Galactic Disk Research Articles

The Pristine survey. XXIII. Data Release 1 and an all-sky metallicity catalogue based on Gaia DR3 BP/RPspectro-photometry

We used the spectro-photometric information of $ million stars from Data Release 3 (DR3) to calculate synthetic, narrow-band, metallicity-sensitive $CaHK$ magnitudes that mimic the observations of the Pristine survey, a survey of photometric metallicities of Milky Way stars that has been mapping more than 6,500\,deg$^2$ of the northern sky with the Canada-France-Hawaii Telescope since 2015. These synthetic magnitudes were used for an absolute recalibration of the deeper Pristine photometry and, combined with broadband information, synthetic and Pristine $CaHK$ magnitudes were used to estimate photometric metallicities over the whole sky. The resulting metallicity catalogue is accurate down to $ and is particularly suited for the exploration of the metal-poor Milky Way ($ We make available here the catalogue of synthetic $CaHK_ syn $ magnitudes for all stars with BP/RP information in DR3, as well as an associated catalogue of more than $ million photometric metallicities for high signal-to-noise FGK stars. This paper further provides the first public data release of the Pristine catalogue in the form of higher quality recalibrated Pristine $CaHK$ magnitudes and photometric metallicities for all stars in common with the BP/RP spectro-photometric information in DR3. We demonstrate that, when available, the much deeper Pristine data greatly enhance the quality of the derived metallicities, in particular at the faint end of the catalogue ($G_ BP 16$). Combined, both photometric metallicity catalogues include more than two million metal-poor star candidates ($ phot <-1.0$) as well as more than 200,000 and sim 8,000 very and extremely metal-poor candidates ($ phot <-2.0$ and $<-3.0$, respectively). Finally, we show that these metallicity catalogues can be used efficiently, among other applications, for Galactic archaeology, to hunt for the most metal-poor stars, and to study how the structure of the Milky Way varies with metallicity, from the flat distribution of disk stars to the spheroid-shaped metal-poor halo.

The [Y/Mg] chemical clock in the Galactic disk

Context. Stellar ages are an important parameter in studies of the chemical evolution of the Galaxy. To better estimate these ages, various methods complementary to the conventional isochrone fitting method have been implemented in the past decade. Several recent studies have established the existence of a relationship between chemical clocks and stellar ages. The [Y/Mg] clock is a promising technique, but there are still several open questions, such as its validity for metal-poor stars and differences between the thin and thick disk populations. Aims. Our aim is to study the relationship between the [Y/Mg] chemical clock and stellar ages for a sample of solar-type disk stars and to provide the empirical dating relation(s) for the stellar age determination from their precise chemical abundances. We also studied the effect of metallicity and populations on this chemical clock. Methods. We derived precise stellar atmospheric parameters as well as the elemental abundances of Mg and Y through line-by-line differential spectroscopic analysis for a sample of 48 metal-poor solar-type stars based on high-quality, high-resolution ESO/HARPS spectra. From high-precision Gaia astrometric data, stellar masses and ages were estimated through isochrone fitting using Yonsei-Yale isochrones. A joint analysis of our sample, together with a sample of 185 solar twins and analogues from our previous works, was performed to calibrate the [Y/Mg] chemical clock in the Galactic disk for −0.71 < [Fe/H] < +0.34. Open clusters and stars with asteroseismic ages were used to validate our relations. Results. Two different populations are clearly seen in the [Mg/Fe]−[Fe/H] plane: the thick and thin disks. Thick disk stars show an age-metallicity relation, whereas the thin disk shows a flatter age–metallicity distribution. We find a strong, metallicity–dependent anti-correlation between the [Y/Mg] ratio and the stellar ages of our sample. For the first time in the literature, we report similar correlations for thin and thick disk stars. Conclusions. We find that the [Y/Mg] relation(s) found here for solar-type stars in a wide metallicity range are compatible with those found for solar twins in the literature. Our relation provides high accuracy and precision (0.45 and 0.99 Gyr, respectively) comparable with the best accuracy achieved for solar twins to date.

