Galactic Disk Research Articles

ISMGCC: Finding Gas Structures in Molecular Interstellar Medium Using Gaussian Decomposition and Graph Theory

Molecular line emissions are commonly used to trace the distribution and properties of molecular Interstellar Medium. However, the emissions are heavily blended on the Galactic disk toward the inner Galaxy because of the relatively large line widths and the velocity overlaps of spiral arms. Structure identification methods based on voxel connectivity in Position-Position-Velocity (PPV) data cubes often produce unrealistically large structures, which is the “over-linking” problem. Therefore, identifying molecular cloud structures in these directions is not trivial. We propose a new method based on Gaussian decomposition and graph theory to solve the over-linking problem, named InterStellar Medium Gaussian Component Clustering (ISMGCC). Using the Milky Way Imaging Scroll Painting (MWISP) 13CO(1–0) data in the range of 13.°5 ≤ l ≤ 14.°5, ∣b∣ ≤ 0.°5, and −100 ≤ V lsr ≤ +200 km s−1, our method identified three hundred molecular gas structures with at least 16 pixels. These structures contain 92% of the total flux in the raw data cube and show single-peaked line profiles on more than 93% of their pixels. The ISMGCC method could distinguish gas structures in crowded regions and retain most of the flux without global data clipping or assumptions on the structure geometry, meanwhile, allowing multiple Gaussian components for complicated line profiles.

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

We examine the prevalence of truncated star-forming discs in the Virgo cluster down to M* ≃ 107 M⊙. This work makes use of deep, high-resolution imaging in the Hα+[N II] narrow-band from the Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE) and optical imaging from the Next Generation Virgo Survey (NGVS). To aid in the understanding of the effects of the cluster environment on star formation in Virgo galaxies, we take a physically motivated approach to define the edge of the star-forming disc via a drop-off in the radial specific star formation rate profile. A comparison with the expected sizes of normal galactic discs provides a measure of how truncated star-forming discs are in the cluster. We find that truncated star-forming discs are nearly ubiquitous across all regions of the Virgo cluster, including beyond the virial radius (0.974 Mpc). The majority of truncated discs at large cluster-centric radii are of galaxies likely on their first infall. As the intra-cluster medium density is low in this region, it is difficult to explain this population with solely ram-pressure stripping. A plausible explanation is that these galaxies are undergoing starvation of their gas supply before ram-pressure stripping becomes the dominant quenching mechanism. A simple model of starvation shows that this mechanism can produce moderate disc truncations within 1−2 Gyr. This model is consistent with ‘slow-then-rapid’ or ‘delayed-then-rapid’ quenching, whereby the early starvation mode drives disc truncations without significant change to the integrated star formation rate, and the later ram-pressure stripping mode rapidly quenches the galaxy. The origin of starvation may be in the group structures that exist around the main Virgo cluster, which indicates the importance of understanding pre-processing of galaxies beyond the cluster virial radius.

A Possible Formation Scenario of the Gaia BH1: Inner Binary Merger in Triple Systems

Based on astrometric measurements and spectral analysis from Gaia DR3, two quiescent black hole (BH) binaries, Gaia BH1 and BH2, have been identified. Their origins remain controversial, particularly for Gaia BH1. By considering a rapidly rotating (ω/ω crit = 0.8) and strongly magnetized (B 0 = 5000 G) merger product, we find that, at typical Galactic metallicity, the merger product can undergo efficient chemically homogeneous evolution. This results in the merger product having a significantly smaller radius during its evolution compared to that of a normally evolving massive star. Under the condition that the initial triple stability is satisfied, we use the Multiple Stellar Evolution code and the MESA code to identify an initial hierarchical triple that can evolve into Gaia BH1. It initially consists of three stars with masses of 9.03 M ⊙, 3.12 M ⊙, and 1 M ⊙, with inner and outer orbital periods of 2.21 days and 121.92 days, and inner and outer eccentricities of 0.41 and 0.45, respectively. This triple initially experiences triple evolution dynamics instability (TEDI) followed by Roche lobe overflow (RLOF). During RLOF, the inner orbit shrinks, and tidal effects gradually suppress the TEDI. Eventually, the inner binary undergoes a merger through contact (or collision). Finally, using models of rapidly rotating and strongly magnetic stars, along with standard core-collapse supernova (SN) or failed supernova (FSN) models, we find that a postmerger binary (PMB) consisting of an 12.11 M ⊙ merger product and a 1 M ⊙ companion star (originally an outer tertiary) can avoid RLOF. After an SN or FSN with a low ejected mass of ∼0.22 M ⊙ and a low kick velocity ( 46−33+25kms−1 or 9−8+16kms−1 ), the PMB can form Gaia BH1 in the Galactic disk.

