Milky Way Research Articles

Peculiarities of the chemical enrichment of metal-poor stars in the Milky Way Galaxy

The oldest stars in the Milky Way are metal-poor with Fe/H $<$ -- 1.0, displaying peculiar elemental abundances compared to solar values. The relative variations in the chemical compositions among stars is also increasing with decreasing stellar metallicity, allowing for the pure signature of unique nucleosynthesis processes to be revealed. The study of the $r$-process is, for instance, one of the main goals of stellar archaeology and metal-poor stars exhibit an unexpected complexity in the stellar production of the $r$-process elements in the early Galaxy. In this work, we report the atmospheric parameters, main dynamic properties, and the abundances of four metal-poor stars: HE 1523-–0901, HD 6268, HD 121135, and HD 195636 (--1.5 $>$ Fe/H The abundances were derived from spectra obtained with the HRS echelle spectrograph at the Southern African Large Telescope, using both local and non-local thermodynamic equilibrium (LTE and NLTE) approaches, with the average error between 0.10 and 0.20 dex. Based on their kinematical properties, we show that HE 1523--0901 and HD 195636 are halo stars with typical high velocities. In particular, HD 121135 displays a peculiar kinematical behaviour, making it unclear whether it is a halo or an accreted star. Furthermore, HD 6268 is possibly a rare prototype of very metal-poor thick disk stars. The abundances derived for our stars are compared with theoretical stellar models and with other stars with similar metallicity values from the literature. HD 121135 is Al-poor and Sc-poor, compared to stars observed in the same metallicity range (--1.62 $>$ Fe/H $>$--1.12). The most metal-poor stars in our sample, HE 1523 -- 0901, HD 6268, and HD 195636, exhibit anomalies that are better explained by supernova models from fast-rotating stellar progenitors for elements up to the Fe group. Compared to other stars in the same metallicity range, their common biggest anomaly is represented by the low Sc abundances. If we consider the elements beyond Zn, HE 1523--0901 can be classified as an r-II star, HD 6268 as an r-I candidate, and HD 195636 and HD 121135 exhibiting a borderline $r$-process enrichment between limited-r and r-I star. Significant relative differences are observed between the $r$-process signatures in these stars.

Evolutionary nature of puffed-up stripped star binaries and their occurrence in stellar populations

The majority of massive stars are formed in multiple systems, and at some point during their life, they interact with their companions via mass transfer. This interaction typically leads to the primary shedding its outer envelope, resulting in the formation of a "stripped star". Classically, stripped stars are expected to quickly contract to become hot and UV-bright helium stars. Surprisingly, recent optical spectroscopic surveys have unveiled many stripped stars that are much larger and cooler, appearing "puffed up" and overlapping with the Main Sequence (MS) in the Hertzsprung-Russell diagram. Here, we study the evolutionary nature of puffed-up stripped (PS) stars and the duration of this enigmatic phase using the stellar-evolution code MESA. We computed grids of binary models at four metallicities: Solar (Z$_ odot $ = 0.017), Large Magellanic Cloud (LMC, Z=0.0068), Small Magellanic Cloud (SMC, Z = 0.0034), and Z = 0.1Z$_ Contrary to previous assumptions, we find that stripped stars regain thermal equilibrium shortly after the end of mass transfer and maintain it during most of the PS phase. Their further contraction towards hot and compact He stars is determined by the rate at which the residual H-rich envelope is depleted, with the main agents being H-shell burning (dominant for M lesssim 50 and mass-loss in winds. The duration of the PS star phase is approximately 10<!PCT!> of the core-He burning lifetime (1<!PCT!> total lifetime) and up to 100 times more than the thermal timescale. It decreases with increasing mass and luminosity and increases with metallicity. We explored several factors influencing PS star lifetimes: orbital period, mass ratio, winds, and semiconvection. We further carried out a simple binary population synthesis estimation, finding that sim 0.5 – 0.7<!PCT!> of all the stars with log (L/L$_ > 3.7 may, in fact, be PS stars. Our results indicate that tens to hundreds of PS stars in post-interaction binaries may be hiding in the MS population, disguised as 'normal' stars: sim 100 (sim 280) in the SMC (LMC) and sim 1500 in the Milky Way . Their true nature may be revealed by spectroscopically measured low surface gravities, high N enrichment, and likely slow rotation rates.

