Pc Cm-3 Research Articles

The Galactic distribution of pulsar scattering and the τ–DM relation

ABSTRACT Interstellar radio wave scattering leads to flux density fluctuations and pulse broadening of pulsar signals. However, Galactic distribution and the structure of the scattering medium are still poorly understood. Pulsar pulse broadening data available for a relatively large number of pulsars is well suited for such investigations. We collected an up-to-date sample of publicly available pulsar scattering data and introduced a new quantity – the reduced scattering strength $\tilde{\tau }$ to study the Galactic distribution of pulsar scattering in the Milky Way. We show that the current observations are dominated by two distinct pulsar populations: a local and an inner-Galactic one separated by $\tilde{\tau }=10^{-5.1}$ s cm6 pc−1. The stronger electron density fluctuations associated with the inner-Galactic population naturally explain the observed steepening of pulsar scattering time τ–dispersion measure relation. We measure an inner disc region with $3\, {\rm kpc}\ \lt\ \rm r\ \lt\ 5.5\, {\rm kpc}$ from the Galactic centre to have a scattering scale height of about 0.28 kpc, supporting a correlation between interstellar radio scattering and structures associating with the ionized gas and stellar activities.

Timing and Single-pulse Study of Pulsar J1909+0122 Discovered by CRAFTS

We report the discovery of PSR J1909+0122 by the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) as part of the Commensal Radio Astronomy FAST Survey. PSR J1909+0122 has a spin period of 1.257 s and a dispersion measure of 186.2 pc cm−3. The averaged pulse profile shows two distinct components. We performed a single-pulse study based on a one-hour observation at 1.25 GHz on 2021 August 23. We used a threshold of 5σ ep to measure the nulling fraction (NF) as 63% ± 1.5%. The longitude-resolved fluctuation spectra and fast Fourier transform spectra of the binary sequences revealed the quasi-periodicity of nulling with a period of 30 rotation periods. We examined the reliability of the periodicity by comparing it to random noise injection. The NF, , and modulation periodicity P M of PSR J1909+0122 were compared with other periodic nulling pulsars, showing that the source of J1909+0122 has the second largest NF in the population. Long-term timing observations over six months were used to derive the phase-connected ephemeris of this pulsar. The measured P and values disfavor dipolar geometry for polar gap models, and the prediction for a space-charge-limited flow model in the case of inverse Compton scattering is only just above the death line. In this work, PSR J1909+0122 has revealed possible correlations between nulling behavior and pulsar properties, which will help to shed light on the pulsar emission mechanism and its temporal evolution in future observations.

The physical and chemical structure of Sagittarius B2

Context. The giant molecular cloud Sagittarius B2 (hereafter SgrB2) is the most massive region with ongoing high-mass star formation in the Galaxy. Two ultra-compact Hii (UCHii ) regions were identified in SgrB2’s central hot cores, SgrB2(M) and SgrB2(N). Aims. Our aim is to characterize the properties of the Hii regions in the entire SgrB2 cloud. Comparing the Hii regions and the dust cores, we aim to depict the evolutionary stages of different parts of SgrB2. Methods. We use the Very Large Array in its A, CnB, and D configurations, and in the frequency band C (~6GHz) to observe the whole SgrB2 complex. Using ancillary VLA data at 22.4 GHz and ALMA data at 96 GHz, we calculated the physical parameters of the UCHii regions and their dense gas environment. Results. We identify 54 UCHii regions in the 6 GHz image, 39 of which are also detected at 22.4 GHz. Eight of the 54 UCHii regions are newly discovered. The UCHii regions have radii between 0.006 pc and 0.04 pc, and have emission measure between 106 pc cm6 and 109 pc cm6. The UCHii regions are ionized by stars of types from B0.5 to O6. We found a typical gas density of ~106–109 cm3 around the UCHii regions. The pressure of the UCHii regions and the dense gas surrounding them are comparable. The expansion timescale of these UCHii regions is determined to be ~104–105 yr. The percentage of the dust cores that are associated with Hii regions are 33%, 73%, 4%, and 1% for SgrB2(N), SgrB2(M), SgrB2(S), and SgrB2(DS), respectively. Two-thirds of the dust cores in SgrB2(DS) are associated with outflows. Conclusions. The electron densities of the UCHii regions we identified are in agreement with that of typical UCHii regions, while the radii are smaller than those of the typical UCHii regions. The dust cores in SgrB2(M) are more evolved than in SgrB2(N). The dust cores in SgrB2(DS) are younger than in SgrB2(M) or SgrB2(N).

