Unit Star Formation Rate Research Articles

Population synthesis of Be X-ray binaries: metallicity dependence of total X-ray outputs

ABSTRACT X-ray binaries (XRBs) are thought to regulate cosmic thermal and ionization histories during the Epoch of Reionization and Cosmic Dawn (z ∼ 5–30). Theoretical predictions of the X-ray emission from XRBs are important for modelling such early cosmic evolution. Nevertheless, the contribution from Be-XRBs, powered by accretion of compact objects from decretion discs around rapidly rotating O/B stars, has not been investigated systematically. Be-XRBs are the largest class of high-mass XRBs (HMXBs) identified in local observations and are expected to play even more important roles in metal-poor environments at high redshifts. In light of this, we build a physically motivated model for Be-XRBs based on recent hydrodynamic simulations and observations of decretion discs. Our model is able to reproduce the observed population of Be-XRBs in the Small Magellanic Cloud with appropriate initial conditions and binary stellar evolution parameters. We derive the X-ray output from Be-XRBs as a function of metallicity in the (absolute) metallicity range Z ∈ [10−4, 0.03] with a large suite of binary population synthesis (BPS) simulations. The simulated Be-XRBs can explain a non-negligible fraction ($\gtrsim 30{{\ \rm per\ cent}}$) of the total X-ray output from HMXBs observed in nearby galaxies for Z ∼ 0.0003–0.02. The X-ray luminosity per unit star formation rate from Be-XRBs in our fiducial model increases by a factor of ∼8 from Z = 0.02 to Z = 0.0003, which is similar to the trend seen in observations of all types of HMXBs. We conclude that Be-XRBs are potentially important X-ray sources that deserve greater attention in BPS of XRBs.

CASCO: Cosmological and AStrophysical parameters from Cosmological simulations and Observations – I. Constraining physical processes in local star-forming galaxies

ABSTRACT We compare the structural properties and dark matter content of star-forming galaxies taken from the Cosmology and Astrophysics with MachinE Learning Simulations (camels) cosmological simulations to the observed trends derived from the SPARC sample in the stellar mass range $[10^{9}, 10^{11}]\, \textrm {M}_{\odot }$, to provide constraints on the value of cosmological and astrophysical (supernova- and active galactic nucleus-related) parameters. We consider the size–, internal DM fraction–, internal DM mass–, and total–stellar mass relations for all the 1065 simulations, all having different cosmological and astrophysical parameters, from the IllustrisTNG, SIMBA, and ASTRID suites of camels, and search for the parameters that minimize the χ2 with respect to the observations. For the IllustrisTNG suite, we find the following constraints for the cosmological parameters: $\Omega _{\textrm {m}} = 0.27_{-0.05}^{+0.01}$, $\sigma _{8} = 0.83_{-0.11}^{+0.08}$, and $S_{8} = 0.78_{-0.09}^{+0.03}$, which are consistent within 1σ with the results from the 9-yr WMAP observations. SN feedback-related astrophysical parameters, which describe the departure of outflow wind energy per unit star formation rate and wind velocity from the reference IllustrisTNG simulations, assume the following values: $A_{\textrm {SN1}} = 0.48_{-0.16}^{+0.25}$ and $A_{\textrm {SN2}} = 1.21_{-0.34}^{+0.03}$, respectively. Therefore, simulations with a lower value of outflow wind energy per unit star formation rate with respect to the reference illustrisTNG simulation better reproduce the observations. Variation of active galactic nucleus feedback parameters, on the other hand, shows negligible effects on the scaling relation trends in the mass range probed. Simulations based on SIMBA and ASTRID suites predict central dark matter masses substantially larger than those observed in real galaxies, which can be reconciled with observations only by requiring values of Ωm inconsistent with cosmological constraints for SIMBA, or simulations characterized by unrealistic galaxy mass distributions for ASTRID.