CLOUDY modeling suggests a diversity of ionization mechanisms for diffuse extraplanar gas

Context. The ionization of diffuse gas located far above the energetic midplane OB stars poses a challenge to the commonly accepted notion that radiation from OB stars is the primary ionization source for gas in galaxies. Aims. We investigated the sources of ionizing radiation, specifically leaking midplane H II regions and/or in situ hot low-mass evolved stars (HOLMES), in extraplanar diffuse ionized gas (eDIG) in a sample of eight nearby (17−52 Mpc) edge-on disk galaxies observed with the Multi Unit Spectroscopic Explorer (MUSE). Methods. We constructed a model for the photoionization of eDIG clouds and the propagation of ionizing radiation through the eDIG using subsequent runs of CLOUDY photoionization code. Our model includes radiation originating both from midplane OB stars and in situ evolved stars and its dilution and processing as it propagates in the eDIG. Results. We fit the model to the data using the vertical line ratio profiles of our sample galaxies, and find that while the ionization by in situ evolved stars is insignificant for most of the galaxies in our sample, it may be able to explain the enhanced high-ionization lines in the eDIG of the green valley galaxy ESO 544−27. Conclusions. Our results show that while leaking radiation from midplane H II regions is the primary ionization source for eDIG, in situ evolved stars can play a significant part in ionizing extraplanar gas in galaxies with low star forming rates.

An Edge-on Regular Disk Galaxy at z = 5.289

While rotation-supported gas disks are known to exist as early as at z ≈ 7, it is still a general belief that stellar disks form late in the Universe. This picture is now being challenged by the observations from the James Webb Space Telescope (JWST), which have revealed a large number of disk-like galaxies that could be at z > 3, with some being candidates at z > 7. As an early formation of stellar disks will greatly impact our theory of galaxy formation and evolution, it is important to determine when such systems first emerged. Here we present D-CEERS-RUBIES-z5289 at z = 5.289 ± 0.001, the second confirmed stellar disk at z > 5, discovered using the archival JWST NIRCam imaging and NIRSpec spectroscopic data. This galaxy has a highly regular edge-on disk morphology, extends to ∼6.2 kpc along its major axis, and has an effective radius of ∼1.3–1.4 kpc. Such a large stellar disk is yet to be produced in numerical simulations. By analyzing its 10-band spectral energy distribution using four different tools, we find that it has a high stellar mass of 109.5–10.0 M ⊙. Its age is in the range of 330–510 Myr, and it has a mild star formation rate of 10–30 M ⊙ yr−1. While the current spectroscopic data do not allow the derivation of its rotation curve, the width of its Hα line from the partial slit coverage on one side of the disk reaches ∼345 km s−1, which suggests that it could have a significant contribution from rotation.

