Molecular gas kinematics and line diagnostics in early-type galaxies: NGC 4710 and NGC 5866

Abstract

We present interferometric observations of CO lines ( 12 CO(1-0, 2-1) and 13 CO(1-0, 2-1)) and dense gas tracers (HCN(1-0), HCO + (1-0), HNC(1-0) and HNCO(4-3)) in two nearby edgeon barred lenticular galaxies, NGC 4710 and NGC 5866, with most of the gas concentrated in a nuclear disc and an inner ring in each galaxy. We probe the physical conditions of a two-component molecular interstellar medium in each galaxy and each kinematic component by using molecular line ratio diagnostics in three complementary ways. First, we measure the ratios of the position-velocity diagrams of different lines, second we measure the ratios of each kinematic component's integrated line intensities as a function of projected position, and third we model these line ratios using a non-local thermodynamic equilibrium radiative transfer code. Overall, the nuclear discs appear to have a tenuous molecular gas component that is hotter, optically thinner and with a larger dense gas fraction than that in the inner rings, suggesting more dense clumps immersed in a hotter more diffuse molecular medium. This is consistent with evidence that the physical conditions in the nuclear discs are similar to those in photo-dissociation regions. A similar picture emerges when comparing the observed molecular line ratios with those of other galaxy types. The physical conditions of the molecular gas in the nuclear discs of NGC 4710 and NGC 5866 thus appear intermediate between those of spiral galaxies and starbursts, while the star formation in their inner rings is even milder.

Multiple-line study of molecular gas in spiral galaxy NGC 2903

“…As revealed from Figure 5, there is degeneracy in T K due to its characteristic 'banana'-shaped ∆χ 2 r contours or inverse n (H 2 ) − T K relationship ( [25,37]; see also appendix C of [15]). The PDF of the kinetic temperature T K also shows a single-peaked distribution peaked at 20 K where the χ 2 result meets with the peak, followed by a steady decrease.…”

“…We compare our best-fitting results (listed in Table 5) to that of literature studies done for the center and along the disc of one spiral and one ETG using similar lines and the same methods. Molecular gas properties along the disc of the spiral galaxy NGC 6946 and ETG NGC 4710 [25][26][27]37] were also studied using the LVG approximation. We found that n (H 2 ) is to be higher in the center of NGC 2903 compared to that found in most of the regions studied in the arms of NGC 6946 [25] and in the center of NGC 6946 [26,27].…”

“…The central region of NGC 4710 was studied using higher resolution (about 6.5'') interferometric observations and a multiple gas component was found, i.e. cold-dense and hot-diffused gas components (see table B1 in [37]). Although the T K found in the center of NGC 2903 is an intermediate between hot-diffused and cold-dense components seen in NGC 4710, the gas is denser than both gas components located in the center of NGC 4710 and the N (CO) is similar.…”

“…R11 versus RD ratios in NGC 2903 and in selected galaxies from the literature. Red, blue, brown, and black filled circles represent starbursts [36], ETGs [37,38], spirals (unpublished data), and NGC 2903, respectively. Lower limit is indicated with arrow.…”

“…We also compared our R D and R 11 ratios to those of a total of 15 galaxies selected from the literature including 4 starbursts and 1 spiral [38], 6 ETGs [37,39], and 4 spirals (unpublished data). The R D versus R 11 ratios of those galaxies together with our target galaxy NGC 2903 are shown in Figure 4.…”