arXiv subjects

J. Michael Shull

Publications and source records attributed to J. Michael Shull.

At least 73 records · Page 4Linked to original sources

arXiv2010

Cosmic Origins Spectrograph Detection of Ne VIII Tracing Warm - Hot Gas Towards PKS 0405-123

We report on the detection of Ne VIII in the HST/Cosmic Origins Spectrograph spectrum of the intervening absorption system at z = 0.495096 towards PKS 0405-123. The high S/N COS spectrum also covers absorption from H I, C III, O III, O IV and O VI associated with this multiphase system. The Ne VIII is detected with high significance in both lines of the doublet, with integrated column densities of log Na(Ne VIII 770) = 13.96 +/- 0.06 dex and log Na(Ne VIII 780) = 14.08 +/- 0.07 dex. We find the origin of Ne VIII consistent with collisionally ionized gas at T ~ 5 x 10^5 K with a large baryonic column density of N(H) ~ 10^{19} - 10^{20} cm^-2. The metallicity in the Ne VIII gas phase is estimated to be [Ne/H] ~ -0.6 +/- 0.3 dex. The intermediate ions such as C III, O III, O IV and H I are consistent with photoionization in lower ionization gas at T ~ 10,000 K. The O V and O VI in this absorber can have contributions from both the photoionized and collisionally ionized gas phases. The absorber is at 180 km/s systematic velocity and 110 kpc projected separation from a M_R = -19.6 galaxy of extended morphology. The collisionally ionized gas at T ~ 5 x 10^5 K points to an origin in multiphase gas embedded in the hot halo of the galaxy, or in a nearby WHIM structure. The high sensitivity UV spectroscopy afforded by COS has opened up new opportunities for discovering large reservoirs of "missing baryons" in the low-z universe through the detection of Ne VIII systems.

astro-ph.CO↗

arXiv2010

Cosmic Origins Spectrograph and FUSE Observations of T ~ 10^5 K Gas In A Nearby Galaxy Filament

We present a detection of a broad Ly-alpha absorber (BLA) with a matching O VI line in the nearby universe. The BLA is detected at z = 0.01028 in the high S/N spectrum of Mrk 290 obtained using the Cosmic Origins Spectrograph. The Ly-alpha absorption has two components, with b(HI) = 55 +/- 1 km/s and b(HI) = 33 +/- 1 km/s, separated in velocity by v ~ 115 km/s. The O VI, detected by FUSE at z = 0.01027, has a b(OVI) = 29 +/- 3 km/s and is kinematically well aligned with the broader HI component. The different line widths of the BLA and OVI suggest a temperature of T = 1.4 x 10^5 K in the absorber. The observed line strength ratios and line widths favor an ionization scenario in which both ion-electron collisions and UV photons contribute to the ionization in the gas. Such a model requires a low-metallicity of -1.7 dex, ionization parameter of log U ~ -1.4, a large total hydrogen column density of N(H) ~ 4 x 10^19 cm^-2, and a path length of 400 kpc. The line of sight to Mrk 290 intercepts at the redshift of the absorber, a megaparsec scale filamentary structure extending over 20 deg in the sky, with several luminous galaxies distributed within 1.5 Mpc projected distance from the absorber. The collisionally ionized gas in this absorber is likely tracing a shock-heated gaseous structure, consistent with a few different scenarios for the origin, including an over-dense region of the WHIM in the galaxy filament or highly ionized gas in the extended halo of one of the galaxies in the filament. In general, BLAs with metals provide an efficient means to study T ~ 10^5 - 10^6 K gas in galaxy halos and in the intergalactic medium. A substantial fraction of the baryons "missing" from the present universe is predicted to be in such environments in the form of highly ionized plasma.

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arXiv2010

HST/COS Observations of the Ly alpha Forest toward the BL Lac Object 1ES1553+113

We present new far-ultraviolet spectra from the Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES1553+113 covering the wavelength range 1135-1795 A. The data show a smooth continuum with a wealth of narrow absorption features arising in the ISM and IGM. These features include 41 Lya absorbers at 0 0.6 for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and gamma-ray sources, 1ES1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES1553+113 has no direct redshift determination. We use intervening Lya absorbers to place a direct limit on the redshift: z_em>0.395 based on a confirmed Lya+OVI absorber and z_em>0.433 based on a single-line detection of Lya. COS/FUV data are only sensitive to Lya absorbers at z<0.47, but we present statistical arguments that z_em<0.58 based on the non-detection of any Lyb absorbers at z>0.4.