Metallicities for more than 10 million stars derived from Gaia BP/RP spectra

The third $Gaia$ Data Release, which includes BP/RP spectra for 219 million sources, has opened a new window into the exploration of the chemical history and evolution of the Milky Way. The wealth of information encapsulated in these data is far greater than their low resolving power ($R would suggest at first glance, as shown in many studies. We zeroed in on the use of these data for the purpose of the detection of "new" metal-poor stars, which are hard to find yet essential for understanding several aspects of the origin of the Galaxy, star formation, and the creation of the elements, among other topics. We strive to refine a metal-poor candidate selection method that was developed with simulated $Gaia$ BP/RP spectra with the ultimate objective of providing the community with both a recipe to select stars for medium and high resolution observations, and a catalog of stellar metallicities. We used a dataset comprised of GALAH DR3 and SAGA database stars in order to verify and adjust our selection method to real-world data. For that purpose, we used dereddening as a means to tackle the issue of extinction, and then we applied our fine-tuned method to select metal-poor candidates, which we thereafter observed and analyzed. We were able to infer metallicities for GALAH DR3 and SAGA stars with color excesses up to $E(B-V)<1.5$ and an uncertainty of $ Fe/H inf 0.36$, which is good enough for the purpose of identifying new metal-poor stars. Further, we selected 26 metal-poor candidates via our method for observations. As spectral analysis showed, 100<!PCT!> of them had $ Fe/H <-2.0$, 57<!PCT!> had $ Fe/H <-2.5$, and 8<!PCT!> had $ Fe/H <-3.0$. We inferred metallicities for these stars with an uncertainty of $ Fe/H inf as was proven when comparing $ Fe/H inf $ to the spectroscopic $ Fe/H $. Finally, we assembled a catalog of metallicities for 10 861 062 stars.

Do spiral arms enhance star formation efficiency?

Spiral arms, as those of our own Milky Way, are some of the most spectacular features in disc galaxies. It has been argued that star formation should proceed more efficiently in spiral arms as a result of gas compression. Yet, observational studies have so far yielded contradictory results. Here, we examine arm/interarm surface density contrasts at sim 100\,pc resolution in 28 spiral galaxies from the PHANGS survey. We find that the arm AND interarm contrast in stellar mass surface density ($ is very modest, typically a few tens of percent. This is much smaller than the contrasts measured for molecular gas ($ mol $) or star formation rate ($ SFR $) surface density, which typically reach a factor of $ sim 3$. However, $ mol $ and $ SFR $ contrasts show a significant correlation with the enhancement in $ suggesting that the small stellar contrast largely dictates the stronger accumulation of gas and star formation. All these contrasts increase for grand-design spirals compared to multi-armed and flocculent systems (and for galaxies with high stellar mass). The median star formation efficiency (SFE) of the molecular gas is $16$<!PCT!> higher in spiral arms than in interarm regions, with a large scatter, and the contrast increases significantly (median SFE contrast $2.34$) for regions of particularly enhanced stellar contrast ($ contrast $>1.97$). The molecular-to-atomic gas ratio ($ mol atom $) is higher in spiral arms, pointing to a transformation of atomic to molecular gas. As a consequence, the total gas contrast ($ mol atom $) slightly drops compared to $ mol $ (median $4$<!PCT!> lower, working at sim kpc resolution), while the SFE contrast increases when we include atomic gas (median $8$<!PCT!> higher than for $ mol $). The contrasts show important fluctuations with galactocentric radius. We confirm that our results are robust against a number of effects, such as spiral mask width, tracers, resolution, and binning. In conclusion, the boost in the SFE of molecular gas in spiral arms is generally modest or absent, except for locations with exceptionally large stellar contrasts.

