Starburst Galaxies Research Articles

The Impact of Mass-dependent Stochasticity at Cosmic Dawn

The James Webb Space Telescope is unveiling a surprising lack of evolution in the number densities of ultraviolet (UV)-selected galaxies at redshift z ≳ 10. At the same time, observations and simulations are providing evidence for highly bursty star formation in high-z galaxies, resulting in significant scatter in their UV luminosities. Galaxies in low-mass dark matter halos are expected to experience most stochasticity due to their shallow potential wells. Here, we explore the impact of a mass-dependent stochasticity using a simple analytical model. We assume that scatter in the M UV–M h relation increases toward lower halo masses, following the decrease in halo escape velocity, σUV∼Mh−1/3 , independent of redshift. Since low-mass halos are more dominant in the early universe, this model naturally predicts an increase in UV luminosity functions (LFs) at high redshifts compared to models without scatter. We make predictions for additional observables, which would be affected by stochasticity and could be used to constrain its amplitude, finding (i) galaxies are less clustered compared to the no-scatter scenario, with the difference increasing at higher-z; (ii) assuming that star-bursting galaxies dominate the ionizing photon budget implies reionization starts earlier and is more gradual compared to the no-scatter case; (iii) at fixed UV magnitude, galaxies should exhibit wide ranges of UV slopes, nebular emission line strengths, and Balmer breaks. Comparing to observations, the mass-dependent stochasticity model successfully reproduces the observed LFs up to z ∼ 12. However, the model cannot match the observed z ∼ 14 LFs, implying additional physical processes enhance star formation efficiency in the earliest galaxies.

The Outflowing [O ii] Nebulae of Compact Starburst Galaxies at z ∼ 0.5

High-velocity outflows are ubiquitous in compact, massive (M * ∼ 1011 M ⊙), z ∼ 0.5 galaxies with extreme star formation surface densities (ΣSFR ∼ 2000 M ⊙ yr−1 kpc−2). We have previously detected and characterized these outflows using Mg ii absorption lines. To probe their full extent, we present Keck/KCWI integral field spectroscopy of the [O ii] and Mg ii emission nebulae surrounding all of the 12 galaxies in this study. We find that [O ii] is more effective than Mg ii in tracing low surface brightness, extended emission in these galaxies. The [O ii] nebulae are spatially extended beyond the stars, with radial extent R 90 between 10 and 40 kpc. The nebulae exhibit nongravitational motions, indicating galactic outflows with maximum blueshifted velocities ranging from −335 to −1920 km s−1. The outflow kinematics correlate with the bursty star formation histories of these galaxies. Galaxies with the most recent bursts of star formation (within the last <3 Myr) exhibit the highest central velocity dispersions (σ ≳ 400 km s−1), while the oldest bursts have the lowest-velocity outflows. Many galaxies exhibit both high-velocity cores and more extended, slower-moving gas indicative of multiple outflow episodes. The slower, larger outflows occurred earlier and have decelerated as they propagate into the circumgalactic medium and mix on timescales ≳50 Myr.

Physical properties of extreme emission-line galaxies at z ∼ 4–9 from the JWST CEERS survey

