Abstract
Gas-rich minor mergers contribute significantly to the gas reservoir of early-type galaxies (ETGs) at low redshift, yet the star formation efficiency (SFE; the star formation rate divided by ...the molecular gas mass) appears to be strongly suppressed following some of these events, in contrast to the more well-known merger-driven starbursts. We present observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of six ETGs, which have each recently undergone a gas-rich minor merger, as evidenced by their disturbed stellar morphologies. These galaxies were selected because they exhibit extremely low SFEs. We use the resolving power of ALMA to study the morphology and kinematics of the molecular gas. The majority of our galaxies exhibit spatial and kinematical irregularities, such as detached gas clouds, warps, and other asymmetries. These asymmetries support the interpretation that the suppression of the SFE is caused by dynamical effects stabilizing the gas against gravitational collapse. Through kinematic modelling we derive high velocity dispersions and Toomre Q stability parameters for the gas, but caution that such measurements in edge-on galaxies suffer from degeneracies. We estimate merger ages to be about 100 Myr based on the observed disturbances in the gas distribution. Furthermore, we determine that these galaxies lie, on average, two orders of magnitude below the Kennicutt–Schmidt relation for star-forming galaxies as well as below the relation for relaxed ETGs. We discuss potential dynamical processes responsible for this strong suppression of star formation surface density at fixed molecular gas surface density.
Abstract
We present ALMA observations of CO isotopologues and high-density molecular tracers (HCN, HCO
+
, CN, etc.) in NGC 7465, an unusually gas-rich early-type galaxy that acquired its cold gas ...recently. In the inner 300 pc, the molecular gas kinematics are misaligned with respect to all other galaxy components; as the gas works its way inward, it is torqued into polar orbits about the stellar kinematically decoupled core (KDC), indicating that the stellar KDC is not related to the current gas accretion event. The galaxy also exhibits unusually high
12
CO/
13
CO line ratios in its nucleus but typical
13
CO/C
18
O ratios. Our calculations show that this result does not necessarily indicate an unusual
12
CO/
13
CO abundance ratio but rather that
12
CO (1–0) is optically thin due to high temperatures and/or large line widths associated with the inner decoupled, misaligned molecular structure. Line ratios of the higher-density tracers suggest that the densest phase of molecular gas in NGC 7465 has a lower density than is typical for nearby galaxies, possibly as a result of the recent gas accretion. All of the observed molecular properties of NGC 7465 are consistent with it having acquired its molecular (and atomic) gas from a spiral galaxy. Further detailed studies of the CO isotopologues in other early-type galaxies would be valuable for investigating the histories of those that may have acquired their gas from dwarfs. Finally, these ALMA data also show an unidentified line source that is probably a background galaxy similar to those found at
z
= 1–3 in blind CO surveys.
ABSTRACT
This is the fourth paper of a series investigating the AGN fuelling/feedback processes in a sample of 11 nearby low-excitation radio galaxies (LERGs). In this paper, we present follow-up ...Atacama Large Millimeter/submillimeter Array (ALMA) observations of one source, NGC 3100, targeting the 12CO(1-0), 12CO(3-2), HCO+(4-3), SiO(3-2), and HNCO(6-5) molecular transitions. 12CO(1-0) and 12CO(3-2) lines are nicely detected and complement our previous 12CO(2-1) data. By comparing the relative strength of these three CO transitions, we find extreme gas excitation conditions (i.e. Tex ≳ 50 K) in regions that are spatially correlated with the radio lobes, supporting the case for a jet–ISM interaction. An accurate study of the CO kinematics demonstrates that although the bulk of the gas is regularly rotating, two distinct non-rotational kinematic components can be identified in the inner gas regions: one can be associated to inflow/outflow streaming motions induced by a two-armed spiral perturbation; the second one is consistent with a jet-induced outflow with vmax ≈ 200 km s−1 and $\dot{M}\lesssim 0.12$ M⊙ yr−1. These values indicate that the jet-CO coupling ongoing in NGC 3100 is only mildly affecting the gas kinematics, as opposed to what expected from existing simulations and other observational studies of (sub-)kpc scale jet–cold gas interactions. HCO+(4-3) emission is tentatively detected in a small area adjacent to the base of the northern radio lobe, possibly tracing a region of jet-induced gas compression. The SiO(3-2) and HNCO(6-5) shock tracers are undetected: this – along with the tentative HCO+(4-3) detection – may be consistent with a deficiency of very dense (i.e. ncrit > 106 cm−3) cold gas in the central regions of NGC 3100.