Four new fast radio bursts discovered in the Parkes 70-cm pulsar survey archive

ABSTRACT We present four new fast radio bursts discovered in a search of the Parkes 70-cm pulsar survey data archive for dispersed single pulses and bursts. We searched dispersion measures (DMs) ranging between 0 and 5000 pc cm−3 with the HEIMDALL and FETCH detection and classification algorithms. All four of the fast radio bursts (FRBs) discovered have significantly larger widths (>50 ms) than almost all of the FRBs detected and catalogued to date. The large pulse widths are not dominated by interstellar scattering or dispersive smearing within channels. One of the FRBs has a DM of 3338 pc cm3, the largest measured for any FRB to date. These are also the first FRBs detected by any radio telescope so far, predating the Lorimer Burst by almost a decade. Our results suggest that pulsar survey archives remain important sources of previously undetected FRBs and that searches for FRBs on time-scales extending beyond ∼100 ms may reveal the presence of a larger population of wide-pulse FRBs.

First Discovery of a Fast Radio Burst at 350 MHz by the GBNCC Survey

Abstract We report the first discovery of a fast radio burst (FRB), FRB 20200125A, by the Green Bank Northern Celestial Cap (GBNCC) Pulsar Survey conducted with the Green Bank Telescope at 350 MHz. FRB 20200125A was detected at a Galactic latitude of 58.°43 with a dispersion measure of 179 pc cm3, while electron density models predict a maximum Galactic contribution of 25 pc cm3 along this line of sight. Moreover, no apparent Galactic foreground sources of ionized gas that could account for the excess DM are visible in multiwavelength surveys of this region. This argues that the source is extragalactic. The maximum redshift for the host galaxy is z max = 0.17, corresponding to a maximum comoving distance of approximately 750 Mpc. The measured peak flux density for FRB 20200125A is 0.37 Jy, and we measure a pulse width of 3.7 ms, consistent with the distribution of FRB widths observed at higher frequencies. Based on this detection and assuming a Euclidean flux density distribution of FRBs, we calculate an all-sky rate at 350 MHz of FRBs sky−1 day−1 above a peak flux density of 0.42 Jy for an unscattered pulse having an intrinsic width of 5 ms, consistent with rates reported at higher frequencies, albeit with large uncertainties. Given the recent improvements in our single-pulse search pipeline, we also revisit the GBNCC survey sensitivity to various burst properties. Finally, we find no evidence of strong interstellar scattering in FRB 20200125A, adding to the growing evidence that some FRBs have circumburst environments where free–free absorption and scattering are not significant.

A survey of the Galactic plane for dispersed radio pulses with the Australian Square Kilometre Array Pathfinder

We report the results from a survey of the Galactic plane for dispersed single pulses using the Australian SKA Pathfinder (ASKAP). We searched for rare bright dispersed radio pulses comprising 160 pointings covering 4800 deg$^2$ of the Galactic plane within |b| $< 7 \deg$, each pointing with an exposure time of 10 hours. We detected one fast radio burst, FRB 180430, and single pulses from 11 pulsars. No rotating radio transients were detected. We detected FRB 180430 in the Galactic plane in the anticentre direction with a fluence of 216$\pm5$Jy ms a dispersion measure (DM) of 264.1 pc cm-3. We estimate the extragalactic DM of the object to be less than 86.7 $ \text{pc} \ \text{cm}^{-3} $ depending on the electron density model. One model suggests that this FRB may be a giant pulse within our galaxy; we discuss how this may not correctly represent the line-of-sight DM. Based on the single detection of FRB 180430 in 3.47 $\times 10^{4}$ deg$^2$ h we derive a FRB event rate in the Galactic plane at the 20 Jy ms threshold to be in the range 2-140 per sky per day at 95% confidence. Despite the necessarily large uncertainties from this single detection, this is consistent with the current ASKAP all-sky detection rate.

