Showing papers by "Marco Spaans published in 2009"

Molecular gas disk structures around active galactic nuclei

Abstract: We present new high-resolution numerical simulations of the interstellar medium (ISM) in a central R ≤ 32 parsecs region around a supermassive black hole (1.3 × 107 M ☉) at a galactic center. Three-dimensional hydrodynamic modeling of the ISM (Wada & Norman 2002) with the nuclear starburst now includes tracking of the formation of molecular hydrogen (H2) out of the neutral hydrogen phase as a function of the evolving ambient ISM conditions with a finer spatial resolution (0.125 pc). In a quasi-equilibrium state, mass fraction of H2 is about 0.4 (total H2 mass is 1.5 × 106 M ☉) of the total gas mass for the uniform far ultra-violet (FUV) with G 0 = 10 in Habing unit. As shown in the previous model, the gas forms an inhomogeneous disk, whose scale height becomes larger in the outer region. H2 forms a thin nuclear disk in the inner 5 pc, which is surrounded by molecular clouds swelled up toward h 10 pc. The velocity field of the disk is highly turbulent in the torus region, whose velocity dispersion is 20 km s–1 on average. Average supernova (SN) rate of 5 × 10–5 yr–1 is large enough to energize these structures. Gas column densities toward the nucleus larger than 1022 cm–2 are observed if the viewing angle is smaller than θ v 50° from the edge-on. However, the column densities are distributed over almost two orders of magnitude around the average for any given viewing angle due to the clumpy nature of the torus. For a stronger FUV (G 0 = 100), the total H2 mass in an equilibrium is only slightly smaller (0.35), a testimony to the strong self-shielding nature of H2, and the molecular gas is somewhat more concentrated in a midplane. Other properties of the ISM are not very sensitive either to the FUV intensity or the SN rate. Finally, the morphology and kinematics of the circum nuclear molecular gas disks emerging from our models are similar to that revealed by recent near infrared observations using VLTI/Keck.

Molecular Gas Disk Structures around AGNs

Abstract: We present new high resolution numerical simulations of the ISM in a central R ~32 parsecs region around a supermassive black hole at a galactic center. Three-dimensional hydrodynamic modeling of the ISM (Wada & Norman 2002) with the nuclear starburst now includes tracking of the formation of molecular hydrogen out of the neutral hydrogen phase. In a quasi equilibrium state, mass fraction of H2 is about 0.4 (total H2 mass is ~1.5 10^6 Msun) of the total gas mass for the uniform far UV (FUV) with G_0 = 10. The gas forms an inhomogeneous disk, whose scale-height becomes larger in the outer region. H2 forms a thin nuclear disk in the inner ~ 5 pc, which is surrounded by molecular clouds swelled up toward h < 10 pc. The velocity field of the disk is highly turbulent in the torus region, whose velocity dispersion is ~ 20 km/s on average. Average supernova rate (SNR) of ~ 5 10^-5/yr is large enough to energize these structures. Gas column densities toward the nucleus larger than 10^22 cm^-2 are observed if the viewing angle is smaller than \theta_v ~ 50 deg from the edge-on. However, the column densities are distributed over almost two orders of magnitude around the average for any given viewing angle due to the clumpy nature of the torus. For a stronger FUV (G_0 =100), the total H2 mass in an equillibrium is only slightly smaller (~ 0.35). Finally the morphology and kinematics of the circumnuclear molecular gas disks emerging from our models is similar to that revealed by recent near infrared observations using VLTI/Keck.

P Cygni Profiles of Molecular Lines Toward Arp 220 Nuclei

Abstract: We report ~100 pc (0farcs3) resolution observations of (sub)millimeter HCO+ and CO lines in the ultraluminous infrared galaxy Arp 220. The lines peak at two merger nuclei, with HCO+ being more spatially concentrated than CO. Asymmetric line profiles with blueshifted absorption and redshifted emission are discovered in HCO+(3-2) and (4-3) toward the two nuclei and in CO(3-2) toward one nucleus. We suggest that these P Cygni profiles are due to ~100 km s–1 outward motion of molecular gas from the nuclei. This gas is most likely outflowing from the inner regions of the two nuclear disks rotating around individual nuclei, clearing the shroud around the luminosity sources there.

