Recent Star Formation History Research Articles

UVIT Study of the MAgellanic Clouds (U-SMAC) – I. Recent star formation history and kinematics of the Shell region in the north-eastern Small Magellanic Cloud

ABSTRACT The interactions between the Magellanic Clouds significantly affect the shape and distribution of the young stellar population, particularly in the periphery of the Small Magellanic Cloud (SMC). We present the first far-UV (FUV) map of the north-east SMC-Shell region using the Ultra Violet Imaging Telescope (UVIT) onboard AstroSat. The detected FUV stars are combined with Gaia Early Data Release 3 data to create a FUV–optical catalogue of ∼14 400 stars. FUV–optical colour-magnitude diagrams are used along with isochrones to estimate the stellar ages. The detected stars are formed in multiple episodes. We identified two episodes of star formation (∼60 and ∼260 Myr ago), where the episode at ∼260 Myr is linked to the recent interaction with the Large Magellanic Cloud (LMC) and the episode at ∼60 Myr is linked to the pericentric passage of the SMC around our Galaxy. The median proper motion (PM) and velocity dispersion are found to be similar to the SMC main body, indicating that this region has not experienced significant tidal effects. The FUV stellar surface density and the dispersion in PM suggest that the extent of the inner SMC in the north-east direction to be around 2.2°. We detect arm-like and arc-like structures in the FUV stellar density map, and their kinematics appear to be similar to the SMC main body. These extended outer features are the spatial stellar overdensities formed over multiple episodes of star formation, but without apparent kinematic distinction.

Most nearby young star clusters formed in three massive complexes.

Efforts to unveil the structure of the local interstellar medium and its recent star-formation history have spanned the past 70 years (refs. 1-6). Recent studies using precise data from space astrometry missions have revealed nearby, newly formed star clusters with connected origins7-12. Nonetheless, mapping young clusters across the entire sky back to their natal regions has been hindered by a lack of clusters with precise radial-velocity data. Here we show that 155 out of 272 (57%) high-quality young clusters13,14 within 1 kiloparsec of the Sun arise from three distinct spatial volumes. This conclusion is based on the analysis of data from the third Gaia release15 and other large-scale spectroscopic surveys. At present, dispersed throughout the solar neighbourhood, their past positions more than 30 million yearsago reveal that these families of clusters each formed in one of three compact, massive star-forming complexes. One of these families includes all of the young clusters near the Sun-the Taurus and Scorpius-Centaurus star-forming complexes16,17. We estimate that more than 200 supernovae were produced from these families and argue that these clustered supernovae produced both the Local Bubble18 and the largest nearby supershell GSH 238+00+09 (ref. 19), both of which are clearly visible in modern three-dimensional dust maps20-22.

Identification of a transition from stochastic to secular star formation around z = 9 with JWST

Star formation histories (SFHs) of early galaxies (6 < z < 12) have been found to be highly stochastic in both simulations and observations, while at z≲6 the presence of a main sequence (MS) of star-forming galaxies implies secular processes at play. In this work we characterise the SFH variability of early galaxies as a function of their stellar mass and redshift. We used the JADES public catalogue and derived the physical properties of the galaxies as well as their SFHs using the spectral energy distribution modelling code CIGALE. To this end, we implemented a non-parametric SFH with a flat prior allowing for as much stochasticity as possible. We used the star formation rate (SFR) gradient, an indicator of the movement of galaxies on the SFR–M* plane, linked to the recent SFH of galaxies. This dynamical approach of the relation between the SFR and stellar mass allows us to show that, at z > 9, 87% of massive galaxies (log(M*/M⊙)≳9) have SFR gradients consistent with a stochastic star formation activity during the last 100 Myr, while this fraction drops to 15% at z < 7. On the other hand, we see an increasing fraction of galaxies with a star formation activity following a common stream on the SFR–M* plane with cosmic time, indicating that a secular mode of star formation is emerging. We place our results in the context of the observed excess of UV emission as probed by the UV luminosity function at z ≳ 10 by estimating σUV, the dispersion of the UV absolute magnitude distribution, to be of the order of 1.2 mag, and compare it with predictions from the literature. In conclusion, we find a transition of star formation mode happening around z ∼ 9: Galaxies with stochastic SFHs dominate at z ≳ 9, although this level of stochasticity is too low to reach those invoked by recent models to reproduce the observed UV luminosity function.