Context. Extreme emission line galaxies (EELGs) are typically characterized by high equivalent widths (EWs) which are driven by elevated specific star formation rates (sSFRs) in low-mass galaxies with subsolar metallicities and little dust. Such extreme systems are exceedingly rare in the local universe, but the number density of EELGs increases with increasing redshift. Such starburst galaxies are currently strongly presumed to be the main drivers of hydrogen reionization over 5.5 &lt; z &lt; 15, which serves to motivate many of the searches for high-z EELGs. Aims. We aim to characterize the physical properties of a sample of ∼730 EELGs at 4 ≲ z &lt; 9 photometrically selected from the CEERS survey using JWST/NIRCam. We validate our method and demonstrate the main physical properties of a subset of EELGs using NIRSpec spectra. Methods. We create synthetic NIRCam observations of EELGs using empirical templates based on ∼2000 local metal-poor starbursts to select EELGs based on color-color criteria. We study their properties based on SED fitting and flux excess from emission lines in the photometric filters. Results. Our sample of EELGs has a mean stellar mass of 107.84 M⊙ with high sSFRs from SED fitting with a mean value of 10−7.03 yr−1. We consider a delayed-τ model for the star formation history and find our sample of EELGs are young with a mean value of the time after the onset of star formation of 45 Myr. We find that they have similar line ratios to local metal-poor starburst galaxies with high log([OIII]/Hβ) ≳ 0.4−1 which indicates that star formation may be the dominant source of ionization in these galaxies. Based on the photometric fluxes and morphologies, we find an increase of EW([OIII]+Hβ) with sSFR and ΣSFR, and a decrease with age and stellar mass. The sample of EELGs can reach ΣSFR &gt; 10 M⊙ yr−1 kpc−2 which indicate they are strong candidates of LyC leakers. Another indirect indicator is the high values of O32 &gt; 5 that can be reached for some galaxies in the sample. This indicates that they may have the conditions to facilitate the escape of ionizing photons.

Evidence for a Fast Soft X-Ray Wind in M82 from XMM-RGS

Starburst wind models predict that metals and energy are primarily carried out of the disk by hot gas (T > 106 K), but the low energy resolution of X-ray CCD observations results in large uncertainties on the mass and energy loading. Here, we present evidence for a fast soft X-ray wind from the prototypical starburst galaxy M82 using deep archival observations from the Reflection Grating Spectrometer on XMM-Newton. After characterizing the complex line-spread function for the spatially extended outflow ( ≈4′ ), we perform emission-line fitting to measure the velocity dispersion, σ v , from O viii (0.65, 0.77 keV), Ne x (1.02 keV), and Mg xii (1.47 keV). For the T ≈ 3 × 106 K gas, O viii yields a velocity dispersion of σv=1160−90+100 km s−1, implying a wind speed that is significantly above the escape velocity (v esc ≲ 450 km s−1). Ne x ( σv=550−150+130 km s−1) and Mg xii (σ v < 370 km s−1) show less velocity broadening than O viii, hinting at a lower wind speed or smaller opening angle on the more compact spatial scales traced by the T ≈ (0.6−1) × 107 K gas. Alternatively, these higher energy emission lines may be dominated by shock-heated gas in the interstellar medium. Future synthesis of these measurements with performance verification observations of the E = 2−12 keV wind in M82 from the Resolve microcalorimeter on the X-ray Imaging and Spectroscopy Mission will inform the phase structure and energy budget of the hot starburst wind.

JWST’s PEARLS: Resolved study of the stellar and dust components in starburst galaxies at cosmic noon

Dusty star-forming galaxies (DSFGs) significantly contribute to the stellar buildup in galaxies during “cosmic noon,” the peak epoch of cosmic star formation. Major mergers and gas accretion are often invoked to explain DSFGs’ prodigious star formation rates (SFRs) and large stellar masses. We conducted a spatially resolved morphological analysis of the rest-frame ultraviolet/near-infrared (∼0.25–1.3 μm) emission in three DSFGs at z ≃ 2.5. Initially discovered as carbon monoxide (CO) emitters by NOrthern Extended Millimeter Array (NOEMA) observations of a bright (S350 μm = 111 ± 10 mJy) Herschel source, we observed them with the James Webb Space Telescope/NIRCam as part of the PEARLS program. The NIRCam data reveal the galaxies’ stellar populations and dust distributions on scales of 250 pc. Spatial variations in stellar mass, SFR, and dust extinction are determined in resolved maps obtained through pixel-based spectral energy distribution fitting. The CO emitters are massive (Mstar ≃ (3 − 30)×1010 M⊙), dusty starburst galaxies with SFRs ranging from 340 to 2500 M⊙ yr−1, positioning them among the most active star-forming galaxies at 2 &lt; z &lt; 3. Notably, they belong to the ∼1.5% of the entire JWST population with extremely red colors. Their morphologies are disk like (Sérsic index n ≃ 1), with effective radii of 2.0–4.4 kpc, and exhibit substructures such as clumps and spiral arms. The galaxies have dust extinctions up to AV = 5–7 mag extending over several kiloparsecs with asymmetric distributions that include off-center regions resembling bent spiral arms and clumps. The near-infrared dust-attenuation curve in these sources deviates from standard laws, possibly implying different dust–star geometries or dust grain properties than commonly assumed in starburst galaxies. The proximity (&lt; 5″) of galaxies with consistent redshifts, strong color gradients, an overall disturbed appearance, asymmetric dust obscuration, and widespread star formation collectively favor interactions (minor mergers and flybys) as the mechanism driving the CO galaxies’ exceptional SFRs. The galaxies’ large masses and rich environment hint at membership in two proto-structures, as initially inferred from their association with a Planck-selected high-z source.

