The compact and, with $${\sim }4.3\pm 0.3\times 10^6$$
 M
 $$_{\odot }$$
 , very massive object located at the center of the Milky Way is currently the very best candidate for a supermassive black hole (SMBH) in our immediate vicinity. The strongest evidence for this is provided by measurements of stellar orbits, variable X-ray emission, and strongly variable polarized near-infrared emission from the location of the radio source Sagittarius A* (SgrA*) in the middle of the central stellar cluster. Simultaneous near-infrared and X-ray observations of SgrA* have revealed insights into the emission mechanisms responsible for the powerful near-infrared and X-ray flares from within a few tens to one hundred Schwarzschild radii of such a putative SMBH. If SgrA* is indeed a SMBH it will, in projection onto the sky, have the largest event horizon and will certainly be the first and most important target for very long baseline interferometry observations currently being prepared by the event horizon telescope (EHT). These observations in combination with the infrared interferometry experiment GRAVITY at the very large telescope interferometer and other experiments across the electromagnetic spectrum might yield proof for the presence of a black hole at the center of the Milky Way. The large body of evidence continues to discriminate the identification of SgrA* as a SMBH from alternative possibilities. It is, however, unclear when the ever mounting evidence for SgrA* being associated with a SMBH will suffice as a convincing proof. Additional compelling evidence may come from future gravitational wave observatories. This manuscript reviews the observational facts, theoretical grounds and conceptual aspects for the case of SgrA* being a black hole. We treat theory and observations in the framework of the philosophical discussions about “(anti)realism and underdetermination”, as this line of arguments allows us to describe the situation in observational astrophysics with respect to supermassive black holes. Questions concerning the existence of supermassive black holes and in particular SgrA* are discussed using causation as an indispensable element. We show that the results of our investigation are convincingly mapped out by this combination of concepts.

/pdf/the-milky-way-s-supermassive-black-hole-how-good-a-case-is-2pw4snti7o.pdf

The Milky Way’s Supermassive Black Hole: How Good a Case Is It?

The advent of high-angular resolution IR and sub-mm interferometry allows for spatially-resolved observations of the parsec-scale environment of active galactic nuclei (AGN), commonly referred to as the “torus.” While molecular lines show the presence of large, massive disks, the IR observations appear to be dominated by a strong polar component that has been interpreted as a dusty wind. This paper aims at using characteristics shared by AGN in each of the wavebands and a set of simple physical principles to form a unifying view of these seemingly contradictory observations: Dusty molecular gas flows in from galactic scales of ∼100 pc to the sub-parsec environment via a disk with small to moderate scale height. The hot, inner part of the disk puffs up due to IR radiation pressure and unbinds a large amount of the inflowing gas from the black hole’s gravitational potential, providing the conditions to launch a wind driven by the radiation pressure from the AGN. The dusty wind feeds back mass into the galaxy at a rate of the order of ∼ 0.1− 100M /yr, depending on AGN luminosity and Eddington ratio. Angle-dependent obscuration as required by AGN unification is provided by a combination of disk, wind, and wind launching region.

/pdf/redefining-the-torus-a-unifying-view-of-agns-in-the-infrared-5av4g0b6rz.pdf

Redefining the Torus: A Unifying View of AGNs in the Infrared and Submillimeter

We present CO(1-0) and CO(2-1) maps of the starburst/Seyfert1 galaxy NGC2782 obtained with the IRAM interferometer, at 2. 1×1. '' 5 and 0. 7×0. '' 6 resolution respectively. The CO emission is aligned along the stellar nuclear bar of radius �1kpc, configured in an elongated structure with two spiral arms at high pitch angle � 90 ◦ . At the extremity of the nuclear bar, the CO changes direction to trace two more extended spiral features at a lower pitch angle. These are the beginning of two straight dust lanes, which are aligned parallel to an oval distortion, reminiscent of a primary bar, almost perpendicular to the nuclear one. The two embedded bars appear in Spitzer IRAC near-infrared images, and HST color images, although highly obscured by dust in the latter. We compute the torques exerted by the stellar bars on the gas, and find systematically negative average torques down to the resolution limit of the images, providing evidence of gas inflow tantalizingly close to the nucleus of NGC2782. We propose a dynamical scenario based on numerical simulations to interpret coherently the radio, optical, and molecular gas features in the center of the galaxy. Star formation is occurring in a partial ring at �1.3kpc radius corresponding to the Inner Lindblad Resonance (ILR) of the primary bar; this ring-like structure encircles the nuclear bar, and is studded with Hemission. The gas traced by CO emission is driven inward by the gravity torques of the decoupled nuclear bar, since most of it is inside its corotation. N-body simulations, including gas dissipation, predict the secondary bar decoupling, the formation of the elongated ring at the �1kpc-radius ILR of the primary bar, and the gas inflow to the ILR of the nuclear bar at a radius of �200-300 pc. The presence of molecular gas inside the ILR of the primary bar, transported by a second nuclear bar, is a potential "smoking gun"; the gas there is certainly fueling the central starburst, and in a second step could fuel directly the AGN.