Elevated Hot Gas and High-mass X-Ray Binary Emission in Low-metallicity Galaxies: Implications for Nebular Ionization and Intergalactic Medium Heating in the Early Universe

High-energy emission associated with star formation has been proposed as a significant source of interstellar medium (ISM) ionization in low-metallicity starbursts and an important contributor to the heating of the intergalactic medium (IGM) in the high-redshift (z ≳ 8) universe. Using Chandra observations of a sample of 30 galaxies at D ≈ 200–450 Mpc that have high specific star formation rates of 3–9 Gyr−1 and metallicities near Z ≈ 0.3Z ⊙, we provide new measurements of the average 0.5–8 keV spectral shape and normalization per unit star formation rate (SFR). We model the sample-combined X-ray spectrum as a combination of hot gas and high-mass X-ray binary (HMXB) populations and constrain their relative contributions. We derive scaling relations of /SFR = 40.19 ± 0.06 and /SFR significantly elevated compared to local relations. The HMXB scaling is also somewhat higher than –SFR-Z relations presented in the literature, potentially due to our galaxies having relatively low HMXB obscuration and young and X-ray luminous stellar populations. The elevation of the hot gas scaling relation is at the level expected for diminished attenuation due to a reduction of metals; however, we cannot conclude that an –SFR-Z relation is driven solely by changes in ISM metal content. Finally, we present SFR-scaled spectral models (both emergent and intrinsic) that span the X-ray-to-IR band, providing new benchmarks for studies of the impact of ISM ionization and IGM heating in the early universe.

HST grism spectroscopy of z ∼ 3 massive quiescent galaxies

Tracing the emergence of the massive quiescent galaxy (QG) population requires the build-up of reliable quenched samples by distinguishing these systems from red, dusty star-forming sources. We present Hubble Space Telescope WFC3/G141 grism spectra of ten quiescent galaxy candidates selected at 2.5 < z < 3.5 in the COSMOS field. Spectroscopic confirmation for the whole sample is obtained within one to three orbits through the detection of strong spectral breaks and Balmer absorption lines. When their spectra are combined with optical to near-infrared photometry, star-forming solutions are formally rejected for the entire sample. Broad spectral indices are consistent with the presence of young A-type stars, which indicates that the last major episode of star formation has taken place no earlier than ∼300–800 Myr prior to observation. This confirms clues from their post-starburst UVJ colors. Marginalising over three different slopes of the dust attenuation curve, we obtain young mass-weighted ages and an average peak star formation rate (SFR) of ∼103 M⊙ yr−1 at zformation ∼ 3.5. Although mid- and far-IR data are too shallow to determine the obscured SFR on a galaxy-by-galaxy basis, the mean stacked emission from 3 GHz data constrains the level of residual-obscured SFR to be globally below 50 M⊙ yr−1, three times below the scatter of the coeval main sequence. Alternatively, the very same radio detection suggests a widespread radio-mode feedback by active galactic nuclei (AGN) four times stronger than in z ∼ 1.8 massive QGs. This is accompanied by a 30% fraction of X-ray luminous AGN with a black hole accretion rate per unit SFR enhanced by a factor of ∼30 with respect to similarly massive QGs at lower redshift. The average compact, high Sérsic index morphologies of the galaxies in this sample, coupled with their young mass-weighted ages, suggest that the mechanisms responsible for the development of a spheroidal component might be concomitant with (or preceding) those causing their quenching.

On the X-Ray Spectral Energy Distributions of Star-forming Galaxies: The 0.3–30 keV Spectrum of the Low-metallicity Starburst Galaxy VV 114

Abstract Binary population synthesis combined with cosmological models suggests that X-ray emission from star-forming galaxies, consisting primarily of emission from X-ray binaries (XRBs) and the hot interstellar medium (ISM), could be an important, and perhaps dominant, source of heating of the intergalactic medium prior to the epoch of reionization. However, such models rely on empirical constraints for the X-ray spectral energy distributions (SEDs) of star-forming galaxies, which are currently lacking for low-metallicity galaxies. Using a combination of Chandra, XMM-Newton, and NuSTAR observations, we present new constraints on the 0.3–30 keV SED of the low-metallicity starburst galaxy VV 114, which is known to host several ultraluminous X-ray sources (ULXs) with luminosities above 1040 erg s−1. We use an archival Chandra observation of VV 114 to constrain the contributions to the X-ray SED from the major X-ray-emitting components of the galaxy and newly acquired, nearly simultaneous XMM-Newton and NuSTAR observations to extend the spectral model derived from Chandra to cover the 0.3–30 keV range. Using our best-fit galaxy-wide spectral model, we derive the 0.3–30 keV SED of VV 114, which we find is dominated by emission from the XRB population, and in particular ULXs, at energies >1.5 keV, and which we find to have an elevated galaxy-integrated X-ray luminosity per unit star formation rate relative to higher-metallicity star-forming galaxies. We discuss our results in terms of the effect of metallicity on XRB populations and the hot ISM, as well as the importance of X-ray emission from star-forming galaxies in the high-redshift universe.