Massive star cluster formation

The mode of star formation that results in the formation of globular clusters and young massive clusters is difficult to constrain through observations. We present models of massive star cluster formation using the TORCH framework, which uses the Astrophysical MUltipurpose Software Environment (AMUSE) to couple distinct multi-physics codes that handle star formation, stellar evolution and dynamics, radiative transfer, and magnetohydrodynamics. We upgraded TORCH by implementing the N-body code PETAR, thereby enabling TORCH to handle massive clusters forming from 106 M⊙ clouds with ≥105 individual stars. We present results from TORCH simulations of star clusters forming from 104, 105, and 106 M⊙ turbulent spherical gas clouds (named M4, M5, M6) of radius R = 11.7 pc. We find that star formation is highly efficient and becomes more so at a higher cloud mass and surface density. For M4, M5, and M6 with initial surface densities 2.325 × 101,2,3 M⊙ pc−2, after a free-fall time of tff = 6.7,2.1,0.67 Myr, we find that ∼30%, 40%, and 60% of the cloud mass has formed into stars, respectively. The end of simulation-integrated star formation efficiencies for M4, M5, and M6 are ϵ⋆ = M⋆/Mcloud = 36%, 65%, and 85%. Observations of nearby clusters similar in mass and size to M4 have instantaneous star formation efficiencies of ϵinst ≤ 30%, which is slightly lower than the integrated star formation efficiency of M4. The M5 and M6 models represent a different regime of cluster formation that is more appropriate for the conditions in starburst galaxies and gas-rich galaxies at high redshift, and that leads to a significantly higher efficiency of star formation. We argue that young massive clusters build up through short efficient bursts of star formation in regions that are sufficiently dense (Σ ≥ 102 M⊙ pc−2) and massive (Mcloud ≥ 105 M⊙). In such environments, stellar feedback from winds and radiation is not strong enough to counteract the gravity from gas and stars until a majority of the gas has formed into stars.

Are all starbursts equal? Star-formation-rate profiles in merger versus secular starbursts

Aims. Galaxy interactions can trigger drastic changes in the resolved star-forming properties of their constituents, but it remains unclear as to whether those changes are discernible from secular starburst triggers. In this Letter we investigate whether or not post-merger galaxies create unique star-forming trends on a kiloparsec scale. Methods. We present radial trends in star-formation-rate (SFR) surface density (ΣSFR) for 150 post-merger galaxies with moderate to extremely heightened global SFRs using observations from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. We juxtapose these profiles with those of noninteracting galaxies (excluding both galaxy pairs and post-merger galaxies) with similarly enhanced global SFRs. Results. Post-merger galaxies have a much stronger central starburst than isolated galaxies with similar global star-formation enhancements. Indeed, isolated starburst galaxies (SBs) lack a marked central enhancement and instead show a fairly uniform enhancement in ΣSFR with radius. Moreover, the difference in central star formation between post-merger galaxies and noninteracting galaxies is more radially extended and pronounced when the global enhancement in star formation is larger. We conclude that post-merger galaxies create a unique signature in their resolved star-forming properties that is distinct from secular processes that can trigger similar global SFR enhancements.