Detecting boosted dark photons with gaseous detectors

We search for indirect signals of O(keV) dark matter annihilating or decaying into O(eV) dark photons. These dark photons will be highly boosted, predominantly transversely polarized, have decay lengths larger than the Milky Way, and can be absorbed by neutrino or dark matter experiments at a rate dependent on the photon-dark photon kinetic mixing parameter and the optical properties of the experiment. We show that current experiments cannot probe new parameter space, but future large-scale gaseous detectors with low backgrounds (i.e., CYGNUS, NEXT, PANDAX-III) may be sensitive to this signal when the annihilation cross section is especially large. Published by the American Physical Society 2024

HI discs of L* galaxies as probes of the baryonic physics of galaxy evolution

Abstract Understanding what shapes the cold gas component of galaxies, which both provides the fuel for star formation and is strongly affected by the subsequent stellar feedback, is a crucial step towards a better understanding of galaxy evolution. Here, we analyse the HI properties of a sample of 46 Milky Way halo-mass galaxies, drawn from cosmological simulations (EMP-Pathfinder and Firebox). This set of simulations comprises galaxies evolved self-consistently across cosmic time with different baryonic sub-grid physics: three different star formation models [constant star formation efficiency (SFE) with different star formation eligibility criteria, and an environmentally-dependent, turbulence-based SFE] and two different feedback prescriptions, where only one sub-sample includes early stellar feedback. We use these simulations to assess the impact of different baryonic physics on the HI content of galaxies. We find that the galaxy-wide HI properties agree with each other and with observations. However, differences appear for small-scale properties. The thin HI discs observed in the local Universe are only reproduced with a turbulence-dependent SFE and/or early stellar feedback. Furthermore, we find that the morphology of HI discs is particularly sensitive to the different physics models: galaxies simulated with a turbulence-based SFE have discs that are smoother and more rotationally symmetric, compared to those simulated with a constant SFE; galaxies simulated with early stellar feedback have more regular discs than supernova-feedback-only galaxies. We find that the rotational asymmetry of the HI discs depends most strongly on the underlying physics model, making this a promising observable for understanding the physics responsible for shaping the interstellar medium of galaxies.

A Sagittarius-like simulated dwarf spheroidal galaxy from TNG50

The Sagittarius dwarf spheroidal (Sgr dSph) galaxy provides one of the most convincing examples of tidal interaction between satellite galaxies and the Milky Way (MW). The main body of the dwarf was recently demonstrated to have an elongated, prolate, bar-like shape and to possess some internal rotation. Whether these features are temporary results of the strong tidal interaction at the recent pericenter passage or are due to a disky progenitor is a matter of debate. I present an analog of Sgr selected among bar-like galaxies from the TNG50 simulation of the IllustrisTNG project. The simulated dwarf is initially a disky galaxy with mass exceeding $10^ $ M$_ and evolves around a MW-like host on a tight orbit with seven pericenter passages and a period of about 1 Gyr. At the second pericenter passage, the disk transforms into a bar and the bar-like shape of the stellar component is preserved until the end of the evolution. The morphological transformation is accompanied by strong mass loss, leaving a dwarf with a final mass of below $ M$_ The gas is lost completely and the star formation ceases at the third pericenter passage. At the last pericenters, the dwarf possesses a bar-like shape, a little remnant rotation, and the metallicity gradient, which are consistent with observations. The more concentrated metal-rich stellar population rotates faster and has a lower velocity dispersion than the more extended metal-poor one. The metallicity distribution evolves so that the most metal-poor stars are stripped first, which explains the metallicity gradient detected in the Sgr stream. This study demonstrates that a dSph galaxy with properties akin to the Sgr dwarf can form from a disky progenitor with a mass of above $ M$_ by tidal evolution around the MW in the cosmological context.