EROSITA (eRASS1) study of the Canis Major overdensity: Developing a multi-wavelength algorithm for classifying faint X-ray sources

Context. Using data from eROSITA (extended Roentgen Survey with an Imaging Telescope Array) on board Spektrum-Roentgen- Gamma (Spektr-RG, SRG) taken during the first eROSITAall-sky survey (eRASS1), we performed the first X-ray classification and population study in the field of the Canis Major overdensity (CMa OD), which is an elliptical-shaped stellar overdensity located at l = -240°, b = −80°. Aims. This study aims to identify the X-ray sources in CMa OD. We developed a classification algorithm using multi-wavelength criteria as a preliminary method for the classification of faint X-ray sources, specifically in regions with a high source number density. Methods. We used the brightness of the multi-wavelength counterparts (mainly from infrared and optical catalogues), along with the X-ray flux and X-ray hardness ratios (HRs) to classify the sources. Results. Out of a total number of 8311 X-ray sources, we classified 1029 sources as Galactic stars and binaries in the foreground, 946 sources as active galactic nuclei (AGNs) and galaxies in the background, and 435 sources with stellar counterparts that may belong to either the MW or CMa OD. Among the sources with a stellar counterpart, we identified 34 symbiotic star candidates, plus 335 sources, of which the infrared (IR) counterparts have properties of M-giants in CMa OD. Moreover, there is a known high-mass X-ray binary (HMXB, 4U 0728-25) in the field of our study; according to the Gaia parallax of its companion, it appears to be a member of CMa OD. There is also a recently detected transient low-mass X-ray binary (LMXB, SRGt J071522.1-191609) is also present; it may be a member of CMa OD based on its companion, which is most likely highly absorbed and is thus located behind the Galactic disk. In addition, we present the X-ray luminosity function (XLF) of members and candidate members of CMa OD. It is dominated by sources with luminosities of <2 × 1032–1033 erg s−1 in the energy range of 0.2–2.3 keV. These sources are expected to be either accreting white dwarfs or quiescent LMXBs.

Physical Parameters and Magnetic Activity of M-type Stars Based on the LAMOST DR9, SDSS, and TESS Surveys

We analyze a catalog comprising 781,232 spectra from 641,095 M dwarf stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low-resolution spectroscopic data release 9. Based on the equivalent width of the Hα line, we ascertain the activity properties of the stars, identifying a total of 107,134 active stars, with 31,175 exhibiting Hα variations. Furthermore, we establish a positive correlation between starspot activity from Transiting Exoplanet Survey Satellite light curves, and chromospheric activity observed through LAMOST spectra on M dwarf stars. Utilizing LAMOST and Gaia data, we map the two-dimensional distribution of active fractions across all M dwarfs in the Milky Way based on Hα and Hβ lines, confirming a decrease in the active fraction as the distance above the Galactic disk increases. Additionally, we investigate the relationship between chromospheric activity and absolute height above the Galactic disk in various M subtypes. Our findings reveal distinct trends: for M0 to M5 dwarf stars, the active fraction of Hα and Hβ lines rapidly decreases within the 0–300 pc range. In the 300–500 pc range, M0 to M4 dwarf stars exhibit a gradual increase, followed by a decrease in the 500–1000 pc range. Conversely, M5 dwarf stars show no significant gradual increase in the 300–500 pc range and decrease in the 500–1000 pc range. More data will be needed to confirm the phenomenon.