The Impact of Realistic Foreground and Instrument Models on 21 cm Epoch of Reionization Experiments

Abstract Predictions for the ability of 21 cm interferometric experiments to discriminate Epoch of Reionization (EoR) signal models are typically limited by the simplicity of data models, whereby foreground signals and characteristics of the instrument are often simplified or neglected. To move toward more realistic scenarios, we explore the effects of applying more realistic foreground and instrument models to the 21 cm signal, as well as the ability to estimate astrophysical parameters with these additional complexities. We use a highly optimized version of 21cmFAST, integrated into 21cmMC, to generate lightcones of the brightness temperature fluctuation for Bayesian parameter estimation. We include a statistical point-source foreground model and an instrument model based on the Murchison Widefield Array scaled in observation time to have an effective sensitivity similar to the future Square Kilometre Array. We also extend the current likelihood prescription to account for the presence of beam convolution and foregrounds, the two-dimensional power spectrum (PS), and the correlation of PS modes. We use frequency bands between 150 and 180 MHz to constrain the ionizing efficiency (ζ), the minimum virial temperature of halos ( ), the soft X-ray emissivity per unit star formation rate (SFR) (L X /SFR), and the X-ray energy threshold (E 0). We find that the inclusion of realistic foregrounds and instrumental components biases the parameter constraints, due to unaccounted for cross-power between the EoR signal, foregrounds, and thermal noise. This causes estimates of ζ to be biased by up to 5σ, but the estimates of T vir, L X /SFR and E 0 remain unaffected and are all within 1σ.

High-mass X-ray binaries in nearby metal-poor galaxies: on the contribution to nebular He ii emission

ABSTRACT Despite significant progress both observationally and theoretically, the origin of high-ionization nebular He ii emission in galaxies dominated by stellar photoionization remains unclear. Accretion-powered radiation from high-mass X-ray binaries (HMXBs) is still one of the leading proposed explanations for the missing He+-ionizing photons, but this scenario has yet to be conclusively tested. In this paper, we present nebular line predictions from a grid of photoionization models with input spectral energy distributions containing the joint contribution of both stellar atmospheres and a multicolour disc model for HMXBs. This grid demonstrates that HMXBs are inefficient producers of the photons necessary to power He ii, and can only boost this line substantially in galaxies with HMXB populations large enough to power X-ray luminosities of 1042 erg s−1 per unit star formation rate (SFR). To test this, we assemble a sample of 11 low-redshift star-forming galaxies with high-quality constraints on both X-ray emission from Chandra and He ii emission from deep optical spectra, including new observations with the MMT. These data reveal that the HMXB populations of these nearby systems are insufficient to account for the observed He ii strengths, with typical X-ray luminosities or upper limits thereon of only 1040–1041 erg s−1 per SFR. This indicates that HMXBs are not the dominant source of He+ ionization in these metal-poor star-forming galaxies. We suggest that the solution may instead reside in revisions to stellar wind predictions, softer X-ray sources, or very hot products of binary evolution at low metallicity.

The X-Ray Luminosity Function of Ultraluminous X-Ray Sources in Collisional Ring Galaxies

Abstract Ring galaxies are fascinating laboratories: a catastrophic impact between two galaxies (one not much smaller than the other) has produced fireworks, especially in the larger one, when hit roughly perpendicularly to the plane. We analyze the point sources produced by the starburst episode following the impact in the rings of seven galaxies and determine their X-ray luminosity function (XLF). In total, we detect 63 sources, of which 50 have luminosity L X ≥ 1039 erg s−1, classifying them as ultraluminous X-ray sources (ULXs). We find that the total XLF is not significantly different from XLFs derived for other kinds of galaxies, with a tendency of having a larger fraction of high X-ray luminosity objects. Both the total number of ULXs and the number of ULXs per unit star formation rate are found in the upper envelope of the more normal galaxies distribution. Further analysis would be needed to address the issue of the nature of the compact component in the binary system.