The recent star formation histories of nearby galaxies on resolved scales

ABSTRACT Star formation histories (SFHs) of galaxies are affected by a variety of factors, both external (field versus cluster/group) and internal [presence of a bar and active galactic nucleus (AGN), morphological type]. In this work, we extend our previous study and apply the $\langle \mathrm{SFR}_{\textnormal {5}} \rangle \big / \langle \mathrm{SFR}_{\textnormal {200}} \rangle$ metric to a sample of 11 nearby galaxies with Multi-Unit Spectroscopic Explorer observations. Based on a combination of H α and ultraviolet photometry, $\langle \mathrm{SFR}_{\textnormal {5}} \rangle \big / \langle \mathrm{SFR}_{\textnormal {200}} \rangle$ is sensitive to star formation time-scales of ∼5–200 Myr and therefore measures the present-day rate of change in the star formation rate (SFR), dSFR/dt. Within this limited galaxy sample, we do not observe systematic variations between the global value of $\langle \mathrm{SFR}_{\textnormal {5}} \rangle \big / \langle \mathrm{SFR}_{\textnormal {200}} \rangle$ and the presence of an AGN, stellar bar, or group or cluster membership. Within some of the individual galaxies, we, however, observe significant differences in $\langle \mathrm{SFR}_{\textnormal {5}} \rangle \big / \langle \mathrm{SFR}_{\textnormal {200}} \rangle$ between the arm and interarm regions. In half of the galaxies, the recent SFH of both arm and interarm regions has been very similar. However, in the galaxies with higher bulge-to-total light ratios and earlier morphological type, the SFR is declining more rapidly in the interarm regions. This decline in SFR is not a result of low molecular gas surface density or a decrease in the star formation efficiency, implying that other factors are responsible for this SFR decrease.

Resolving the Mechanical and Radiative Feedback in J1044+0353 with Keck Cosmic Web Imager Spectral Mapping

We present integral field spectroscopy toward and around J1044+0353, a rapidly growing, low-metallicity galaxy that produces extreme [O iii] line emission. A new map of the O32 flux ratio reveals a density-bounded ionization cone emerging from the starburst. The interaction of the hydrogen-ionizing radiation, produced by the very young starburst, with a cavity previously carved out by a galactic outflow, whose apex lies well outside the starburst region, determines the pathway for global Lyman continuum (LyC) escape. In the region within a few hundred parsecs of the young starburst, we demonstrate that superbubble breakthrough and blowout contribute distinct components to the [O iii] line profile: broad and very broad emission line wings, respectively. We draw attention to the large [O iii] luminosity of the broad component and argue that this emission comes from photoionized, superbubble shells rather than a galactic wind as is often assumed. The spatially resolved He ii λ4686 nebula appears to be photoionized by young star clusters. Stellar wind emission from these stars is likely the source of line wings detected on the He ii line profile. This broader He ii component indicates slow stellar winds, consistent with an increase in stellar rotation (and a decrease in effective escape speed) at the metallicity of J1044+0353. At least in J1044+0353, the recent star formation history plays a critical role in generating a global pathway for LyC escape, and the anisotropic escape would likely be missed by direct observations of the LyC.

The Star Formation History in Local Group Galaxies. I. Ten Dwarf Galaxies

The star formation histories (SFHs) of galaxies provide valuable insights into galaxy evolution and stellar physics. Understanding the SFHs enables the study of chemical enrichment of galaxies, star formation triggered by interactions, and the behavior of various stellar populations. This work investigates the SFHs of ten dwarf galaxies in the Local Group (LG), which spans a wide range of types, masses, luminosities, and metallicities. The analysis is based on our new sample of the member stars in the LG after removing foreground dwarf stars by using information from the near-infrared color–color diagram and Gaia astrometry. The sample includes the most complete and pure red supergiants and asymptotic giant branch stars to gain valuable insights into the recent SFHs of the galaxies. A fitting method based on the color–magnitude diagram (CMD) is introduced to measure the SFH. The Padova isochrones are used to generate initial model CMDs, accounting for photometric errors and completeness through star-field simulations to match the completeness and error distributions of the observed CMDs. Subsequently, the SFHs, distance modulus, and metallicity of the ten dwarf galaxies are determined by fitting the CMDs. The results indicate that the star formation rates of dwarf irregulars show a gradual increase, while those of dwarf ellipticals exhibit a gradual decrease from the past to the present. Furthermore, this work shows that the star formation activity in dwarf ellipticals persisted up to 30 Myr ago. A significant increasing feature in the SFH of NGC 6822 reveals star formation activity triggered by an interaction event.