/pdf/molecular-gas-in-nuclei-of-galaxies-nuga-ix-the-decoupled-4s95y01eh0.pdf

Molecular Gas in NUclei of GAlaxies (NUGA) - IX. The decoupled bars and gas inflow in NGC 2782

In this paper we create a complete gravity torque map of the disk of the LINER/Seyfert 1.9 galaxy NGC 4579. We quantify the efficiency of angular momentum transport and search for signatures of secular evolution in the fueling process from r ∼ 15 kpc down to the inner r ∼ 50 pc around the active galactic nucleus (AGN). We use both the 1− 0a nd 2−1 line maps of 12 CO obtained with the Plateau de Bure Interferometer (PdBI) as part of the NUclei of Galaxies-(NUGA)-project. The CO(1−0) and CO(2−1) PdBI maps at 2.0 �� × 1.3 �� and 1.0 �� × 0.6 �� resolution, respectively, include the short spacing correction provided by IRAM-30 m data. We derive the stellar potential from an NIR (K band) wide field image of the galaxy. The K-band image, which reveals a large-scale stellar bar and a weak nuclear oval, together with a high-resolution Hi map of NGC 4579 obtained with the Very Large Array (VLA), allow us to extend the gravity torque analysis to the outer r ≥ 2 kpc disk. The bulk of the gas response traced by the new CO PdBI+30 m maps of NGC 4579 presented here follows the expected gas flow pattern induced by the bar potential in the presence of two inner Lindblad resonances (ILR) at r ∼ 500 pc and r ∼ 1.3 kpc. The morphology of the outer disk traced by Hi suggests that the neutral gas is currently piling up in a pseudo-ring formed by two winding spiral arms that are morphologically decoupled from the bar structure. The pseudo-ring is located inside the bar corotation (rCR ∼ 6 kpc) and close to the predicted position of the ultra harmonic resonance (UHR) (rUHR ∼ 3.8 kpc). The derived gravity torque budget in NGC 4579 shows that inward gas flow is occurring on different spatial scales in the disk. In the outer disk, the decoupling of the spiral allows the gas to efficiently populate the UHR region, and thus produce net gas inflow on intermediate scales. The corotation barrier seems to be overcome by secular evolution processes. The gas in the inner disk (r ≤ 2 kpc) is efficiently funneled by gravity torques down to r ∼ 300 pc. Closer to the AGN (r < 200 pc), gas feels negative torques due to the combined action of the large-scale bar and the inner oval. The two m = 2 modes act in concert to produce net gas inflow down to r ∼ 50 pc, providing clear smoking gun evidence of inward gas transport on short dynamical timescales (∼1−3 rotation periods).

/pdf/molecular-gas-in-nuclei-of-galaxies-nuga-xi-a-complete-59d7wr3yh6.pdf

Molecular gas in NUclei of GAlaxies (NUGA) - XI. A complete gravity torque map of NGC 4579: new clues to bar evolution

We present CO(1-0) and CO(2-1) maps of the starburst/Seyfert 1 galaxy NGC 2782, obtained with the IRAM interferometer. The CO emission is aligned along the stellar nuclear bar of radius 1 kpc, configured in an elongated structure with two spiral arms at high pitch angle. At the extremity of the nuclear bar, the CO changes direction to trace two more extended spiral features at a lower pitch angle. These are the beginning of two straight dust lanes, which are aligned parallel to an oval distortion, reminiscent of a primary bar, almost perpendicular to the nuclear one. The two embedded bars appear in Spitzer IRAC near-infrared images, and HST color images, although highly obscured by dust in the latter. We compute the torques exerted by the stellar bars on the gas, and find systematically negative average torques down to the resolution limit of the images, providing evidence of gas inflow tantalizingly close to the nucleus of NGC 2782. The observations are well reproduced by numerical simulations, including gas dissipation, which predict the secondary bar decoupling, the formation of an elongated ring at the 1 kpc-radius Inner Lindblad Resonance (ILR) of the primary bar, and the gas inflow to the ILR of the nuclear bar. The presence of molecular gas inside the ILR of the primary bar, transported by a second nuclear bar, is a potential ``smoking gun''; the gas there is certainly fueling the central starburst, and in a second step could fuel directly the AGN.