Preheating of the early universe by radiation from high-mass X-ray binaries

Using a reliably measured intrinsic (i.e. corrected for absorption effects) present-day luminosity function of high-mass X-ray binaries (HMXBs) in the 0.25-2 keV energy band per unit star-formation rate, we estimate the preheating of the early Universe by soft X-rays from such systems. We find that X-ray irradiation, mainly executed by ultraluminous and supersoft ultraluminous X-ray sources with luminosity L> 10^39 erg/s, could significantly heat (T>T_cmb, where T_cmb is the temperature of the cosmic microwave background) the intergalactic medium by z~10 if the specific X-ray emissivity of the young stellar population in the early Universe was an order of magnitude higher than at the present epoch (which is possible due to the low metallicity of the first galaxies) and the soft X-ray emission from HMXBs did not suffer strong absorption within their galaxies. This makes it possible to observe the 21 cm line of neutral hydrogen in emission from redshifts z<10.

The intrinsic collective X-ray spectrum of luminous high-mass X-ray binaries

Using a sample of two hundred luminous (L_unabs>10^38 erg/s, where L_unabs is the unabsorbed 0.25-8 keV luminosity) high-mass X-ray binary (HMXB) candidates found with Chandra in 27 nearby galaxies, we have constructed the collective X-ray spectrum of HMXBs in the local Universe per unit star formation rate, corrected for observational biases associated with intrinsic diversity of HMXB spectra and X-ray absorption in the interstellar medium. This spectrum is well fit by a power law with a photon index Gamma=2.1+/-0.1 and is dominated by ultraluminous X-ray sources with L_unabs>10^39 erg/s. Hard sources (those with the 0.25-2 keV to 0.25-8 keV flux ratio of <0.6) dominate above ~2 keV, while soft and supersoft sources (with the flux ratios of 0.6-0.95 and >0.95, respectively) at lower energies. The derived spectrum probably represents the angle-integrated X-ray emission of the near- and super-critically accreting stellar mass black holes and neutron stars in the local Universe. It provides an important constraint on supercritical accretion models and can be used as a reference spectrum for calculations of the X-ray preheating of the Universe by the first generations of X-ray binaries.

Bright end of the luminosity function of high-mass X-ray binaries: contributions of hard, soft and supersoft sources

Using a spectral analysis of bright Chandra X-ray sources located in 27 nearby galaxies and maps of star-formation rate (SFR) and ISM surface densities for these galaxies, we constructed the intrinsic X-ray luminosity function (XLF) of luminous high-mass X-ray binaries (HMXBs), taking into account absorption effects and the diversity of HMXB spectra. The XLF per unit SFR can be described by a power law dN/d log L=2.0(L/10^39 erg/s)^(-0.6) (M_Sun/yr)^(-1) from L=10^38 to 10^40.5 erg/s, where L is the unabsorbed luminosity at 0.25-8 keV. The intrinsic number of luminous HMXBs per unit SFR is a factor of ~2.3 larger than the observed number reported before. The intrinsic XLF is composed of hard, soft and supersoft sources (defined here as those with the 0.25-2 keV to 0.25-8 keV flux ratio of <0.6, 0.6-0.95 and >0.95, respectively) in ~2:1:1 proportion. We also constructed the intrinsic HMXB XLF in the soft X-ray band (0.25-2 keV). Here, the numbers of hard, soft and supersoft sources prove to be nearly equal. The cumulative present-day 0.25-2 keV emissivity of HMXBs with luminosities between 10^38 and 10^40.5 erg/s is ~5 10^39 erg/s (M_Sun/yr)^(-1), which may be relevant for studying the X-ray preheating of the early Universe.