Implications for Galactic Electron Density Structure from Pulsar Sightlines Intersecting H ii Regions

Recent radio surveys have revealed pulsars with dispersion and scattering delays induced by ionized gas that are larger than the rest of the observed pulsar population, in some cases with electron column densities (or dispersion measures, DMs) larger than the maximum predictions of Galactic electron density models. By cross-matching the observed pulsar population against H ii region catalogs, we show that the majority of pulsars with DM > 600 pc cm−3 and scattering delays τ(1 GHz) > 10 ms lie behind H ii regions, and that H ii region intersections may be relevant to as much as a third of the observed pulsar population. The fraction of the full pulsar population with sightlines intersecting H ii regions is likely larger. Accounting for H ii regions resolves apparent discrepancies where Galactic electron density models place high-DM pulsars beyond the Galactic disk. By comparing emission measures inferred from recombination line observations to pulsar DMs, we show that H ii regions can contribute tens to hundreds of parsecs per cubic centimeter in electron column density along a pulsar line of sight. We find that nearly all pulsars with significant excess (and deficit) scattering from the mean τ–DM relation are spatially coincident with known discrete ionized gas structures, including H ii regions. Accounting for H ii regions is critical to the interpretation of radio dispersion and scattering measurements as electron density tracers, both in the Milky Way and in other galaxies.

Very Extended Ionized Gas Discovered around NGC 1068 with the Circumgalactic Hα Spectrograph

We have performed wide-field, ultra-low-surface-brightness Hα emission-line mapping around NGC 1068 with the newly commissioned Circumgalactic Hα Spectrograph. NGC 1068 is notable for its active galactic nucleus, which globally ionizes gas in the disk and halo. Line-emitting diffuse ionized gas is distributed throughout the galactic disk and large-scale ionized filaments are found well beyond the disk, aligned with the cone angle of the central jet. We report the discovery of a new ribbon of ionized gas around NGC 1068 beyond even the known outer filamentary structure, located 20 kpc from the galaxy. The Hα surface brightness of this ribbon is on the order of the bright telluric lines, ranging from 4 to 16 R, with fainter regions on the order of the sky background continuum. Unlike previous extended emission, the ribbon is not as well aligned with the current axis of the central jet. It is not associated with any galactic structure or known tidal features in the halo of NGC 1068, though it may originate from a larger distribution of unmapped neutral atomic or molecular gas in the halo. The morphology of the ribbon emission in Hα is correlated with extended UV emission around NGC 1068. Hα-to-UV flux ratios in the ribbon are comparable to extended emission-line ratios in the halos of NGC 5128, NGC 253, and M82. The Hα excess in the ribbon gas suggests ionization by slow shocks or a mixture of in situ star formation and photoionization and collisional ionization processes.

Quiescent black hole X-ray binaries as multi-messenger sources

The origin of Galactic cosmic rays (CRs) is unknown even though they have traditionally been connected to supernovae based on energetic arguments. In the past decades, Galactic black holes in X-ray binaries (BHXBs) have been proposed as candidate sources of CRs, which revises the CR paradigm. BHXBs launch two relativistic jets during their outbursts, but recent observations suggested that these jets may be launched even during quiescence. A0620−00 is a well-studied object that shows indications of jet emission. We study the simultaneous radio-to-X-ray spectrum of this source that was detected while the source was in quiescence to better constrain the jet dynamics. Because most BHXBs spend their lifetimes in quiescence (qBHXBs), we used the jet dynamics of A0620−00 to study a population of 105 such sources distributed throughout the Galactic disc, and a further 104 sources that are located in the boxy bulge around the Galactic centre. While the contribution to the CR spectrum is suppressed, we find that the cumulative intrinsic emission of qBHXBs from both the boxy bulge and from the Galactic disc adds to the diffuse emission that various facilities detected from radio to TeV γ rays. We examined the contribution of qBHXBs to the Galactic diffuse emission and investigated the possibility of SKA, INTEGRAL, and CTAO to detect individual sources in the future. Finally, we compare the predicted neutrino flux to the recently presented Galactic diffuse neutrino emission by IceCube.