Abstract
As part of the mm-Wave Interferometric Survey of Dark Object Masses project we present an estimate of the mass of the supermassive black hole (SMBH) in the nearby fast-rotating early-type ...galaxy NGC4429, that is barred and has a boxy/peanut-shaped bulge. This estimate is based on Atacama Large Millimeter/submillimeter Array (ALMA) cycle-2 observations of the 12CO(3-2) emission line with a linear resolution of ≈13 pc (0.18 arcsec × 0.14 arcsec). NGC4429 has a relaxed, flocculent nuclear disc of molecular gas that is truncated at small radii, likely due to the combined effects of gas stability and tidal shear. The warm/dense 12CO(3-2) emitting gas is confined to the inner parts of this disc, likely again because the gas becomes more stable at larger radii, preventing star formation. The gas disc has a low velocity dispersion of 2.2$^{+0.68}_{-0.65}$ km s−1. Despite the inner truncation of the gas disc, we are able to model the kinematics of the gas and estimate a mass of (1.5 ± $0.1^{+0.15}_{-0.35}$) × 108 M⊙ for the SMBH in NGC4429 (where the quoted uncertainties reflect the random and systematic uncertainties, respectively), consistent with a previous upper limit set using ionized gas kinematics. We confirm that the V-band mass-to-light ratio changes by ≈30 per cent within the inner 400 pc of NGC4429, as suggested by other authors. This SMBH mass measurement based on molecular gas kinematics, the sixth presented in the literature, once again demonstrates the power of ALMA to constrain SMBH masses.
In this work, we present CO(1–0) and CO(2–1) observations of a pilot sample of 15 early-type galaxies (ETGs) drawn from the MASSIVE galaxy survey, a volume-limited integral-field spectroscopic study ...of the most massive ETGs (M
* ≳ 1011.5 M⊙) within 108 Mpc. These objects were selected because they showed signs of an interstellar medium and/or star formation. A large amount of gas (>2 × 108 M⊙) is present in 10 out of 15 objects, and these galaxies have gas fractions higher than expected based on extrapolation from lower mass samples. We tentatively interpret this as evidence that stellar mass-loss and hot halo cooling may be starting to play a role in fuelling the most massive galaxies. These MASSIVE ETGs seem to have lower star formation efficiencies (SFE = SFR/M
H2) than spiral galaxies, but the SFEs derived are consistent with being drawn from the same distribution found in other lower mass ETG samples. This suggests that the SFE is not simply a function of stellar mass, but that local, internal processes are more important for regulating star formation. Finally, we used the CO line profiles to investigate the high-mass end of the Tully–Fisher relation (TFR). We find that there is a break in the slope of the TFR for ETGs at high masses (consistent with previous studies). The strength of this break correlates with the stellar velocity dispersion of the host galaxies, suggesting it is caused by additional baryonic mass being present in the centre of massive ETGs. We speculate on the root cause of this change and its implications for galaxy formation theories.
We study the origin of the wide distribution of angles between the angular momenta of the stellar and gas components, αG, S, in early-type galaxies (ETGs). We use the GALFORM model of galaxy ...formation, set in the Λ cold dark matter framework, and coupled it with a Monte Carlo simulation to follow the angular momenta flips driven by matter accretion on to haloes and galaxies. We consider a gas disc to be misaligned with respect to the stellar body if αG,S > 30 deg. By assuming that the only sources of misalignments in galaxies are galaxy mergers, we place a lower limit of 2–5 per cent on the fraction of ETGs with misaligned gas/stellar components. These low fractions are inconsistent with the observed value of ≈42 ± 6 per cent in ATLAS3D. In the more general case, in which smooth gas accretion in addition to galaxy mergers can drive misalignments, our calculation predicts that ≈46 per cent of ETGs have αG, S > 30 deg. In this calculation, we find correlations between αG, S and stellar mass, cold gas fraction and star formation rate, such that ETGs with high masses, low cold gas fractions and low star formation rates are more likely to display aligned cold gas and stellar components. We confirm these trends observationally for the first time using ATLAS3D data. We argue that the high fraction of misaligned gas discs observed in ETGs is mostly due to smooth gas accretion (e.g. cooling from the hot halo of galaxies) which takes place after most of the stellar mass of the galaxy is in place and comes misaligned with respect to the stellar component. Galaxies that have accreted most of their cold gas content prior to the time where most of the stellar mass was in place show aligned components.