Beware the recent past: a bias in spectral energy distribution modelling due to bursty star formation

ABSTRACT We investigate how the recovery of galaxy star formation rates (SFRs) using energy-balance spectral energy distribution (SED) fitting codes depends on their recent star formation histories (SFHs). We use the magphys and prospector codes to fit 6706 synthetic SEDs of simulated massive galaxies at 1 < z < 8 from the Feedback in Realistic Environments project. We identify a previously unknown systematic error in the magphys results due to bursty star formation: the derived SFRs can differ from the truth by as much as 1 dex, at large statistical significance (>5σ), depending on the details of their recent SFH. SFRs inferred using prospector with non-parametric SFHs do not exhibit this trend. We show that using parametric SFHs (pSFHs) causes SFR uncertainties to be underestimated by a factor of up to 5×. Although this undoubtedly contributes to the significance of the systematic, it cannot explain the largest biases in the SFRs of the starbursting galaxies, which could be caused by details of the stochastic prior sampling or the burst implementation in the magphys libraries. We advise against using pSFHs and urge careful consideration of starbursts when SED modelling galaxies where the SFR may have changed significantly over the last ∼100 Myr, such as recently quenched galaxies, or those experiencing a burst. This concern is especially relevant, e.g. when fitting JWST observations of very high redshift galaxies.

Post-starburst Galaxies in SDSS-IV MaNGA: Two Broad Categories of Evolutionary Pathways

We study the size–mass relation (SMR) and recent star formation history (SFH) of post-starburst (PSB) galaxies in the local Universe using spatially resolved spectroscopy from the final data release of MaNGA. Our sample includes 489 PSB galaxies: 94 cPSB galaxies with central PSB regions, 85 rPSB galaxies with ringlike PSB regions, and 310 iPSB galaxies with irregular PSB regions. When compared to control galaxies of similar star formation rate, redshift, and mass, a similar SMR is found for all types of PSB samples except the cPSB galaxies, which have smaller sizes at intermediate masses ( 9.5≲log10(M*/M⊙)≲10.5 ). The iPSB galaxies in the star-forming sequence (iPSB-SF) show no/weak gradients in D n (4000), EW(Hδ A ), and EW(Hα), consistent with the global star-forming status of this type of galaxy, while the quiescent iPSB (iPSB-Q) sample shows negative gradients in D n (4000) and positive gradients in EW(Hδ A ), indicating older stellar populations in the inner regions. Both the cPSB and rPSB samples show positive gradients in D n (4000) and negative gradients in EW(Hδ A ), indicating younger stellar populations in the inner regions. These results imply that the four types of PSB galaxies can be broadly divided into two distinct categories in terms of evolutionary pathway: (1) iPSB-SF and iPSB-Q, which have SMRs and SFHs similar to control galaxies, preferring an inside-out quenching process, and (2) rPSB and cPSB, which appear to be different stages of the same event and likely to follow the outside-in quenching process driven by disruption events such as mergers that result in a more compact structure as quenching proceeds.

The star formation histories of galaxies in different stages of pre-processing in the Fornax A group