Simulations of galaxies in an expanding Universe with modified Newtonian dynamics (MOND) and with modified gravitational attractions (MOGA)

The stability of galaxies is either explained by the existence of dark matter or caused by a modification of Newtonian acceleration (MOND). Here, we show that the modification of the Newtonian dynamics can equally well be obtained by a modification of Newton’s law of universal gravitational attraction (MOGA) when Newton’s inverse square attraction from a distant object is replaced with an inverse attraction. This modification is often proposed in the standard model, and with the modification of the attraction caused by dark matter. The recently derived algorithm, Eur Phys J Plus, 137:99, 2022; Class Quantum Grav, 39:225006, 2022, for classical celestial dynamics is used to simulate models of the Milky Way in an expanding universe and with either MOND or MOGA. The simulations show that the galaxies with MOND dynamics are unstable, whereas MOGA stabilizes the galaxies. The rotation velocities for objects in galaxies with classical Newtonian dynamics decline inversely proportional to the square root of the distance r to the center of the galaxy. But the rotation velocities is relatively independent of r for MOGA and qualitatively in agreement with experimentally determined rotation curves for galaxies in the Universe. The modification of the attractions may be caused by the masses of the objects in the central part of the galaxy by the lensing of gravitational waves from far-away objects in the galaxy.

Molecular gas scaling relations for local star-forming galaxies in the low-M_* regime

We derived molecular gas fractions ($f_ mol =M_ mol /M_*$) and depletion times ($ mol = M_ mol SFR $) for 353 galaxies representative of the local star-forming population with $10^ \,M_ < M_* < \,M_ drawn from the ALLSMOG and xCOLDGASS surveys of CO(2-1) and CO(1-0) line emission. By adding constraints from low-mass galaxies and upper limits for CO non-detections, we find the median molecular gas fraction of the local star-forming population to be constant at $ f_ mol $, challenging previous reports of increased molecular gas fractions in low-mass galaxies. Above $M_* \,M_ we find the $f_ mol $ versus $M_*$ relation to be sensitive to the selection criteria for star-forming galaxies. We tested the robustness of our results against different prescriptions for the CO-to-H$_2$ conversion factor and different selection criteria for star-forming galaxies. The depletion timescale $ mol $ weakly depends on $M_*$, following a power law with a best-fit slope of . This suggests that small variations in specific star formation rate ($ sSFR=SFR /M_*$) across the local main sequence of star-forming galaxies with $M_* < 10^ \,M_ are mainly driven by differences in the efficiency of converting the available molecular gas into stars. We tested these results against a possible dependence of $f_ mol $ and $ mol $ on the surrounding (group) environment of the targets by splitting them into centrals, satellites, and isolated galaxies, and find no significant variation between these populations. We conclude that the group environment is unlikely to have a large systematic effect on the molecular gas content of star-forming galaxies in the local Universe.

Satellite group infall into the Milky Way: Exploring the Crater-Leo case with new HST proper motions

Within Lambda cold dark matter (Lambda CDM) simulations, Milky Way-like galaxies accrete some of their satellite galaxies in groups of 3-5 members rather than individually, and this has been suggested as a possible mechanism driving the formation of satellite planes. Objects accreted in groups are expected to share similar specific total energy and angular momentum, and to also have identical orbital planes and directions. Looking at observations of Milky Way satellites, the dwarf galaxies Leo II, IV, V, and Crater II, and the star cluster Crater 1 were proposed to be a vestige of group infall. The suggested `Crater-Leo group' shows a coherent distance gradient and all these objects align along a great circle on the sky. We used proper motion data to investigate whether the phase-space distribution of the members of the proposed group are indeed consistent with group infall. To further investigate this possibility, we used Gaia Data Release 3 (DR3) and new Hubble Space Telescope (HST) proper motions -- namely, $( alpha* 0.0523)$ mas $ for Leo IV and $( alpha* 0.1704)$ mas $ for Leo V -- to derive accurate orbital properties for the proposed group objects. In addition, we explored other possible members of this putative association. par Leo II, Leo IV, and Crater 1 show orbital properties consistent with those we predict from assuming group infall. However, our results suggest that Crater II was not accreted with the rest of the objects. If confirmed with increasingly accurate proper motions in the future, the Crater-Leo objects would appear to constitute the first identified case of a cosmologically expected, typical group infall event, as opposed to the highly hierarchical Magellanic Cloud system.