Detection of decametre-wavelength pulsed radio emission of 40 known pulsars

The study of pulsars at the lowest radio frequencies observable from the ground (10-30 MHz) is complicated by strong interstellar (dispersion, scattering) and ionospheric (scintillation, refraction) propagation effects, as well as intense Galactic background noise and interference. However, it permits us to measure interstellar plasma parameters (the effects of which increase by a power of two to >4 times the wavelength), the spectrum and the pulse profile at low frequencies more accurately. Up to now, only ˜10 pulsars have been successfully detected at these frequencies. The recent upgrade of the receivers at the Ukrainian T-shaped Radio telescope, second modification (UTR-2) has increased its sensitivity and motivated a new search for pulsed radio emissions. In this work we carried out a survey of known pulsars with declination above -10°, period >0.1 s and dispersion measure (DM) < 30 pc cm-3, i.e. a sample of 74 sources. Our goal was either to detect pulsars not recorded before in the decametre range or to identify factors that prevent their detection. As a result, we have detected the radio emission of 40 pulsars, i.e. 55 per cent of the observed sample. For 30 of them, this was a first detection at these frequencies. Parameters of their average profiles have been calculated, including the intrinsic widening of the pulse (not due to interstellar scattering) with decreasing frequency. Furthermore, two pulsars beyond the selected DM (B0138+59 with DM ≈ 35 pc cm-3 and B0525+21 with DM ≈51 pc cm-3) were also detected. Our results indicate that there is still room to detect new transient and pulsed sources with low-frequency observations.

FOUR HIGHLY DISPERSED MILLISECOND PULSARS DISCOVERED IN THE ARECIBO PALFA GALACTIC PLANE SURVEY

We present the discovery and phase-coherent timing of four highly dispersed millisecond pulsars (MSPs) from the Arecibo PALFA Galactic plane survey: PSRs J1844+0115, J1850+0124, J1900+0308, and J1944+2236. Three of the four pulsars are in binary systems with low-mass companions, which are most likely white dwarfs, and which have orbital periods on the order of days. The fourth pulsar is isolated. All four pulsars have large dispersion measures (DM > 100 pc cm-3), are distant (> 3.4 kpc), faint at 1.4 GHz (< 0.2 mJy), and are fully recycled (with spin periods P between 3.5 and 4.9 ms). The three binaries also have very small orbital eccentricities, as expected for tidally circularized, fully recycled systems with low-mass companions. These four pulsars have DM/P ratios that are among the highest values for field MSPs in the Galaxy. These discoveries bring the total number of confirmed MSPs from the PALFA survey to fifteen. The discovery of these MSPs illustrates the power of PALFA for finding weak, distant MSPs at low-Galactic latitudes. This is important for accurate estimates of the Galactic MSP population and for the number of MSPs that the Square Kilometer Array can be expected to detect.

FIVE NEW MILLISECOND PULSARS FROM A RADIO SURVEY OF 14 UNIDENTIFIED FERMI -LAT GAMMA-RAY SOURCES

We have discovered five millisecond pulsars (MSPs) in a survey of 14 unidentified Fermi-LAT sources in the southern sky using the Parkes radio telescope. PSRs J0101-6422, J1514-4946, and J1902-5105 reside in binaries, while PSRs J1658-5324 and J1747-4036 are isolated. Using an ephemeris derived from timing observations of PSR J0101-6422 (P =2.57 ms, DM=12 pc cm-3), we have detected {\gamma}-ray pulsations and measured its proper motion. Its {\gamma}-ray spectrum (a power law of {\Gamma} = 0.9 with a cutoff at 1.6 GeV) and efficiency are typical of other MSPs, but its radio and {\gamma}-ray light curves challenge simple geometric models of emission. The high success rate of this survey-enabled by selecting {\gamma}-ray sources based on their detailed spectral characteristics-and other similarly successful searches indicate that a substantial fraction of the local population of MSPs may soon be known.