ABSTRACT We study the recent star formation histories of 10 galaxies in the Fornax A galaxy group, on the outskirts of the Fornax cluster. The group galaxies are gas-rich, and their neutral atomic hydrogen (H i) was studied in detail with observations from the MeerKAT telescope. This allowed them to be classified into different stages of pre-processing (early, ongoing, advanced). We use long-slit spectra obtained with the South African Large Telescope (SALT) to analyse stellar population indicators to constrain quenching time-scales and to compare these to the H i gas content of the galaxies. The H α equivalent width, EW(H α), suggest that the pre-processing stage is closely related to the recent (<10 Myr) specific Star Formation Rate (sSFR). The early-stage galaxy (NGC 1326B) is not yet quenched in its outer parts, while the ongoing-stage galaxies mostly have a distributed population of very young stars, though less so in their outer parts. The galaxies in the advanced stage of pre-processing show very low recent sSFR in the outer parts. Our results suggest that NGC 1326B, FCC 35, and FCC 46 underwent significantly different histories from secular evolution during the last Gyr. The fact that most galaxies are on the secular evolution sequence implies that pre-processing has a negligible effect on these galaxies compared to secular evolution. We find EW(H α) to be a useful tool for classifying the stage of pre-processing in group galaxies. The recent sSFR and H i morphology show that galaxies in the Fornax A vicinity are pre-processing from the outside in.

Significance mode analysis (SigMA) for hierarchical structures

We present a new clustering method, significance mode analysis (SigMA), for extracting co-spatial and co-moving stellar populations from large-scale surveys such as ESA Gaia. The method studies the topological properties of the density field in the multidimensional phase space. We validated SigMA on simulated clusters and find that it outperforms competing methods, especially in cases where many clusters are closely spaced. We applied the new method to Gaia DR3 data of the closest OB association to Earth, Scorpio-Centaurus (Sco-Cen), and find more than 13 000 co-moving young objects, about 19% of which have a substellar mass. SigMA finds 37 co-moving clusters in Sco-Cen. These clusters are independently validated by their narrow Hertzsprung-Russell diagram sequences and, to a certain extent, by their association with massive stars too bright for Gaia, and are hence unknown to SigMA. We compared our results with similar recent work and find that the SigMA algorithm recovers richer populations, is able to distinguish clusters with velocity differences down to about 0.5 km s−1, and reaches cluster volume densities as low as 0.01 sources pc−3. The 3D distribution of these 37 coeval clusters implies a larger extent and volume for the Sco-Cen OB association than typically assumed in the literature. Additionally, we find the association more actively star-forming and dynamically complex than previously thought. We confirm that the star-forming molecular clouds in the Sco-Cen region, namely, Ophiuchus, L134/L183, Pipe Nebula, Corona Australis, Lupus, and Chamaeleon, are part of the Sco-Cen association. The application of SigMA to Sco-Cen demonstrates that advanced machine learning tools applied to the superb Gaia data allows an accurate census of the young populations to be constructed, which in turn allows us to quantify their dynamics and recreate the recent star formation history of the local Milky Way.

Spatially Resolved Recent Star Formation History in NGC 6946

The nearby face-on star-forming spiral galaxy NGC 6946 is known as the Fireworks Galaxy due to its hosting an unusually large number of supernovae. We analyze its resolved near-ultraviolet stellar photometry measured from images taken with the Hubble Space Telescope’s Wide Field Camera 3 with the F275W and F336W filters. We model the color–magnitude diagrams of the UV photometry to derive the spatially resolved star formation history of NGC 6946 over the last 25 Myr. From this analysis, we produce maps of the spatial distribution of the young stellar population and measure the total recent star formation rate (SFR) of nearly the entire young stellar disk. We find the global SFR(age ≤25 Myr) = . Over this period, the SFR is initially very high ( 16–25 Myr ago), then monotonically decreases to in the last 10 Myr. This decrease in global SFR over the last 25 Myr is consistent with measurements made with other SFR indicators. We discuss in detail two of the most active regions of the galaxy, which we find are responsible for 3% and 5% of the total star formation over the past 6.3 Myr.

