Search

Integral-field spectroscopy in the near-infrared (NIR) is a powerful tool to analyze the gaseous and stellar distributions and kinematics, as well as the excitation mechanisms in the centers of galaxies. The unique combination of NIR and sub-mm data at comparable high angular resolution, which has just been possible with SINFONI and ALMA, allows to trace warm and cold gas reservoirs. Only the NIR gives an unobscured view to the center and allows to study the conditions and impact of star formation in the centers of galaxies in a spatially resolved way. Here, we present recent studies of nearby Seyferts and low-luminosity QSOs performed by our group. [ABSTRACT FROM AUTHOR]

Context. There are a number of faint compact infrared excess sources in the central stellar cluster of the Milky Way. Their nature and origin is unclear. In addition to several isolated objects of this kind there is a small but dense cluster of comoving sources (IRS13N) located ∼3" west of SgrA" just 0.5" north of the bright IRS13E cluster of Wolf-Rayet and O-type stars. Based on the analysis of their color and brightness, there are two main possibilities: (1) they may be dust-embedded stars older than a few Myr; or (2) very young, dusty stars with ages younger than 1 Myr. Aims. We present a first Ks-band identification and proper motions of the IRS13N members, the high-velocity dusty S-cluster object (DSO, also referred to as G2), and other infrared excess sources in the central field. Goal is to constrain the nature of these source. Methods. The L'- (3.8 μm) Ks- (2.2 μm) and H-band (1.65 μm) observations were carried out using the NACO adaptive optics system at the ESO VLT. Proper motions were obtained by linear fitting of the stellar positions extracted by StarFinder as a function of time, weighted by positional uncertainties, and by Gaussian fitting from high-pass filtered and deconvolved images. We also present results of near-infrared (NIR) H- and Ks-band ESO-SINFONI integral field spectroscopy of the Galactic center cluster ISR13N. Results. We show that within the uncertainties, the positions and proper motions of the IRS13N sources in Ks- and L'-band are identical. The HK-sL' colors then indicate that the bright L'-band IRS13N sources are indeed dust-enshrouded stars rather than core-less dust clouds. The proper motions also show that the IRS13N sources are not strongly gravitationally bound to each other. Combined with their NIR colors, this implies that they have been formed recently. For the DSO we obtain proper motions and a Ks-L'-color. Conclusions. Most of the compact L'-band excess emission sources have a compact H- or Ks-band counterpart and therefore are likely stars with dust shells or disks. Our new results and orbital analysis from our previous work favor the hypothesis that the infrared excess IRS13N members and other dusty sources close to SgrA" are young dusty stars and that star formation at the Galactic center (GC) is a continuously ongoing process. For the DSO the color information indicates that it may be a dust cloud or a dust-embedded star. [ABSTRACT FROM AUTHOR]

In contrast to the predictions of the unified model, some X-ray unobscured Seyfert 2 galaxies have been discovered in the last decade. One of them, the starburst/Seyfert composite galaxy IRAS 01072+4954 (z = 0.0236), has a typical Type-1 X-ray emission, while its optical spectrum resembles an H II galaxy and lacks the expected broad lines. We performed near-infrared integral-field observations of this object with the aim to determine the nature of its nuclear emission and to find indications for the existence or absence of a broad-line region. Several reasons have been proposed to explain this peculiar emission. We studied the validity of these hypotheses, including the possibility for this galaxy to be a True-Seyfert 2. We found little obscuration toward the nucleus Av = 2.5 mag, and a nuclear star-formation rate ΣSFR < 11.6 Mʘ yr-1 kpc-2, which is below the average in Seyferts. Unresolved hot-dust emission with T∼1150 K seems to indicate the presence of a torus with its axis close to the line of sight. We found that IRAS 01072+4954 hosts a low-mass black hole with an estimated mass of MBH ∼ 105 Mʘ. Its bolometric luminosity is Lbol ≃2.5 × 1042 erg s-1, which yields a high accretion rate with an Eddington ratio λEdd ≃0.2. If the relations found in more massive systems also apply to this case, then IRAS 01072+4954 should show broad emission lines with a FWHMbroad ∼ (400-600) km s-1. Indeed, some indications for such narrow broad-line components are seen in our data, but the evidence is not yet conclusive. This source therefore does not seem to be a True-Seyfert 2, but an extreme case of a narrow-line Seyfert 1, which, due to the faintness of the active nucleus, does not have strong Fe II emission in the optical. [ABSTRACT FROM AUTHOR]

ABSTRACT Several massive black holes exhibit flux variability on time-scales that correspond to source sizes of the order of few Schwarzschild radii. We survey the potential of near-infrared and X-ray polarimetry to constrain physical properties of such black hole systems, namely, their spin and inclination. We have focused on a model where an orbiting hotspot is embedded in an accretion disc. A new method of searching for the time-lag between orthogonal polarization channels is developed and applied to an ensemble of hotspot models that samples a wide range of parameter space. We found that the hotspot model predicts signatures in polarized light which are in the range to be measured directly in the near future. However, our estimations are predicted upon the assumption of a Keplerian velocity distribution inside the flow where the dominant part of the magnetic field is toroidal. We also found that if the right model of the accretion flow can be chosen for each source (e.g. on the basis of magnetohydrodynamic simulations), then the black hole spin and inclination can be constrained to a small two-dimensional area in the spin-inclination space. The results of the application of the method to the available near-infrared polarimetric data of Sagittarius A* (Sgr A*) are presented. It is shown that even with the currently available data, the spin and inclination of Sgr A* can be constrained. Next generations of near-infrared and X-ray polarimeters should be able to exploit this tool. [ABSTRACT FROM AUTHOR]