A Multitransition CO Study of the Antennae Galaxies NGC 4038/9

Abstract

For the Antennae interacting galaxy pair, we have obtained high quality, fully sampled 12 CO J=1-0 and 3-2 maps of the regions surrounding the nuclei and the area of overlap between the two galaxies. The maps possess an angular resolution of 15 ′′ or 1.5 kpc, so far the highest resolution maps available at both the J=1-0 and 3-2 transitions. In addition, 12 CO J=2-1 data have been obtained for the positions of the two nuclei as well as in part of the overlap region with 20 ′′ angular resolution. The 12 CO J=1-0, 2-1, 3-2 emission all peak in an off-nucleus region adjacent to where the two disks overlap. Use of the conventional X factor yields ∼ 4 × 10 9 M ⊙ molecular gas mass in the overlap region. It is difficult to understand how such a large amount of molecular gas can be accumulated in this region given the relatively short lifetime of molecular clouds and the limited period of time for this region to form.Line emission at 13 CO J=2-1 and 3-2 is detected at selected points in the two nuclei and the overlap region. Both the 12 CO/ 13 CO J=2-1 and 3-2 integrated intensity ratios are remarkably high in the overlap region. This is the first published case in which such high 12 CO/ 13 CO J=2-1 and 3-2 ratios are found outside a galactic nucleus. Detailed LVG modeling indicates that the 12 CO and 13 CO emission originate in different spatial components. The 12 CO emission may originate within a non-virialized low density gas component with a large velocity gradient. Assuming a CO-to-H 2 abundance ratio of 10 −4 , the X factor given by the LVG model is an order of magnitude lower than the conventional value for molecular clouds in the Milky Way, but it scales inversely as the assumed value for this ratio. Accordingly, we suggest the possibility that the strong CO emission in the overlap region of the Antennae galaxies is associated with increased radiative efficiency, possibly caused by a large velocity dispersion within the individual molecular clouds.A comparison of the CO J=3-2 emission with the SCUBA 850 µm continuum in the Antennae galaxies shows that the CO line emission contributes globally 46% of the 850 µm continuum flux and that the ratio of 12 CO J=3-2 to SCUBA 850 µm flux varies by a factor of two across the system. After correcting for the 12 CO J=3-2 contamination, the dust emission at 850 µm detected by SCUBA is consistent with the thermal emission from a single warm dust component with a mass of 1.7 ×10 7 M ⊙ .

A survey of submillimeter C and CO lines in nearby galaxies

“…For NGC 4038, Zhu et al (2003) proposed a one component model (N( 12 CO)/∆v = 3.4 × 10 16 cm −2 /km s −1 , n(H 2 ) = 1−4×10 3 cm −3 and T K = 43 K) in good agreement with our study except for the n(H 2 ) value. For Overlap, the one component model proposed by Zhu et al (2003) has N( 12 CO)/∆v ≈ 1.0 × 10 16 cm −2 /km s −1 , n(H 2 ) = 1−5 × 10 3 cm −3 and T K = 33 K for their (0 ; 10 ) offset positions which almost corresponds to our Overlap position. The agreement of the former study with our values is not as good as for NGC 4038 but Zhu et al (2003) used a higher CO abundance ratio ( 12 CO/ 13 CO = 60) than we do (40), which may be at the origin of the differences.…”

“…For T K , we agree better with the value proposed by Eckart et al (1990) than with the one proposed by Israel & Baas (2003). For NGC 4038, Zhu et al (2003) proposed a one component model (N( 12 CO)/∆v = 3.4 × 10 16 cm −2 /km s −1 , n(H 2 ) = 1−4×10 3 cm −3 and T K = 43 K) in good agreement with our study except for the n(H 2 ) value. For Overlap, the one component model proposed by Zhu et al (2003) has N( 12 CO)/∆v ≈ 1.0 × 10 16 cm −2 /km s −1 , n(H 2 ) = 1−5 × 10 3 cm −3 and T K = 33 K for their (0 ; 10 ) offset positions which almost corresponds to our Overlap position.…”

“…The ∆v values used for the target galaxies are reported in Table 3. The 12 CO 13 CO abundance ratio is set to 40 (see Table 3) for IC 10, IC 342, NGC 4038, Overlap, M 83 and NGC 6946, the value used in previous LVG modelling works on the same galaxies (for IC 342 in Mauersberger & Henkel 1993;Henkel et al 1998; for NGC 4038 in Zhu et al 2003, for M 83 in Mauersberger & Henkel 1993and for NGC 6946, in Israel & Baas 2001. But for IC 10, Petitpas & Wilson (1998a) used a 12 CO 13 CO abundance ratio of 50 and for Overlap, Zhu et al (2003) used two different abundance ratios (40 and 60).…”

ALMA CO and VLT/SINFONI H2observations of the Antennae overlap region: mass and energy dissipation

“…To measure fluxes we used a clipped cube where all pixel values <2σ (6 mJy beam −1 ) were set to zero. To quantify the missing flux, we constructed spectra at the center of the overlap region with the angular resolution of the single-dish JCMT and HHT CO(3−2) observations of Zhu et al (2003) and Schulz et al (2007), respectively. We found about half the total flux of the single-dish data.…”

Warm Molecular Gas in Galaxy-Galaxy Merger NGC 6090

“…The CO emission in NGC 6090 is more similar to the Antennae galaxies (NGC 4038/9). Although the optical morphology of NGC 6090 and the Antennae galaxies are different, the CO (1-0), (2-1), and (3-2) emissions all peak at the overlap region (Wilson et al 2000;Gao et al 2001;Zhu et al 2003). Even though most of the CO (3-2) flux in NGC 6090 comes from the overlap region, the weak radio continuum indicates that the history of star formation may not be very long.…”