The Calibration of Polycyclic Aromatic Hydrocarbon Dust Emission as a Star Formation Rate Indicator in the AKARI NEP Survey

Polycyclic aromatic hydrocarbon (PAH) dust emission has been proposed as an effective extinction-independent star formation rate (SFR) indicator in the mid-infrared, but this may depend on conditions in the interstellar medium. The coverage of the AKARI/Infrared Camera (IRC) allows us to study the effects of metallicity, starburst intensity, and active galactic nuclei on PAH emission in galaxies with f ν (L18W) ≲ 19 AB mag. Observations include follow-up, rest-frame optical spectra of 443 galaxies within the AKARI North Ecliptic Pole survey that have IRC detections from 7 to 24 μm. We use optical emission line diagnostics to infer SFR based on Hα and [O ii]λ λ3726, 3729 emission line luminosities. The PAH 6.2 μm and PAH 7.7 μm luminosities (L(PAH 6.2 μm) and L(PAH 7.7 μm), respectively) derived using multiwavelength model fits are consistent with those derived from slitless spectroscopy within 0.2 dex. L(PAH 6.2 μm) and L(PAH 7.7 μm) correlate linearly with the 24 μm dust-corrected Hα luminosity only for normal, star-forming “main-sequence” galaxies. Assuming multilinear correlations, we quantify the additional dependencies on metallicity and starburst intensity, which we use to correct our PAH SFR calibrations at 0 < z < 1.2 for the first time. We derive the cosmic star formation rate density (SFRD) per comoving volume from 0.15 ≲ z ≲ 1. The PAH SFRD is consistent with that of the far-infrared and reaches an order of magnitude higher than that of uncorrected UV observations at z ∼ 1. Starburst galaxies contribute ≳0.7 of the total SFRD at z ∼ 1 compared to main-sequence galaxies.

Ne v] emission from a faint epoch of reionization-era galaxy: evidence for a narrow-line intermediate-mass black hole

ABSTRACT Here, we present high spectral resolution ${\it JWST}$ NIRSpec observations of GN 42437, a low-mass (log(M$_\ast /{\rm M}_\odot)=7.9$), compact ($r_e \lt 500$pc), extreme starburst galaxy at $z=5.59$ with 13 emission-line detections. GN 42437 has a low metallicity (5–10 per cent Z$_\odot$) and its rest-frame H$\alpha$ equivalent width suggests nearly all of the observed stellar mass formed within the last 3 Myr. GN 42437 has an extraordinary 7$\sigma$ significant [Ne v] 3427 Å detection. The [Ne v] line has a rest-frame equivalent width of $11\pm 2$Å, [Ne v]/H$\alpha =0.04\pm 0.007$, [Ne v]/[Ne iii] 3870Å $= 0.26\pm 0.04$, and [Ne v]/He ii 4687Å $ = 1.2\pm 0.5$. Ionization from massive stars, shocks, or high-mass X-ray binaries cannot simultaneously produce these [Ne v] and other low-ionization line ratios. Reproducing the complete nebular structure requires both massive stars and accretion on to a black hole. We do not detect broad lines nor do the traditional diagnostics indicate that GN 42437 has an accreting black hole. Thus, the very high-ionization emission lines powerfully diagnose faint narrow-line black holes at high redshift. We approximate the black hole mass in a variety of ways as log(M$_{\rm BH}/{\rm M}_\odot) \sim 5{-}7$. This black hole mass is consistent with local relations between the black hole mass and the observed velocity dispersion, but significantly more massive than the stellar mass would predict. Very high-ionization emission lines may reveal samples to probe the formation and growth of the first black holes in the universe.