Filling in the blanks

Context. In the solar neighbourhood, only ∼2% of stars in the Gaia survey have a line-of-sight velocity (vlos) contained within the RVS catalogue. These limitations restrict conventional dynamical analysis, such as finding and studying substructures in the stellar halo. Aims. We aim to present and test a method to infer a probability density function (PDF) for the missing vlos of a star with 5D information within 2.5 kpc. This technique also allows us to infer the probability that a 5D star is associated with the Milky Way’s stellar Disc or the stellar Halo, which can be further decomposed into known stellar substructures. Methods. We use stars from the Gaia DR3 RVS catalogue to describe the local orbital structure in action space. The method is tested on a 6D Gaia DR3 RVS sample and a 6D Gaia sample crossmatched to ground-based spectroscopic surveys, stripped of their true vlos. The stars predicted vlos, membership probabilities, and inferred structure properties are then compared to the true 6D equivalents, allowing the method’s accuracy and limitations to be studied in detail. Results. Our predicted vlos PDFs are statistically consistent with the true vlos, with accurate uncertainties. We find that the vlos of Disc stars can be well-constrained, with a median uncertainty of 26 km s−1. Halo stars are typically less well-constrained with a median uncertainty of 72 km s−1, but those found likely to belong to Halo substructures can be better constrained. The dynamical properties of the total sample and subgroups, such as distributions of integrals of motion and velocities, are also accurately recovered. The group membership probabilities are statistically consistent with our initial labelling, allowing high-quality sets to be selected from 5D samples by choosing a trade-off between higher expected purity and decreasing expected completeness. Conclusions. We have developed a method to estimate 5D stars’ vlos and substructure membership. We have demonstrated that it is possible to find likely substructure members and statistically infer the group’s dynamical properties.

Illuminating the Incidence of Extraplanar Dust Using Ultraviolet Reflection Nebulae with GALEX

Circumgalactic dust grains trace the circulation of mass and metals between star-forming regions and gaseous galactic halos, giving insight into feedback and tidal stripping processes. We perform a search for ultraviolet (UV) reflection nebulae produced by extraplanar dust around 551 nearby (D < 100 Mpc), edge-on disk galaxies using archival near-ultraviolet and far-ultraviolet images from the Galaxy Evolution Explorer (GALEX), accounting for the point-spread function (FWHM = 4″–5″). We detect extraplanar emission ubiquitously in stacks of galaxies binned by morphology and star formation rate, with scale heights of h h = 1–2.3 kpc and ≈10% of the total (reddened) flux in the galaxy found beyond the B-band isophotal level of μ B = 25 mag arcsec−2. This emission is detected in 7% of the individual galaxies, and an additional one-third have at least 5% of their total flux found beyond μ B = 25 mag arcsec−2 in a disk component. The extraplanar luminosities and colors are consistent with reflection nebulae rather than stellar halos and indicate that, on average, disk galaxies have an extraplanar dust mass of 5%–15% of that in their interstellar medium. This suggests that recycled material composes at least a third of the inner circumgalactic medium (R < 10 kpc) in ∼L* galaxies.

(Re)mind the gap: A hiatus in star formation history unveiled by APOGEE DR17

Context. Analysis of several spectroscopic surveys indicates the presence of a bimodality between the disc stars in the abundance ratio space of [α/Fe] versus [Fe/H]. The two stellar groups are commonly referred to as the high-α and low-α sequences. Some models capable of reproducing such a bimodality invoke the presence of a hiatus in the star formation history in our Galaxy, whereas other models explain the two sequences by means of stellar migration. Aims. Our aim is to show that the existence of the gap in the star formation rate between high-α and low-α is evident in the stars of APOGEE DR17, if one plots [Fe/α] versus [α/H], confirming previous suggestions. We then try to interpret the data by means of detailed chemical models. Methods. We compare the APOGEE DR17 red giant stars with the predictions of a detailed chemical evolution model based on the two-infall paradigm, taking into account also the possible accretion of dwarf satellites. Results. The APOGEE DR17 abundance ratios [Fe/α] versus [α/H] exhibit a sharp increase in [Fe/α] at a nearly constant [α/H] (where α elements considered are Mg, Si, O) during the transition between the two disc phases. This observation strongly supports the hypothesis that a hiatus in star formation occurred during this evolutionary phase. Notably, the most pronounced growth in the [Fe/α] versus [α/H] relation is observed for oxygen, as this element is exclusively synthesised in core-collapse supernovae. The revised version of the two-infall chemical evolution model proposed in this study reproduces the APOGEE DR17 abundance ratios better than before. Particularly noteworthy is the model’s ability to predict the hiatus in the star formation between the two infalls of gas, which form the thick and thin disc, respectively, and thus generate abundance ratios compatible with APOGEE DR17 data. Conclusions. We show that the signature of a hiatus in the star formation is imprinted in the APOGEE DR17 abundance ratios. A chemical model predicting a pause in the star formation of a duration of roughly 3.5 Gyr, and in which the high-α disc starts forming from pre-enriched gas by a previous encounter with a dwarf galaxy, could well explain the observations

