Pei-Ying Hsieh

Submillimeter ALMA Observations of the Dense Gas in the Low-Luminosity Type-1 Active Nucleus of NGC1097

We present the first 100 pc scale view of the dense molecular gas in the central ~ 1.3 kpc region of the type-1 Seyfert NGC 1097 traced by HCN (J=4-3) and HCO+ (J=4-3) lines afforded with ALMA band 7. This galaxy shows significant HCN enhancement with respect to HCO+ and CO in the low-J transitions, which seems to be a common characteristic in AGN environments. Using the ALMA data, we study the characteristics of the dense gas around this AGN and search for the mechanism of HCN enhancement. We find a high HCN (J=4-3) to HCO+ (J=4-3) line ratio in the nucleus. The upper limit of the brightness temperature ratio of HCN (v2=1^{1f}, J=4-3) to HCN (J=4-3) is 0.08, which indicates that IR pumping does not significantly affect the pure rotational population in this nucleus. We also find a higher HCN (J=4-3) to CS (J=7-6) line ratio in NGC 1097 than in starburst galaxies, which is more than 12.7 on the brightness temperature scale. Combined from similar observations from other galaxies, we tentatively suggest that this ratio appears to be higher in AGN-host galaxies than in pure starburst ones similar to the widely used HCN to HCO+ ratio. LTE and non-LTE modeling of the observed HCN and HCO+ lines using J=4-3 and 1-0 data from ALMA, and J=3-2 data from SMA, reveals a high HCN to HCO+ abundance ratio (5 < [HCN]/[HCO+] < 20: non-LTE analysis) in the nucleus, and that the high-J lines (J=4-3 and 3-2) are emitted from dense (10^{4.5} < n_H2 [/cc] < 10^6), hot (70 < Tkin [K] < 550) regions. Finally we propose that the high temperature chemistry is more plausible to explain the observed enhanced HCN emission in NGC 1097 than the pure gas phase PDR/XDR chemistry.

Multimolecule ALMA observations toward the Seyfert 1 galaxy NGC 1097

Context. The nearby Sy 1 galaxy NGC 1097 represents an ideal laboratory for exploring the molecular chemistry in the surroundings of an active galactic nucleus (AGN). Aims. Exploring the distribution of di erent molecular species allows us to understand the physical processes a ecting the interstellar medium both in the AGN vicinity and in the outer star forming molecular ring. Methods. We carried out 3 mm ALMA observations that include seven di erent molecular species, namely HCN, HCO + , CCH, CS, HNCO, SiO, HC3N, and SO, as well as the 13 C isotopologues of the first two. Spectra were extracted from selected positions and all species were imaged over the central 2 kpc ( 30 00 ) of the galaxy at a resolution of 2.2 00 1:5 00 (150 pc 100 pc). Results. HCO + and CS appear to be slightly enhanced in the star forming ring. CCH shows the largest variations across NGC 1097 and is suggested to be a good tracer of both obscured and early stage star formation. HNCO, SiO, and HC3N are significantly enhanced in the inner circumnuclear disk surrounding the AGN. Conclusions. Di erences in the molecular abundances are observed between the star forming ring and the inner circumnuclear disk. We conclude that the HCN/HCO + and HCN/CS di erences observed between AGN-dominated and starburst (SB) galaxies are not due to a HCN enhancement due to X-rays, but rather this enhancement is produced by shocked material at distances of 200 pc from the AGN. Additionally, we claim that lower HCN/CS is a combination of a small underabundance of CS in AGNs, together with excitation e ects, where a high density gas component ( 10 6 cm 3 ) may be more prominent in SB galaxies. However, the most promising are the di erences found among the dense gas tracers that, at our modest spatial resolution, seem to outline the physical structure of the molecular disk around the AGN. In this picture, HNCO probes the well-shielded gas in the disk, surrounding the dense material moderately exposed to the X-ray radiation traced by HC3N. Finally SiO might be the innermost molecule in the disk structure.

MILKY WAY SUPERMASSIVE BLACK HOLE: DYNAMICAL FEEDING FROM THE CIRCUMNUCLEAR ENVIRONMENT

The supermassive black hole (SMBH), Sgr A*, at the Galactic center is surrounded by a molecular circumnuclear disk (CND) lying between 1.5 and 4 pc radii. The irregular and clumpy structures of the CND suggest dynamical evolution and episodic feeding of gas toward the central SMBH. New sensitive data from the Submillimeter Array and Green Bank Telescope reveal several >5-10 pc scale molecular arms, which either directly connect to the CND or may penetrate inside the CND. The CND appears to be the convergence of the innermost parts of large-scale gas streamers, which are responding to the central gravitational potential well. Rather than being a quasi-stationary structure, the CND may be dynamically evolving, incorporating inflow via streamers, and feeding gas toward the center.