New dark matter analysis of milky way dwarf satellite galaxies with madhatv2

New dark matter analysis of milky way dwarf satellite galaxies with madhatv2

Studying [CII] emission in low-mass galaxies at z ∼ 7

Abstract We report on a $\rm {[CII]}_{158\mu \rm {m}}$ search using the Atacama Large Millimeter/submillimeter Array (ALMA) on three lensed, confirmed Lyα emitting galaxies at z ∼ 7. Our targets are ultra-violet (UV) faint systems with stellar masses on the order of M* ∼ 109M⊙. We detect a single [CII] line emission (4σ) from the brightest (L ∼ 2.4 × 1010L⊙) galaxy in our sample, MACS0454-1251. We determine a systemic redshift (z[CII] = 6.3151 ± 0.0005) for MACS0454-1251 and measure a Lyα velocity offset of $\Delta v \approx 300 \pm 70 \rm {km\, s}^{-1}$. The remaining two galaxies we detect no [CII] but provide 3σ upper limits on their [CII] line luminosities which we use to investigate the $L_{\textrm {[CII]}} - \rm {SFR}$ relation. Overall our single [CII] detection shows agreement with the relation for local dwarf galaxies. Our [CII] deficient galaxies could potentially be exhibiting low metallicities (Z < Z⊙). Another possible explanation for weaker [CII] emission could be strong feedback from star formation disrupting molecular clouds. We do not detect continuum emission in any of the sources, placing upper limits on their dust masses. Assuming a single dust temperature of $T_{d}=35 \rm {K}$ dust masses (Mdust) range from <4.8 × 107M⊙ to 2.3 × 108M⊙. Collectively, our results suggest faint reionization era sources could be metal poor and/or could have strong feedback suppressing [CII] emission.

Detailed Mapping of the Galactic Disk Structure in the solar neighborhood through LAMOST K Dwarfs

Abstract The Galactic disk is one of the main components of the Milky Way, which contributes most of the luminosity. Its structure is essential for understanding the formation and evolution of the Milky Way. Using 174,443 K-type dwarf stars observed by both LAMOST and Gaia DR3, we study the disk density profile in the local volume within 1,200 pc. In the azimuthal dimension, we find strong asymmetric signal of the thin disk. The surface density and the scale height of the southern disk significantly change versus the azimuthal angle at the same galactocentric distance R. Meanwhile, in the vertical dimension, the scale height of the northern disk has quite different trend than that of the southern one. The scale height of the southern disk shows a decreasing trend with φ ∼ −2.5○, and change to an increasing one with φ ∼ 5.0○. Meanwhile, the scale height of the northern disk has a consistently smaller increase. Finally, we divide the entire sample into three subsamples based on metallicity and all three subsamples show significant non-axisymmetric and north-south asymmetric signals in the Galactic disk. Furthermore, we find that the scale height of the metal-poor ([Fe/H] < -0.4 dex) subsample in the northern disk is greater than that of the metal-rich ([Fe/H] > -0.1 dex) subsample. However, in the southern disk, the scale height exhibits varying relationships across different metallicity slices.