On the Origin of the 107 K Hot Emitting Gas in the Circumgalactic Medium of the Milky Way

The presence of the ≈106 K gas in the circumgalactic medium of the Milky Way (MW) has been well established. However, the location and the origin of the newly discovered hot gas at “supervirial (SV)” temperatures of ≈107 K have been puzzling. This hot gas has been detected in both absorption and emission; here, we focus on the emitting gas only. We show that both the “virial” and the SV temperature gas, as observed in emission, occupy disk-like extraplanar regions, in addition to the diffuse virial temperature gas filling the halo of the MW. We perform idealized hydrodynamical simulations to show that the ≈107 K emitting gas is likely to be produced by stellar feedback in and around the Galactic disk. We further show that the emitting gas at both SV and virial temperatures in the extraplanar regions is metal enriched and is not in hydrostatic equilibrium with the halo but is continuously evolving.

Evaluating the gravitational wave detectability of globular clusters and the Magellanic Clouds for LISA

We used the stellar evolution code bpass and the gravitational wave (GW) simulation code legwork to simulate populations of compact binaries that may be detected by the future space-based GW detector LISA. Specifically, we simulate the Magellanic Clouds and binary populations mimicking several globular clusters, neglecting dynamical effects. We find that a handful of sources should be detectable in each of the Magellanic Clouds, but for globular clusters the amount of detectable sources will likely be less than one each. We compared our results to earlier research and find that our predicted numbers are several dozen times lower than both the results from calculations that used the stellar evolution code bse and take dynamical effects into account, and results from calculations that used the stellar evolution code SeBa for the Magellanic Clouds. Earlier research that compared bpass models for GW sources in the Galactic disk with bse models found a similarly sized discrepancy. We determine that this discrepancy is caused by differences between the stellar evolution codes, particularly in the treatment of mass transfer and common-envelope events in binaries: in bpass mass transfer is more likely to be stable and tends to lead to less orbital shrinkage in the common-envelope phase than in other codes. This difference results in fewer compact binaries with periods short enough to be detected by LISA existing in the bpass population. For globular clusters, we conclude that the impact of dynamical effects is uncertain based on the literature, but the differences in stellar evolution have an effect of a factor of 20 to 40 on the number of detectable binaries.

The disc origin of the Milky Way bulg. The high velocity dispersion of metal-rich stars at low latitudes

Previous studies of the chemo-kinematic properties of stars in the Galactic bulge have revealed a puzzling trend. Along the bulge minor axis, and close to the Galactic plane, metal-rich stars display a higher line-of-sight velocity dispersion compared to metal-poor stars, while at higher latitudes metal-rich stars have lower velocity dispersions than metal-poor stars, similar to what is found in the Galactic disc. In this work, we re-examine this issue, by studying the dependence of line-of-sight velocity dispersions on metallicity and latitude in APOGEE Data Release 17, confirming the results of previous works. We then analyse an $N$-body simulation of a Milky Way-like galaxy, also taking into account observational biases introduced by the APOGEE selection function. We show that the inversion in the line-of-sight velocity dispersion-latitude relation observed in the Galactic bulge -- where the velocity dispersion of metal-rich stars becomes greater than that of metal-poor stars as latitude decreases -- can be reproduced by our model. We show that this inversion is a natural consequence of a scenario in which the bulge is a boxy or peanut-shaped structure, whose metal-rich and metal-poor stars mainly originate from the thin and thick disc of the Milky Way, respectively. Due to their cold kinematics, metal-rich, thin disc stars are efficiently trapped in the boxy, peanut-shaped bulge, and at low latitudes show a strong barred morphology, which -- given the bar orientation with respect to the Sun-Galactic centre direction -- results in high velocity dispersions that are larger than those attained by the metal-poor populations. Extremely metal-rich stars in the Galactic bulge, which have received renewed attention in the literature, do follow the same trends as those of the metal-rich populations. The line-of-sight velocity-latitude relation observed in the Galactic bulge for metal-poor and metal-rich stars are thus both an effect of the intrinsic nature of the Galactic bulge (i.e. mostly secular) and of the angle at which we observe it from the Sun.

