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Abstract: We have conducted a deep, complete HI survey, using Arecibo-ALFA, of a fieldcentered on the nearby, isolated galaxy, NGC 2903, which is similar to theMilky Way in its properties. The field size was 150 kpc x 260 kpc and the finalvelocity range spanned from 100 to 1133 km-s. The ALFA beams have been mappedas a function of azimuth and cleaned from each azimuth-specific cube prior toforming final cubes. The final HI data are sensitive down to an HI mass of 2 x10^5 Mo and column density of 2 x 10^{17} cm^{-2} at the 3sigma x 2deltaVlevel, where sigma is the rms noise level and deltaV is the velocityresolution. NGC 2903 is found to have an HI envelope that is larger thanpreviously known, extending to at least 3 times the optical diameter of thegalaxy. Our search for companions yields one new discovery with an HI mass of2.6 x 10^6 Mo. The companion is 64 kpc from NGC 2903 in projection, is likelyassociated with a small optical galaxy of similar total stellar mass, and isdark matter dominated, with a total mass >10^8 Mo. In the region surveyed,there are now two known companions: our new discovery and a previously knownsystem that is likely a dwarf spheroidal, lacking HI content. If HI constitutes1% of the total mass in all possible companions, then we should have detected230 companions, according to LCDM predictions. Consequently, if this number ofdark matter clumps are indeed present, then they contain less than 1% HIcontent, possibly existing as very faint dwarf spheroidals or as starless,gasless dark matter clumps.

Author: J. A. Irwin, G. L. Hoffman, K. Spekkens, M. P. Haynes, R. Giovanelli, S. M. Linder, B. Catinella, E. Momjian, B. S. Koribalski, J


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