arXiv subjects

A. Eckart

Publications and source records attributed to A. Eckart.

At least 145 records · Page 8Linked to original sources

arXiv2014

Future mmVLBI Research with ALMA: A European vision

Very long baseline interferometry at millimetre/submillimetre wavelengths (mmVLBI) offers the highest achievable spatial resolution at any wavelength in astronomy. The anticipated inclusion of ALMA as a phased array into a global VLBI network will bring unprecedented sensitivity and a transformational leap in capabilities for mmVLBI. Building on years of pioneering efforts in the US and Europe the ongoing ALMA Phasing Project (APP), a US-led international collaboration with MPIfR-led European contributions, is expected to deliver a beamformer and VLBI capability to ALMA by the end of 2014 (APP: Fish et al. 2013, arXiv:1309.3519). This report focuses on the future use of mmVLBI by the international users community from a European viewpoint. Firstly, it highlights the intense science interest in Europe in future mmVLBI observations as compiled from the responses to a general call to the European community for future research projects. A wide range of research is presented that includes, amongst others: - Imaging the event horizon of the black hole at the centre of the Galaxy - Testing the theory of General Relativity an/or searching for alternative theories - Studying the origin of AGN jets and jet formation - Cosmological evolution of galaxies and BHs, AGN feedback - Masers in the Milky Way (in stars and star-forming regions) - Extragalactic emission lines and astro-chemistry - Redshifted absorption lines in distant galaxies and study of the ISM and circumnuclear gas - Pulsars, neutron stars, X-ray binaries - Testing cosmology - Testing fundamental physical constants

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arXiv2014

Molecular line emission in NGC1068 imaged with ALMA. I An AGN-driven outflow in the dense molecular gas

We investigate the fueling and the feedback of star formation and nuclear activity in NGC1068, a nearby (D=14Mpc) Seyfert 2 barred galaxy, by analyzing the distribution and kinematics of the molecular gas in the disk. We have used ALMA to map the emission of a set of dense molecular gas tracers (CO(3-2), CO(6-5), HCN(4-3), HCO+(4-3) and CS(7-6)) and their underlying continuum emission in the central r ~ 2kpc of NGC1068 with spatial resolutions ~ 0.3"-0.5" (~ 20-35pc). Molecular line and dust continuum emissions are detected from a r ~ 200pc off-centered circumnuclear disk (CND), from the 2.6kpc-diameter bar region, and from the r ~ 1.3kpc starburst (SB) ring. Most of the emission in HCO+, HCN and CS stems from the CND. Molecular line ratios show dramatic order-of-magnitude changes inside the CND that are correlated with the UV/X-ray illumination by the AGN, betraying ongoing feedback. The gas kinematics from r ~ 50pc out to r ~ 400pc reveal a massive (M_mol ~ 2.7 (+0.9, -1.2) x 10^7 Msun) outflow in all molecular tracers. The tight correlation between the ionized gas outflow, the radio jet and the occurrence of outward motions in the disk suggests that the outflow is AGN-driven. The outflow rate estimated in the CND, dM/dt ~ 63 (+21, -37) Msun yr^-1, is an order of magnitude higher than the star formation rate at these radii, confirming that the outflow is AGN-driven. The power of the AGN is able to account for the estimated momentum and kinetic luminosity of the outflow. The CND mass load rate of the CND outflow implies a very short gas depletion time scale of <=1 Myr.