SUBMILLIMETER-HCN DIAGRAM FOR ENERGY DIAGNOSTICS IN THE CENTERS OF GALAXIES

Compiling data from literature and the Atacama Large Millimeter/submillimeter Array archive, we show enhanced HCN(4-3)/HCO+(4-3) and/or HCN(4-3)/CS(7-6) integrated intensity ratios in circumnuclear molecular gas around active galactic nuclei (AGNs) compared to those in starburst (SB) galaxies (submillimeter HCN. enhancement). The number of sample galaxies is significantly increased from our previous work. We expect that this feature could potentially be an extinction-free energy diagnostic tool of nuclear regions of galaxies. Non-LTE radiative transfer modelings of the above molecular emission lines involving both collisional and radiative excitation, as well as a photon trapping effect, were conducted to investigate the cause of the high line ratios in AGNs. As a result, we found that enhanced abundance ratios of HCN to HCO+ and HCN to CS in AGNs as compared to SB galaxies by a factor of a few to even greater than or similar to 10 are a plausible explanation for the submillimeter HCN. enhancement. However, a counterargument of a systematically higher gas density in AGNs than in SB galaxies can also be a plausible scenario. Although we cannot fully distinguish. these two scenarios at this moment owing to an insufficient amount of multi-transition, multi-species data, the former scenario is indicative of abnormal chemical composition in AGNs. Regarding the actual mechanism to realize the composition, we suggest that it is difficult with conventional gas-phase X-ray-dominated region ionization models to reproduce the observed high line ratios. We might have to take into account other mechanisms such as neutral-neutral reactions that are efficiently activated in high-temperature environments and/or mechanically heated regions to further understand the high line ratios in AGNs.

PROBING CIRCUMNUCLEAR ENVIRONMENTS WITH THE HCN(J = 3-2) AND HCO+(J = 3-2) LINES: CASE OF NGC 1097

We present the first interferometric HCN(J = 3-2) and HCO+(J = 3-2) maps in the circumnuclear region of NGC 1097, obtained with the Submillimeter Array. The goal is to study the characteristics of the dense gas associated with the starburst ring/Seyfert nucleus. With these transitions, we suppress the diffuse low density emission in the nuclear region. We detect and resolve the individual compact giant molecular cloud associations (GMAs) in the 1.4 kpc circumnuclear starburst ring and within the 350 pc nuclear region. The nucleus is brighter than the ring in both lines, and contributes ~20% and ~30% to the total detected HCO+(J = 3-2) and HCN(J = 3-2) flux, within the central 1.4 kpc. The intensity ratios of HCN(J = 3-2)/HCO+(J = 3-2) are roughly unity in the GMAs of the starburst ring. However, this ratio is up to ~2 in the nuclear region. From the HCN(J = 3-2)/HCN(J = 1-0) ratio of ≤0.2 in the nucleus, we infer that the nuclear HCN(J = 3-2) emission might be optically thin. The HCO+(J = 3-2) and HCN(J = 3-2) show correlations with 12CO(J = 3-2) and the 24 μm emission. The tight correlations of HCN(J = 3-2), HCO+(J = 3-2), and 24 μm emission in the starburst ring suggest that the dense molecular gas and the dust are from the same origins of star-forming regions. On the other hand, the HCN(J = 3-2) emission of the nucleus is significantly enhanced, indicating mechanisms other than star formation, such as active galactic nucleus activities. A self-consistent check of the fractional abundance enhanced by X-ray ionization chemistry of the nucleus is possible with our observations.

Hydrodynamical Simulations of the Barred Spiral Galaxy NGC 1097

NGC 1097 is a nearby barred spiral galaxy believed to be interacting with the elliptical galaxy NGC 1097A located to its northwest. It hosts a Seyfert 1 nucleus surrounded by a circumnuclear starburst ring. Two straight dust lanes connected to the ring extend almost continuously out to the bar. The other ends of the dust lanes attach to two main spiral arms. To provide a physical understanding of its structural and kinematical properties, two-dimensional hydrodynamical simulations have been carried out. Numerical calculations reveal that many features of the gas morphology and kinematics can be reproduced provided that the gas flow is governed by a gravitational potential associated with a slowly rotating strong bar. By including the self-gravity of the gas disk in our calculation, we have found the starburst ring to be gravitationally unstable, which is consistent with the observation in Hsieh et al. Our simulations show that the gas inflow rate is 0.17 M-circle dot yr(-1) into the region within the starburst ring even after its formation, leading to the coexistence of both a nuclear ring and a circumnuclear disk. (Less)

SDSS-IV MaNGA-resolved Star Formation and Molecular Gas Properties of Green Valley Galaxies: A First Look with ALMA and MaNGA