Chemical evolution of a young super star cluster at the Sunburst Arc

ABSTRACT Recent observations of high-redshift galaxies have revealed starburst galaxies with excessive amounts of nitrogen, well above that expected in standard evolutionary models. The Sunburst Arc galaxy, particularly its young and massive star cluster, represents the closest ($z=2.4$) and brightest of these as a strongly lensed object. In this work, we study the chemical history of this star cluster to determine the origin of the elevated gas-phase nitrogen using a chemical evolution model. Our model includes the enrichment of OB stars through stellar winds and core-collapse supernovae assuming that massive stars ($M\gt 25$ $\mathrm{ M}_\odot$) collapse directly into black holes at the end of their lives. We fit the model parameters to the observed chemical abundances of the Sunburst Arc cluster: O/H, C/O, and N/O. We find that the observed chemical abundances can be explained by models featuring intense star formation events, characterized by rapid gas accretion and high star formation efficiencies. Additionally, the stellar population contributing to the gas enrichment must exclude Wolf–Rayet stars. These conditions might be present in other nitrogen-rich objects as their similar chemical abundances suggest a common history. As previous studies have proposed the presence of Wolf–Rayet stars in the new nitrogen-rich objects, further research using chemodynamic modeling is necessary to ascertain the true nature of these objects.

The properties and kinematics of HCN emission across the closest starburst galaxy NGC 253 observed with ALMA

Context. Investigating molecular gas tracers, such as hydrogen cyanide (HCN), to probe higher densities than CO emission across nearby galaxies remains challenging. This is due to the large observing times required to detect HCN at a high sensitivity and spatial resolution. Although approximate kiloparsec scales of HCN maps are available for tens of galaxies, higher-resolution maps still need to be available. Aims. We aim to study the properties of molecular gas, the contrast in intensity between two tracers that probe different density regimes (the HCN(1–0)/CO(2–1) ratio), and their kinematics across NGC 253, one of the closest starburst galaxies. With its advanced capabilities, the Atacama Large Millimeter/submillimeter Array (ALMA) can map these features at a high resolution across a large field of view and uncover the nature of such dense gas in extragalactic systems. Methods. We present new ALMA Atacama Compact Array and Total Power (ACA+TP) observations of the HCN emission across NGC 253. The observations cover the inner 8.6′ of the galaxy disk at a spatial resolution of 300 pc. Our study examines the distribution and kinematics of the HCN-traced gas and its relationship with the bulk molecular gas traced by CO(2–1). We analyze the integrated intensity and mean velocity of HCN and CO along each line of sight. We also used the SCOUSE software to perform spectral decomposition, which considers each velocity component separately. Results. We find that the denser molecular gas traced by HCN piles up in a ring-like structure at a radius of 2 kpc. The HCN emission is enhanced by two orders of magnitude in the central 2 kpc regions, beyond which its intensity decreases with increasing galactocentric distance. The number of components in the HCN spectra shows a robust environmental dependence, with multiple velocity features across the center and bar. The HCN spectra exhibit multiple velocity features across the center and bar, which shows a robust environmental dependence. We have identified an increase in the HCN/CO ratio in these regions, corresponding to a velocity component likely associated with a molecular outflow. We have also discovered that the ratio between the total infrared luminosity and dense gas mass, which is an indicator of the star formation efficiency of dense gas, is anticorrelated with the molecular gas surface density up to approximately 200 M⊙ pc−2. However, beyond this point, the ratio starts to increase. Conclusions. We argue that using information about spectroscopic features of molecular emission is an important aspect of understanding molecular properties in galaxies.

Sizes and Stellar Masses of the Little Red Dots Imply Immense Stellar Densities

The “Little Red Dots” (LRDs) are red and compact galaxies detected in JWST deep fields, mainly in the redshift range z = 4–8. Given their compactness and the inferred stellar masses in the hypothesis that LRDs are starburst galaxies, the implied stellar densities are immense. This Research Note uses an extensive catalog of LRDs from the PRIMER and the COSMOS-Web surveys to investigate these densities. We find a median (upper limit) on the effective radius of 80 pc, which leads to median (lower limit) values of the core density of ∼104 M ⊙ pc−3, and individual densities as high as ∼108 M ⊙ pc−3, which is ∼10 times higher than the density necessary for runaway collisions to take place. For ∼35% of the LRDs investigated, the lower limits are higher than the highest stellar densities observed in any system in any redshift range.