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arXiv2014

ALMA observations of feeding and feedback in nearby Seyfert galaxies: an AGN-driven outflow in NGC 1433

We report ALMA observations of CO(3-2) emission in the Seyfert 2 double-barred galaxy NGC1433, at the unprecedented spatial resolution of 0.5"=24 pc. Our aim is to probe AGN feeding and feedback phenomena through the morphology and dynamics of the gas inside the central kpc. The CO map, which covers the whole nuclear region (nuclear bar and ring), reveals a nuclear gaseous spiral structure, inside the nuclear ring encircling the nuclear stellar bar. This gaseous spiral is well correlated with the dusty spiral seen in Hubble Space Telescope images. The nuclear spiral winds up in a pseudo-ring at 200 pc radius, which might correspond to the inner ILR. Continuum emission is detected at 0.87 mm only at the very centre, and its origin is more likely thermal dust emission than non-thermal emission from the AGN. It might correspond to the molecular torus expected to exist in this Seyfert 2 galaxy. The HCN(4-3) and HCO+(4-3) lines were observed simultaneously, but only upper limits are derived, with a ratio to the CO(3-2) line lower than 1/60 at 3sigma, indicating a relatively low abundance of very dense gas. The kinematics of the gas over the nuclear disk reveal rather regular rotation only slightly perturbed by streaming motions due to the spiral; the primary and secondary bars are too closely aligned with the galaxy major or minor axis to leave a signature in the projected velocities. Near the nucleus, there is an intense high-velocity CO emission feature redshifted to 200 km/s (if located in the plane), with a blue-shifted counterpart, at 2" (100 pc) from the centre. While the CO spectra are quite narrow in the centre, this wide component is interpreted as an outflow involving a molecular mass of 3.6 10^6 Mo and a flow rate 7 Mo/yr. The flow could be in part driven by the central star formation, but is mainly boosted by the AGN through its wind or radio jets.

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arXiv2014

ALMA reveals the feeding of the Seyfert 1 nucleus in NGC 1566

We report ALMA observations of CO(3-2) emission in the Seyfert 1 galaxy NGC 1566, at a spatial resolution of 25 pc. Our aim is to investigate the morphology and dynamics of the gas inside the central kpc, and to probe nuclear fueling and feedback phenomena. NGC 1566 has a nuclear bar of 1.7 kpc radius and a conspicuous grand design spiral starting from this radius. The ALMA field of view, of diameter 0.9 kpc, lies well inside the nuclear bar and reveals a molecular trailing spiral structure from 50 to 300~pc in size, which is contributing to fuel the nucleus, according to its negative gravity torques. The spiral starts with a large pitch angle from the center and then winds up in a pseudo-ring at the inner Lindblad resonance (ILR) of the nuclear bar. This is the first time that a trailing spiral structure is clearly seen driving the gas inwards inside the ILR ring of the nuclear bar. This phenomenon shows that the massive central black hole has a significant dynamical influence on the gas, triggering its fueling. The gaseous spiral is well correlated with the dusty spiral seen through extinction in HST images, and also with a spiral feature emitting 0.87mm continuum. This continuum emission must come essentially from cold dust heated by the interstellar radiation field. The HCN(4-3) and HCO+(4-3) lines were simultaneously mapped and detected in the nuclear spiral. The HCO+(4-3) line is 3 times stronger than the HCN(4-3), as expected when star formation excitation dominates over active galactic nucleus (AGN) heating. The CO(3-2)/HCO+(4-3) integrated intensity ratio is \sim 100. The molecular gas is in remarkably regular rotation, with only slight non-circular motions at the periphery of the nuclear spiral arms. These perturbations are quite small, and no outflow nor AGN feedback is detected.

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arXiv2013

Nuclei of Seyfert galaxies and QSOs - Central engine and conditions of star formation. Workshop summary and open questions

Observationally established correlations between black hole mass and host galaxy structural/dynamical properties (like the M_BH - sigma relation) give support to the idea of an intimate link between the growth of black holes (BHs) and their host galaxies. Active galactic nuclei represent a poorly understood phase (or phases) in the life of a galaxy, during which the BH growth is directly observable. With the advent of wide-field surveys and high angular resolution instruments, it is now possible to observe the source of energy in Seyfert galaxies and QSOs, and conduct both statistical and detailed on-object studies. Combining these two perspectives gives rise to new research questions and methods. This Workshop, aimed at discussing these questions and gathering the astronomical community working on AGN, their feeding and feedback mechanisms, and their relations to the host galaxies. The purpose of this summary paper is to condense in a few pages some of the ideas and discussion points that were considered during the Workshop. We would also like to call the attention to some open questions that are still matter of debate and drive the research efforts in the field.

