Central Spiral Structure of Molecular Gas in Maffei 2

Distribution and kinematics of molecular gas in the central region of the barred spiral galaxy Maffei 2 were investigated using a data set of 12CO(1-0), 12CO(2-1), CS(2-1) lines and 103 GHz continuum. We found that the offset ridges along the kpc-scale bar continue to the central spiral structure embedded in the weak oval structure which is regarded as x2 orbit in the bar potential. The spiral structure continues toward the center diverging from the oval structure. The size of these structures is less than R ~ 100 pc. The mass concentration within R = 35 pc is estimated to be 2 X 10^8 Mo. The high mass concentration is consistent with theoretical predictions concerning the creation of such a nuclear spiral structure. A comparison with the tracers of dense gas and star-forming region suggests that the dense molecular gas traced by CS(2-1) line is formed at the crossing points of x1 and x2 orbits and the star-forming region appears after 2 X 10^5 yr which is comparable with the free-fall time of the dense gas traced by the CS line (~ 10^5 cm^-3).Comment: 7 pages, 10 figures, Publication of Astronomical Society of Japan, in pres

Diffuse and Gravitationally Stable Molecular Gas in the Post-Starburst Galaxy NGC 5195

The Nobeyama Millimeter Array (NMA) has been used to make aperture synthesis CO(1-0) observations of the post-starburst galaxy NGC 5195. CO(1-0) and HCN(1-0) observations of NGC 5195 using the Nobeyama 45 m telescope are also presented. High-resolution (1".9 x 1".8 or 86 pc x 81 pc at D = 9.3 Mpc) NMA maps show a strong concentration of CO emission toward the central a few 100 pc region of NGC 5195, despite the fact that the current massive star formation is suppressed there. The HCN-to-CO integrated intensity ratio on the brightness temperature scale, R_{HCN/CO}, is about 0.02 within the central r < 400 pc region. This R_{HCN/CO} is smaller than those in starburst regions by a factor of 5 - 15. These molecular gas properties would explain why NGC 5195 is in a post-starburst phase; most of the dense molecular cores (i.e., the very sites of massive star formation) have been consumed away by a past starburst event, and therefore a burst of massive star formation can no longer last, although a large amount of low density gas still exists. We propose that dense molecular gas can not be formed from remaining diffuse molecular gas because the molecular gas in the center of NGC 5195 is too stable to form dense cores via gravitational instabilities of diffuse molecular gas.Comment: 26 pages, 10 figures, PASJ, vol. 54, in press. For the preprint with high resolution figures, see http://www.nro.nao.ac.jp/library/report/list.html or http://www.ioa.s.u-tokyo.ac.jp/~kkohno/n5195/all.ps.g

The Radio Continuum, Far-Infrared Emission, And Dense Molecular Gas In Galaxies

A tight linear correlation is established between the HCN line luminosity and the radio continuum (RC) luminosity for a sample of 65 galaxies (from Gao & Solomon's HCN survey), including normal spiral galaxies and luminous and ultraluminous infrared galaxies (LIRGs/ULIRGs). After analyzing the various correlations among the global far-infrared (FIR), RC, CO, and HCN luminosities and their various ratios, we conclude that the FIR-RC and FIR-HCN correlations appear to be linear and are the tightest among all correlations. The combination of these two correlations could result in the tight RC-HCN correlation we observed. Meanwhile, the non-linear RC-CO correlation shows slightly larger scatter as compared with the RC-HCN correlation, and there is no correlation between ratios of either RC/HCN-CO/HCN or RC/FIR-CO/FIR. In comparison, a meaningful correlation is still observed between ratios of RC/CO-HCN/CO. Nevertheless, the correlation between RC/FIR and HCN/FIR also disappears, reflecting again the two tightest FIR-RC and FIR-HCN correlations as well as suggesting that FIR seems to be the bridge that connects HCN with RC. Interestingly, despite obvious HCN-RC and RC-CO correlations, multi-parameter fits hint that while both RC and HCN contribute significantly (with no contribution from CO) to FIR, yet RC is primarily determined from FIR with a very small contribution from CO and essentially no contribution from HCN. These analyses confirm independently the former conclusions that it is practical to use RC luminosity instead of FIR luminosity, at least globally, as an indicator of star formation rate in galaxies including LIRGs/ULIRGs, and HCN is a much better tracer of star-forming molecular gas and correlates with FIR much better than that of CO.Comment: 11 ApJ pages, 7 figures; ApJ in pres