SPITZERCHARACTERIZATION OF DUST IN THE IONIZED MEDIUM OF THE LARGE MAGELLANIC CLOUD

A systematic investigation of dust emission associated with the ionized gas has so far been performed only in our Galaxy and for wavelengths longer than 60 {\mu}m. Newly available Spitzer data now offer the opportunity to carry out a similar analysis in the Large Magellanic Cloud (LMC). By cross-correlating Spitzer SAGE (Surveying the Agents of a Galaxy's Evolution) data with the ATCA/Parkes HI 21-cm data, the NANTEN 12CO (J=1-0) data, and both the SHASSA H{\alpha} and the Parkes 6-cm data, we investigate the physical properties of dust associated with the different phases of the gas (atomic, molecular and ionized). In particular, we study the presence and nature of dust from 3.6 to 160 {\mu}m and for various regimes of the ionized gas, spanning emission measures (EM) from \sim 1 pc cm-6 (diffuse component) to \sim 10^3 pc cm-6 (HII regions). Using a dust emission model, and testing our results with several radiation field spectra, we show that dust in the ionized gas is warmer than dust associated with other phases (atomic and molecular). We also find a decrease of the polycyclic aromatic hydrocarbons (PAH) relative abundance with respect to big grains (BGs), as well as an increase of the near infrared (NIR) continuum. These three results (e.g. warmer temperature, decrease of PAH abundance and increase of the NIR continuum) are found consistently for all regimes of the ionized gas. On the contrary, the molecular phase appears to provide favorable conditions for the survival of PAHs. Furthermore, the very small grain (VSG) relative abundance tends to increase in the ionized phase, especially in bright HII regions. Last but not least, our analysis shows that the emissivity of dust associated with the ionized gas is lower in the LMC than in our Galaxy, and that this difference is not accounted for by the lower metallicity of the LMC.

LOCATING THE YOUNGEST H II REGIONS IN M82 WITH 7 mm CONTINUUM MAPS

We present 7 mm Very Large Array continuum images of the starburst galaxy M82. On arcsecond scales, two-thirds of the 7 mm continuum consists of free-free emission from H?II regions. In the subarcsecond resolution map, we identify 14 compact sources, including 9 bright H?II regions with N lyc > 1051 s?1. Four of the H?II regions have rising spectra, implying emission measures >108 cm?6 pc. Except for one compact source with peculiar features, all other compact radio sources are found in dust lanes and do not have optical or near-infrared continuum counterparts. Four regions of extended, high brightness (EM >107 cm-6 pc) radio emission are found in our high-resolution map, including some as large as ~2'', or 30 pc, representing either associations of small H?II regions, or sheet-like structures of denser gas. The good correlation between 7 mm emission and Spitzer IRAC 8 ?m continuum-removed polycyclic aromatic hydrocarbon (PAH) feature suggests that PAH emission may track the recently formed OB stars. We find an excellent correlation between molecular gas and star formation, particularly dense gas traced by HCN, down to the ~45 pc scale in M82. We also find star formation efficiencies (SFEs) of 1%-10% on the same scale, based on CO maps. The highest SFE are found in regions with the highest dense gas fractions.

Very Large Array Observations of Candidate High-Mass Protostellar Objects at 7 Millimeters

We present radio-continuum observations at 7 mm made using the Very Large Array toward three massive star-forming regions thought to be in very early stages of evolution, selected from the 2002 sample of Sridharan et al. Emission was detected toward all three sources (IRAS 18470-0044, IRAS 19217+1651, and IRAS 23151+5912). We find that in all cases the 7 mm emission corresponds to thermal emission from ionized gas. The regions of ionized gas associated with IRAS 19217+1651 and IRAS 23151+5912 are hypercompact, with diameters of 0.009 and 0.0006 pc and emission measures of 7.0 × 108 and 2.3 × 109 pc cm-6, respectively.