/pdf/molecular-gas-in-nuclei-of-galaxies-nuga-ix-the-decoupled-3m69xc905f.pdf

Molecular Gas in NUclei of GAlaxies (NUGA): IX. The decoupled bars and gas inflow in NGC 2782

We present the frst VLTI observations of targets in the Galactic Center region. The encouraging results are establishing the scientifb capabilities of infrared interferometry in the outstanding but - for interferometric means - distant and faint dust obscured Central Parsec. We successfully combined the light of two 8m-class telescopes in K and N band with ESO's AMBER, and MIDI beam combiners. The enigmatic dust-forming and spectrally featureless source IRS 3 could be clearly resolved into two components with different physical properties. We estimated lower limits of the 10 μm sizes of the most prominent Northern Arm bow-shocks IRS 1W, 10W, and 8. Investigations on the suitability of IRS 7 as a phase reference for long baseline interferometry at NIR, wavelengths are underway.

First infrared VLTI fringes on Galactic Center sources

Current and future opportunities for interferometric observations of the Galactic Center in the near- and mid-infrared (NIR/MIR) wavelength domain are highlighted. Main emphasis is being put on the Large Binocular Telescope (LBT) and the Very Large Telescope Interferometer (VLTI). The Galactic Center measurements of stellar orbits and strongly variable NIR and X-ray emission from Sagittarius A* (SgrA*) at the center of the Milky Way have provided the strongest evidence so far that the dark mass concentration at this position is associated with a super massive black hole. Similar dark mass concentrations seen in many galactic nuclei are most likely super massive black holes as well. High angular resolution interferometric observations in the NIR/MIR will provide key information on the central massive black hole and the stellar cluster it is embedded in. These observations have already started: Recent results on the luminous dust enshrowded star IRS3 using MIDI at the VLTI are presented and future scientific possibilities in the GC using MIDI at the VLTI in the MIR and GRAVITY in the NIR are highlighted. As a NIR wide field interferometric imager offering an angular resolution of about 10 milliarcseconds LINC/NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging galactic nuclei including the center of the Milky Way.

Interferometric observations of the galactic center: LBT and VLTI

We investigate the interplay between active nuclei and their surrounding ISM by interferometric radio continuum and mm line observations at high angular resolution and high sensitivity of a sample of active galaxies (e.g. NGC3718), carried out with MERLIN, EVN and the IRAM Plateau de Bure interferometer. They are part of an ongoing international study-NUGA (NUclei of GAlaxies)-about the nature, i.e. basically the fueling mechanisms, of nearby active galaxies. The mm line observations provide information about the distribution and kinematics of the molecular gas in these galaxies, which are regarded to be an important part of the matter feeding the central engine. We find a variety of molecular gas structures in our sample ranging from spirals, bars, warped disks, rings to asymmetric disks. The MERLIN/EVN measurements at 18 cm, as well as 6 cm, allow us to investigate the nature of the central engine itself. Also, the radio observations unveil extended structures on all observed angular scales (from VLA to MERLIN and EVN), like jets, diffuse components, as well as compact features. Moreover, the spectra are flat to steep, depending on the respective source components. The combination of the mm line and radio continuum results allow us to discuss possible interactions between the observed jets and the surrounding interstellar medium.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Active galaxies-parsec scale radio emission and the surrounding ISM

The research presented in this paper has
been financially supported in part by the SFB 494. Stephane Leon is partially supported by DGI Grant AYA 2002-03338
and Junta de Andalucia.

/pdf/molecular-gas-in-nuclei-of-galaxies-nuga-iii-the-warped-4tvefb8mhm.pdf

Molecular gas in NUclei of GAlaxies (NUGA): III. The warped LINER NGC 3718

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Papers

First infrared VLTI fringes on Galactic Center sources

Interferometric observations of the galactic center: LBT and VLTI

Active galaxies-parsec scale radio emission and the surrounding ISM

Molecular gas in NUclei of GAlaxies (NUGA): III. The warped LINER NGC 3718