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arXiv2009

Broad HI Absorbers as Metallicity-Independent Tracers of the Warm-Hot Intergalactic Medium

Thermally broadened Ly alpha absorbers (BLAs) offer an alternative method to highly-ionized metal lines for tracing the WHIM. We compile a catalog of reliable BLA candidates along seven AGN sight lines from a larger set of Lya absorbers observed by HST/STIS. We compare our measurements based on independent reduction and analysis of the data to those published by other research groups. Purported BLAs are grouped into probable (15), possible (48) and non-BLA (56) categories. We infer a line frequency (dN/dz)_BLA=18+-11, comparable to observed OVI absorbers. There is significant overlap between BLA and OVI absorbers (20-40%) and we find that OVI detections in BLAs are found closer to galaxies than OVI non-detections. Based on 164 measured COG HI line measurements, we statistically correct the observed line widths via a Monte- Carlo simulation. Gas temperature and neutral fraction f(HI) are inferred from these statistically-corrected line widths and lead to a distribution of total hydrogen columns. We find Omega_BLA=(6.3+1.1-0.8)x10^-3. There are a number of critical systematic assumptions implicit in this calculation, and we discuss how each affects our results and those of previously published work. Taking our value, current OVI and BLA surveys can account for ~20% of the baryons in the local universe. Finally, we present new, high-S/N observations of several of the BLA candidate lines from Early Release Observations made by the Cosmic Origins Spectrograph on HST.

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arXiv2009

Highly Ionized Envelopes of High Velocity Clouds

We present recent results on highly ionized gas in Galactic High-Velocity Clouds (HVCs), originally surveyed in OVI (Sembach et al. 2003). In a new FUSE/HST survey of SiII/III/IV (Shull et al. 2009) toward 37 AGN, we detected SiIII (lambda 1206.500 A) absorption with a sky coverage fraction 81 +/- 5% (61 HVCs along 30 of 37 high-latitude sight lines). The SiIII (lambda 1206.500 A) line is typically 4-5 times stronger than OVI (lambda 1031.926 A). The mean HVC column density of perhaps 10^19 cm^-2 of low-metallicity (0.1 - 0.2 Z_sun) ionized gas in the low halo. Recent determinations of HVC distances allow us to estimate a total reservoir of ~10^8 M_sun. Estimates of infall velocities indicate an infall rate of around 1 M_sun yr^-1, comparable to the replenishment rate for star formation in the disk. HVCs appear to be sheathed by intermediate-temperature gas (10^4.0 - 10^4.5 K) detectable in SiIII and SiIV, as well as hotter gas seen in OVI and other high ions. To prepare for HST observations of 10 HVC-selected sight lines with the Cosmic Origins Spectrograph (COS), we compile FUSE/STIS spectra of these ions, plus FeIII, CIII, CIV, and SIV. Better constraints on the physical properties of HVC envelopes and careful treatment of HVC kinematics and infall rates should come from high-quality (S/N ~ 30-40) COS data.

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arXiv2009

A Large Reservoir of Ionized Gas in the Galactic Halo: Ionized Silicon in High-Velocity and Intermediate-Velocity Clouds

The low Galactic halo is enveloped by a sheath of ionized, low-metallicity gas, which can provide a substantial (1 M_sun/yr) cooling inflow to replenish star formation in the disk. Using absorption spectra from the HST and FUSE toward 37 active galactic nuclei at high latitude, we detect widespread interstellar SiIII 1206.5 absorption: 61 high-velocity clouds (HVCs) along 30 sight lines and 22 intermediate-velocity clouds (IVCs) along 20 sight lines. We find a segregation of redshifted and blueshifted absorbers across the Galactic rotation axis at l=180, consistent with a lag in the rotation velocity above the Galactic plane. The HVC sky coverage is large (81+-5% for 30/37 directions) with SiIII optical depth typically 4-5 times that of OVI 1032. The mean HVC column density per sight line, =13.42+-0.21, corresponds to total column density N_HII~6x10^18)/(Z_Si/0.2Z_sun) of ionized low-metallicity gas, similar to that inferred in OVI. This reservoir could total 10^8 M_sun and produce a mass infall rate ~1 M_sun/yr. By modeling SiII, SiIII, SiIV, and HI in a subset of absorbers, we constrain the mean photoionization parameter in the low halo, = -3.0 (+0.3,-0.4), approximately ten times lower than observed in the low-redshift intergalactic medium. The metallicities in some HVCs, derived from [SiII/HI], are 10-30% solar, whereas values found from all three silicon ions are lower in the pure-photoionization models. Thse formally lower metallicities are highly uncertain, since some of the higher ions may be collisionally ionized. The mean Si metallicities, = -2.1(+1.1,-0.3) in 17 HVCs and -1.0(+0.6,-1.0) in 19 IVCs, are somewhat uncertain owing to ionization modeling, but consistent with the median photometric metallicity, [Fe/H]=-1.46+-0.30, for ~2e5 halo F/G stars in SDSS.

