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Session 70 - Cosmic Backgrounds and the Distant Universe.
Oral session, Thursday, June 11
Presidio,
We present the results of deep, high-dispersion Keck/LRIS spectroscopy on the brightest ``U-drop'' in the Hubble Deep Field. This z=2.80 galaxy, HDF\,4-555.1, has been dubbed ``the Hot Dog'' because of its highly-elongated morphology. Our LRIS optical spectra (\lambda/\Delta\lambda\sim 1000) sample the rest-frame UV (1090 - 1890\,Åa region devoid of strong forbidden emission lines. However, many of the resonance lines associated with hot stellar winds do appear, although their P Cygni profiles are less extended in velocity than those observed in some local star-bursts. The overall rest-UV continuum indicates that the Hot Dog is an actively star-forming galaxy, albeit with internal dust extinction of E(B-V)\approx 0.1^m, while the presence of high-ionization N\scriptsize V in emission demands at least some O-stars. Ly\alpha is completely suppressed, and the absorption profile can be fit by a combination of stellar photospheric absorption and a modest interstellar hydrogen column of N(H\,I)\approx 10^20\, cm^-2 (a borderline damped system). The Hot Dog exhibits some of the strongest interstellar absorption features seen in the Lyman-break population.
We also serendipitously discovered in our long-slit spectroscopy a Ly\alpha line emission galaxy (W_0\approx 30\,Åat a redshift within 1000\, km\,s^-1 of the Hot Dog, and a projected separation of 17.6\,h^-1\,kpc (h=H_\circ/100 km\,s^-1\,Mpc^-1, q_\circ=0.5). The flux in the Ly-alpha line of this companion galaxy is 3\times 10^-17\,erg\,cm^-2\,s^-1, and the rest-UV continuum suggests a star formation rate of SFR_ UV\approx 1.4\,h^-2\,M_ødot\,yr^-1.
Finally, our spatially-resolved spectroscopy reveals Mg\scriptsize II \lambda\lambda 2796/2803\,Å\ and Fe\scriptsize II \lambda 2600\,Å\ absorption by a foreground system at z=1.239. The absorption is only pronounced in the northern component of the Hot Dog, and arises from a galaxy 2\,arcsec away. The absence of strong Mg\scriptsize II absorption in the southern component enables us to constrain the physical size of the Lyman limit system halo of this galaxy to be r<13.5\,h^-1\,kpc.