HCN Survey of Normal Spiral, IR-luminous and Ultraluminous Galaxies

We report systematic HCN J=1-0 (and CO) observations of a sample of 53 infrared (IR) and/or CO-bright and/or luminous galaxies, including seven ultraluminous infrared galaxies, nearly 20 luminous infrared galaxies, and more than a dozen of the nearest normal spiral galaxies. This is the largest and most sensitive HCN survey of galaxies to date. All galaxies observed so far follow the tight correlation between the IR luminosity $L_{\rm IR}$ and the HCN luminosity $L_{\rm HCN}$ initially proposed by Solomon, Downes, & Radford, which is detailed in a companion paper. We also address here the issue of HCN excitation. There is no particularly strong correlation between $L_{\rm HCN}$ and the 12$\mu$m luminosity; in fact, of all the four \IRAS bands, the 12$\mu$m luminosity has the weakest correlation with the HCN luminosity. There is also no evidence of stronger HCN emission or a higher ratio of HCN and CO luminosities $L_{\rm HCN}/L_{\rm CO}$ for galaxies with excess 12$\mu$m emission. This result implies that mid-IR radiative pumping, or populating, of the J=1 level of HCN by a mid-IR vibrational transition is not important compared with the collisional excitation by dense molecular hydrogen. Furthermore, large velocity gradient calculations justify the use of HCN J=1-0 emission as a tracer of high-density molecular gas (\approxgt 3\times 10^4/\tau cm$^{-3}$) and give an estimate of the mass of dense molecular gas from HCN observations. Therefore, $L_{\rm HCN}$ may be used as a measure of the total mass of dense molecular gas, and the luminosity ratio $L_{\rm HCN}/L_{\rm CO}$ may indicate the fraction of molecular gas that is dense.Comment: ApJS, May issue (final version, 12 pages + 5 figures + 4 tables). Fig. 2a,b,c and Fig. 3a,b are in GIF format due to the space limitation of astro-ph. Added an error-bar in both Fig. 4 and Fig. 5a. Please see the companion paper by Gao & Solomon (Paper II, astro-ph/0310339) for the detailed analysis and implications of this HCN surve

The Star Formation Rate and Dense Molecular Gas in Galaxies

(abridged) HCN luminosity Lhcn is a tracer of DENSE molecular gas, n(H_2) >~ 3x10^4 cm^{-3}, associated with star-forming giant molecular cloud (GMC) cores. We present the results and analysis of our survey of HCN emission from 65 infrared galaxies including 9 ultraluminous infrared galaxies (ULIGs, Lir>10^{12}Lsun), 22 luminous infrared galaxies (LIGs, 10^{11}<Lir<~10^{12}Lsun) and 34 normal spiral galaxies with lower IR luminosity (most are large spiral galaxies). We have measured the global HCN line luminosity and the observations are reported in Gao and Solomon (2003, Paper I). This paper analyzes the relationships between the total far-IR luminosity a tracer of the star formation rate, the global HCN line luminosity a measure of the total DENSE molecular gas content, and the CO luminosity a measure of the total molecular content. We find a tight linear correlation between the IR and HCN luminosities Lir and Lhcn (in the log-log plot) with a correlation coefficient R=0.94. The IR--HCN linear correlation is valid over 3 orders of magnitude including ULIGs. The direct consequence of the linear IR--HCN correlation is that the star formation law in terms of DENSE molecular gas content has a power law index of 1.0. The global star formation rate is linearly proportional to the mass of dense molecular gas in normal spiral galaxies, LIGs, and ULIGs. This is strong evidence in favor of star formation as the power source in ultraluminous galaxies since the star formation in these galaxies appears to be normal and expected given their high mass of dense star-forming molecular gas.Comment: ApJ, May 1 issue (final version, 20 pages + 9 figures + 3 tables + appendix/refs.). Please see Gao & Solomon (Paper I, astro-ph/0310341) for the presentation and discussion of the HCN surve

HI and CO observations of Arp 104: a spiral-elliptical interacting pair

We present data probing the spatial and kinematical distribution of both the atomic (HI) and molecular (CO) gas in NGC 5218, the late-type barred spiral galaxy in the spiral-elliptical interacting pair, Arp 104. We consider these data in conjunction with far-infrared and radio continuum data, and N-body simulations, to study the galaxies interactions, and the star formation properties of NGC 5218. We use these data to assess the importance of the bar and tidal interaction on the evolution of NGC 5218, and the extent to which the tidal interaction may have been important in triggering the bar. The molecular gas distribution of NGC 5218 appears to have been strongly affected by the bar; the distribution is centrally condensed with a very large surface density in the central region. The N-body simulations indicate a timescale since perigalacticon of approximately 3 x 10**8 yr, which is consistent with the interaction having triggered or enhanced the bar potential in NGC 5218, leading to inflow and the large central molecular gas density observed. Whilst NGC 5218 appears to be undergoing active star formation, its star formation efficiency is comparable to a `normal' SBb galaxy. We propose that this system may be on the brink of a more active phase of star formation.Comment: 15 pages, accepted by MNRA