12CO-To- 13CO Ratios, Carbon Budget, and X Factors
Total Page：16
File Type：pdf, Size：1020Kb
Astronomy & Astrophysics manuscript no. 34198corrac c ESO 2020 January 15, 2020 Central molecular zones in galaxies: 12CO-to- 13CO ratios, carbon budget, and X factors F.P. Israel1 Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, the Netherlands Accepted December 19. 2019 ABSTRACT We present ground-based measurements of 126 nearby galaxy centers in 12CO and 92 in 13CO in various low-J transitions. More than 60 galaxies were measured in at least four lines. The average relative intensities of the first four 12CO J transitions are 1.00 : 0.92 : 0.70 : 0.57. In the first three J transitions, the average 12CO-to- 13CO intensity ratios are 13.0, 11.6, and 12.8, with individual values in any transition ranging from 5 to 25. The sizes of central CO concentrations are well defined in maps, but poorly determined by multi-aperture photometry. On average, the J=1-0 12CO fluxes increase linearly with the size of the observing beam, but CO emission covers only a quarter of the HI galaxy disks. Using radiative transfer models (RADEX), we derived model gas parameters. The assumed carbon elemental abundances and carbon gas depletion onto dust are the main causes of uncertainty. The new CO data and published [CI] and [CII] data imply that CO, C◦, and C+ each represent about one-third of the gas-phase carbon in the molecular 21 −2 interstellar medium. The mean beam-averaged molecular hydrogen column density is N(H2) = (1.5 ± 0.2) × 10 cm . Galaxy center CO-to- H2 conversion factors are typically ten times lower than the ‘standard’ Milky Way X◦ disk value, with a mean X(CO) = 19 −2 −1 19 −2 −1 (1.9 ± 0.2) × 10 cm / K km s and a dispersion 1.7 × 10 cm / K km s . The corresponding [CI]- H2 factor is five times higher than X(CO), with X[CI] = (9±2)×1019 cm−2/ K km s−1. No unique conversion factor can be determined for [CII]. The low molecular gas content of galaxy centers relative to their CO intensities is explained in roughly equal parts by high central gas-phase carbon abundances, elevated gas temperatures, and large gas velocity dispersions relative to the corresponding values in galaxy disks. Key words. Galaxies: galaxies: centers – interstellar medium: molecules – millimeter lines – CO observations 1. Introduction surveys in higher 12CO transitions are fewer in number and usu- ally only sample the nucleus (J=2-1; Braine etal. 1993a; Al- The aim of this paper is to determine the carbon budget and the brecht etal. 2007; J=3-2: Mauersberger etal. (1999); Yao etal. amount of molecular hydrogen in the centers of nearby galax- 2003; Mao et al. 2010). The survey by Dumke etal. (2001) and ies as accurately as possible, based on extensive new observa- especially the James Clerk Maxwell Telescope (JCMT) legacy tions and current chemical and radiative transfer models. The survey of nearby galaxies (NGLS: Wilson etal. 2012; Mok etal. bright inner disks of late-type galaxies contain massive concen- 2016) are exceptional because they provide maps of almost 100 trations of circumnuclear molecular hydrogen gas. These reser- galaxies in the J=3-2 transition, many of them in the Virgo clus- voirs feed central black holes, outflows, and bursts of star for- ter. Specific surveys of Virgo cluster galaxies have also been mation. Before their crucial role in inner galaxy evolution can published by Stark etal. (1986, J=1-0), Kenney and Young be understood and evaluated, the physical characteristics of the (1988, J=1-0), and Hafok & Stutzki (2003, J=2-1 and J=3-2). gas must be determined. Cool and quiescent molecular hydro- 12 gen (H ) gas is difficult to detect, and studies of the molecular The CO lines in the survey are optically thick and cannot 2 be used to measure molecular gas column densities or masses. interstellar medium (ISM) in galaxies rely on the observation of 12 tracers such as continuum emission from thermal dust or line Even the analysis of a whole ladder of multiple CO tran- emission from the CO molecule. CO is one of the most common sitions either fails to break the degeneracy between H2 den- sity, kinetic temperature, and CO column density and leaves the molecules in the ISM after H2, even though its relative abun- arXiv:2001.04975v1 [astro-ph.GA] 14 Jan 2020 dance is only about 10−5. It has become the instrument of choice mass an