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arXiv2013

Young Stellar Objects close to Sgr A*

We aim at modelling small groups of young stars such as IRS 13N, 0.1 pc away from Sgr A*, which is suggested to contain a few embedded massive young stellar objects. We perform hydrodynamical simulations to follow the evolution of molecular clumps orbiting about a $4\times10^6 ~ M_{\odot}$ black hole, to constrain the formation and the physical conditions of such groups. We find that, the strong compression due to the black hole along the orbital radius vector of clumps evolving on highly eccentric orbits causes the clumps densities to increase to higher than the tidal density of Sgr A*, and required for star formation. This suggests that the tidal compression from the black hole could support star formation. Additionally, we speculate that the infrared excess source G2/DSO approaching Sgr A* on a highly eccentric orbit could be associated with a dust enshrouded star that may have been formed recently through the mechanism supported by our models.

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arXiv2013

Are Narrow Line Seyfert 1s a special class of Active Galactic Nuclei?

No. Due to their apparently extreme optical to X-ray properties, Narrow Line Seyfert 1s (NLSy1s) have been considered a special class of active galactic nuclei (AGN). Here, we summarize observational results from different groups to conclude that none of the characteristics that are typically used to define the NLSy1s as a distinct group - from the, nowadays called, Broad Line Seyfert 1s (BLSy1s) - is unique, nor ubiquitous of these particular sources, but shared by the whole Type 1 AGN. Historically, the NLSy1s have been distinguished from the BLSy1s by the narrow width of the broad Hbeta emission line. The upper limit on the full width at half maximum of this line is 2000 km/s for NLSy1s, while in BLSy1s it can be of several thousands of km/s. However, this border has been arbitrarily set and does not correspond to the change of any physical property. All observed parameters in Type 1 AGN cover continues ranges of values, which does not allow to infer the existence of two different kind of populations with FWHM(Hbeta,broad) = 2000 km/s as dividing point. We argue that the usage of this velocity limit to define samples of NLSy1s galaxies - as it is usually done in comparative studies -, together with the well known observational biases, naturally favors the selection of sources with low black hole masses and high Eddington ratios that are hosted by blue spiral galaxies. Therefore selection biases might be responsible for the reported differences between NLSy1 and BLSy1 sources.

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arXiv2013

High-resolution observations of SDSS J080800.99+483807.7 in the optical and radio domains. A possible example of jet-triggered star formation

Double-lobe radio galaxies are ideally suited to investigate the interaction of the individual components of the radio structure with the intergalactic medium and the interstellar medium of the host galaxy. SDSS J080800.99+483807.7 has been serendipitously discovered in MERLIN 18 cm observations to be a double-lobed radio galaxy. Because it is an optically faint source, basic information like redshift, linear size, and structure has been incomplete until now. Furthermore, there are no spectra of this source available in any databases. The goal of this work is to derive the main physical properties of SDSS J080800.99+483807.7 and study the possible interaction between the radio jets and the interstellar medium of the host galaxy. To achieve this goal, we used optical spectroscopy and radio interferometry. The radio data were obtained with MERLIN at 18 cm and the optical data with the Multi-Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT). The redshift of the galaxy is z = 0.2805 +/- 0.0003, resulting in a linear size of the observed radio structure of ~ 26.3 kpc. The optical line emission as well as the infrared and radio continuum emission suggest a high star-formation activity. In addition, we estimated the mass of the central black hole to be log (MBH/Msun) ~ 6.9. The MODS spectrum and the optical images from Sloan Digital Sky Survey suggest that SDSS J080800.99+483807.7 is an elliptical host galaxy. In combination with the overall radio structure, we argue that the star formation could be the result of the back-flow along the jet and the interstellar medium of the host.

astro-ph.CO↗

arXiv2013

Is IRAS 01072+4954 a True-Seyfert 2? Hints from Near Infrared Integral Field Spectroscopy