Discovery of Five Recycled Pulsars in a High Galactic Latitude Survey

We present five recycled pulsars discovered during a 21 cm survey of approximately 4150 deg2 between 15° and 30° from the Galactic plane using the Parkes radio telescope. One new pulsar, PSR J1528-3146, has a 61 ms spin period and a massive white dwarf companion. Like many recycled pulsars with heavy companions, the orbital eccentricity is relatively high (~0.0002), consistent with evolutionary models that predict less time for circularization. The four remaining pulsars have short spin periods (3 ms < P < 6 ms); three of these have probable white dwarf binary companions and one (PSR J2010-1323) is isolated. PSR J1600-3053 is relatively bright for its dispersion measure of 52.3 pc cm-3 and promises good timing precision thanks to an intrinsically narrow feature in its pulse profile, resolvable through coherent dedispersion. In this survey, the recycled pulsar discovery rate was 1 per 4 days of telescope time or 1 per 600 deg2 of sky. The variability of these sources implies that there are more millisecond pulsars that might be found by repeating this survey.

VLBA Identification of the Milliarcsecond Active Nucleus in the Seyfert Galaxy NGC 4151

The Seyfert galaxy NGC 4151 has been imaged at resolution better than 0.1 pc using a VLBI array consisting of the Very Long Baseline Array and three 100 m class telescopes. A flat-spectrum 3 mJy source with a monochromatic radio power of ~1037 ergs s-1 has been detected, apparently at the location of the active galactic nucleus (AGN) and its central black hole. The radio source has a minimum brightness temperature of 2.1 × 108 K and a size upper limit of 0.035 pc, about 10 times the diameter of the broad-line region and 15,000 times the diameter of the black hole's event horizon. An additional flat-spectrum component located within a parsec of the apparent nucleus is likely to be a knot in the inner radio jet. The presence of some steep-spectrum radio emission within 0.1 pc of the galaxy nucleus limits the emission measure of a possible ionized torus to a maximum value of 108 cm-6 pc. If the hard X-ray source in NGC 4151 is associated with the radio AGN, its radio–to–X-ray ratio is less than 10-5, putting NGC 4151 securely in the radio-quiet class of AGNs. The radio image reveals a 0.2 pc two-sided base to the well-known arcsecond radio jet. The apparent speeds of the jet components relative to the radio AGN are less than 0.050c and less than 0.028c at respective nuclear distances of 0.16 and 6.8 pc. These are the lowest speed limits yet found for a Seyfert galaxy and indicate nonrelativistic jet motions, possibly due to thermal plasma, on a scale only an order of magnitude larger than the broad-line region.

Observations of Diffuse Extreme-Ultraviolet Emission with theCosmic Hot Interstellar Plasma Spectrometer(CHIPS)

The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) was designed to study diffuse emission from hot gas in the local interstellar cavity in the wavelength range 90-265 A. Between launch in 2003 January and early 2004, the instrument was operated in narrow-slit mode, achieving a peak spectral resolution of about 1.4 A FWHM. Observations were carried out preferentially at high Galactic latitudes; weighted by observing time, the mean absolute value of the Galactic latitude for all narrow-slit observations combined is about 45°. The total integration time is about 13.2 Ms (74% day, 26% night). In the context of a standard collisional ionization equilibrium plasma model, the CHIPS data set tight constraints on the emission measure at temperatures between 105.55 and 106.4 K. At 106.0 K, the 95% upper limit on the emission measure is about 0.0004 cm-6 pc for solar-abundance plasma with a foreground neutral hydrogen column of 2 × 1018 cm-2. This constraint, derived primarily from limits on the extreme ultraviolet emission lines of highly ionized iron, is well below the range for the local hot bubble estimated previously from soft X-ray studies. If the pattern of elemental depletion in the hot gas follows that observed in much denser interstellar clouds, the gas-phase abundance of iron, relative to other heavy elements that contribute more to the soft X-ray emission, might be much lower than solar. However, to support the emission measures inferred previously from X-ray data would require depletions much higher than the moderate values reported previously for hot gas. Excluding the He II Lyman lines, which are known to be primarily terrestrial in origin, the brightest feature we find in the integrated spectrum is an Fe IX line at 171.1 A. The sky-averaged flux of the feature is about 6 photons cm-2 s-1 sr-1, a flux that exceeds the 1 σ shot noise significantly but is comparable to the systematic uncertainty. We find bright 171.1 A emission (flux greater than 10 photons cm-2 s-1 sr-1 and S/N >2) in about 10% of the observing time. However, these bright observations overwhelmingly select for daytime (96% of 1.3 Ms). Thus, a local rather than interstellar origin for much of the 171.1 A emission seems likely.