HD and H-2 formation in low-metallicity dusty gas clouds at high redshift

Abstract: Context. HD and H-2 molecules play important roles in the cooling of primordial and very metal-poor gas at high redshift. Aims. Grain surface and gas phase formation of HD and H-2 are investigated to assess the importance of trace amounts of dust, 10(-5)-10(-3) Z(circle dot), in the production of HD and H-2. Methods. We consider carbonaceous and silicate grains and include both physisorption and chemisorption, tunneling, and realistic grain surface barriers. We find that, for a collapsing gas cloud environment with coupled chemical and thermal balance, dust abundances as small as 10(-5) solar lead to a strong boost in the H-2 formation rate due to surface reactions. As a result of this enhancement in H-2, HD is formed more efficiently in the gas phase through the D+ + H-2 reaction. Direct formation of HD on dust grains cannot compete well with this gas phase process for dust temperatures below 150 K. We also derive up-to-date analytic fitting formulae for the grain surface formation of H-2 an HD, including the different binding energies of H and D. Results. Grain surface reactions are crucial to the availability of H-2 and HD in very metal-poor environments. Above metallicities of 10(-5) solar, the grain surface route dominates the formation of H-2, which in turn drives the formation of HD in the gas phase. At dust temperatures above 150 K, laboratory experiments and theoretical modeling suggest that H-2 formation on grains is suppressed while HD formation on grains is not.

HD and H2 formation in low-metallicity dusty gas clouds at high redshift

Abstract: Context: The HD and H2 molecules play important roles in the cooling of primordial and very metal-poor gas at high redshift. Aims: Grain surface and gas phase formation of HD and H2 is investigated to assess the importance of trace amounts of dust, 10^{-5}-10^{-3} Zo, in the production of HD and H2. Methods: We consider carbonaceous and silicate grains and include both physisorption and chemisorption, tunneling, and realistic grain surface barriers. We find, for a collapsing gas cloud environment with coupled chemical and thermal balance, that dust abundances as small as 10^{-5} solar lead to a strong boost in the H2 formation rate due to surface reactions. As a result of this enhancement in H2, HD is formed more efficiently in the gas phase through the D+ + H2 reaction. Direct formation of HD on dust grains cannot compete well with this gas phase process for dust temperatures below 150 K. We also derive up-to-date analytic fitting formulae for the grain surface formation of H2 and HD, including the different binding energies of H and D. Results: Grain surface reactions are crucial to the availability of H2 and HD in very metal-poor environments. Above metallicities of 10^{-5} solar, the grain surface route dominates the formation of H2, which in turn, drives the formation of HD in the gas phase. At dust temperatures above 150 K, laboratory experiments and theoretical modelling suggest that H2 formation on grains is suppressed while HD formation on grains is not.

Millimetron-a large Russian-European submillimeter space observatory

Abstract: Millimetron is a Russian-led 12 m diameter submillimeter and far-infrared space observatory which is included in the Space Plan of the Russian Federation for launch around 2017. With its large collecting area and state-of-the-art receivers, it will enable unique science and allow at least one order of magnitude improvement with respect to the Herschel Space Observatory. Millimetron will be operated in two basic observing modes: as a single-dish observatory, and as an element of a ground-space very long baseline interferometry (VLBI) system. As single-dish, angular resolutions on the order of 3 to 12 arc sec will be achieved and spectral resolutions of up to a million employing heterodyne techniques. As VLBI antenna, the chosen elliptical orbit will provide extremely large VLBI baselines (beyond 300,000 km) resulting in micro-arc second angular resolution.

Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations

Abstract: Photon-dominated regions (PDRs) are expected to show a layered structure in molecular abundances and emerging line emission, which is sensitive to the physical structure of the region as well as the UV radiation illuminating it. We aim to study this layering in the Orion Bar, a prototypical nearby PDR with a favorable edge-on geometry. We present new maps of 2 by 2 arcminute fields at 14-23 arcsecond resolution toward the Orion Bar in the SO 8_8-9_9, H2CO 5_(1,5)-4_(1,4), 13CO 3-2, C2H 4_(9/2)-3_(7/2) and 4_(7/2)-3_(5/2), C18O 2-1 and HCN 3-2 transitions. The data reveal a clear chemical stratification pattern. The C2H emission peaks close to the ionization front, followed by H2CO and SO, while C18O, HCN and 13CO peak deeper into the cloud. A simple PDR model reproduces the observed stratification, although the SO emission is predicted to peak much deeper into the cloud than observed while H2CO is predicted to peak closer to the ionization front than observed. In addition, the predicted SO abundance is higher than observed while the H2CO abundance is lower than observed. The discrepancies between the models and observations indicate that more sophisticated models, including production of H2CO through grain surface chemistry, are needed to quantitatively match the observations of this region.

High-resolution HNC 3-2 SMA observations of Arp 220

Abstract: Aims. We study the properties of the nuclear molecular gas of the ultra luminous merger Arp 220 and effects of the nuclear source on gas excitation and chemistry. Specifically, our aim is to investigate the spatial location of the luminous HNC 3-2 line emission and address the underlying cause of its unusual brightness. Methods. We present high resolution observations of HNC J=3-2 with the submillimeter array (SMA). Results. We find luminous HNC 3-2 line emission in the western part of Arp 220, centred on the western nucleus, while the eastern side of the merger shows relatively faint emission. A bright (36 K at $0\hbox{$.\!\!^{\prime\prime}$ }4$ resolution), narrow (60 ${\rm km~s}^$) emission feature emerges from the western nucleus, superposed on a broader spectral component. A possible explanation is weak maser emission through line-of-sight amplification of the background continuum source. There is also a more extended HNC 3-2 emission feature north and south of the nucleus. This feature resembles the bipolar OH maser morphology around the western nucleus. Substantial HNC abundances are required to explain the bright line emission from this warm environment - even when the high gas column density towards the western nucleus is taken into account. We discuss this briefly in the context of an X-ray affected chemistry and radiative excitation. Conclusions. The luminous and possibly amplified HNC emission of the western nucleus of the Arp 220 merger reflects the unusual, and perhaps transient environment of the starburst/AGN activity there. The faint HNC line emission towards Arp 220-east reveals a real difference in physical conditions between the two merger nuclei.

Probing X-ray irradiation in the nucleus of NGC 1068 with observations of high-J lines of dense gas tracers

Abstract: Context. Single-dish observations of molecular tracers have suggested that both star formation and an AGN can drive the gas chemistry of the central similar to kpc of active galaxies. The irradiation by UV photons from an starburst or by X-rays from an AGN is expected to produce different signatures in molecular chemistry, which existing data on low-J lines cannot distinguish, as they do not trace gas at high temperature and density. Depending on the angular scale of a galaxy, the observed low-J lines can be dominated by the emission coming from the starburst ring rather than from the central region. Aims. With the incorporation of high-J molecular lines, we aim to constrain the physical conditions of the dense gas in the central region of the Seyfert 2 galaxy NGC 1068 and to determine signatures of the AGN or the starburst contribution. Methods. We used the James Clerk Maxwell Telescope to observe the J = 4-3 transition of HCN, HNC, and HCO(+), as well as the CN N(J) = 2(5/2) - 1(3/2) and NJ = 3(5/2) - 2(5/2), in NGC 1068. We estimate the excitation conditions of HCN, HNC, and CN, based on the line intensity ratios and radiative transfer models. We discuss the results in the context of models of irradiation of the molecular gas by UV light and X-rays. Results. A first-order estimate leads to starburst contribution factors of 0.58 and 0.56 for the CN and HCN J = 1-0 lines, respectively. We find that the bulk emission of HCN, HNC, CN, and the high-J HCO+ emerge from dense gas (n(H(2)) = 10(5) cm(-3)). However, the low-J HCO+lines (dominating the HCO+ column density) trace less dense (n(H(2)) 30 K) gas than the other molecules. We also find that the HNC/HCN and CN/HCN line intensity ratios decrease with increasing rotational quantum number J. Conclusions. The HCO(+) J = 4-3 line intensity, compared with the lower transition lines and with the HCN J = 4-3 line, support the influence of a local XDR environment. The estimated N(CN)/N(HCN) similar to 1-4 column density ratios are indicative of an XDR/AGN environment with a possible contribution of grain-surface chemistry induced by X-rays or shocks.

