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تسجيل مستخدم جديدMultiple CO lines in SMM J16359+6612 -- Further evidence for a merger
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Using the IRAM 30m telescope, we report the detection of the CO(3--2), CO(4--3), CO(5--4) and CO(6--5) lines in the gravitational lensed submm galaxy SMM J16359+6612 at z=2.5. The CO lines have a double peak profile in all transitions. From a Gaussian decomposition of the spectra we show that the CO line ratios, and therefore the underlying physical conditions of the gas, are similar for the blue and the redshifted component. The CO line Spectral Energy Distribution (SED; i.e. flux density vs. rotational quantum number) turns over already at the CO(5--4) transition which shows that the molecular gas is less excited than in nearby starburst galaxies and high--z QSOs. This difference mainly arises from a lower average H2 density, which indicates that the gas is less centrally concentrated than in nuclear starburst regions in local galaxies. We suggest that the bulk of the molecular gas in SMM J16359+6612 may arise from an overlap region of two merging galaxies. The low gas density and clear velocity separation may reflect an evolutionary stage of the merger event that is in between those seen in the Antennae and in the more evolved ultraluminous infrared galaxies (ULIRGs) like e.g. Mrk231.
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We report the detection of CO ($J$=3$to$2) line emission from all three multiple images (A,B and C) of the intrinsically faint ($simeq$ 0.8 mJy) submillimeter-selected galaxy SMM J16359+6612. The brightest source of the submm continuum emission (B) a
lso corresponds to the brightest CO emission, which is centered at $z$=2.5168, consistent with the pre-existing redshift derived from Ha. The observed CO flux in the A, B and C images is 1.2, 3.5 and 1.6 Jy kms respectively, with a linewidth of $500pm 100$ kms. After correcting for the lensing amplification, the CO flux corresponds to a molecular gas mass of $sim 2times 10 ^{10} h_{71}^{-2}$ Msun, while the extent of the CO emission indicates that the dynamical mass of the system $sim9times10^{10}$ Msun. Two velocity components are seen in the CO spectra; these could arise from either a rotating compact ring or disk of gas, or merging substructure. The star formation rate in this galaxy was previously derived to be $sim$100--500 Msun yr. If all the CO emission arises from the inner few kpc of the galaxy and the galactic CO-to-H$_2$ conversion factor holds, then the gas consumption timescale is a relatively short 40 Myr, and so the submm emission from SMM J16359+6612 may be produced by a powerful, but short-lived circumnuclear starburst event in an otherwise normal and representative high-redshift galaxy.
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