HII galaxies as standard candles: Evolutionary corrections

Over the past decade, the relation between the Balmer-line luminosity of HII galaxies and the velocity width of the emission lines, the relation, has been painstakingly calibrated as a cosmological distance indicator with seemingly spectacular results: the Hubble constant and the energy density of dark energy obtained using the indicator agree remarkably well with the values from canonical indicators. Since most of the luminosity of these young compact starburst galaxies is emitted by a few narrow emission lines, they can be observed with good precision up to redshifts of $z with JWST, making the indicator a potentially unique cosmological probe. However, the precision of the method remains too low to effectively constrain the relevant cosmological parameters, notably the equation of state of dark energy. The scatter of the relation is significantly larger than the random observational errors, so we do not have a good handle on the systematics of the method. In a previous paper, we posited that since the ionizing radiation of these young galaxies fades rapidly over timescales of only a few million years, age differences could be the main underlying cause of the scatter. In this paper, we explore several different ways of explaining the scatter of the correlation, but without success. We show that the majority of HII galaxies are powered by multiple starbursts of slightly different ages, and therefore that the equivalent widths are not reliable chronometers to correct the luminosities for evolution. Thus, it is not likely that the accuracy of the distance indicator can be improved in the near future. Since we do not fully understand either the systematics or the underlying physics of the relation, using large samples of distant HII galaxies may or may not improve the accuracy of the method.

Unraveling the Mystery of the Low CO-to-H2 Conversion Factor in Starburst Galaxies: RADEX Modeling of the Antennae

CO emission has been widely used as a tracer of molecular gas mass. However, it is a long-standing issue to accurately constrain the CO-to-H2 conversion factor (α CO) that converts CO luminosity to molecular gas mass, especially in starburst galaxies. We present the first resolved α CO modeling results with multiple Atacama Large Millimeter/submillimeter Array CO and 13CO transition observations at both giant molecular cloud (GMC) scale at 150 pc and kiloparsec scale for one of the closest starburst mergers, the Antennae. By combining our CO modeling results and measurements of 350 GHz dust continuum, we find that most GMCs in the Antennae have α CO values approximately four times smaller than the commonly adopted Milky Way value (4.3). We find that α CO at GMC scales shows a strong dependence on CO intensity, 13CO/CO ratio, and GMC velocity dispersion, which is consistent with various theoretical and simulation predictions. Specifically, we suggest that the 13CO/CO line ratio and the velocity dispersion can be used to infer α CO in starburst regions. By applying our modeled α CO in GMC analyses, we find that GMCs in the Antennae are less gravitationally bound than in normal spiral galaxies, which is more consistent with what is predicted by merger simulations. At kiloparsec scale, we find that our modeled α CO values are smaller than the modeled α CO at GMC scale by 40%, which can be due to inclusion of a diffuse gas component with lower α CO values. We find a similar correlation of α CO and CO intensity at kiloparsec scales to that at GMC scales.

Constraining the hadronic properties of star-forming galaxies above 1 GeV with 15-years Fermi-LAT data