Probing X-ray irradiation in the nucleus of NGC 1068 with observations of high-J lines of dense gas tracers

Abstract: With the incorporation of high-J molecular lines, we aim to constrain the physical conditions of the dense gas in the central region of the Seyfert 2 galaxy NGC 1068 and to determine signatures of the AGN or the starburst contribution. We used the James Clerk Maxwell Telescope to observe the J=4-3 transition of HCN, HNC, and HCO+, as well as the CN N_J=2_{5/2}-1_{3/2} and N_J=3_{5/2}-2_{5/2}, in NGC 1068. We estimate the excitation conditions of HCN, HNC, and CN, based on the line intensity ratios and radiative transfer models. We find that the bulk emission of HCN, HNC, CN, and the high-J HCO+ emerge from dense gas n(H_2)>=10^5 cm^-3). However, the low-J HCO+ lines (dominating the HCO+ column density) trace less dense (n(H_2) 30 K) gas than the other molecules. The HCO+ J=4-3 line intensity, compared with the lower transition lines and with the HCN J=4-3 line, support the influence of a local XDR environment. The estimated N(CN)/N(HCN)~1-4 column density ratios are indicative of an XDR/AGN environment with a possible contribution of grain-surface chemistry induced by X-rays or shocks.

CHAMP+ observations of warm gas in M17 SW

Abstract: Since the main cooling lines of the gas phase are important tracers of the interstellar medium in Galactic and extragalactic sources, proper and detailed understanding of their emission, and the ambient conditions of the emitting gas, is necessary for a robust interpretation of the observations. With high resolution (7"-9") maps (~3x3 pc^2) of mid-J molecular lines we aim to probe the physical conditions and spatial distribution of the warm (50 to few hundred K) and dense gas (n(H_2)>10^5 cm^-3) across the interface region of M17 SW nebula. We have used the dual color multiple pixel receiver CHAMP+ on APEX telescope to obtain a 5'.3x4'.7 map of the J=6-5 and J=7-6 transitions of 12CO, the 13CO J=6-5 line, and the {^3P_2}-{^3P_1} 370 um fine-structure transition of [C I] in M17 SW. LTE and non-LTE radiative transfer models are used to constrain the ambient conditions. The warm gas extends up to a distance of ~2.2 pc from the M17 SW ridge. The 13CO J=6-5 and [C I] 370 um lines have a narrower spatial extent of about 1.3 pc along a strip line at P.A=63 deg. The structure and distribution of the [C I] {^3P_2}-{^3P_1} 370 um map indicate that its emission arises from the interclump medium with densities of the order of 10^3 cm^-3. The warmest gas is located along the ridge of the cloud, close to the ionization front. An LTE approximation indicates that the excitation temperature of the embedded clumps goes up to ~120 K. The non-LTE model suggests that the kinetic temperature at four selected positions cannot exceed 230 K in clumps of density n(H_2)~5x10^5 cm^-3, and that the warm T_k>100 K and dense (n(H_2)>10^4 cm^-3) gas traced by the mid-J 12CO lines represent just about 2% of the bulk of the molecular gas. The clump volume filling factor ranges between 0.04 and 0.11 at these positions.