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arXiv2009

The Cosmic Origins Spectrograph and the Future of Ultraviolet Astronomy

I describe the capabilities of the Cosmic Origins Spectrograph, scheduled for May 2009 installation on the Hubble Space Telescope. With a factor-of-ten increase in far-UV throughput for moderate resolution spectroscopy, COS will enable a range of scientific programs that study hot stars, AGN, and gas in the interstellar medium, intergalactic medium, and galactic halos. We also plan a large-scale HST Spectroscopic Legacy Project for QSO absorption lines, galactic halos, and AGN outflows. Studies of next-generation telescopes for UV/O astronomy are now underway, including small, medium, and large missions to fill the imminent ten-year gap between the end of Hubble and a plausible launch of the next large mission. Selecting a strategy for achieving these goals will involve hard choices and tradeoffs in aperture, wavelength, and capability.

astro-ph.IM↗

arXiv2008

Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample

We report total abundances and related parameters for the full sample of the FUSE survey of molecular hydrogen in 38 translucent lines of sight. New results are presented for the "second half" of the survey involving 15 lines of sight to supplement data for the first 23 lines of sight already published. We assess the correlations between molecular hydrogen and various extinction parameters in the full sample, which covers a broader range of conditions than the initial sample. In particular, we are now able to confirm that many, but not all, lines of sight with shallow far-UV extinction curves and large values of the total-to-selective extinction ratio, $R_V$ = $A_V$ / $E(B-V)$ -- characteristic of larger than average dust grains -- are associated with particularly low hydrogen molecular fractions ($f_{\rm H2}$). In the lines of sight with large $R_V$, there is in fact a wide range in molecular fractions, despite the expectation that the larger grains should lead to less H$_2$ formation. However, we see specific evidence that the molecular fractions in this sub-sample are inversely related to the estimated strength of the UV radiation field and thus the latter factor is more important in this regime. We have provided an update to previous values of the gas-to-dust ratio, $N$(H$_{\rm tot}$)/$E(B-V)$, based on direct measurements of $N$(H$_2$) and $N$(H I). Although our value is nearly identical to that found with Copernicus data, it extends the relationship by a factor of 2 in reddening. Finally, as the new lines of sight generally show low to moderate molecular fractions, we still find little evidence for single monolithic "translucent clouds" with $f_{\rm H2}$ $\sim$ 1.

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arXiv2008

Constraints on First-Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background

We examine the constraints on high-redshift star formation, ultraviolet and X-ray pre-ionization, and the epoch of reionization at redshift z_r, inferred from the recent WMAP-5 measurement, tau_e = 0.084 +/- 0.016, of the electron scattering optical depth of the cosmic microwave background (CMB). Half of this scattering can be accounted for by the optical depth, tau_e = 0.04-0.05, of a fully ionized intergalactic medium (IGM) at z < z_GP = 6-7, consistent with Gunn-Peterson absorption in neutral hydrogen. The required additional optical depth, Delta-tau_e = 0.03 +/- 0.02 at z > z_GP, constrains the ionizing contributions of first light sources. WMAP-5 also measured a significant increase in small-scale power, which lowers the required efficiency of star formation and ionization from mini-halos. Early massive stars (UV radiation) and black holes (X-rays) can produce a partially ionized IGM, adding to the residual electrons left from incomplete recombination. Inaccuracies in computing the ionization history, x_e(z), and degeneracies in cosmological parameters (Omega_m, Omega_b, sigma_8, n_s) add systematic uncertainty to the measurement and modeling of $τ_e$. From the additional optical depth from sources at z > z_GP, we limit the star-formation efficiency, the rate of ionizing photon production for Pop III and Pop II stars, and the photon escape fraction, using standard histories of baryon collapse, minihalo star formation, and black-hole X-ray preionization.