Asymmetric drift in MaNGA: Mass and radially-dependent stratification rates in galaxy disks

Abstract We measure the age-velocity relationship from the lag between ionized gas and stellar tangential speeds in ∼500 nearby disk galaxies from MaNGA in SDSS-IV. Selected galaxies are kinematically axisymmetric. Velocity lags are asymmetric drift, seen in the Milky Way’s (MW) solar neighborhood and other Local Group galaxies; their amplitude correlates with stellar population age. The trend is qualitatively consistent in rate ($\dot{\sigma }$) with a simple power-law model where σ ∝ tb that explains the dynamical phase-space stratification in the solar neighborhood. The model is generalized based on disk dynamical times to other radii and other galaxies. We find in-plane radial stratification parameters σ0, r (dispersion of the youngest populations) in the range of 10-40 km s−1 and 0.2 < br < 0.5 for MaNGA galaxies. Overall br increases with galaxy mass, decreases with radius for galaxies above 10.4 dex (M⊙) in stellar mass, but is ∼constant with radius at lower mass. The measurement scatter indicates the stratification model is too simple to capture the complexity seen in the data, unsurprising given the many possible astrophysical processes that may lead to stellar population dynamical stratification. Nonetheless, the data show dynamical stratification is broadly present in the galaxy population, with systematic trends in mass and density. The amplitude of the asymmetric drift signal is larger for the MaNGA sample than the MW, and better represented in the mean by what is observed in the disks of M31 and M33. Either typical disks have higher surface-density or, more likely, are dynamically hotter (hence thicker) than the MW.

Three-dimensional stellar orbits due to off-centered dark matter halo at the center of the disc galaxies

Stellar orbits and the evolution of galaxies are intertwined processes that have long-term implications on each other. This paper studies how stellar orbits at the galaxy’s central region are disturbed by an asymmetric dark matter halo potential. Evidence from the observations and simulations in the Milky Way type galaxy suggests that the center of the dark matter halo could be off-centered by a few parsecs concerning the center of the core. The equations of motion of stars in the core of galaxies are expressed in terms of three-dimensional perturbed potential arising from the offset halo. The central region’s azimuthal variation in the effective potential is obtained and the first-order epicyclic theory is used to solve for the orbits. The magnitude of this perturbation potential grows at small radii and exhibits m=1 azimuthal fluctuations. In the central region, within 3 kpc radius, even a small halo offset of 300 pc can cause a surprisingly strong spatial and kinematical lopsidedness. A planar orbit, initially assumed to be in disc plane, tends to leave the plane giving rise to non-planar configuration. Furthermore, as long as the halo offset persists, the central region will stay lopsided. The dark matter halo would significantly impact the dynamic development of this region and could help fuel the active galactic nucleus.

Autumn's Sky

This is an image of the night sky in the northern hemisphere’s autumn. If you look carefully, you can see the Milky Way, the constellation Cassiopeia, and even the Milky Way’s biggest neighbor, the Andromeda Galaxy.

FRBs from rapid spindown neutron stars

Abstract A fast radio burst (FRB) localized to a globular cluster (GC) challenges FRB models involving ordinary young magnetars. In this paper, we examine the rapid spindown millisecond neutron star (NS) scenario, which favours the dynamic environment in GCs. Fast spindown corresponds to a larger magnetic field than regular millisecond pulsars, which empirically favours giant pulse (GP) emission. The kinetic energy in millisecond NSs can readily exceed the magnetic energy in magnetars. The high inferred isotropic luminosity of most FRBs is challenging to explain in spin-down powered pulsars. A recent observation of a GP from the Crab pulsar, on the other hand, suggests highly Doppler-beamed emission, making the required energy orders of magnitude smaller than estimated with isotropic assumptions. Considering this strong beaming effect, GPs from a recycled pulsar with a modest magnetic field could explain the energetics and burst rates for a wide range of FRBs. The short life span accounts for a paucity of bright FRBs in the Milky Way neighbourhood. We point out that tidal disruption spin-up from a main sequence star can provide sufficient accretion rate to recycle a NS with mild magnetic field. It can also explain the observed source density and the spatial offset in the GC for FRB 20200120E. Frequency variation in the scattering tail for some of the brightest FRBs is expected in this scenario.

