizing signs of recent evolution in and early types separately. This approach proves to be powerful in identifying galaxies that are currently being altered by an environmental interaction: early-type galaxies that have either been newly transformed or prodded back into an active phase, and spiral galaxies where star formation is being suppressed or enhanced all stand out in our sample. We begin by using variations in the early-type galaxy population as indicators of recent activity. Because ellipticals and S0s form such a homogeneous class in the local universe, we are sensitive to even very subtle signatures of recent and current environmental interactions. This study has yielded two key results: By constructing the Fundamental Plane (FP) of Cl 0024, we observe that elliptical and S0 galaxies exhibit a high scatter in their FP residuals, which occurs only among galaxies in the cluster core, suggesting a turbulent assembly history for Cl 0024 early types. Near the Virial radius of Cl 0024, we observe a number of compact, intermediate-mass ellipticals undergoing a burst of star formation or weak AGN activity, indicated by strong [O II] emission; their locations may mark the minimum radius at which merging is effective in each cluster. While E+S0 galaxies do prove to be sensitive indicators of environmental interaction, it is the spiral galaxies that, of course, host the bulk of star formation within and around these clusters. We therefore probe for kinematic disturbances in spiral disks by measuring resolved rotation curves from optical emission lines, and constructing the Tully-Fisher relation for across Cl 0024 and MS 0451. We find that the cluster Tully-Fisher relation exhibits significantly higher scatter than the field relation. In probing for the origin of this difference, we find that the central mass densities of star-forming exhibit a sharp break near the cluster Virial radius, with in the cluster outskirts exhibiting significantly lower densities. We argue that these results considered together demonstrate that cluster are kinematically disturbed by their environment, likely due to galaxy-galaxy interactions (harassment). We then discuss our most powerful method of tracking galaxy evolution across Cl 0024 and MS 0451: identifying and studying transition galaxies-galaxies whose stellar populations or dynamical states indicate a recent or ongoing change in morphology or star formation rate. Such galaxies are often revealed by star formation histories that seem to be at odds with the galaxy morphologies: for example, spiral galaxies with no signs of star formation, or elliptical galaxies that do show signs of star formation. We identify and study one such class of objects, the spirals in Cl 0024. These objects exhibit no emission lines in their spectra, suggesting a lack of star formation, yet are surprisingly detected in the UV, revealing the presence of young stars. By modeling the different temporal sensitivities of UV and spectroscopic data to recent activity, we show that star formation in Cl 0024 passive has decayed on timescales of less than 1 Gyr, consistent with the action of gas starvation. We then build on and link together our previous indications of galaxy evolution at work, aiming to piece together a more comprehensive picture of how cluster galaxies are affected by their environment at intermediate redshift. To accomplish this, we document what we believe to be the first direct evidence for the transformation of into S0s: through an analysis of their stellar populations and recent star formation rates, we link the passive spiral galaxies in both clusters to their eventual end states as newly generated cluster S0 galaxies. Differences between the two clusters in both the timescales and spatial location of this conversion process allow us to evaluate the relative importance of several proposed physical mechanisms that could be responsible for the transformation. Combined with other diagnostics that are sensitive to either ICM-driven galaxy evolution or galaxy-galaxy interactions, we describe a self-consistent picture of galaxy evolution in clusters. We find that spiral galaxies within infalling groups have already begun a slow process of conversion into S0s primarily via gentle galaxy-galaxy interactions that act to quench star formation. The fates of upon reaching the core of the cluster depend heavily on the cluster ICM, with rapid conversion of all remaining into S0s via ram-pressure stripping in clusters where the ICM is dense. In the presence of a less-dense ICM, the conversion continues at a slower pace, with galaxy-galaxy interactions continuing to play a role along with starvation by the ICM. We conclude that the buildup of the local S0 population through the transformation of spiral galaxies is a heterogeneous process that nevertheless proceeds robustly across a variety of different environments from cluster outskirts to cores.

We present a JWST imaging survey of I Zw 18, the archetypal extremely metal-poor, star-forming (SF), blue compact dwarf galaxy. With an oxygen abundance of only ∼3% Z ⊙, it is among the lowest-metallicity systems known in the local Universe, and is, therefore, an excellent accessible analog for the galactic building blocks which existed at early epochs of ionization and star formation. These JWST data provide a comprehensive infrared (IR) view of I Zw 18 with eight filters utilizing both Near Infrared Camera (F115W, F200W, F356W, and F444W) and Mid-Infrared Instrument (F770W, F1000W, F1500W, and F1800W) photometry, which we have used to identify key stellar populations that are bright in the near- and mid-IR. These data allow for a better understanding of the origins of dust and dust-production mechanisms in metal-poor environments by characterizing the population of massive, evolved stars in the red supergiant (RSG) and asymptotic giant branch (AGB) phases. In addition, it enables the identification of the brightest dust-enshrouded young stellar objects (YSOs), which provide insight into the formation of massive stars at extremely low metallicities typical of the very early Universe. This paper provides an overview of the observational strategy and data processing, and presents first science results, including identifications of dusty AGB, RSG, and bright YSO candidates. These first results assess the scientific quality of JWST data and provide a guide for obtaining and interpreting future observations of the dusty and evolved stars inhabiting compact dwarf SF galaxies in the local Universe.

We review the formation and evolution of fossil groups and clusters from both the theoretical and the observational points of view. In the optical band, these systems are dominated by the light of the central galaxy. They were interpreted as old systems that had enough time to merge all the M* galaxies within the central one. During the last two decades, many observational studies were performed to prove the old and relaxed state of fossil systems. The majority of these studies that spans a wide range of topics including halos global scaling relations, dynamical substructures, stellar populations, and galaxy luminosity functions seem to challenge this scenario. The general picture that can be obtained by reviewing all the observational works is that the fossil state could be transitional. Indeed, the formation of the large magnitude gap observed in fossil systems could be related to internal processes rather than an old formation.

The cosmological numerical simulations tell us that accretion of external metal-poor gas drives star formation (SF) in galaxy disks. One the best pieces of observational evidence supporting this prediction is the existence of low-metallicity star-forming regions in relatively high-metallicity host galaxies. The SF is thought to be fed by metal-poor gas recently accreted. Since the gas accretion is stochastic, there should be galaxies with all the properties of a host but without the low-metallicity starburst. These galaxies have not been identified yet. The exception may be UGC 2162, a nearby ultra-diffuse galaxy (UDG) that combines low surface brightness and relatively high metallicity. We confirm the high metallicity of UGC 2162 ( ) using spectra taken with the 10 m GTC telescope. UGC 2162 has the stellar mass, metallicity, and star formation rate surface density expected for a host galaxy in between outbursts. This fact suggests a physical connection between some UDGs and metal-poor galaxies, which may be the same type of object in a different phase of the SF cycle. UGC 2162 is a high-metallicity outlier of the mass–metallicity relation, a property shared by the few UDGs with known gas-phase metallicity.