Multiwavelength Characterization of the High-mass X-Ray Binary Population of M33

We present multiwavelength characterization of 65 high-mass X-ray binary (HMXB) candidates in M33. We use the Chandra ACIS survey of M33 (ChASeM33) catalog to select hard X-ray point sources that are spatially coincident with UV-bright point-source optical counterparts in the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region catalog, which covers the inner disk of M33 at near-IR, optical, and near-UV wavelengths. We perform spectral energy distribution fitting on multiband photometry for each point-source optical counterpart to measure its physical properties including mass, temperature, luminosity, and radius. We find that the majority of the HMXB companion star candidates are likely B-type main-sequence stars, suggesting that the HMXB population of M33 is dominated by Be X-ray binaries (Be-XRBs), as is seen in other Local Group galaxies. We use spatially resolved recent star formation history maps of M33 to measure the age distribution of the HMXB candidate sample and the HMXB production rate for M33. We find a bimodal distribution for the HMXB production rate over the last 80 Myr, with a peak at ∼10 and ∼40 Myr, which match theoretical formation timescales for the most massive HMXBs and Be-XRBs, respectively. We measure an HMXB production rate of 107–136 HMXBs/(M ⊙ yr−1) over the last 50 Myr and 150–199 HMXBs/(M ⊙ yr−1) over the last 80 Myr. For sources with compact object classifications from overlapping NuSTAR observations, we find a preference for giant/supergiant companion stars in black hole HMXBs and main-sequence companion stars in neutron star HMXBs.

Evolution of radioactive elements in the LMC: predictions for future γ-ray surveys

ABSTRACT Short-lived radionuclides, such as 26Al and 60Fe, are tracers of star formation. Therefore, their abundances can unravel the recent star formation history of the host galaxy. In view of future γ-ray surveys, we predict the masses and fluxes of these two elements in the Large Magellanic Cloud (LMC) using new chemical evolution models. Our best model reproduces the abundance patterns of [α/Fe] ratios, the gas mass, the average metallicity, the present time supernova and nova rates observed in LMC. We show three main results: (i) the best model for the LMC suggests a star formation rate very mild at the beginning with a recent burst, and a Salpeter-like initial mass function. (ii) The predicted mass of 26Al is 0.33 M⊙, 2/3 produced by massive stars and 1/3 by novae. (iii) The predicted mass of 60Fe is 0.44 M⊙, entirely produced by massive stars. This result suggests a larger fraction of 60Fe, at variance with the Milky Way. The explanation for this lies in the adopted initial mass function, that for the LMC contains more massive stars than for the Milky Way. These predictions can be useful for the COSI-SMEX mission planned for launch in 2027. The expected γ-ray line fluxes for the 1.809 MeV line of 26Al and the 1.173 and 1.332 MeV lines of 60Fe are in the range of (0.2–$2.7)\times 10^{-6}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$ and (0.7–$2.8)\times 10^{-7}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$, respectively. This new instrument could have the sensitivity to detect the upper end of the predicted 1.8 MeV flux within its nominal two-yr mission.

The spatial correlation of high-mass X-ray binaries and young star clusters in nearby star-forming galaxies

ABSTRACT We present an analysis of the two-point spatial correlation functions of high-mass X-ray binary (HMXB) and young star cluster (YSC) populations in M31 and M33. We find evidence that HMXBs are spatially correlated with YSCs to a higher degree than would be expected from random chance in both galaxies. When supplemented with similar studies in the Milky Way, Small Magellanic Cloud, and NGC 4449, we find that the peak value of the spatial correlation function correlates strongly with the specific star formation rate of the host galaxy. We additionally perform an X-ray stacking analysis of 211 non-X-ray detected YSCs in M31 and 463 YSCs in M33. We do not detect excess X-ray emission at the stacked cluster locations down to 3σ upper limits of ∼1033 erg s−1 (0.35–8 keV) in both galaxies, which strongly suggests that dynamical formation within YSCs is not a major HMXB formation channel. We interpret our results in the context of (1) the recent star formation histories of the galaxies, which may produce differences in the demographics of compact objects powering the HMXBs, and (2) the differences in natal kicks experienced by compact objects during formation, which can eject newly formed HMXBs from their birth clusters.