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arXiv2008

Dust sputtering by Reverse Shocks in Supernova Remnants

We consider sputtering of dust grains, believed to be formed in cooling supernovae ejecta, under the influence of reverse shocks. In the regime of self-similar evolution of reverse shocks, we can follow the evolution of ejecta density and temperature analytically as a function of time in different parts of the ejecta, and calculate the sputtering rate of graphite and silicate grains embedded in the ejecta as they encounter the reverse shock. Through analytic (1D) calculations, we find that a fraction of dust mass ($ 1\hbox{--}20$% for silicates and %$\le 5$% for graphites) can be sputtered by reverse shocks, the fraction varying with the grain size distribution and the steepness of the density profile of the ejecta mass. It is expected that many more grains will get sputtered in the region between the forward and reverse shocks, so that our analytical results provide a lower limit to the destroyed fraction of dust mass.

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arXiv2008

The Low-z Intergalactic Medium. III. HI and Metal Absorbers at z<0.4

We conduct an ultraviolet (HST and FUSE) spectroscopic survey of HI (Lyman lines) and seven metal ions (OVI, NV, CIV, CIII, SiIV, SiIII, FeIII) in the low-redshift intergalactic medium (IGM) at z<0.4. We analyzed 650 Lya absorbers over redshift pathlength Delta z=5.27, detecting numerous absorbers: 83 OVI systems, 39 CIII, 53 SiIII, 24 CIV, 24 NV, and so on. Our survey yields distributions in column density and estimates of the IGM baryon content and metallicities of C, N, O in the IGM. In the low-z IGM, we have accounted for ~40% of the baryons: 30% in the photoionized Lya forest and 10% in the (T=10^5-6 K) warm-hot intergalactic medium (WHIM) traced by OVI. Statistical metallicities of C, N, O ions are consistent with the canonical (z=0) value of 10% solar, with considerable scatter. Improved statistics for weak OVI absorbers allows us to estimate Omega_WHIM/Omega_b=0.073+-0.008 down to logN_OVI=13.4. NV absorption is well-correlated with OVI and both ions show similarly steep power-law indices dN/dz N^-beta with beta_OVI beta_NV 2 while beta_HI=1.7. We conclude that OVI and NV are reliable tracers of the portion of the WHIM at T=10^5-6 K. CIV may be present in both collisional and photoionized phases; N_CIV correlates poorly with both N_HI and N_OVI and beta_HI<beta_CIV<beta_OVI. The ions CIII, SiIII, and SiIV are well correlated with HI and show patterns typical of photoionization. Adjacent ion stages of the same element (CIII/IV and SiIII/IV) provide useful constraints on the photoionization parameter, logU=-1.5+-0.5. Comparison of SiIV and CIV with high-z surveys shows a modest increase in line density, consistent with increasing IGM metallicity at recent epochs.

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arXiv2008

The Fate of the First Galaxies. III. Properties of Primordial Dwarf Galaxies and their Impact on the Intergalactic Medium

In two previous papers, we presented simulations of the first galaxies in a representative volume of the Universe. The simulations are unique because we model feedback-regulated galaxy formation, using time-dependent, spatially-inhomogeneous radiative transfer coupled to hydrodynamics. Here, we study the properties of simulated primordial dwarf galaxies with masses <2x10^8 Msolar and investigate their impact on the intergalactic medium. While many primordial galaxies are dark, about 100--500 per comoving Mpc^3 are luminous but relatively faint. They form preferentially in chain structures, and have low surface brightness stellar spheroids extending to 20% of the virial radius. Their interstellar medium has mean density n_H~10--100 cm^-3, metallicity Z~ 0.01--0.1 Zsolar and can sustain a multi-phase structure. With large scatter, the mean efficiency of star formation scales with halo mass, \propto M_dm^2, independent of redshift. Because of feedback, halos smaller than a critical mass, M_crit(z), are devoid of most of their baryons. More interestingly, we find that dark halos have always a smaller M_crit(z) than luminous ones. Metal enrichment of the intergalactic medium is inhomogeneous, with only a 1%--10% volume filling factor of enriched gas with [Z/H]>-3.0 and 10%--50% with [Z/H]>-5.0. At z=10, the fraction of stars with metallicity Z<10^-3 Zsolar is 10^-6 of the total stellar mass. Although detections of high-redshift dwarf galaxies with the James Webb Space Telescope will be a challenge, studies of their fossil records in the local Universe are promising because of their large spatial density.