Different massive black hole mass – host galaxy scaling relations suggest that the growth of massive black holes is entangled with the evolution of their host galaxies. The number of measured black ...hole masses is still limited and additional measurements are necessary to understand the underlying physics of this apparent coevolution. We add six new black hole mass (MBH) measurements of nearby fast rotating early-type galaxies to the known black hole mass sample, namely NGC 584, NGC 2784, NGC 3640, NGC 4570, NGC 4281, and NGC 7049. Our target galaxies have effective velocity dispersions (σe) between 170 and 245 km s−1, and thus this work provides additional insight into the black hole properties of intermediate-mass early-type galaxies. We combined high-resolution adaptive-optics SINFONI data with large-scale MUSE, VIMOS and SAURON data from ATLAS3D to derive two-dimensional stellar kinematics maps. We then built both Jeans Anisotropic Models and axisymmetric Schwarzschild models to measure the central black hole masses. Our Schwarzschild models provide black hole masses of (1.3 ± 0.5) × 108 M⊙ for NGC 584, (1.0 ± 0.6) × 108 M⊙ for NGC 2784, (7.7 ± 5) × 107 M⊙ for NGC 3640, (5.4 ± 0.8) × 108 M⊙ for NGC 4281, (6.8 ± 2.0) × 107 M⊙ for NGC 4570, and (3.2 ± 0.8) × 108 M⊙ for NGC 7049 at 3σ confidence level, which are consistent with recent MBH−σe scaling relations. NGC 3640 has a velocity dispersion dip and NGC 7049 a constant velocity dispersion in the center, but we can clearly constrain their lower black hole mass limit. We conclude our analysis with a test on NGC 4570 taking into account a variable mass-to-light ratio (M/L) when constructing dynamical models. When considering M/L variations linked mostly to radial changes in the stellar metallicity, we find that the dynamically determined black hole mass from NGC 4570 decreases by 30%. Further investigations are needed in the future to account for the impact of radial M/L gradients on dynamical modeling.
The nearby dwarf galaxy NGC 404 harbors a low-luminosity active galactic nucleus powered by the lowest-mass (<150,000 M ) central massive black hole (MBH), with a dynamical mass constraint, currently ...known, thus providing a rare low-redshift analog to the MBH "seeds" that formed in the early universe. Here, we present new imaging of the nucleus of NGC 404 at 12-18 GHz with the Karl G. Jansky Very Large Array (VLA) and observations of the CO(2-1) line with the Atacama Large Millimeter/Submillimeter Array (ALMA). For the first time, we have successfully resolved the nuclear radio emission, revealing a centrally peaked, extended source spanning 17 pc. Combined with previous VLA observations, our new data place a tight constraint on the radio spectral index and indicate an optically thin synchrotron origin for the emission. The peak of the resolved radio source coincides with the dynamical center of NGC 404, the center of a rotating disk of molecular gas, and the position of a compact, hard X-ray source. We also present evidence for shocks in the NGC 404 nucleus from archival narrowband HST imaging, Chandra X-ray data, and Spitzer mid-infrared spectroscopy, and discuss possible origins for the shock excitation. Given the morphology, location, and steep spectral index of the resolved radio source, as well as constraints on nuclear star formation from the ALMA CO(2-1) data, we find the most likely scenario for the origin of the radio source in the center of NGC 404 to be a radio outflow associated with a confined jet driven by the active nucleus.
Massive early-type galaxies (ETGs) commonly have gas discs which are kinematically misaligned with the stellar component. These discs feel a torque from the stars and the angular momentum vectors are ...expected to align quickly. We present results on the evolution of a misaligned gas disc in a cosmological simulation of a massive ETG from the feedback in realistic environments project. This galaxy experiences a merger which, together with a strong galactic wind, removes most of the original gas disc. The galaxy subsequently reforms a gas disc through accretion of cold gas, but it is initially 120° misaligned with the stellar rotation axis. This misalignment persists for about 2 Gyr before the gas–star misalignment angle drops below 20°. The time it takes for the gaseous and stellar components to align is much longer than previously thought, because the gas disc is accreting a significant amount of mass for about 1.5 Gyr after the merger, during which the angular momentum change induced by accreted gas dominates over that induced by stellar torques. Once the gas accretion rate has decreased sufficiently, the gas disc decouples from the surrounding halo gas and realigns with the stellar component in about six dynamical times. During the late evolution of the misaligned gas disc, the centre aligns faster than the outskirts, resulting in a warped disc. We discuss the observational consequences of the long survival of our misaligned gas disc and how our results can be used to calibrate merger rate estimates from observed gas misalignments.
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