GOODS-ALMA 2.0: Last gigayear star formation histories of the so-called starbursts within the main sequence

Recently, a population of compact main sequence (MS) galaxies exhibiting starburst-like properties were identified in the GOODS-ALMA blind survey at 1.1 mm. Several evolution scenarios were proposed to explain their particular physical properties (e.g., compact size, low gas content, short depletion time). In this work, we aim to study the star formation history (SFH) of the GOODS-ALMA galaxies to understand whether or not the so-called ‘starbursts (SBs) in the MS’ galaxies exhibit a star formation activity over the last gigayear that is different from that of the MS galaxies and that could explain their specificity. We use the CIGALE SED modelling code to which we add nonparametric SFHs. To compare the recent SFHs of the galaxies quantitatively, we define a parameter, the star formation rate (SFR) gradient, which provides the angle showing the direction that a galaxy has followed in the SFR-versus-stellar-mass plane over a given period. We show that SB in the MS galaxies show positive or weak negative gradients over the last 100, 300, and 1000 Myr, which is at odds with a scenario where these galaxies would be transitioning from the SB region at the end of a strong starburst phase. Normal GOODS-ALMA galaxies and “SB in the MS” galaxies show the same SFR gradient distribution, meaning that they have similar recent SFHs despite their different properties (compactness, low depletion time). The “SBs in the MS” galaxies manage to maintain their star-formation activity, allowing them to stay within the MS. This points toward a diversity of galaxies within a complex MS.

Constraints on Fluctuating Star Formation Rates for Intermediate-mass Galaxies with Hα and UV Luminosities

We study the recent star formation histories (SFHs) of 575 intermediate-mass galaxies (IMGs, 109 ≤ M/M ⊙ ≤ 1010) in COSMOS at 0.3 < z < 0.4 by comparing their Hα and UV luminosities. These two measurements trace star formation rates (SFRs) on different timescales and together reveal fluctuations in recent activity. We compute L Hα from Magellan IMACS spectroscopy, while L UV is derived from rest-frame 2800 Å photometry. Dust corrections are applied to each band independently. We compare the deviation of L Hα and L UV from their respective star-forming sequences (i.e., and ), and after accounting for observational uncertainties we find a small intrinsic scatter between the two quantities (σ δ ≲ 0.03 dex). This crucial observational constraint precludes strong fluctuations in the recent SFHs of IMGs: simple linear SFH models indicate that a population of IMGs would be limited to only factors of ≲2 change in SFR over 200 Myr and ≲30% on shorter timescales of 20 Myr. No single characteristic SFH for IMGs, such as an exponentially rising/falling burst, can reproduce the individual and joint distribution of and . Instead, an ensemble of SFHs is preferred. Finally, we find that IMG SFHs predicted by recent hydrodynamic simulations, in which feedback drives rapid and strong SFR fluctuations, are inconsistent with our observations.

Searching for Conformity Across Cosmic Time with Local Group and Local Volume Star Formation Histories

Conformity denotes the correlation of properties between pairs of galaxies as a function of separation. Correlations between properties such as the star formation rate (SFR), stellar mass, and specific star formation rate (sSFR) have implications for the impact of environment upon galaxy formation and evolution. Conformity between primary galaxies and satellites within the same dark matter halo has been well documented in simulations and observations. However, the existence of conformity at greater distances—known as two-halo conformity—remains uncertain. We investigate whether galaxies in the Local Volume to a distance of 4 Mpc show conformity by examining the SFR, sSFR, stellar mass, and quenched fraction as a function of physical separation. Making use of the star formation histories of these galaxies, we then extend this analysis back in time to offer the first probe of conformity inside our past light cone. At the present day, we find that the stellar mass or sSFR of a galaxy correlates with the median SFR of neighboring galaxies at a separation of 2–3 Mpc. At a lookback time of 1 Gyr, we find a correlation with the quenched fraction of neighboring galaxies, again at a 2–3 Mpc separation. These signals of conformity likely arise from the differences between the recent star formation histories of Local Group dwarf galaxies and those outside the Local Group. As current and future missions including JWST, Rubin, and Roman expand the sample of Local Volume galaxies, tests of conformity using star formation histories will provide an important tool for exploring spatiotemporal correlations between galaxies.