Probing the Disk Kinematics of M31 with DESI

M31’s stellar rotation curve and disk line-of-sight dispersion is presented using kinematic data from a novel DESI M31 survey in combination with similar datasets. After removing the foreground, the disk region is isolated and the velocity field deprojected. A flat ring model is fit to the velocity field of M31 out to 30 kpc using a maximum-likelihood routine. The stellar rotation curve is found to flatten to ∼220 km/s, and a disk line-of-sight velocity dispersion of ∼60 km/s is derived. The results extend M31’s rotation curve to 30 kpc and support the picture in which M31 has thicker disk than the Milky Way.

Accreting Neutron Stars in 3D General-relativistic Magnetohydrodynamic Simulations: Jets, Magnetic Polarity, and the Interchange Slingshot

Abstract Accreting neutron stars differ from black holes by the presence of the star’s own magnetic field, whose interaction with the accretion flow is a central component in understanding these systems’ disk structure, outflows, jets, and spin evolution. It also introduces an additional degree of freedom, as the stellar dipole can have any orientation relative to the inner disk’s magnetic field. We present a suite of 3D general-relativistic magnetohydrodynamic simulations in which we investigate the two extreme polarities, with the dipole field being either parallel or antiparallel to the initial disk field, in both the accreting and propeller states. When the magnetosphere truncates the disk near or beyond the corotation radius, most of the system’s properties, including the relativistic jet power, are independent of the star–disk relative polarity. However, when the disk extends well inside the corotation radius, in the parallel orientation the jet power is suppressed and the inner disk is less dense and more strongly magnetized. We suggest a physical mechanism that may account for this behavior—the interchange slingshot—and discuss its astrophysical implications. When the star is in the rapidly accreting regime, which in most cases will be associated with strong spin-up, we expect large observational differences between the two magnetic orientations. This may be reflected in increased variability as the accretion flow drags in successive magnetic structures of varying polarity.

Morphology of Molecular Clouds at Kiloparsec Scale in the Milky Way: Shear-induced Alignment and Vertical Confinement

The shape of the cold interstellar molecular gas is determined by several processes, including self-gravity, tidal force, turbulence, magnetic field, and galactic shear. Based on the 3D dust extinction map derived by Vergely et al., we identify a sample of 550 molecular clouds within 3 kpc of the solar vicinity in the Galactic disk. Our sample contains clouds whose size ranges from parsec to kiloparsec, which enables us to study the effect of Galactic-scale processes, such as shear, on cloud evolution. We find that our sample clouds follow a power-law mass–size relation of M∝32.00Rmax1.77 , M∝20.59RS2.04 , and M∝14.41RV2.29 , where Rmax is the major axis-based cloud radius, R S is the area-based radius, and R V is the volume-based radius, respectively. These clouds have a mean constant surface density of ∼7 M ⊙ pc−2 and follow a volume density–size relation of ρ∝2.60Rmax−0.55 . As cloud size increases, their shapes gradually transition from ellipsoidal to disk-like to bar-like structures. Large clouds tend to have a pitch angle of 28°−45°, where the angle is measured concerning the Galactic tangential direction. These giant clouds also tend to stay parallel to the Galactic disk plane and are confined within the Galactic molecular gas disk. Our results show that large molecular clouds in the Milky Way can be shaped by Galactic shear and confined in the vertical direction by gravity.

Can Supernovae from Runaway Stars Mimic the Signs of Absorbing “Supervirial” Gas?

The recent detection of large column density absorption lines from highly ionized gas in a few directions through the circumgalactic medium (CGM) of the Milky Way (MW) has been puzzling. The inferred temperature from these absorption lines far exceeds the virial temperature of the MW, and the column densities are also too large to be easily explained. In this paper, we propose a novel idea to explain these observations and claim that they may not have originated from the CGM, but from a totally different type of source, namely, stellar ejecta from supernovae (SNe) above the Galactic disk that happen to lie in the line of sight to the background quasars. About ∼20% of massive OB stars (progenitors of core-collapse supernovae) are known to be runaway stars that have high ejection velocities near the Galactic plane and can end up exploding as SNe above the Galactic disk. We show that the associated reverse shock in the supernova remnant in the early nonradiative phase can heat the ejecta to temperatures of ≳107 K and can naturally explain the observed high column density of ions in the observed “supervirial” phase along with α-enriched supersolar abundance that is typical of core-collapse supernovae. However, SNe from runaway stars has a covering fraction of ≲0.7% and thus can only explain the observations along limited sightlines.

