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Register a new userStructure and Kinematics of Molecular Disks in Fast-Rotator Early-Type Galaxies
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Lisa M. Young
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We present interferometric observations resolving the CO emission in the four gas-rich lenticular galaxies NGC 3032, NGC 4150, NGC 4459, and NGC 4526, and we compare the CO distribution and kinematics to those of the stars and ionized gas. Counterrotation documents an external origin for the gas in at least one case (NGC 3032), and the comparisons to stellar and ionized gas substructures in all four galaxies offer insights into their formation histories. The molecular gas is found in kpc-scale disks with mostly regular kinematics and average surface densities of 100 to 200 msunsqpc. The disks are well aligned with the stellar photometric and kinematic axes. In the two more luminous Virgo Cluster members NGC 4459 and NGC 4526 the molecular gas shows excellent agreement with circular velocities derived independently from detailed modeling of stellar kinematic data. There are also two puzzling instances of disagreements between stellar kinematics and gas kinematics on sub-kpc scales. In the inner arcseconds of NGC 3032 the CO velocities are significantly lower than the inferred circular velocities, and the reasons may possibly be related to the external origin of the gas but are not well understood. In addition, the very young population of stars in the core of NGC 4150 appears to have the opposite sense of rotation from the molecular gas.
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Spectroscopic observations of three lenticular (S0) galaxies (NGC 1167, NGC 4150, and NGC 6340) and one SBa galaxy (NGC 2273) have been taken with the 6-m telescope of the Special AstrophysicalObservatory of the Russian Academy of Sciences aimed to study the structure and kinematic properties of early-type disk galaxies. The radial profiles of the stellar radial velocities and the velocity dispersion are measured. N-body simulations are used to construct dynamical models of galaxies containing a stellar disk, bulge, and halo. The masses of individual components are estimated formaximum-mass disk models. A comparison of models with estimated rotational velocities and the stellar velocity dispersion suggests that the stellar disks in lenticular galaxies are overheated; i.e., there is a significant excess velocity dispersion over the minimum level required to maintain the stability of the disk. This supports the hypothesis that the stellar disks of S0 galaxies were subject to strong gravitational perturbations. The relative thickness of the stellar disks in the S0 galaxies considered substantially exceed the typical disk thickness of spiral galaxies.
Observations of galaxy isophotes, longs-slit kinematics and high-resolution photometry suggested a possible dichotomy between two distinct classes of E galaxies. But these methods are expensive for large galaxy samples. Instead, integral-field spectroscopic can efficiently recognize the shape, dynamics and stellar population of complete samples of early-type galaxies (ETGs). These studies showed that the two main classes, the fast and slow rotators, can be separated using stellar kinematics. We showed there is a dichotomy in the dynamics of the two classes. The slow rotators are weakly triaxial and dominate above $M_{rm crit}approx2times10^{11} M_odot$. Below $M_{rm crit}$, the structure of fast rotators parallels that of spiral galaxies. There is a smooth sequence along which, the metals content, the enhancement in $alpha$-elements, and the weight of the stellar initial mass function, all increase with the CENTRAL mass density slope, or bulge mass fraction, while the molecular gas fraction correspondingly decreases. The properties of ETGs on galaxy scaling relations, and in particular the $(M_{ast}, R_{rm e})$ diagram, and their dependence on environment, indicate two main independent channels for galaxy evolution. Fast rotators ETGs start as star forming disks and evolve trough a channel dominated by gas accretion, bulge growth and quenching. While slow rotators assemble near the center of massive halos via intense star formation at high redshift, and remain as such for the rest of their evolution via a channel dominated by gas poor mergers. This is consistent with independent studies of the galaxies redshift evolution.
We present interferometric observations of CO lines (12CO(1-0, 2-1) and 13CO(1-0, 2-1)) and dense gas tracers (HCN(1-0), HCO+(1-0), HNC(1-0) and HNCO(4-3)) in two nearby edge-on 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 components 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 NGC4710 and NGC5866 thus appear intermediate between those of spiral galaxies and starbursts, while the star formation in their inner rings is even milder.
We present interferometric observations of HI in nine slow rotator early-type galaxies of the Atlas3D sample. With these data, we now have sensitive HI searches in 34 of the 36 slow rotators. The aggregate detection rate is 32% $pm$ 8%, consistent with previous work; however, we find two detections with extremely high HI masses, whose gas kinematics are substantially different from what was previously known about HI in slow rotators. These two cases (NGC 1222 and NGC 4191) broaden the known diversity of HI properties in slow rotators. NGC 1222 is a merger remnant with prolate-like rotation and, if it is indeed prolate in shape, an equatorial gas disc; NGC 4191 has two counterrotating stellar discs and an unusually large HI disc. We comment on the implications of this disc for the formation of $2sigma$ galaxies. In general, the HI detection rate, the incidence of relaxed HI discs, and the HI/stellar mass ratios of slow rotators are indistinguishable from those of fast rotators. These broad similarities suggest that the HI we are detecting now is unrelated to the galaxies formation processes and was often acquired after their stars were mostly in place. We also discuss the HI nondetections; some of these galaxies that are undetected in HI or CO are detected in other tracers (e.g. FIR fine structure lines and dust). The question of whether there is cold gas in massive galaxies scoured nuclear cores still needs work. Finally, we discuss an unusual isolated HI cloud with a surprisingly faint (undetected) optical counterpart.
I have combined the Emsellem et al. ATLAS3D rotation measures of a large sample of early-type galaxies with HST-based classifications of their central structure to characterize the rotation velocities of galaxies with cores. Core galaxies rotate slowly, while power-law galaxies (galaxies that lack cores) rotate rapidly, confirming the analysis of Faber et al. Significantly, the amplitude of rotation sharply discriminates between the two types in the -19 > Mv > -22 domain over which the two types coexist. The slow rotation in the small set of core galaxies with Mv > -20, in particular, brings them into concordance with the more massive core galaxies. The ATLAS3D fast-rotating and slow-rotating early-type galaxies are essentially the same as power-law and core galaxies, respectively, or the Kormendy & Bender two families of elliptical galaxies based on rotation, isophote shape, and central structure. The ATLAS3D fast rotators do include roughly half of the core galaxies, but their rotation-amplitudes are always at the lower boundary of that subset. Essentially all core galaxies have ATLAS3D rotation-amplitudes lambda_(R_e/2) <= 0.25, while all galaxies with lambda_(R_e/2) > 0.25 and figure eccentricity > 0.2 lack cores. Both figure rotation and the central structure of early-type galaxies should be used together to separate systems that appear to have formed from wet versus dry mergers.
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