The spatially resolved correlation between [NII] 205μm line emission and the 24μm continuum in nearby galaxies

A correlation between the 24 {\mu}m continuum and the [NII] 205 {\mu}m line emission may arise if both quantities trace the star formation activity on spatially-resolved scales within a galaxy, yet has so far only been observed in the nearby edge-on spiral galaxy NGC 891. We therefore assess whether the [NII] 205 - 24 {\mu}m emission correlation has some physical origin or is merely an artefact of line-of-sight projection effects in an edge-on disc. We search for the presence of a correlation in Herschel and Spitzer observations of two nearby face-on galaxies, M51 and M83, and the interacting Antennae galaxies NGC 4038 and 4039. We show that not only is this empirical relationship also observed in face-on galaxies, but also that the correlation appears to be governed by the star formation rate (SFR). Both the nuclear starburst in M83 and the merger-induced star formation in NGC 4038/9 exhibit less [NII] emission per unit SFR surface density than the normal star-forming discs. These regions of intense star formation exhibit stronger ionization parameters, as traced by the 70/160 {\mu}m far-infrared colour, that suggest the presence of higher ionization lines that may become more important for gas cooling, thereby reducing the observed [NII] 205 {\mu}m line emission in regions with higher star formation rates. Finally, we present a general relation between the [NII] 205 {\mu}m line flux density and SFR density for normal star-forming galaxies, yet note that future studies should extend this analysis by including observations with wider spatial coverage for a larger sample of galaxies.

Pitfalls when observationally characterizing the relative formation rates of stars and stellar clusters in galaxies

Abstract Stars generally form in aggregates, some of which are bound (‘clusters’) while others are unbound and disperse on short (${\sim }10 {\,{\rm Myr}}$) time-scales (‘associations’). The fraction of stars forming in bound clusters (Γ) is a fundamental outcome of the star formation process. Recent observational and theoretical work has suggested that Γ increases with the gas surface density (Σ) or star formation rate (SFR) surface density (ΣSFR), both within galaxies and between different ones. However, a recent paper by Chandar et al. has challenged these results, showing that the total number of stellar aggregates per unit SFR does not vary systematically with the host galaxy's absolute SFR. In this Letter, we show that no variations are expected when no distinction is made between bound and unbound aggregates, because the sum of these two fractions should be close to unity. We also demonstrate that any scaling of Γ with the absolute SFR is much weaker than with ΣSFR, due to the mass–radius–SFR relation of star-forming ‘main-sequence’ galaxies. The environmental variation of Γ should therefore be probed as a function of area-normalized quantities, such as Σ or ΣSFR. We present a set of guidelines for meaningful observational tests of cluster formation theories and show that these resolve the reported discrepancy.

Metallicity dependence of high-mass X-ray binary populations

High-mass X-ray binaries (HMXBs) might have contributed a non-negligible fraction of the energy feedback to the interstellar and intergalactic media at high redshift, becoming important sources for the heating and ionization history of the Universe. However, the importance of this contribution depends on the hypothesized increase in the number of HMXBs formed in low-metallicity galaxies and in their luminosities. In this work we test the aforementioned hypothesis, and quantify the metallicity dependence of HMXB population properties. We compile from the literature a large set of data on the sizes and X-ray luminosities of HMXB populations in nearby galaxies with known metallicities and star formation rates. We use Bayesian inference to fit simple Monte Carlo models that describe the metallicity dependence of the size and luminosity of the HMXB populations. We find that HMXBs are typically ten times more numerous per unit star formation rate in low-metallicity galaxies (12 + log(O/H) < 8, namely < 20% solar) than in solar-metallicity galaxies. The metallicity dependence of the luminosity of HMXBs is small compared to that of the population size. Our results support the hypothesis that HMXBs are more numerous in low-metallicity galaxies, implying the need to investigate the feedback in the form of X-rays and energetic mass outflows of these high-energy sources during cosmic dawn.