The Distance to Dobashi 6193—A Dark Cloud Shadowing the eROSITA Bubbles

We focus on a prominent gas cloud, Dobashi 6193, toward the western base of the northern eROSITA bubble (eRObub). It can potentially put a tight distance constraint on the eRObubs and clarify the emissions of the Loop I superbubble from the eRObubs. We found its distance to be 700 pc based on Gaia DR3 stellar extinction and parallax measurements, making it one of the most distant clouds shadowing the eRObubs well above the Galactic disk.

A Potential Dynamical Origin of the Galactic Disk Warp: The Gaia–Sausage–Enceladus Major Merger

Previous studies have revealed that the Galactic warp is a long-lived, nonsteady, and asymmetric structure. There is a need for a model that accounts for the warp’s long-term evolution. Given that this structure has persisted for over 5 Gyr, its timeline may coincide with the completion of the Gaia–Sausage–Enceladus (GSE) merger. Recent studies indicate that the GSE, the significant merger of our Galaxy, was likely a gas-rich merger and the large amount of gas introduced could have created a profound impact on the Galactic morphology. This study utilizes GIZMO simulation code to construct a gas-rich GSE merger. By reconstructing the observed characteristics of the GSE, we successfully reproduce the disk warp and capture nearly all of its documented features, which align closely with observational data from both stellar and gas disks. This simulation demonstrates the possibility that a single major merger could generate the Galactic warp amplitude and precession. Furthermore, the analysis of the warp’s long-term evolution may offer more clues into the formation history of the Milky Way.

Assessing the robustness of the Galactic rotation curve inferred from the Jeans equations using Gaia DR3 and cosmological simulations

Several authors have recently applied Jeans modelling to -based datasets to infer the circular velocity curve for the Milky Way. These works have consistently found evidence for a continuous decline in the rotation curve beyond $ kpc $, which may indicate the existence of a light dark matter (DM) halo. Using a large sample of Gaia DR3 data, we aim to derive the rotation curve of the Milky Way using the Jeans equations, and to quantify the role of systematic effects, both in the data and those inherent to the Jeans methodology under the assumptions of axisymmetry and time independence. We used data from the Gaia DR3 radial velocity spectrometer sample, supplemented with distances inferred through Bayesian frameworks, to determine the radial variation of the second moments of the velocity distribution for stars close to the Galactic plane. We used these profiles to determine the rotation curve using the Jeans equations under the assumption of axisymmetry and explored how they vary with azimuth and position above and below the plane of the Galactic disc. We applied the same methodology to an N-body simulation of a Milky Way-like galaxy impacted by a satellite akin the Sagittarius dwarf, and to the Auriga suite of cosmological simulations. The circular velocity curve we infer for the Milky Way is consistent with previous findings out to $ 15$ kpc, where our statistics are robust. Due to the larger number of stars in our sample, we are able to reveal evidence of disequilibrium and deviations from axisymmetry closer in. For example, we find that the second moment of $V_R$ flattens out at $R kpc, and that the second moment of $V_ is different above and below the plane for $R kpc. Our exploration of the simulations indicates that these features are typical of galaxies that have been perturbed by external satellites. From the simulations, we also estimate that the difference between the true circular velocity curve and that inferred from Jeans equations can be as high as $15<!PCT!>$, but that it is likely of the order of $10<!PCT!>$ for the Milky Way. This is higher than the systematic uncertainties associated with the observations or those linked to most modelling assumptions when using the Jeans equations. However, if the density of the tracer population were truncated at large radii instead of being exponential as often assumed, this could lead to the erroneous conclusion of a steeply declining rotation curve. We find that steady-state axisymmetric Jeans modelling becomes less robust at large radii, indicating that particular caution must be exercised when interpreting the rotation curve inferred in those regions. A more careful and sophisticated approach may be necessary for precision measurements of the DM content of our Galaxy.