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arXiv2007

Critical Metallicities for Second-Generation Stars

The first massive stars may influence the formation of second-generation stars, in part by their metal enrichment of the surrounding gas. We investigate the "critical metallicity", defined as the the value, Z_crit, at which primordial gas cools more efficiently by fine-structure lines of O I (63.18 microns, Si II 34.8 microns, Fe II (25.99 and 35.35 microns), and C II (157.74 microns) than by either H I or H2 line emission. We explore the time-dependent thermodynamics and fragmentation of cooling gas at redshifts z = 10-30, seeded by trace heavy elements expelled from early supernovae. Because different modes of nucleosynthesis (alpha-process, Fe-group) produce abundance ratios far from solar values, these early stellar populations are likely to be influenced by O, Si, and Fe cooling. Our models also include radiative coupling of the fine structure lines and H2 to the cosmic microwave background (CMB), which sets a temperature floor (70-80K at z = 25-30) that may increase the Jeans mass. The H2 forms from catalytic effects of electrons left over from the recombination epoch or produced during virialization. These electrons form the H^- ion (H + e -> H- + gamma), which in turn forms H2 through associative detachment (H- + H -> H2 + e). In virialized halos at z = 10-30, the gas densities (n = 1-100 cm^{-3}) are well below the critical densities, n_cr = 10^{5-6} cm^{-3}, at which (O, Si, Fe) fine-structure lines reach LTE populations and produce their most efficient cooling. Thus, Z_crit may initially exceed 0.01 Z_sun at n = 1-100 cm^{-3}, and then drop to 10^{-3.5} Z_sun at n = 10^6 cm^{-3}, where the Jeans mass may be imprinted on the stellar mass function. Primordial clouds of 10^8 M_sun at 0.01 Z_sun and 200K will produce redshifted fine structure lines, with fluxes between 10^{-22} and 10^{-21} Watts/m^2 at z = 4.

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arXiv2007

The Metallicity of Intergalactic Gas in Cosmic Voids

We have used the Hubble/STIS and FUSE archives of ultraviolet spectra of bright AGN to identify intergalactic Lya absorbers in nearby (z < 0.1) voids. From a parent sample of 651 Lya absorbers, we identified 61 void absorbers located more than 1.4/h_70 Mpc from the nearest L* or brighter galaxy. Searching for metal absorption in high-quality (S/N > 10) spectra at the location of three diagnostic metal lines (O VI 1032, C IV 1548, Si III 1206), we detected no metal lines in any individual absorber, or in any group of absorbers using pixel co-addition techniques. The best limits on metal-line absorption in voids were set using four strong Lya absorbers with N(H I) > 10^{14} cm^-2, with 3-sigma equivalent-width limits ranging from 8 mA (O VI), 7-15 mA (C IV), and 4-10 mA (Si III). Photoionization modeling yields metallicity limits Z < 10^{-1.8+/-0.4} Z_sun, from non-detections of C IV and O VI, some 6 times lower than those seen in Lya and OVI absorbers at z < 0.1. Although the void Lya absorbers could be pristine material, considerably deeper spectra are required to rule out a universal metallicity floor produced by bursts of early star formation, with no subsequent star formation in the voids. The most consistent conclusion derived from these low-z results, and similar searches at z = 3-5, is that galaxy filaments have increased their mean IGM metallicity by factors of 30-100 since z = 3.

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arXiv2006

What is the Total Deuterium Abundance in the Local Galactic Disk?

Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, together with spectra from the Copernicus and IMAPS instruments, reveal an unexplained very wide range in the observed deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk beyond the Local Bubble. We argue that spatial variations in the depletion of deuterium onto dust grains can explain these local variations in the observed gas-phase D/H ratios. We present a variable deuterium depletion model that naturally explains the constant measured values of D/H inside the Local Bubble, the wide range of gas-phase D/H ratios observed in the intermediate regime (log N(H I} = 19.2-20.7), and the low gas-phase D/H ratios observed at larger hydrogen column densities. We consider empirical tests of the deuterium depletion hypothesis: (i) correlations of gas-phase D/H ratios with depletions of the refractory metals iron and silicon, and (ii) correlation with the molecular hydrogen rotational temperature. Both of these tests are consistent with deuterium depletion from the gas phase in cold, not recently shocked, regions of the ISM, and high gas-phase D/H ratios in gas that has been shocked or otherwise heated recently. We argue that the most representative value for the total (gas plus dust) D/H ratio within 1 kpc of the Sun is >=23.1 +/- 2.4 (1 sigma) parts per million (ppm). This ratio constrains Galactic chemical evolution models to have a very small deuterium astration factor, the ratio of primordial to total (D/H) ratio in the local region of the Galactic disk, which we estimate to be f_d <= 1.19 +/-0.16 (1 sigma) or <= 1.12 +/- 0.14 (1 sigma) depending on the adopted light element nuclear reaction rates.

