arXiv subjects

J. C. Howk

Publications and source records attributed to J. C. Howk.

41 records · Page 3Linked to original sources

arXiv2001

Far Ultraviolet Spectroscopy of the Intergalactic and Interstellar Absorption Toward 3C 273

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the molecular, neutral atomic, weakly ionized, and highly ionized components of the interstellar and intergalactic material toward the quasar 3C 273. We identify Ly-beta absorption in 8 of the known intergalactic Ly-alpha absorbers along the sight line with rest-frame equivalent widths greater than 50 mA. Refined estimates of the H I column densities and Doppler parameters (b) of the clouds are presented. We find a range of b = 16-46 km/s. We detect multiple H I lines (Ly-beta - Ly-theta) in the 1590 km/s Virgo absorber and estimate log N(H I) = 15.85, ten times more H I than in all of the other absorbers along the sight line combined. The Doppler width of this absorber, b ~ 16 km/s, implies T < 15,000 K. We detect O VI absorption at 1015 km/s at the 2-3 sigma level that may be associated with hot, X-ray emitting gas in the Virgo Cluster. We detect weak C III and O VI absorption in the IGM at z=0.12007; this absorber is predominantly ionized and has N(H+)/N(H I) > 4000/Z, where Z is the metallicity. Strong Galactic interstellar O VI is present between -100 and +100 km/s, with an additional high-velocity wing containing about 13% of the total O VI between +100 and +240 km/s. The broad O VI absorption wing likely traces the expulsion of hot gas out of the Galactic disk into the halo. A limit on the amount of diffuse O VI emission present 3.5 arc minutes off the sight line combined with the observed O VI column density toward 3C 273, implies n_e < 0.02 cm^-3 and P/k < 11,500 cm^-3 K for an assumed temperature of 3x10^5 K. (abbreviated version)

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arXiv2001

Observations of O VI Emission from the Diffuse Interstellar Medium

We report the first Far Ultraviolet Spectroscopic Explorer (FUSE) measurements of diffuse O VI (lambda,lambda 1032,1038) emission from the general diffuse interstellar medium outside of supernova remnants or superbubbles. We observed a 30arcsec x 30arcsec region of the sky centered at l = 315 and b = -41. From the observed intensities (2930+/-290(random)+/-410(systematic) and 1790+/-260(random)+/-250(systematic) photons/cm/cm/s/sr in the 1032 and 1038 Angstrom emission lines, respectively), derived equations, and assumptions about the source location, we calculate the intrinsic intensity, electron density, thermal pressure, and emitting depth. The intensities are too large for the emission to originate solely in the Local Bubble. Thus, we conclude that the Galactic thick disk and lower halo also contribute. High velocity clouds are ruled out because there are none near the pointing direction. The calculated emitting depth is small, indicating that the O VI-bearing gas fills a small volume. The observations can also be used to estimate the cooling rate of the hot interstellar medium and constrain models. The data also yield the first intensity measurement of the C II 3s2 S1/2 to 2p2 P3/2 emission line at 1037 Angstroms and place upper limits on the intensities of ultraviolet line emission from C I, C III, Si II, S III, S IV, S VI, and Fe III.

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arXiv2000

No diffuse H2 in the metal deficient galaxy I Zw 18

The metal deficient starburst galaxy IZw18 has been observed with FUSE in a search for H2 molecules. The spectrum obtained with an aperture covering the full galaxy shows no absorption lines of diffuse H2 at the radial velocity of the galaxy. The upper limit for the diffuse H2 column density is found to be very low: N(H2) <~ 10^15 cm-2 (10 sigma), unlike our Galaxy where H2 is generally present for even low HI column densities. Although the HI column density is here as high as N(HI) ~ 2.10^21 cm-2, we observe 2N(H2)/N(HI)<< 10^-6. We cannot exclude the possibility that some H2 could be in very dense, small and discrete clumps which cannot be detected with the present observation. However, the remarkable absence of diffuse H2 in this metal-poor galaxy can be explained by the low abundance of dust grains (needed to form this molecule from H-atoms), the high ultraviolet flux and the low density of the HI cloud surrounding the star-forming regions. Thus having eliminated diffuse H2 as a significant contributor to the total mass, it appears that the gas of the galaxy is dominated by HI, and that the high dynamical mass is not composed of cold and diffuse baryonic dark matter.

