198 research outputs found
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.Comment: Submitted to ApJ, 27 pages in ApJ format (figure and discussion
added
The ISM Interactions of a Runaway LBV Nebula in the LMC
New observations of the Magellanic Cloud Luminous Blue Variable candidate
S119 (HD269687) show the relationship of the star to its environs. Echelle
spectroscopy and high-resolution HST imagery reveal an expanding bubble
centered on the star. This bubble appears in both Halpha and [NII] and is
noticeably brighter on the near (blue-shifted) side. The systemic velocity of
both the expanding bubble and the star itself (as seen by the very broad Halpha
emission feature in the stellar spectrum) is V_hel=160 km/s whereas the
velocity of the superposed LMC ISM is 250-300 km/s. ISM absorption features
seen in FUSE spectra reveal components at both stellar and LMC velocities. Thus
we conclude that S119 is located within the LMC ISM and that the bubble is
interacting strongly with the ISM in a bow shock.Comment: 5 pages in EmulateApJ format, 3 figures Accepted by ApJL See
http://fuse.pha.jhu.edu/~danforth/s119
HST-COS Observations of AGN. I. Ultraviolet Composite Spectra of the Ionizing Continuum and Emission Lines
The ionizing fluxes from quasars and other active galactic nuclei (AGN) are
critical for interpreting the emission-line spectra of AGN and for
photoionization and heating of the intergalactic medium. Using ultraviolet
spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space
Telescope (HST), we have directly measured the rest-frame ionizing continua and
emission lines for 22 AGN. Over the redshift range 0.026 < z < 1.44, COS
samples the Lyman continuum and many far-UV emission lines (Lya 1216, C IV
1549, Si IV/OIV] 1400, N V 1240, O VI 1035). Strong EUV emission lines with
14-22 eV excitation energies (Ne VIII 770,780, Ne V 569, O II 834, O III 833,
702, O IV 788,608,554, O V 630, N III 685) suggest the presence of hot gas in
the broad emission-line region. The rest-frame continuum, F_nu ~ nu^{alpha_nu},
shows a break at wavelengths below 1000 A, with spectral index alpha_nu = -0.68
+/- 0.14 in the FUV (1200-2000 A) steepening to alpha_nu = -1.41 +/- 0.21 in
the EUV (500-1000 A). The COS EUV index is similar to that of radio-quiet AGN
in the 2002 HST/FOS survey (alpha_nu = -1.57 +/- 0.17). We see no Lyman edge
(tau_HI < 0.03) or He I 584 emission in the AGN composite. Our 22 AGN exhibit a
substantial range of FUV/EUV spectral indices and a correlation with AGN
luminosity and redshift, likely due to observing below the 1000 A break.Comment: 17 pages, 10 figs, accepted to Astrophysical Journal (revised AGN
luminosities and fluxes, updated figures
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