The Brown Dwarf Kinematics Project (BDKP). VI. Ultracool Dwarf Radial and Rotational Velocities from SDSS/APOGEE High-resolution Spectroscopy

We present precise measurements of radial (RV) and projected rotational ( vsini ) velocities of a sample of 258 M6 to L2 dwarfs with multiepoch, high-resolution (λ/Δλ = 22,500), near-infrared (1.514–1.696 μm) spectroscopic observations reported in the Apache Point Observatory Galactic Evolution Experiment Data Release 17. The spectra were modeled using a Markov Chain Monte Carlo forward-modeling method, which achieved median precisions of σ RV = 0.4 km s−1 and σvsini = 1.1 km s−1. One-half of our sample (138 sources) are previously known members of nearby young clusters and moving groups, and we identified three new kinematic members of the Argus or Carina Near moving groups, 2MASS J05402570+2448090, 2MASS J14093200+4138080, and 2MASS J21272531+5553150. Excluding these sources, we find that the majority of our sample have kinematics consistent with the Galactic thin disk, and 11 sources are associated with the intermediate thin/thick disk. The field sample has a velocity dispersion of 38.2 ± 0.3 km s−1, equivalent to an age of 3.30 ± 0.19 Gyr based on empirical age–velocity dispersion relations, and a median vsini of 17 km s−1. For 172 sources with multiepoch observations, we identified 37 as having significant RV variations, and determined preliminary orbit parameters for 26 sources with four or more epochs, nine of which are short-period binary candidates. For 40 sources with photometric variability periods from the literature less than 5 days and vsini > 20 km s−1, we find a decline in projected radii ( Rsini ) with age congruent with evolutionary models. Finally, we also present multiepoch RV and vsini measurements for additional 444 candidate ultracool dwarfs.

Galactic outflows in different geometries

In the gravitational potential well of a galaxy, the behavior of outflows has been studied in different geometries i.e. constant flux tube, divergent and extreme divergent environments. First of all, we describe the hydrodynamic model by which we approach the outflows system. This model treats all the waves as fluids and applies fluid mechanics to them(cosmic rays, thermal plasma & Alfvén wave). Whereas outflows mean the gas regime above and below the center of the galactic disk, and we have taken it as a system where interaction between cosmic rays and plasma occurs. After discussing the hydrodynamic three-fluid model, we define three geometrical structures which can be acquired by outflows and relate these structures with thermal plasma, cosmic and Alfvén waves pressures, thus seeking for its behavior. The solution of the system is categorically divided into three branches, supersonic, subsonic and unphysical solutions.The most interesting of all is the transonic solution, a solution that evolves from subsonic to supersonic. First of all, we discuss only thermal plasma outflows and explain their evolution in three cases of outflows environments, followed by the addition of cosmic waves and Alfvén waves. After discussing three fluid outflows in different structures, we compared the results of thermal outflows with three fluid outflows and see how the addition of waves in the system affects the dynamics of the system. At last, the findings of this paper are discussed.