Evolution of HiiRegions inside Hot Molecular Cores

We study the evolution of H II regions inside accreting hot molecular cores (HMCs). The H II regions are being pushed by a strong stellar wind from a massive young star at the center. We consider a range of pressures characteristic of HMCs. We find that thermal conductivity reduces to a thin shell the bubble of hot shocked stellar wind so that the expansion of the H II region is driven by the momentum of the stellar wind. The H II region can expand beyond the boundary of the HMC at ~0.1 pc, in t 5 × 104 yr. Just before reaching the HMC boundary, it has equatorial emission measures EMeq ~ 3 × 107 cm-6 pc, characteristic of ultracompact H II regions. In fact, these ionized regions will evolve from hypercompact to ultracompact H II regions as they expand inside the HMC. We also study the effect of the magnitude of the thermal conductivity coefficient on the evolution of the shocked stellar wind shell and calculate its hard X-ray emission. Because of the oscillating cooling instability of the shell, the X-ray emission is variable. This work addresses the birth and evolution of H II regions inside the HMCs where massive stars are born.

On the contribution of charge-exchange induced X-ray emission in the ISM and ICM

X-ray emission can be generated by charge-exchange (CXE) between highly charged ions of a hot plasma and neutral species of an interacting cool/warm gas, a phenomenon recently observed in the case of the solar wind interaction with cometary, interstellar, and geocoronal neutrals (Lisse et al. [CITE]; Cox [CITE]; Cravens [CITE], [CITE]). Charge-exchange processes are included in most theoretical models of hot interstellar plasmas interacting with partially neutral gas, resulting in a modification of the physical states of the gases in the interaction region. However, the contribution of the charge-exchange induced X-ray emission produced at these interfaces has hitherto not been considered to be significant. The detailed calculations performed by Wise & Sarazin ([CITE]), motivated by the observations of X-ray emission following charge-exchange in laboratory fusion devices, have shown that the emission is negligible in the case of an SNR fast shock. Our goal here is to investigate its relative importance in different astrophysical cases. We simplistically consider interfaces between partially neutral and hot gas in the following cases: (a) a supernova blast wave propagating in a neutral (or partially ionized) ISM (b) a galactic wind engulfing a halo dense cloud; (c) a high velocity cloud (HVC) moving through the halo, and (c) a dense cloud moving in intra-cluster gas. Although the phenomenon is restricted to the very narrow envelopes defined by the mean free path of the neutrals through the hot plasma, it is easy to show that its efficiency is such that the volume emissivity from these interfaces can be orders of magnitude above the emissivity of the hot gas itself, and, more important, that its relative contribution increases with decreasing hot gas density. As a consequence it should be at maximum in mixing layers in very low density hot gas. Our preliminary results suggest that the charge-exchange X-ray emission from the interface does not contribute significantly in case (a), in agreement with Wise & Sarazin, except marginally for lines of sight tangent to the interfaces, but that it can be a non-negligible fraction of the hot gas emission in cases (b–d). Detailed self-consistent models of plasma and neutral atom distributions in conduction fronts or contact discontinuities are needed for better estimates. In the easiest test case of HVCs, there is a good agreement between our predicted equivalent emission measure cm-6 pc for complex C-type clouds and their soft X-ray emission measured by ROSAT (e.g. Kerp et al. [CITE]). If confirmed by more realistic calculations, a contribution of the CXE emission may play a role in a number of astrophysical cases: (i) tight correlations between Hα and X-ray patterns; (ii) enhanced X-ray limb brightening at interfaces; (iii) spectral and abundance “anomalies”, because charge-exchange spectra differ from thermal spectra and contain only emission lines, resulting in some biases when they are interpreted with classical models. Interestingly, interaction between cold IS clouds and the intra-cluster gas finally represents the most favorable case for the CXE emission, with the following potential consequences. At low spectral resolution, CXE emission can mimic global hot gas cooling. However, it is not in fact a global cooling, but instead accelerated cooling restricted to small physical areas, associated with the relative motions of the neutral clouds. In galaxy clusters, if CXE emission is a non-negligible contributor to the total X-ray diffuse emission, it may help reduce the need for cooling flows.