undetermined quantity, or samples only a small frac- in the investigation of the molecular ISM because it is compar- tion of the total gas content in the higher J transitions. Con- atively easy to detect and traces molecular gas already at low sequently, most molecular gas masses quoted in the literature densities and temperatures. are critically dependent on an assumed value for the relation Following the first detections in the mid-1970s, numerous between velocity-integrated CO line intensity and H2 column galaxies have been observed in various transitions of CO and density, XCO = N(H2)/I(CO). Unfortunately, this so-called X- its isotopologue 13CO. Substantial surveys have been conducted factor does not follow from basic physical considerations. In- in the J=1-0 transition of 12CO (e.g., Stark et al. 1987; Braine stead, its empirically estimated value is rather sensitive to as- etal. 1993a; Sage 1993; Young etal. 1995; Elfhag et al. 1996; sumptions made in the process, and it varies depending on au- Nishiyama & Nakai 2001; Sauty et al. 2003; Albrecht et al. thor and method. The most reliable method uses gamma-ray 2007; Kuno etal. 2007). These surveys sample the nucleus and observations to trace hydrogen nuclei, and a useful overview sometimes also a limited number of disk positions. Extensive of X values thus obtained can be found in Table E.1 of Remy etal. (2017). The empirically determined X values implicitly in- Send offprint requests to: F.P. Israel clude both H2 gas mixed with CO and H2 gas that contains no Article number, page 1 of 41 A&A proofs: manuscript no. 34198corrac Table 1. Galaxy sample NGC Dist. lgFIR lgLFIR D25 NGC Dist. lgFIR lgLFIR Size NGC Dist. lgFIR lgLFIR Size −2 −2 −2 IC Mpc Wm L⊙ ’ IC Mpc Wm L⊙ ’ IC Mpc Wm L⊙ ’ (1) (2) (3) (4) (5) (1) (2) (3) (4) (5) (1) (2) (3) (4) (5) N 134 21.5 -11.88 10.26 8.1x2.6 N2993 35.9 -12.28 10.31 1.3x0.9 N4666 27.5 -11.74 10.62 4.6x1.3 N 253 3.4 -10.42 10.13 25x7.4 N3034 5.9 -10.28 10.74 11x4.3 N4736 4.8 -11.50 9.35 11x9.1 N 275 23.6 -12.56 9.67 1.5x1.1 N3044 20.4 -12.25 9.86 5.7x0.6 N4826∗ 3.8 -11.66 9.34 10x5.4 N 278 11.3 -11.87 9.76 2.2x2.1 N3079∗ 20.7 -11.60 10.51 7.9x1.4 N4835 23.9 -12.00 10.24 4.0x0.9 N 300 1.9 -11.77 8.26 22x16 I2554 16.4 -12.04 9.87 3.2x1.5 N4945∗ 4.4 -10.67 10.10 20x3.8 N 470 31.7 -12.44 10.04 2.8x1.7 N3175 13.6 -12.10 9.65 5.0x1.3 N5033∗ 17.2 -12.00 9.95 11x5.0 N 520 30.5 -11.79 10.66 4.5x1.8 N3227∗ 20.3 -12.32 9.77 5.4x3.6 N5055 8.3 -11.66 9.66 13x7.2 N 613 19.7 -11.90 10.17 5.5x4.2 N3256 37.0 -11.71 10.91 3.8x2.1 N5135∗ 57.7 -12.04 10.96 2.6x1.8 N 628 9.9 -12.60 8.87 11x9.5 N3281∗ 44.7 -12.50 10.28 3.3x1.8 N5194∗ 9.1 -11.59 9.81 11x6.9 N 660 12.2 -11.46 10.20 9.1 N3310 19.2 -11.79 10.26 3.1x2.4 N5218 46.5 -12.38 10.43 1.9x1.0 N 695 130 -12.36 11.35 0.8x0.7 N3351 9.0 -12.00 9.39 3.1x2.9 N5236 4.0 -11.22 9.46 13x12 N 891 9.4 -11.53 9.90 14x2.5 N3504 27.8 -11.98 10.39 2.7x2.2 Circ∗ 2.9 -10.92 9.48 6.9x3.0 N 908 19.9 -12.00 10.08 6.0x2.6 N3556 14.2 -11.81 9.97 8.7x2.2 N5433 67.8 -12.44 10.70 1.6x0.4 N 972 21.4 -11.75 10.39 3.4x1.7 N3593 5.6 -11.97 9.01 5.75 I4444 23.1 -11.95 10.26 1.7x1.4 Maff2 3.1 -11.23 9.23 5.8x1.6 N3620 20.4 -11.61 10.49 2.8x1.1 N5643∗ 14.4 -11.93 9.87 4.6x4.0 N1055 13.4 -11.84 9.89 7.6x2.7 N3621 6.5 -11.96 9.15 12x7.1 N5713 31.3 -11.95 10.52 2.8x2.5 N1068∗ 15.2 -11.04 10.80 7.1x6.0 N3627 6.5 -11.61 9.50 9.1x4.2 N5775 28.9 -11.97 10.43 4.2x1.0 N1084 18.6 -12.33 9.69 3.3x1.2 N3628 8.5 -11.54 9.80 15x3.0 N6000 31.0 -11.71 10.75 1.9x1.6 N1097∗ 16.5 -11.81 10.10 9.3x6.6 N3690 48.5 -11.32 11.53 2.9x2.1 N6090 126 -12.47 11.21 1.7x0.7 N1317 25.8 -12.62 9.68 2.8x2.4 N3783 36.1 -12.76 9.83 1.9x1.7 N6215 20.2 -11.83 10.26 2.1x1.8 N1365∗ 21.5 -11.36 10.78 11x6.2 N3982∗ 21.8 -12.37 9.79 1.7x1.5 N6221∗ 19.3 -11.65 10.40 3.5x2.5 I342 3.8 -11.36 9.28 21x21 N4030 26.4 -11.95 10.37 4.3 N6240 109 -11.96 11.59 2.1x1.1 N1433∗ 13.3 -12.55 9.18 6.5x5.9 N4038 23.3 -11.65 10.56 5.2x3.1 N6300∗ 14.0 -12.02 9.75 4.5x3.0 N1448 14.7 -12.22 9.59 7.6x1.7 N4039 23.3 -11.65 10.56 3.1x1.6 N6744 10.7 -12.55 8.99 20x13 N1482 25.4 -11.79 10.50 2.5x1.4 N4051∗ 12.9 -12.27 9.43 5.2x3.9 N6764 38.5 -12.44 10.21 2.3x1.3 N1559 16.3 -11.83 10.07 3.5x2.0 N4102 17.3 -11.62 10.34 2.8x1.2 N6810 28.8 -11.99 10.41 3.2x0.9 N1566∗ 19.4 -12.02 10.04 8.3x6.6 N4254 39.8 -11.78 10.90 5.4x4.7 N6946 5.5 -11.46 9.50 12x9.8 N1614 64.2 -11.82 11.28 1.3x1.1 N4258∗ 8.0 ..