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 HII 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 such peculiar emission. We studied the validity of such hypotheses, including the possibility for it to be True-Seyfert~2. We found little obscuration towards the nucleus A_V = 2.5 mag, and a nuclear star-formation rate Sigma_SFR < 11.6 Msun 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 M_BH ~ 10^5 Msun and an upper limit of 2.5x10^6 Msun. Its bolometric luminosity is L_bol ~ 2.5x10^{42} erg/s, which yields a high accretion rate with an Eddington ratio ~ 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 FWHM_{broad} ~(400-600) km/s. Indeed, some indications for such narrow broad-line components are seen in our data, but the evidence is not yet conclusive. This source thus seems not 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 FeII emission in the optical.

astro-ph.CO↗

arXiv2013

Discovery of an Intermediate Mass Black Hole at the center of the starburst/Seyfert composite galaxy IRAS 01072+4954

The starburst/Seyfert composite galaxy IRAS 01072+4954 (z=0.0236) is an enigmatic source that combines a Seyfert~1-like X-ray emission with a starburst optical spectrum that lacks broad line emission. We performed high angular resolution observations of the central kiloparsec of this galaxy in the near-infrared. Combining our data with 2MASS images of the whole galaxy, we obtain and model the surface brightness profile. We find indications for the presence of an elongated bar-like structure in both data sets. We also model the line of sight velocity distribution of the stars in the bulge. The derived photometrical and kinematical parameters of the bulge are used to evaluate the black hole mass through scaling relations. We find that all reliable estimations of the black hole mass are consistent with the presence of an intermediate mass black hole with a mass of ~10^5 solar masses (or less).

astro-ph.CO↗

arXiv2013

The Central 3 kpc of NGC 5850

NGC 5850 is a nearby (z=0.0085) early type spiral galaxy classified as LINER. It is considered as a prototype double-barred system. Our optical Integral Field Spectroscopic (IFS) data of the central 21x19 arcsec^2 of NGC 5850 show extended LINER-like emission which we ascribe to the presence of a hot and evolved stellar population, possibly together with a faint AGN. Additionally NGC 5850 shows extended `composite' ionization patterns, likely to stem from a mixture of LINER-like ionization and photoionization by star formation. The kinematics of the gas deviates strongly from a simple rotational structure.

astro-ph.GA↗

arXiv2012

Near-infrared proper motions and spectroscopy of infrared excess sources at the Galactic Center

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 we find a small but dense cluster of co-moving sources (IRS13N) about 3" west of SgrA* just 0.5" north of the bright IRS13E cluster of WR and O-type stars. Based on their color and brightness, there are two main possibilities: (1) they may be dust embedded stars older than few Myr, or (2) extremely young, dusty stars with ages less than 1Myr. We present fist H- and Ks-band identifications or proper motions of the IRS13N members, the high velocity dusty S-cluster object (DSO), and other infrared excess sources in the central field. We also present results of NIR H- and Ks-band ESO-SINFONI integral field spectroscopy of ISR13N. We show that within the uncertainties, the proper motions of the IRS13N sources in Ks- and L'-band are identical. This indicates that the bright L'-band IRS13N sources are indeed dust enshrouded stars rather than core-less dust clouds. The proper motions show that the IRS13N sources are not strongly gravitationally bound to each other implying that they have been formed recently. We also present a first H- and Ks-band identification as well as proper motions and HKsL'-colors of a fast moving DSO which was recently found in the cluster of high speed S-stars that surround the super-massive black hole Sagittarius A* (SgrA*). 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 very young dusty stars and that star formation at the GC is a continuously ongoing process.

astro-ph.IM↗

arXiv2012

Classifying Radio Emitters from the Sloan Digital Sky Survey: Spectroscopy and Diagnostics