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arXiv2006

Does the Milky Way Produce a Nuclear Galactic Wind?

We detect high-velocity absorbing gas using Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer medium resolution spectroscopy along two high-latitude AGN sight lines (Mrk 1383 and PKS 2005-489) above and below the Galactic Center (GC). These absorptions are most straightforwardly interpreted as a wind emanating from the GC which does not escape from the Galaxy's gravitational potential. Spectra of four comparison B stars are used to identify and remove foreground velocity components from the absorption-line profiles of O VI, N V, C II, C III, C IV, Si II, Si III, and Si IV. Two high-velocity (HV) absorption components are detected along each AGN sight line, three redshifted and one blueshifted. Assuming that the four HV features trace a large-scale Galactic wind emanating from the GC, the blueshifted absorber is falling toward the GC at a velocity of 250 +/- 20 km/s, which can be explained by "Galactic fountain" material that originated in a bound Galactic wind. The other three absorbers represent outflowing material; the largest derived outflow velocity is +250 +/- 20 km/s, which is only 45% of the velocity necessary for the absorber to escape from its current position in the Galactic gravitational potential. All four HV absorbers are found to reach the same maximum height above the Galactic plane (|z_max| = 12 +/- 1 kpc), implying that they were all ejected from the GC with the same initial velocity. The derived metallicity limits of >10-20% Solar are lower than expected for material recently ejected from the GC unless these absorbers also contain significant amounts of hotter gas in unseen ionization stages.

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arXiv2006

Optical Structure and Proper-Motion Age of the Oxygen-rich Supernova Remnant 1E 0102-7219 in the Small Magellanic Cloud

We present new optical emission-line images of the young SNR 1E 0102-7219 (E0102) in the SMC obtained with the HST Advanced Camera for Surveys (ACS). E0102 is a member of the oxygen-rich class of SNRs showing strong oxygen, neon , and other metal-line emissions in its optical and X-ray spectra, and an absence of H and He. The progenitor of E0102 may have been a Wolf-Rayet star that underwent considerable mass loss prior to exploding as a Type Ib/c or IIL/b SN. The ejecta in this SNR are fast-moving (V > 1000 km/s) and emit as they are compressed and heated in the reverse shock. In 2003, we obtained optical [O III], H-alpha, and continuum images with the ACS Wide Field Camera. The [O III] image captures the full velocity range of the ejecta, and shows considerable high-velocity emission projected in the middle of the SNR that was Doppler-shifted out of the narrow F502N bandpass of a previous Wide Field and Planetary Camera 2 image from 1995. Using these two epochs separated by ~8.5 years, we measure the transverse expansion of the ejecta around the outer rim in this SNR for the first time at visible wavelengths. From proper-motion measurements of 12 ejecta filaments, we estimate a mean expansion velocity for the bright ejecta of ~2000 km/s and an inferred kinematic age for the SNR of \~2050 +/- 600 years. The age we derive from HST data is about twice that inferred by Hughes et al.(2000) from X-ray data, though our 1-sigma error bars overlap. Our proper-motion age is consistent with an independent optical kinematic age derived by Eriksen et al.(2003) using spatially resolved [O III] radial-velocity data. We derive an expansion center that lies very close to X-ray and radio hotspots, which could indicate the presence of a compact remnant (neutron star or black hole).

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arXiv2005

The Radiative Transport of Dust in Primordial Galaxies and Second-Generation Star Formation

We investigate the radiative transport of dust in primordial galaxies in the presence of the UV radiation field from the first metal-free stars. We find that dust created in the first supernova (SN) explosions can be driven through the interior of the SN remnant to accumulate in the SN shells, where second-generation stars may form from compressed cooling gas. This scenario requires metal-free stars to form continuously over timescales of up to 10 Myr, consistent with recent estimates. Silicate and graphite grains, as well as iron-bearing magnetites, are transported to the shells for reasonable parameter assumptions, but their relative yields from primordial SNe is an important factor in the resulting abundance ratios. We compare the results of segregated grain transport with the current nucleosynthetic data on extremely metal-poor Galactic halo stars. Fossil signatures of this process may already have been detected in those iron-poor stars with enhanced carbon and silicate elements such as magnesium, silicon and oxygen. We discuss the implications of our results for the transition from first- to second-generation star formation in primordial galaxies, and the role played by the radiative transport of dust in this process.

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