A COMPREHENSIVE X-RAY AND MULTIWAVELENGTH STUDY OF THE COLLIDING GALAXY PAIR NGC 2207/IC 2163

We present a comprehensive study of the total X-ray emission from the colliding galaxy pair NGC2207/IC2163, based on Chandra, Spitzer, and GALEX data. We detect 28 ultra-luminous X-ray sources (ULXs), 7 of which were not detected previously due to X-ray variability. Twelve sources show significant long-term variability, with no correlated spectral changes. Seven sources are transient candidates. One ULX coincides with an extremely blue star cluster (B-V = -0.7). We confirm that the global relation between the number and luminosity of ULXs and the integrated star formation rate (SFR) of the host galaxy also holds on local scales. We investigate the effects of dust extinction and/or age on the X-ray binary (XRB) population on sub-galactic scales. The distributions of Nx and Lx are peaked at L(IR)/L(NUV)~1, which may be associated with an age of ~10 Myr for the underlying stellar population. We find that ~1/3 of the XRBs are located in close proximity to young star complexes. The luminosity function of the X-ray binaries is consistent with that typical for high-mass X-ray binaries, and appears unaffected by variability. We disentangle and compare the X-ray diffuse spectrum with that of the bright XRBs. The hot interstellar medium dominates the diffuse X-ray emission at E<1 keV, has a temperature kT=0.28 (+0.05/-0.04) keV and intrinsic 0.5-2 keV luminosity of 7.9e+40 erg/s, a factor of ~2.3 higher than the average thermal luminosity produced per unit SFR in local star-forming galaxies. The total X-ray output of NGC2207/IC2163 is 1.5e+41 erg/s, and the corresponding total integrated SFR is 23.7 Msol/yr.

X-ray emission from star-forming galaxies – III. Calibration of the LX-SFR relation up to redshift z ≈ 1.3

We investigate the relation between total X-ray emission from star-forming galaxies and their star formation activity. Using nearby late-type galaxies and ULIRGs from Paper I and star-forming galaxies from Chandra Deep Fields, we construct a sample of 66 galaxies spanning the redshift range z~0-1.3 and the star-formation rate (SFR) range ~0.1-10^3 M_sun/yr. In agreement with previous results, we find that the Lx-SFR relation is consistent with a linear law both at z=0 and for the z=0.1-1.3 CDF galaxies, within the statistical accuracy of ~0.1 in the slope of the Lx-SFR relation. For the total sample, we find a linear scaling relation Lx/SFR~(4.0\pm 0.4)x10^{39}(erg/s)/(Msun/yr), with a scatter of ~0.4 dex. About ~2/3 of the 0.5-8 keV luminosity generated per unit SFR is expected to be due to HMXBs. We find no statistically significant trends in the mean Lx/SFR ratio with the redshift or star formation rate and constrain the amplitude of its variations by <0.1-0.2 dex. These properties make X-ray observations a powerful tool to measure the star formation rate in normal star-forming galaxies that dominate the source counts at faint fluxes.

Cosmic ray driven outflows from high-redshift galaxies

We study winds in high-redshift galaxies driven by a relativistic cosmic ray (proton) component in addition to the hot thermal gas component. Cosmic rays (CRs) are likely to be efficiently generated in supernova shocks inside galaxies. We obtain solutions of such CR-driven free winds in a gravitational potential of the Navarro–Frenk–White form, relevant to galaxies. CRs naturally provide the extra energy and/or momentum input to the system, needed for a transonic wind solution in a gas with adiabatic index γ= 5/3. We show that CRs can effectively drive winds even when the thermal energy of the gas is lost due to radiative cooling. These wind solutions predict an asymptotic wind speed closely related to the circular velocity of the galaxy. Furthermore, the mass outflow rate per unit star formation rate (ηw) is predicted to be ∼0.2–0.5 for massive galaxies, with masses M∼ 1011–1012 M⊙. We show ηw to be inversely proportional to the square of the circular velocity. Magnetic fields at the μG levels are also required in these galaxies to have a significant mass loss. A large ηw for small mass galaxies implies that CR-driven outflows could provide a strong negative feedback to the star formation in dwarf galaxies. Further, our results will also have important implications to the metal enrichment of the intergalactic medium. These conclusions are applicable to the class of free wind models where the source region is confined to be within the sonic point.