The Magellanic Clouds Are Very Rare in the IllustrisTNG Simulations

The Large and Small Magellanic Clouds (LMC and SMC) form the closest interacting galactic system to the Milky Way, therewith providing a laboratory to test cosmological models in the local Universe. We quantify the likelihood for the Magellanic Clouds (MCs) to be observed within the ΛCDM model using hydrodynamical simulations of the IllustrisTNG project. The orbits of the MCs are constrained by proper motion measurements taken by the Hubble Space Telescope and Gaia. The MCs have a mutual separation of dMCs=24.5kpc and a relative velocity of vMCs=90.8kms−1, implying a specific phase-space density of fMCs,obs≡(dMCs·vMCs)−3=9.10×10−11km−3s3kpc−3. We select analogues to the MCs based on their stellar masses and distances in MW-like halos. None of the selected LMC analogues have a higher total mass and lower Galactocentric distance than the LMC, resulting in &gt;3.75σ tension. We also find that the fMCs distribution in the highest resolution TNG50 simulation is in 3.95σ tension with observations. Thus, a hierarchical clustering of two massive satellites like the MCs in a narrow phase-space volume is unlikely in ΛCDM, presumably because of short merger timescales due to dynamical friction between the overlapping dark matter halos. We show that group infall led by an LMC analogue cannot populate the Galactic disc of satellites (DoS), implying that the DoS and the MCs formed in physically unrelated ways in ΛCDM. Since the 20∘ alignment of the LMC and DoS orbital poles has a likelihood of P=0.030 (2.17σ), adding this χ2 to that of fMCs gives a combined likelihood of P=3.90×10−5 (4.11σ).

A spectroscopic and kinematic survey of fast hot subdwarfs

Hot subdwarfs (sdO/B) are the stripped helium cores of red giants formed via binary interactions. Close hot subdwarf binaries with massive white dwarf companions have been proposed as possible progenitors of thermonuclear supernovae type Ia (SN Ia). If the supernova is triggered by stable mass transfer from the helium star, the companion should survive the explosion and should be accelerated to high velocities. The hypervelocity star US 708 is regarded as the prototype for such an ejected companion. To find more of those objects we conducted an extensive spectroscopic survey. Candidates for such fast stars have been selected from the spectroscopic database of the Sloan Digital Sky Survey (SDSS) and several ground-based proper-motion surveys. Follow-up spectroscopy has been obtained with several 4m- to 10m-class telescopes. Combining the results from quantitative spectroscopic analyses with space-based astrometry from Gaia Early Data Release 3 (EDR3) we determined the atmospheric and kinematic parameters of 53 fast hot subdwarf stars. None of these stars is unbound to the Galaxy, although some have Galactic restframe velocities close to the Galactic escape velocity. 21 stars are apparently single objects, that crossed the Galactic disc within their lifetimes in the sdO/B stage and could be regarded as potential candidates for the SN Ia ejection scenario. However, the properties of the full sample are more consistent with a pure old Galactic halo population. We therefore conclude that the fast sdO/B stars we found are likely to be extreme halo stars.

Metallicity-dependent kinematics and orbits in the Milky Way’s nuclear stellar disc

Context. The nuclear stellar disc (NSD) is a flat and dense stellar structure at the centre of the Milky Way. Previous work has identified the presence of metal-rich and metal-poor stars in the NSD, suggesting that they have different origins. The recent publication of photometric, metallicity, proper motion, and orbital catalogues allows the NSD stellar population to be characterised with unprecedented detail. Aims. We aim to explore the proper motions and orbits of NSD stars with different metallicities to assess whether they have different origins and to better understand the metallicity distribution in the NSD. Methods. We distinguished between metal-rich and metal-poor stars by applying a Gaussian mixture model, as done in previous work, and analysed the proper motions, orbits, and spatial distribution of stars with different metallicities. Results. We find that metal-rich stars exhibit a lower velocity dispersion, suggesting that they trace a kinematically cooler component compared to metal-poor ones. Furthermore, z-tube orbits are predominant among metal-rich stars, while chaotic/box orbits are more common among metal-poor ones. We also find that metal-rich and metal-poor stars show a similar extinction and are present throughout the analysed regions. As a secondary result, we detected a metallicity gradient in the metal-rich population with higher metallicity towards the centre of the NSD and a tentative gradient for the metal-poor stars, which is consistent with previous studies that did not distinguish between the two populations. Conclusions. Our results suggest that metal-rich stars trace the NSD, whereas metal-poor ones are related to the Galactic bar and probably constitute Galactic bar interlopers and/or are NSD stars that originated from accreted clusters. The detected metallicity gradients aligns with the currently accepted inside-out formation of the NSD.