Star-forming and starburst galaxies (SFGs and SBGs) are considered to be powerful emitters of non-thermal γ-rays and neutrinos, due to their intense phases of star-formation activity, which should confine high-energy Cosmic-Rays (CRs) inside their environments. On this regard, the Fermi-LAT collaboration has found a correlation between the γ-ray and infrared luminosities for a sample of local sources. Yet, the physics behind these non-thermal emission is still under debate. We provide novel constraints on the tight relation between γ-rays and star formation rate (SFR) exploiting 15 years of public Fermi-LAT data. Thus, we probe the calorimetric fraction Fcal of high-energy protons in SFGs and SBGs, namely, the fraction of high-energy protons actually producing high-energy γ-rays and neutrinos. Further, we extrapolate this information to their diffuse γ-ray and neutrino emissions constraining their contribution to the extra-galactic gamma-ray background (EGB) and the diffuse neutrino flux. Using the publicly-available fermitools, we analyse 15.3 years of γ-ray between 1-1000 GeV data for 70 sources, 56 of which were not previously detected. We relate this emission to a theoretical model for SBGs in order to constrain Fcal for each source and then study its correlation with the star formation rate of the sources. Firstly, we find at 4σ level an indication of γ-ray emission for other two SBGs, namely M 83 and NGC 1365. By contrast, we find that, even with the new description of background, the significance for the γ-ray emission of M 33 (initially reported as discovered) still stands at ~ 4σ (as already reported by previous works).Along with previous findings, the flux of each detected source is consistent with a ~ E -2.3/2.4 spectrum, compatible with the injected CR flux inferred in the Milky-Way. We also notice that the correlation between Fcal and the SFR is in accordance with the expected scaling relation for CR escape dominated by advection. We remark that undiscovered sources strongly constrain Fcal at 95% CL, providing fundamental information when we interpret the results as common properties of SFGs and SBGs. Finally, we find that these sources might contribute (12 ± 3)% to the EGB, while the corresponding diffuse neutrino flux strongly depends on the spectral index distribution along the source class.

The roles of environment and interactions on the evolution of red and blue galaxies in the EAGLE simulation

We study the evolution of the red and blue galaxies from z=3 to z=0 using the EAGLE simulation. The galaxies in the blue cloud and the red sequence are separated at each redshift using a scheme based on Otsu's method. Our analysis shows that the two populations have small differences in the local density and the clustering strength until z=2, after which the red galaxies preferentially occupy the denser regions and exhibit a significantly stronger clustering than the blue galaxies. The significant disparities in cold gas mass and specific star formation rate (sSFR) observed before z=2 suggest that factors beyond environmental influences may also contribute to the observed dichotomy. Interacting galaxy pairs at a given separation exhibit a higher SFR at increasing redshifts, which may be linked to the rising gas fractions at higher redshift. As redshift decreases, the SFR decreases across all separations, suggesting a gradual depletion of the cold gas reservoir. At pair separations < 50 kpc, an anomalous increase in the SFR among paired galaxies in isolation around z ~ 2 suggests that environmental effects begin to dominate at this redshift, thereby increasing the rate of galaxy interactions and the occurrence of starburst galaxies. We observe a substantial decrease in the blue fraction in paired galaxies starting from z=1 to the present. However, the decrease in the blue fraction in paired galaxies with their second nearest neighbour at a distance greater than 500 kpc continues until z=0.5, after which the blue fraction begins to increase.

First Detection of CO Isotopologues in a High-redshift Main-sequence Galaxy: Evidence of a Top-heavy Stellar Initial Mass Function

Recent observations and theories have presented a strong challenge to the universality of the stellar initial mass function (IMF) in extreme environments. A notable example has been found for starburst conditions, where evidence favors a top-heavy IMF, i.e., there is a bias toward massive stars compared to the IMF that is responsible for the stellar mass function and elemental abundances observed in the Milky Way. Local starburst galaxies have star formation rates similar to those in high-redshift main-sequence galaxies, which appear to dominate the stellar mass budget at early epochs. However, the IMF of high-redshift main-sequence galaxies is yet to be probed. Since 13CO and C18O isotopologues are sensitive to the IMF, we have observed these lines toward four strongly lensed high-redshift main-sequence galaxies using the Atacama Large Millimeter/submillimeter Array. Of our four targets, SDSS J0901+1814, at z ≈ 2.26, is seen clearly in 13CO and C18O, the first detection of CO isotopologues in the high-redshift main-sequence galaxy population. The observed 13C/18O ratio, 2.4 ± 0.8, is significantly lower than that of local main-sequence galaxies. We estimate the isotope ratio, oxygen abundance, and stellar mass using a series of chemical evolution models with varying star formation histories and IMFs. All models favor an IMF that is more top-heavy than that of the Milky Way. Thus, as with starburst galaxies, main-sequence galaxies in the high-redshift Universe have a greater fraction of massive stars than a Milky Way IMF would imply.