Letting Pulsars Sing: Sonification With Granular Synthesis

An astronomy sonification project has been initiated to create sound and music from the data of pulsars in space. Pulsars are formed when some stars burn out all of their fuel and emit electromagnetic radiation, which hits earth periodically as the pulsar rotates. Each pulsar has unique characteristics. The source of the data is the online Pulsar Catalog from the Australian National Telescope Facility. The first result is a stereo fixed media composition, From Orion to Cassiopeia, which reveals a sweep of much of the Milky Way, displaying audio for many of the known pulsars. Galactic longitude, rotation speed, pulse width, mean flux density, age, and distance are mapped to granular synthesis parameters. Sound event duration, amplitude, amount of reverberation, grain rate, grain duration, grain frequency, and panning are controlled by the data. The piece was created with the new SGRAN2() instrument in the RTcmix music programming language.

The Gaia-ESO Survey: No sign of multiple stellar populations in open clusters from their sodium and oxygen abundances

The light element (anti-)correlations shown by globular clusters (GCs) are the main spectroscopic signature of multiple stellar populations. These internal abundance variations provide us with fundamental constraints on the formation mechanism of stellar clusters. Using $Gaia$-ESO, the largest and most homogeneous survey of open clusters (OCs), we intend to check whether these stellar aggregates display the same patterns. Based on previous studies of many GCs, several young and massive clusters in the Magellanic Clouds, as well as a few OCs, we do not expect to find any anti-correlation, given the low mass of Milky Way OCs. We used the results based on UVES spectra of stars in $Gaia$-ESO to derive the distribution of Na and O abundances and see whether they show an unexplained dispersion or whether they are anti-correlated. By selecting only high-probability members with high-precision stellar parameters, we ended up with more than 700 stars in 74 OCs. We examined the O-Na distribution in 28 OCs with at least 4 stars available as well as the Na distribution in 24 OCs, with at least 10 stars available. We find that the distribution of Na abundances is compatible with a single-value population, within the errors. The few apparent exceptions can be explained by differences in the evolutionary phase (main sequence and giant post first dredge-up episode) or by difficulties in analysing low gravity giants. We did not find any indication of an Na-O anti-correlation in any of the clusters for which O has been derived. Based on the very small spread we find, OCs maintain the status of single stellar populations. However, a definitive answer requires studying more elements and larger samples covering different evolutionary phases. This will be possible with the next generation of large surveys.

The relationship between the turnover frequency and photo-ionization in radio sources

ABSTRACT We investigate the connection between the turnover frequency in the radio spectrum, νTO, and the rate of ionizing ultraviolet photons, $Q_{\rm {H{\small I}}}$, in extragalactic sources. From a large, optically selected sample, we find νTO to be correlated with $Q_{\rm {H\,{\small I}}}$ in sources that exhibit a turnover. The significance of the correlation decreases when we include power-law radio sources as limits, by assuming that the turnover frequency occurs below the lowest value observed. However, the power-law fit sources are less well sampled across the band and so these may just be contributing noise to the data. Given that the observed νTO–$Q_{\rm {H{\small I}}}$ correlation is purely empirical, we use the ionizing photon rate to obtain the electron density in a free–free absorption model. For each of the constant, exponential, constant plus exponential (Milky Way), and spherical models of the gas distribution, there is also an increase in the turnover frequency with ionizing photon rate. Furthermore, for a given gas mass, we find that the turnover frequency is anticorrelated with the scale-factor of the gas density. While other mechanisms, such as ageing electrons or synchrotron self-absorption, may be required to reproduce the spectral indices, for an exponential scale-factor similar to the linear size, this simple free–free absorption model reproduces the turnover-size correlation seen in radio sources.