Apostle–auriga: effects of stellar feedback subgrid models on the evolution of angular momentum in disc galaxies

ABSTRACT Utilizing the apostle–auriga simulations, which start from the same zoom-in initial conditions of Local Group-like systems, but run with different galaxy formation subgrid models and hydrodynamic solvers, we study the impact of stellar feedback models on the evolution of angular momentum in disc galaxies. At $z = 0$, auriga disc galaxies tend to exhibit higher specific angular momenta compared to their cross-matched apostle counterparts. By tracing the evolution history of the Lagrangian mass tracers of the in-situ star particles in the $z = 0$ galaxies, we find that the specific angular momentum distributions of the gas tracers from the two simulations at the halo accretion time are relatively similar. The present-day angular momentum difference is mainly driven by the physical processes occurring inside dark matter haloes, especially galactic fountains. Due to the different subgrid implementations of stellar feedback processes, auriga galaxies contain a high fraction of gas that has gone through recycled fountain (${\sim } 65$ per cent) which could acquire angular momentum through mixing with the high angular momentum circumgalactic medium (CGM). In apostle, however, the fraction of gas that has undergone the recycled fountain process is significantly lower (down to ${\sim } 20$ per cent for Milky Way-sized galaxies) and the angular momentum acquisition from the CGM is marginal. As a result, the present-day auriga galaxies overall have higher specific angular momenta.

CHANG-ES. XXXIII. A 20 kpc radio bubble in the halo of the star-forming galaxy NGC 4217

Cosmic rays may be dynamically very important in driving large-scale galactic winds. Edge-on galaxies give us an outsider's view of radio haloes, and of their extra-planar cosmic-ray electrons and magnetic fields. We present a new radio continuum imaging study of the nearby edge-on galaxy NGC\,4217. We examine the distribution of extra-planar cosmic rays and magnetic fields. We observed it with both the Jansky Very Large Array (JVLA) in the $S$ band (2--4\,GHz) and the LOw Frequency ARray (LOFAR) at 144\,MHz. We measured vertical intensity profiles and exponential scale heights. We re-imaged both the JVLA and LOFAR data at matched angular resolution in order to measure radio spectral indices between 144\,MHz and 3\,GHz. Confusing point-like sources were subtracted prior to imaging. We then fitted intensity profiles with cosmic-ray electron advection models, using an isothermal wind model that is driven by a combination of pressure from the hot gas and cosmic rays. We discover a large-scale radio halo on the north-western side of the galactic disc. The morphology is reminiscent of a bubble extending up to 20\,kpc from the disc. We find spectral ageing in the bubble, which allowed us to measure the advection speeds of the cosmic-ray electrons, which accelerate from 300 to $600\ $. Assuming energy equipartition between the cosmic rays and the magnetic field, we estimate the bubble may have been inflated by a modest 10\,<!PCT!> of the kinetic energy injected by supernovae over its dynamical timescale of 35\,Myr. While no active galactic nucleus (AGN) has been detected, such activity in the recent past cannot be ruled out. Non-thermal bubbles with sizes of tens of kiloparsecs may be a ubiquitous feature of star-forming galaxies, and if so this would demonstrate the influence of feedback. Determining possible contributions by AGN feedback will require deeper observations.

