Probing IGM accretion on to faint Lyα emitters at z ∼ 2.8

Observing the signature of accretion from the intergalactic medium (IGM) on to galaxies at z ∼ 3 requires the detection of faint (L ≪ L*) galaxies embedded in a filamentary matrix of low-density ($\rho \lt 100\ \overline{\rho }$), metal-poor gas (Z ∼ 10−2.5 Z⊙) coherent over hundreds of kpc. We study the gaseous environment of three Lyα emitters (LAEs) at z = 2.7 − 2.8, found to be aligned in projection with a background QSO over ∼250 kpc along the slit of a long-slit spectrum. The lack of detection of the LAEs in deep continuum images and the low inferred Lyα luminosities show the LAEs to be intrinsically faint, low-mass galaxies ($L\lesssim 0.1\, L^*$, $M_\mathrm{star}\lesssim 0.1\, M^*$). An echelle spectrum of the QSO reveals strong Lyα absorption within ±200 km s−1 from the LAEs. Our absorption line analysis leads to $\rm{H\,{\small I}}$ column densities in the range of log $N\mathrm{(\rm{H\,{\small I}})}/\mbox{${\rm cm^{-2}}$}=16\!-\!18$. Associated absorption from ionic metal species $\rm{C\,{\small IV}}$ and $\rm{Si\,{\small IV}}$ constrains the gas metallicities to ∼0.01 solar if the gas is optically thin, and possibly as low as ∼0.001 solar if the gas is optically thick, assuming photoionization equilibrium. While the inferred metallicities are at least a factor of 10 lower than expected metallicities in the interstellar medium (ISM) of these LAEs, they are consistent with the observed chemical enrichment level in the IGM at the same epoch. Total metal abundances and kinematic arguments suggest that these faint galaxies have not been able to affect the properties of their surrounding gas. The projected spatial alignment of the LAEs, together with the kinematic quiescence and correspondence between the LAEs and absorbing gas in velocity space, suggests that these observations probe a possible filamentary structure. Taken together with the blue-dominant Lyα emission line profile of one of the objects, the evidence suggests that the absorbing gas is part of an accretion stream of low-metallicity gas in the IGM.

A Supernova Remnant in the Dwarf HO IX

The dwarf irregular galaxy Ho IX (DDO 66) is a satellite of the giant spiral M81. Triggered by an interaction with M81 (Hopp & Schulte-Ladbeck, 1987 AA 187, 5), Ho IX may have recently undergone a burst of star formation. On very deep Calar Alto 3.5m telescope R images, we detected a ring-like nebula in the northeastern part of Ho IX, which is situated near to the giant CO molecular cloud recently discussed by Henkel et al. (1993 AA 273, L15). The structure of the nebula resembles supernova remnants like CTB 80. No HI-features can be detected at its location, especially no hole. It shows a blue central point source (R=21.7). A Lucy deconvolution of the 0.9″ seeing images indicates further faint point sources, perhaps a small stellar cluster. An optical long slit spectrum of the nebula shows emission lines with ratios as in old supernova remnants while it excludes normal HII regions, planetary nebula, or the LMC X-1 type nebula. The position of the central object coincides with a bright and variable ROSAT point source which was already detected by the Einstein spacecraft. The X-ray spectrum is rather steep and shows strong intrinsic absorption. The nature of the X-ray source and its possible connection to the optical nebula is not yet understood. Miller (1995, ApJ 446, L75) who independetly found the nebula interpretated his spectra as a supershell surrounding an OB association. Our optical nebula line ratios severely deviates from Miller's. Our ratios as the X-ray spectrum point more to a SNR at the M81 distance, most probably belonging to Ho IX.