Water and ammonia abundances in S140 with the Odin satellite

Abstract: Aims. We investigate the effect of the physical environment on water and ammonia abundances across the S140 photodissociation region (PDR) with an embedded outflow. Methods. We used the Odin satellite to obtain strip maps of the ground-state rotational transitions of ortho-water and ortho-ammonia, as well as CO(5-4) and (13)CO(5-4) across the PDR, and H(2)(18)O in the central position. A physi-chemical inhomogeneous PDR model was used to compute the temperature and abundance distributions for water, ammonia, and CO. A multi-zone escape probability method then calculated the level populations and intensity distributions. These results are compared to a homogeneous model computed with an enhanced version of the RADEX code. Results. H(2)O, NH(3), and (13)CO show emission from an extended PDR with a narrow line width of similar to 3 kms(-1). Like CO, the water line profile is dominated by outflow emission, but mainly in the red wing. Even though CO shows strong self-absorption, no signs of self-absorption are seen in the water line. The H(2)(18)O molecule is not detected. The PDR model suggests that the water emission arises mainly from the surfaces of optically thick, high-density clumps with n(H(2)) greater than or similar to 10(6) cm(-3) and a clump water abundance, with respect to H(2), of 5 x 10(-8). The mean water abundance in the PDR is 5 x 10(-9) and between similar to 4 x 10(-8)-4 x 10(-7) in the outflow derived from a simple two-level approximation. The RADEX model points to a somewhat higher average PDR water abundance of 1 x 10(-8). At low temperatures deep in the cloud, the water emission is weaker, likely due to adsorption onto dust grains, while ammonia is still abundant. Ammonia is also observed in the extended clumpy PDR, likely from the same high density and warm clumps as water. The average ammonia abundance is about the same as for water: 4 x 10(-9) and 8 x 10(-9) given by the PDR model and RADEX, respectively. The differences between the models most likely arise from uncertainties in density, beam-filling, and volume-filling of clumps. The similarity of water and ammonia PDR emission is also seen in the almost identical line profiles observed close to the bright rim. Around the central position, ammonia also shows some outflow emission, although weaker than water in the red wing. Predictions of the H(2)O 1(1,0)- 1(0,1) and 1(1,1) - 0(0,0) antenna temperatures across the PDR are estimated with our PDR model for the forthcoming observations with the Herschel Space Observatory.

A Sideband-Separating Mixer Upgrade for ALMA Band 9

Abstract: The ALMA band 9 (600-720 GHz) receiver cartridge, as currently being produced, features two single-ended (dual sideband) SIS mixers in orthogonal polarisations In the case of spectral line observations in the presence of atmospheric backgound, the integration time to reach a certain desired signal to noise level can be reduced by about a factor of two by rejecting the unused sideband A proof-of-concept sideband separating mixer has been successfully designed and produced over the past few years, the results of which have been presented earlier at this conference At the time that the band 9 cartridge got its final form, however, this mixer was not yet ready for series production Here, we present a design study that investigates the feasibility of upgrading the current ALMA band 9 cartridge to full dual-polarisation sideband separating capability, with minimal impact on the overall structure of the cartridge The goal is to re-use as many of the parts and techniques of the existing cartridge as possible to provide a minimal-cost upgrade path, to be available in a couple of years

Diagnosing the ISM in star-forming regions

Abstract: A report on studies using the observed line ratios of high-density molecular tracers to diagnose the physics and chemistry of the ISM in star-formation environments.

Detecting the First Quasars with ALMA

Abstract: We show that ALMA is the first telescope that can probe the dust-obscured central region of quasars at z > 5 with a maximum resolution of ~ 30 pc employing the 18 km baseline.We explore the possibility of detecting the first quasars with ALMA (Schleicher, Spaans, & Klessen 2009). For this purpose, we adopt the Seyfert 2 galaxy NGC 1068 as a reference system and calculate the expected fluxes if this galaxy were placed at high redshift. This choice is motivated by the detailed observations available for this system and the absence of any indication for an evolution in metallicity in high-redshift quasars. It is a conservative choice due to the moderate column densities in NGC 1068, leading to moderate fluxes.