IllustrisTNG in the HSC-SSP: No Shortage of Thin Disk Galaxies in TNG50

We perform a thorough analysis of the projected shapes of nearby galaxies in both observations and cosmological simulations. We implement a forward-modeling approach to overcome the limitations in previous studies, which hinder accurate comparisons between observations and simulations. We measure axis ratios of z = 0 (snapshot 99) TNG50 galaxies from their synthetic Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) images and compare them with those obtained from real HSC-SSP images of a matched galaxy sample. Remarkably, the comparison shows excellent agreement between the observations and the TNG50 simulation, challenging previous claims that ΛCDM models underproduced the abundance of thin galaxies. Specifically, for galaxies with stellar masses 10≤log(M⋆/M☉)≤11.5 , we find ≲0.1σ tensions between the observations and the simulation, a stark contrast to the previously reported ≳10σ tensions. We reveal that low-mass galaxies (M⋆≲109.5M☉) in TNG50 are thicker than their observed counterparts in HSC-SSP and attribute this to the spurious dynamical heating effects that artificially puff up galaxies. We also find that, despite the overall broad agreement, TNG50 galaxies are more concentrated than the HSC-SSP ones at the low- and high-mass end of the stellar mass range of 9.0≤log(M⋆/M☉)≤11.2 and are less concentrated at intermediate stellar masses. But we argue that the higher concentrations of the low-mass TNG50 galaxies are not likely the cause of their thicker/rounder appearances. Our study underscores the critical importance of conducting mock observations of simulations and applying consistent measurement methodologies to facilitate proper comparison with observations.

Anatomy of the Star Formation in a Tidally Disturbed Disk Galaxy: NGC 3718

We present a UV, optical, and near-IR study of the star-forming complexes in the nearby peculiar galaxy NGC 3718, using Ultraviolet Imaging telescope, Galaxy Evolution Explorer, Spitzer, and DECam Legacy Survey imaging data. The galaxy has a disturbed optical morphology owing to the multiple tidal arms, the warped disk, and the prominent curved dust lanes, but in the near-IR it appears to be a bulge-dominated galaxy. Its disturbed morphology makes it an excellent case to study star formation in a tidally disturbed galaxy that may have undergone a recent minor merger. To study the distribution and properties of the star-forming clumps (SFCs), we divided the galaxy within the R 25 (B band) radius into three parts—the upper, central, and lower regions. Using the UV band images, we investigated the warped star-forming disk, the extended tidal arms, and the distribution and sizes of the 182 SFCs. Their distribution is 49, 60, and 73 in the galaxy’s upper, central, and lower regions, respectively. We determined the UV color, star formation rates (SFRs), star formation density (ΣSFR), and ages of the SFCs. The central disk of the galaxy has a larger mean ΣSFR that is ∼3.3 and ∼1.6 times higher than the upper and lower regions, respectively. We also find that the SFCs in the central disk are older than those in the tidal arms. Our study thus shows that minor mergers can trigger the inside-out growth of galaxy disks, where the younger SFCs are in outer tidal arms and not in the inner disk.

A Break in the Size–Stellar Mass Relation: Evidence for Quenching and Feedback in Dwarf Galaxies