The Radio Continuum of the Metal‐deficient Blue Compact Dwarf Galaxy SBS 0335−052

We present new Very Large Array observations at five frequencies, from 1.4 to 22 GHz, of the extremely low metallicity blue compact dwarf SBS 0335-052. The radio spectrum shows considerable absorption at 1.49 GHz and a composite thermal plus nonthermal slope. After fitting the data with a variety of models, we find the best-fitting geometry to be one with free-free absorption homogeneously intermixed with the emission of both thermal and nonthermal components. The best-fitting model gives an emission measure EM ~ 8 × 107 pc cm-6 and a diameter of the radio-emitting region D ≈ 17 pc. The inferred density is ne ~ 2000 cm-3. The thermal emission comes from an ensemble of ~9000 O7 stars, with a massive star formation rate (≥5 M☉) of 0.13-0.15 yr-1 and a supernova (SN) rate of 0.006 yr-1. We find evidence for ionized gas emission from stellar winds, since the observed Brα line flux significantly exceeds that inferred from the thermal radio emission. The nonthermal fraction at 5 GHz is ~0.7, corresponding to a nonthermal luminosity of ~2 × 1020 W Hz-1. We derive an equipartition magnetic field of ~0.6-1 mG and a pressure of ~3 × 10-8 to 1 × 10-7 dynes cm-2. Because of the young age and compact size of the starburst, it is difficult to interpret the nonthermal radio emission as resulting from diffusion of SN-accelerated electrons over 107-108 yr timescales. Rather, we attribute the nonthermal radio emission to an ensemble of compact SN remnants expanding in a dense interstellar medium. If the radio properties of SBS 0335-052 are representative of star formation in extremely low metallicity environments, derivations of the star formation rate from the radio continuum in high-redshift primordial galaxies need to be reconsidered. Moreover, photometric redshifts inferred from "standard" spectral energy distributions could be incorrect.

The Nature of the Massive Young Stars in W75 N

We have observed the W75 N massive star forming region in SiO (J = 2-1 and J = 1-0) at 3''-5'' resolution and in 6 cm, 2 cm, and 7 mm continuum emission at 14-02 resolution. The abundance ratio of [SiO]/[H2] ~ 5-7 × 10-11 is typical for what is expected in the ambient component of molecular clouds with active star formation. The SiO morphology is diffuse and centered on the positions of the ultracompact H II regions—no collimated, neutral jet was discovered. The ionized gas surrounding the protostars has emission measures ranging from 1 to 15 × 106 pc cm-6, densities from 0.4 to 5 × 104 cm-3, and derived spectral types of the central ionizing stars ranging from B0.5 to B2. Most of the detected sources have spectral indices that suggest optically thin to moderately optically thick H II regions produced by a central ionizing star. The spread in ages between the oldest and youngest early-B protostars in the W75 N cluster is 0.1-5 × 106 yr. This evolutionary timescale for W75 N is consistent with that found for early-B stars born in clusters forming more massive stars (M* > 25 M☉).

The Cosmic Dispersion Measure from Gamma-Ray Burst Afterglows: Probing the Reionization History and the Burst Environment

We show a possible way to measure the column density of free electrons along the light path, the so-called dispersion measure (DM), from the early [~415(ν/1 GHz)-2(DM/105 pc cm-3) s] radio afterglows of the gamma-ray bursts. We find that the proposed Square Kilometer Array can detect bright radio afterglows around the time ~103(ν/160 MHz)-2 s to measure the intergalactic DM (6000 pc cm-3 at redshift z > 6) up to z ~ 30, from which we can determine the reionization history of the universe and identify the missing warm-hot baryons if many DMs can be measured. At low z, the DM in the host galaxy may reach ~105 pc cm-3 depending on the burst environment, which may be probed by the current detectors. Free-free absorption and diffractive scattering may also affect the radio emission at a high density.