Compact ellipticals in the Antlia cluster of galaxies

Compact ellipticals (cEs) are rare objects in the local Universe, but relevant for the evolution of galaxies. Their origin is not completely understood, with evidence equally suggesting that they are relics of early epochs, and remnants of tidal disruptions. This article aims to characterise two cEs, members of the nearby Antlia cluster, providing insights into the formation channels of cEs residing in high-density environments. This dataset consists of archival ACS/HST images and Gemini/GMOS slit spectra. The isophotal analysis of the cEs was compared with their kinematics and stellar population synthesis. The results were also compared with a sample of cEs and `relic' galaxies from the literature, to put them into context. Their characterisations highlight clear differences between the two cEs. Although the stellar populations for both cEs are mainly old, with mass-weighted ages above 10\,Gyr, FS90\,110, the most massive one, has a minor contribution from an intermediate-age population. Besides, it presents rotational evidence, a remarkably bright nucleus for its luminosity, and a clear dichotomy between the inner and outer components. It is probably the remnant of a low-mass disc galaxy that has been tidally disrupted by the close massive elliptical NGC\,3258. On the contrary, FS90\,192 does not present evidence for either rotation or discy isophotes, and its population synthesis leads to a stellar population with ages above 12\,Gyr. It is probably a low-mass analogue to `relic' galaxies.

On the Origin of Quenched but Gas-rich Regions at Kiloparsec Scales in Nearby Galaxies

We use resolved spectroscopy from MaNGA to investigate the significance of both local and global properties of galaxies to the cessation of star formation at kpc scales. Quenched regions are identified from a sample of isolated disk galaxies by a single-parameter criterion D n (4000) - log EW(Hα) >1.6−log2=1.3 , and are divided into gas-rich quenched regions (GRQRs) and gas-poor quenched regions according to the surface density of cold gas (Σgas). Both types of quenched regions tend to be hosted by non-AGN galaxies with relatively high mass (M * ≳ 1010 M ⊙) and red colors (NUV − r ≳ 3), as well as low star formation rate and high central density at fixed mass. They span wide ranges in other properties including structural parameters that are similar to the parent sample, indicating that the conditions responsible for quenching in gas-rich regions are largely independent of the global properties of galaxies. We train random forest classifiers and regressors for predicting quenching in our sample with 15 local/global properties. Σ* is the most important property for quenching, especially for GRQRs. These results strongly indicate the important roles of low-mass hot evolved stars, which are numerous and long-lived in quenched regions and can provide substantial radiation pressure to support the surrounding gas against gravitational collapse. The different feature importance for quenching, as found previously by A. F. L. Bluck et al., is partly due to the different definitions of quenched regions, particularly the different requirements on EW(Hα).

The masses of open star clusters and their tidal tails and the stellar initial mass function

Unresolved binaries have a strong influence on the observed parameters of SC We quantify this influence and compute the resulting mass underestimates and MF N-body simulations of realistic SC were used to investigate the evolution of the binary population in a SC and its tidal tails. Together with an empirically gauged stellar mass-luminosity relation, the results were then used to determine how the presence of binaries changes the photometric mass and MF of the SC and its tails as deduced from observations. T1 which is the tidal tail caused by gas expulsion, contains a larger fraction of binaries than both the SC and T2 which forms after gas expulsion. Additionally T1 has a larger velocity dispersion. Using the luminosity of an unresolved binary, an observer would underestimate its mass. This bias sensitively depends on the companion masses due to the structure of the stellar mass-luminosity relation. Combining the effect of all binaries in the simulation, the total photometric mass of the SC is underestimated by 15<!PCT!>. Dark objects (black holes and neutron stars) increase the difference between the real and observed mass of the SC further. For both the SC and the tails, the observed power-law index of the MF between a stellar mass of 0.3 and 0.7 $M_ is smaller by up to 0.2 than the real one, the real IMF being steeper by this amount. This difference is larger for stars with a larger velocity dispersion or binary fraction. Since the stars formed in SC are the progenitors of the Galactic field stars, this work suggests that the binary fractions of different populations of stars in the Galactic disc will differ as a function of the velocity dispersion. However, the direction of this correlation is currently unclear, and a complete population synthesis will be needed to investigate this effect. Variations in the binary fractions of different clusters can lead to perceived variations of the deduced stellar MF