Mapping stars and gas in nearby galaxies is fundamental for understanding their growth and the impact of their environment. This issue is addressed by comparing the stellar “edges” of galaxies D stellar, defined as the outermost diameter where in situ star formation significantly drops, with the gaseous distribution parameterized by the neutral atomic hydrogen diameter measured at 1 M ⊙ pc−2, D HI. By sampling a broad H i mass range 105 M ⊙ < M HI < 1011 M ⊙, we find several dwarf galaxies with M HI < 109 M ⊙ from the field and Fornax Cluster that are distinguished by D stellar ≫ D HI. For the cluster dwarfs, the average H i surface density near D stellar is ∼0.3 M ⊙ pc−2, reflecting the impact of quenching and outside-in gas removal from ram pressure and tidal interactions. In comparison, D stellar/D HI ranges between 0.5 and 2 in dwarf field galaxies, consistent with the expectations from stellar feedback. Only more massive disk galaxies in the field can thus be characterized by the common assumption that D stellar ≲ D HI. We discover a break in the D stellar–M ⋆ relation at m break ∼ 4 × 108 M ⊙ that potentially differentiates the low-mass regime, where the influence of stellar feedback and environmental processes more prominently regulates the sizes of nearby galaxies. Our results highlight the importance of combining deep optical and H i imaging for understanding galaxy evolution.

A Closer Look at the Extended Edge-on Low-surface Brightness Galaxies

To understand the origin of extended disks of low-surface brightness (LSB) galaxies, we studied in detail four such systems with large disks seen edge on. Two of them are edge-on giant LSB galaxies (gLSBGs) recently identified by our team. The edge-on orientation of these systems boosts their surface brightnesses that provided an opportunity to characterize stellar populations spectroscopically and yielded the first such measurements for edge-on gLSBGs. We collected deep images of one galaxy using the 1.4 m Milanković Telescope, which we combined with the archival Subaru Hyper Suprime-Cam and DESI Legacy Surveys data available for the three other systems, and measured the structural parameters of the disks. We acquired deep long-slit spectra with the Russian 6 m telescope and the 10 m Keck II telescope and estimated stellar population properties in the high-surface brightness and LSB regions as well as the gas-phase metallicity distribution. The gas metallicity gradients are shallow to flat in the range between 0 and −0.03 dex per exponential disk scale length, which is consistent with the extrapolation of the gradient–scale length relation for smaller disk galaxies. Our estimates of stellar velocity dispersion in the LSB disks as well as the relative thickness of the disks indicate the dynamical overheating. Our observations favor mergers as the essential stage in the formation scenario for massive LSB galaxies.

An empirical isochrone archive for nearby open clusters

Context. The ages of star clusters and co-moving stellar groups contain essential information about the Milky Way. Their special properties and placement throughout the galactic disk make them excellent tracers of galactic structure and key components to unlocking its star formation history. Yet, even though the importance of stellar population ages has been widely recognized, their determination remains a challenging task often associated with highly model-dependent and uncertain results. Aims. We propose a new approach to this long-standing problem, which relies on empirical isochrones of known clusters extracted from high-quality observational data. These purely observation-based data products open up the possibility of relative age determination, free of stellar evolution model assumptions. Methods. For the derivation of the empirical isochrones, we used a combination of the statistical analysis tool principal component analysis for preprocessing and the supervised machine learning method support vector regression for curve extraction. To improve the statistical reliability of our result, we defined the empirical isochrone of a color-magnitude diagram (CMD) of a cluster as the median calculated from a set of nboot = 1000 curves derived from bootstrapped data. The algorithm requires no physical priors, is computationally fast, and can easily be generalized over a large range of CMD combinations and evolutionary stages of clusters. Results. We provide empirical isochrones in all Gaia DR2 and DR3 color combinations for 83 nearby clusters (d &lt; 500 pc), which cover an estimated age range of 7 Myr to 3 Gyr. In doing so, we pave the way for a relative comparison between individual stellar populations based on an age-scaling ladder of empirical isochrones of known clusters. Furthermore, due to the exceptional precision of the available observational data, we report accurate lower main sequence empirical isochrones for many clusters in our sample, which are of special interest as this region is known to be especially complex to model. We validate our method and results by comparing the extracted empirical isochrones to cluster ages in the literature. We also investigate the added information that empirical isochrones covering the lower main sequence can provide on case studies of the IC 4665 cluster and the Meingast 1 stream. Conclusions. The archive of empirical isochrones offers a novel approach to validating age estimates and can be used as an age-scaling ladder or age brackets for new populations and serve as calibration data for further constraining stellar evolution models.