We study the role of cold gas in quenching star formation in the green valley by analyzing ALMA 12CO (1–0) observations of three galaxies with resolved optical spectroscopy from the MaNGA survey. We present resolution-matched maps of the star formation rate and molecular gas mass. These data are used to calculate the star formation efficiency (SFE) and gas fraction ( fgas) for these galaxies separately in the central “bulge” regions and outer disks. We find that, for the two galaxies whose global specific star formation rate (sSFR) deviates most from the star formation main sequence, the gas fraction in the bulges is significantly lower than that in their disks, supporting an “inside-out” model of galaxy quenching. For the two galaxies where SFE can be reliably determined in the central regions, the bulges and disks share similar SFEs. This suggests that a decline in fgas is the main driver of lowered sSFR in bulges compared to disks in green valley galaxies. Within the disks, there exist common correlations between the sSFR and SFE and between sSFR and fgas on kiloparsec scales—the local SFE or fgas in the disks declines with local sSFR. Our results support a picture in which the sSFR in bulges is primarily controlled by fgas, whereas both SFE and fgas play a role in lowering the sSFR in disks. A larger sample is required to confirm if the trend established in this work is representative of the green valley as a whole.

Submillimeter Array12CO ([FORMULA][F]J=3-2[/F][/FORMULA]) Interferometric Observations of the Central Region of M51

We present the first interferometric 12CO (J = 3-2) observations (beam size of 39 × 16 or 160 × 65 pc) with the Submillimeter Array (SMA) toward the center of the Seyfert 2 galaxy M51. The image shows a strong concentration at the nucleus and weak emission from the spiral arm to the northwest. The integrated intensity of the central component in 12CO (J = 3-2) is almost twice as high as that in 12CO (J = 1-0), indicating that the molecular gas within an ~80 pc radius of the nucleus is warm (100 K) and dense (~104 cm-3). Similar intensity ratios are seen in shocked regions in our Galaxy, suggesting that these gas properties may be related to active galactic nucleus or starburst activity. The central component shows a linear velocity gradient (~1.4 km s-1 pc-1) perpendicular to the radio continuum jet, similar to that seen in previous observations and interpreted as a circumnuclear molecular disk/torus around the Seyfert 2 nucleus. In addition, we identify a linear velocity gradient (0.7 km s-1 pc-1) along the jet. Judging from the energetics, the velocity gradient can be explained by supernova explosions or energy and momentum transfer from the jet to the molecular gas via interaction, which is consistent with the high-intensity ratio.

Molecular Gas Feeding the Circumnuclear Disk of the Galactic Center

The interaction between a supermassive black hole (SMBH) and the surrounding material is of primary importance in modern astrophysics. The detection of the molecular 2-pc circumnuclear disk (CND) immediately around the Milky Way SMBH, SgrA*, provides an unique opportunity to study SMBH accretion at sub-parsec scales. Our new wide-field CS(2-1) map toward the Galactic center (GC) reveals multiple dense molecular streamers originated from the ambient clouds 20-pc further out, and connecting to the central 2 parsecs of the CND. These dense gas streamers appear to carry gas directly toward the nuclear region and might be captured by the central potential. Our phase-plot analysis indicates that these streamers show a signature of rotation and inward radial motion with progressively higher velocities as the gas approaches the CND and finally ends up co-rotating with the CND. Our results might suggest a possible mechanism of gas feeding the CND from 20 pc around 2 pc in the GC. In this paper, we discuss the morphology and the kinematics of these streamers. As the nearest observable Galactic nucleus, this feeding process may have implications for understanding the processes in extragalactic nuclei.

Molecular gas properties of galaxies: The SMA CO(2-1) B0DEGA legacy project

In the last two decades high-resolution (<5″) CO observations for ~ 150 galaxies have provided a wealth of information about the molecular gas morphologies in the circumnuclear regions. While in samples of “normal” galaxies the molecular gas does not seem to peak toward the nuclear regions for about 50% of the galaxies, barred galaxies and mergers show larger concentrations. However, we do not exactly know from an observational point of view how the molecular gas properties of a galaxy evolve as a result of an interaction. Here we present the SMA CO(2–1) B0DEGA (Below 0 DEgree GAlaxies) legacy project (http://b0dega.iaa.es) in which we are imaging the CO(2–1) line of the circumnuclear regions (1´) of a large (~ 70) sample of nearby IR-bright spiral galaxies, likely interacting, and that still remained unexplored due to its location in the southern hemisphere. We find different molecular gas morphologies, such as rings, nuclear arms, nuclear bars, and asymmetries. We find a centrally peaked concentration in about 85% of the galaxies with typical size scales of about 0.5–1 kpc. This might be related to perturbations produced by recent interactions.