The ALMA-ALPAKA survey. II. Evolution of turbulence in galaxy disks across cosmic time: Difference between cold and warm gas

The gas in the interstellar medium (ISM) of galaxies is supersonically turbulent. Measurements of turbulence typically rely on cold gas emission lines for low-$z$ galaxies and warm ionized gas observations for $z &gt; 0$ galaxies. Studies of warm gas kinematics at $z &gt; 0$ conclude that the turbulence strongly evolves as a function of redshift, due to the increasing impact of gas accretion and mergers in the early Universe. However, recent findings suggest potential biases in turbulence measurements derived from ionized gas at high-$z$, impacting our understanding of turbulence origin, ISM physics and disk formation. We investigate the evolution of turbulence using velocity dispersion (sigma ) measurements from cold gas tracers (i.e., CO CI CII ). The initial dataset comprises 17 galaxy disks with high data quality from the ALPAKA sample, supplemented with galaxies from the literature, resulting in a sample of 57 galaxy disks spanning the redshift range $z = 0 - 5$. This extended sample consists of main-sequence and starburst galaxies with stellar masses $ M_ odot $. The comparison with current Halpha kinematic observations and existing models demonstrates that the velocity dispersion inferred from cold gas tracers differ by a factor of $ 3$ from those obtained using emission lines tracing the warm, ionized gas. We show that stellar feedback is the main driver of turbulence measured from cold gas tracers and the physics of turbulence driving does not appear to evolve with time. This is fundamentally different from the conclusions of studies based on warm gas, which had to consider additional turbulence drivers to explain the high values of sigma . We present a model predicting the redshift evolution of turbulence in galaxy disks, attributing the increase of sigma with redshift to the higher energy injected by supernovae due to the elevated star-formation rate in high-$z$ galaxies. This supernova-driven model suggests that turbulence is lower in galaxies with lower stellar mass compared to those with higher stellar mass. Additionally, it forecasts the evolution of sigma in Milky-Way like progenitors.

Fine-grained photometric classification using multi-model fusion method with redshift estimation

The modern sky surveys accelerates astronomical data collection. We proposed a multi-model fusion method aimed at comprehensive and fine-grained astronomical source classification. This method incorporates a redshift estimation model using the mixture density network into a source classification model. Based on 1.2 million sources from the SDSS and the ALLWISE, we performed three-class experiments for stars, quasars, and galaxies, four-class experiments to further classify galaxies into normal and emission-line galaxies (NGs; ELGs), and seven-class experiments where ELG were refined into active galactic nuclei (AGNs), broad-line galaxies (BLs), star-forming galaxies (SFs), and starburst galaxies (SBs). In all experiments, our proposed method is superior to direct classification. In three- and four-class, we obtains 0.77% and 1.14% improvement in accuracy, demonstrating the effectiveness of adding redshift estimation. Meanwhile, three machine learning algorithms were stacked into one by us to finish fine-grained classification, which achieved an accuracy of 78.5%, with F1 scores of 99.2% for stars, 97% for quasars, 64.3% for NGs, 60.8% for AGNs, 68.3% for BLs, 87.2% for SBs, and 71.3% for SFs. The NMAD and R2 for the redshift estimation part of our method are 0.18 and 0.916, while it has only 2.65% outliers. The method we proposed further mines the information contained in the photometry to achieve comprehensive and fine-grained classification, which will be beneficial for immediate analysis in large-scale surveys. Besides, this method can leverage feature importance to stimulate new insights for astronomers.

A two-step strategy to identify episodic sources of gravitational waves and high-energy neutrinos in starburst galaxies

A two-step strategy to identify episodic sources of gravitational waves and high-energy neutrinos in starburst galaxies