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arXiv2000

FUSE Observations of Interstellar Gas Towards the LMC Star Sk -67 05

We report on measurements of interstellar O VI, H2, P II, Si II, Ar I, and Fe II absorption along the line of sight to Sk -67 05, a B0 Ia star in a diffuse H II region in the western edge of the Large Magellanic Cloud (LMC). We find log N(O VI) = 14.40 +/- 0.04 in the Milky Way (MW) component and, using the C IV column density from previous IUE observations, N(C IV) / N(O VI) = 1.00 +/- 0.16, a value similar to other halo measurements made with FUSE. In the LMC component, log N(O VI) = 13.89 +/- 0.05, and N(C IV) / N(O VI) < 0.4 (3 sigma), since only an upper limit on N(C IV) is available. Along this sightline the LMC is rich in molecular hydrogen, log N(H2) = 19.50 +/- 0.08; in the MW log N(H2) = 14.95 +/- 0.08. A two-component fit for the excitation temperature of the molecular gas in the LMC gives T_01 = 59 +/- 5 K for J=0,1 and T_ex = 800 +/- 330 K for J=3,4,5. For the MW, T_01 = 99 (+30/-20) K; no excitation temperature could be determined for the higher rotational states. The MW and LMC gas-phase [Fe/P] abundances are ~0.6 and ~0.7 dex lower, respectively, than solar system abundances. These values are similar to [Fe/Zn] measurements for the MW and LMC towards SN 1987A.

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arXiv1998

Observational evidence for the accretion of low-metallicity gas onto the Milky Way; metallicity, physical conditions and distance limit for HVC complex C

We present observations of the (field of the) Seyfert galaxy Mark290, which probes the high-velocity cloud (HVC) complex C, one of the largest HVCs. We find that this object has a metallicity of 0.094pm0.020(+0.022-0.019) times solar. We determine a lower limit to its distance of 5 kpc (z>3.5 kpc). If the gas is in thermal equilibrium with a hot halo, then it is also likely that D<30 kpc, putting the HVC in the Galactic Halo, as was the case for HVC complex~A (for which z=2.5--7 kpc). We find that, on this sightline, H+ represents 23pm10% of the hydrogen in the HVC. The total gaseous mass is 2x10^6 (D/5 kpc)^2 Msun and, depending on whether the space velocity is completely radial or vertical, the HVC represents a downward mass flux of ~0.036-0.083 (D/5 kpc) Msun/yr, or 2.9--6.7x10^{-3} (D/5 kpc)^{-1} Msun/yr/kpc^2. The low metallicity and large mass suggest that complex C is unlikely to be part of a Galactic Fountain, but rather represents accreting low-metallicity material. It may be a present-day analogue of the damped Ly-alpha absorbers seen in QSO spectra. Our abundance result provides the first direct observational evidence for the infall of low-metallicity gas on the Milky Way, required in models of galactic chemical evolution. It remains to be seen whether ultimately complex C is a remnant of the formation of the Milky Way (Oort 1970), a gas cloud orbiting the Galaxy (Kerr & Sullivan 1969), a Local Group object (Verschuur 1969, Blitz et al. 1996), the result of tidal interactions between the Magellanic System and the Galaxy >3 Gyr ago (an "Old Magellanic Stream"; Toomre, quoted in Kerr & Sullivan 1969), or was formed when hot, ionized intergalactic gas was compressed by the motion of the Milky Way (Silk et al. 1987).

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