Mapping CO in the Outer Parts of UV Disks CO Detection Beyond the Optical Radius Miroslava Dessauges Observatoire de Genève, Switzerland Françoise Combes Observatoire de Paris (LERMA), France Daniel Pfenniger Observatoire de Genève, Switzerland A multi-wavelength 3D perspective - June 10-13, ESO, Germany

Hints for Molecular Gas in Outer Regions of Galaxies Currently, well established: CO is the strongest in the central regions of spirals and falls off as does the blue light with galactocentric distance (e.g. Young & Scoville 82,91; Young et al. 95) Indirect evidence for molecular gas at large radii: 1) star formation detected in H  and broad-band observations in a few galaxies (Ferguson et al. 98; Cuillandre et al.01; de Block & Walter 03) 2) presence of dust (best example: M31, Cuillandre et al. 01) 3) H 2 as baryonic dark matter reservoir (Pfenniger & Combes 94; Combes & Pfenniger 97; Bureau et al. 99; Revaz et al. 06) GALEX recent results: H  observations still fail to detect a significant population of B (and O) stars  UV-bright disks extending up to 2-3 times the optical radius have been reported in 30% of spirals (Thilker et al. 05; Gil de Paz et al. 05,07)

Detections of CO beyond R 25 external clouds in the Milky Way  Digel et al. 94 NGC 4414  Braine & Herpin 04 CO pointings selected on high N(HI) knots NGC 6946  Braine et al. 07 CO pointings selected on HII regions M63  present work CO pointings selected on UV emission

NUV+FUV composite Optical Observations Target: M63 - NGC 5055  remarkable example of a nearby spiral with an extended UV disk reaching 2.5xR 25  SAbc spiral galaxy seen almost face-on, located at 8.2 Mpc (Kennicutt et al. 03)  shows no interaction and hence is representative of other galaxies  HI disk extends out to 2.8xR 25 with N(HI) > cm -2 out to 2.5xR 25 (Battaglia et al. 06) Gil de Paz et al. 07 Data acquired:  at the IRAM 30m telescope (Pico Veleta, Spain), observations of CO(1-0) and CO(2-1)  strong UV region at 1.3xR 25  radial profile along the major axis out to 1.3xR 25 R 25 = 6.3’ = 15 kpc } spatial sampling = 1 beam size = 22” = 875 pc Gil de Paz et al. 07

Results Strong UV region at 1.3xR 25 (12 points sampled)  CO(1-0) detected at 2-3  at several offset positions  sum of all scans at all offset positions:  detection - FWHM = 23.3 km s -1 - integrated intensity = 0.11±0.02 K km s -1 - H 2 column density = 2.2x10 19 cm -2  N(H 2 ) is 10-20% of N(HI) with the standard X(CO) = 2x10 20 cm -2 / K km s -1 - H 2 mass = 46x10 5 M  with H 2 mass-to-CO luminosity ratio = 4.6 M  / K km s -1 pc 2 (Solomon et al. 87) size = 132’’ x 44’’ = 5.2 kpc x 1.7 kpc CO(1-0) v LSR [km s -1 ] T A * [K]

Results Radial profile out to 1.3xR 25 (24 points sampled)  CO(1-0) detected at 4-5  out to 0.95xR 25 with FWHM = km s -1 (larger than in GMCs)  CO(2-1) detected at 2-3  out to 0.65xR 25 with CO(2-1)/CO(1-0) decreasing from 0.8 to 0.4 (decrease in excitation temperature?) R 25  beyond R 25, I(CO) < 0.03 K km s -1 : 3x more sensitive than CO(1-0) in the UV region  trend for a correlation between CO and UV  much steeper radial decrease of CO than UV brightness

Outputs of Molecular Gas Detection at Large Radii 1) ideal place to study the HI gas origin: is HI a product of SF processes, dissociation of H 2 by UV flux radiation? (Allen et al. 86,04; Smith et al. 00) 2) simplified laboratory for determining the SF threshold: what is the minimum gas density required for SF to occur spontaneously? (Kennicutt 89; Martin & Kennicutt 01) 3) estimation of the H 2 content: can H 2 account for the total baryonic dark matter in galaxies? Detections of CO beyond R 25 external clouds in the Milky Way  Digel et al. 94 NGC 4414  Braine & Herpin 04 NGC 6946  Braine et al. 07