On the fraction of intermediate-mass close binaries that explode as Type Ia supernovae

Type Ia supernova (SNIa) explosions are thought to result from a thermonuclear runaway in CO white dwarfs (WDs) that approach the Chandrasekhar limit, either through accretion from a companion or through a merger with another WD. However, it is unknown which of these channels operates in reality, and what are the details of the process. I compile from the literature observational estimates of the fraction η of intermediate-mass stars that eventually explode as SNeIa, supplement them with several new estimates, and compare them self-consistently. The estimates are based on five different methods, each utilizing some observable related to the SNIa rate, combined with assumptions regarding the initial mass function (IMF): the ratio of SNIa to core-collapse explosions in star-forming galaxies; the SNIa rate per unit star formation rate; the SNIa rate per unit stellar mass; the iron-to-stellar mass ratio in galaxy clusters; and the abundance ratios in galaxy clusters. The five methods indicate that a fraction in the range η ≈ 2‐40 per cent of all stars with initial masses of 3‐8 M� (the progenitors of the WDs generally thought capable of exploding through the above scenarios) explode as SNeIa. A fraction of η ≈ 15 per cent is consistent with all five methods for a range of plausible IMFs. Considering also the binarity fraction among such stars, the fraction of binaries with the appropriate mass ratios, the fraction in close initial orbits, and duplicity (every binary can produce only one SNIa explosion), this implies that nearly every intermediate-mass close binary ends up as an SNIa, or possibly more SNeIa than progenitor systems. Theoretically expected fractions are generally one to two orders of magnitude lower. The problem could be solved if all the observational estimates are in error, or with a ‘middle-heavy’ IMF, or by some mechanism that strongly enhances the efficiency of binary evolution towards SNIa explosion, or by a non-binary origin for SNeIa.

Dust and Atomic Gas in Dwarf Irregular Galaxies of the M81 Group: The SINGS and THINGS View

[abridged] We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M81 group from the SINGS and THINGS surveys. The Spitzer observations provide a first glimpse of the nature of the non-atomic ISM in these metal-poor (Z~0.1 Z_sun), quiescent (SFR~0.001-0.1 M_sun/yr) dwarf galaxies. Dust emission is detected in five out of the seven targets. Most detected dust emission is restricted to HI column densities >1x10^21 cm^-2. Spitzer spectroscopy of two regions in the brightest galaxies (IC 2574 and Holmberg II) show distinctly different spectral shapes. The spectrum of IC 2574 shows aromatic features that are less luminous (relative to the FIR luminosity) compared to an average SINGS spiral galaxy by a factor of \~7 . The aromatic features in Holmberg~II (which has only a slightly lower gas-phase metallicity) are fainter than in IC 2574 by an order of magnitude. This result emphazises that the strength of the aromatic features is not a simple linear function of metallicity. We estimate dust masses of ~10^4-10^6 M_sun for the M81 dwarf galaxies, resulting in an average dust--to--gas ratio (M_dust/M_HI) of ~3x10^-4 (1.5x10^-3 if only the HI that is associated with dust emission is considered); this is an order of magnitude lower than the typical value derived for the SINGS spirals. The dwarf galaxies are underluminous per unit star formation rate at 70um as compared to the more massive galaxies in SINGS by a factor of ~2. However, the average 70um/160um ratio in the sample dwarf galaxies is higher than what is found in the other galaxies of the SINGS sample. This can be explained by a combination of a lower dust content in conjunction with a higher dust temperature in the dwarfs.

The 21-cm forest

We examine the prospects for studying the pre-reionization intergalactic medium (IGM) through the so-called 21-cm forest in spectra of bright high-redshift radio sources. We first compute the evolution of the mean optical depth τ for models that include X-ray heating of the IGM gas, Wouthuysen–Field coupling, and reionization. Under most circumstances, the spin temperature TS grows large well before reionization begins in earnest; this occurs so long as the X-ray luminosity of high-redshift starbursts (per unit star formation rate) is comparable to that in nearby galaxies. As a result, τ≲ 10−3 throughout most of reionization, and background sources must sit well beyond the reionization surface in order to experience absorption that is measurable by square-kilometre class telescopes. H ii regions produce relatively large ‘transmission gaps’ and may therefore still be observable during the early stages of reionization. Absorption from sheets and filaments in the cosmic web fades once TS becomes large and should be rare during reionization. Minihaloes can produce strong (albeit narrow) absorption features. Measuring their abundance would yield useful limits on the strength of feedback processes in the IGM as well as their effect on reionization.