Did the Milky Way just light up? The recent star formation history of the Galactic disc

We map the stellar age distribution (≲1 Gyr) across a 6 kpc × 6 kpc area of the Galactic disc in order to constrain our Galaxy’s recent star formation history. Our modelling draws on a sample that encompasses all presumed disc O-, B-, and A-type stars (∼500 000 sources) with G &lt; 16. To be less sensitive to reddening, we did not forward-model the detailed CMD distribution of these stars; instead, we forward-modelled the K-band absolute magnitude distribution, n(MK), among stars with MK &lt; 0 and Teff &gt; 7000 K at certain positions x in the disc as a step function with five age bins, b(τ | x, α), logarithmically spaced in age from τ = 5 Myr to τ ∼ 1 Gyr. Given a set of isochrones and a Kroupa initial mass function, we sampled b(τ | x, α) to maximise the likelihood of the data n(MK | x, α), accounting for the selection function. This results in a set of mono-age stellar density maps across a sizeable portion of the Galactic disc. These maps show that some, but not all, spiral arms are reflected in overdensities of stars younger than 500 Myr. The maps of the youngest stars (&lt; 10 Myr) trace major star-forming regions. The maps at all ages exhibit an outward density gradient and distinct spiral-like spatial structure, which is qualitatively similar on large scales among the five age bins. When summing over the maps’ areas and extrapolating to the whole disc, we find an effective star formation rate over the last 10 Myr of ≈ 3.3 M⊙ yr−1, higher than previously published estimates that had not accounted for unresolved binaries. Remarkably, our stellar age distribution implies that the star formation rate has been three times lower throughout most of the last Gyr, having risen distinctly only in the very recent past. Finally, we used TNG50 simulations to explore how justified the common identification of local age distribution with global star formation history is: we find that the global star formation rate at a given radius in Milky Way-like galaxies is approximated within a factor of ∼1.5 by the young age distribution within a 6 kpc × 6 kpc area near R⊙.

Molecular Gas Reservoirs in Massive Quiescent Galaxies at z ∼ 0.7 Linked to Late-time Star Formation

We explore how the presence of detectable molecular gas depends on the inferred star formation histories (SFHs) in eight massive, quiescent galaxies at z ∼ 0.7. Half of the sample have clear detections of molecular gas, traced by CO(2–1). We find that the molecular gas content is unrelated to the rate of star formation decline prior to the most recent 1 Gyr, suggesting that the gas reservoirs are not left over from their primary star formation epoch. However, the recent SFHs of CO-detected galaxies demonstrate evidence for secondary bursts of star formation in their last Gyr. The fraction of stellar mass formed in these secondary bursts ranges from f burst ≈ 0.3%–6% and ended between t end-burst ≈ 0–330 Myr ago. The CO-detected galaxies form a higher fraction of mass in the last Gyr () compared to the CO-undetected galaxies (). The galaxies with gas reservoirs have enhanced late-time star formation, highlighting this as a contributing factor to the observed heterogeneity in the gas reservoirs in high-redshift quiescent galaxies. We find that the amount of gas and star formation driven by these secondary bursts are inconsistent with that expected from dry minor mergers, and instead are likely driven by recently accreted gas, i.e., gas-rich minor mergers. This conclusion would not have been made based on SFRUV+IR measurements alone, highlighting the power of detailed SFH modeling in the interpretation of gas reservoirs. Larger samples are needed to understand the frequency of low-level rejuvenation among quiescent galaxies at intermediate redshifts, and to what extent this drives the diversity of molecular gas reservoirs.

The recent star formation history of NGC 628 on resolved scales

ABSTRACT Star formation histories (SFHs) are integral to our understanding of galaxy evolution. We can study recent SFHs by comparing the star formation rate (SFR) calculated using different tracers, as each probes a different time-scale. We aim to calibrate a proxy for the present-day rate of change in SFR, dSFR/dt, which does not require full spectral energy distribution (SED) modelling and depends on as few observables as possible, to guarantee its broad applicability. To achieve this, we create a set of models in cigale and define an SFR change diagnostic as the ratio of the SFR averaged over the past 5 and 200 Myr, $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$, probed by the H α–FUV colour. We apply $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$ to the nearby spiral NGC 628 and find that its star formation activity has overall been declining in the recent past, with the spiral arms, however, maintaining a higher level of activity. The impact of the spiral arm structure is observed to be stronger on $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$ than on the star formation efficiency. In addition, increasing disc pressure tends to increase recent star formation, and consequently $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$. We conclude that $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$ is sensitive to the molecular gas content, spiral arm structure, and disc pressure. The $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$ indicator is general and can be used to reconstruct the recent SFH of any star-forming galaxy for which H α, FUV, and either mid- or far-IR photometry is available, without the need of detailed modelling.