A cross-correlation of the SDSS DR7 with the FIRST radio survey makes it possible to conduct a joined multiwavelength statistical study of radio-optical galaxy properties on a very large number of sources. Our goal is to improve the study of the combined radio-optical data by investigating if there is a correlation between the radio luminosity at 20 cm over the luminosity of the optical Hαline (L[20 cm]/L[Hα]) and line excitation ratios, where the latter provide the spectroscopic classification in Seyferts, low-ionization nuclear emission-line regions (LINERs) and star-forming galaxies. We found that the percentage of detected AGNs (Seyferts and LINERs) or composites is much higher in the optical-radio sample than in the optical sample alone. There is a progressive shift of the sources towards the AGN region of the diagram with increasing L[20 cm]/L[Hα], with an indication for a different behavior for LINERs and Seyferts. The classification appears to slightly depend on the redshift. A comparison with photoionization and shock models shows that the large fraction of LINERs identified in our study have emission lines that may be explained by shocks. The [NII]/Hαvs. equivalent width of the Hαline (WHAN) diagram confirms the LINER classification for most of those that have been identified with the traditional diagnostic diagrams. The correlation between L[20 cm]/L[Hα] and optical emission line ratios suggests the nuclear origin of the emission from the most powerful radio-galaxies.

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arXiv2012

Chandra-HETGS Observations of the Brightest Flare Seen from Sgr A*

Starting in 2012, we began an unprecedented observational program focused on the supermassive black hole in the center of our Galaxy, Sgr A*, utilizing the High Energy Transmission Gratings Spectrometer (HETGS) instrument on the Chandra X-ray Observatory. These observations will allow us to measure the quiescent X-ray spectra of Sgr A* for the first time at both high spatial and spectral resolution. The X-ray emission of Sgr A*, however, is known to flare roughly daily by factors of a few to ten times over quiescent emission levels, with rarer flares extending to factors of greater than 100 times quiescence. Here were report an observation performed on 2012 February 9 wherein we detected what is the highest peak flux and fluence flare ever observed from Sgr A*. The flare, which lasted for 5.6 ks and had a decidedly asymmetric profile with a faster decline than rise, achieved a mean absorbed 2-8 keV flux of (8.5+/-0.9)X10^{-12} erg cm^{-2} s^{-1}. The peak flux was 2.5 times higher, and the total 2-10 keV emission of the event was approximately 10^{39} erg. Only one other flare of comparable magnitude, but shorter duration, has been observed in Sgr A* by XMM-Newton in 2002 October. We perform spectral fits of this Chandra observed flare, and compare our results to the two brightest flares ever observed with XMM-Newton. We find good agreement among the fitted spectral slopes (Gamma~2) and X-ray absorbing columns (N_H~15X10^{22} cm^{-2}) for all three of these events, resolving prior differences (which are most likely due to the combined effects of pileup and spectral modeling) among Chandra and XMM-Newton observations of Sgr A* flares. We also discuss fits to the quiescent spectra of Sgr A*.

astro-ph.HE↗

arXiv2012

Source-intrinsic near-infrared properties of Sgr A*: Total intensity measurements

We present a comprehensive data description for Ks-band measurements of Sgr A*. We characterize the statistical properties of the variability of Sgr A* in the near-infrared, which we find to be consistent with a single-state process forming a power-law distribution of the flux density. We discover a linear rms-flux relation for the flux-density range up to 12 mJy on a timescale of 24 minutes. This and the power-law flux density distribution implies a phenomenological, formally non-linear statistical variability model with which we can simulate the observed variability and extrapolate its behavior to higher flux levels and longer timescales. We present reasons why data with our cadence cannot be used to decide on the question whether the power spectral density of the underlying random process shows more structure at timescales between 25 min and 100 min compared to what is expected from a red noise random process.

astro-ph.HE↗

arXiv2012

Flare emission from Sagittarius A*

Based on Bremer et al. (2011) and Eckart et al. (2012) we report on simultaneous observations and modeling of the millimeter, near-infrared, and X-ray flare emission of the source Sagittarius A* (SgrA*) associated with the super-massive black hole at the Galactic Center. We study physical processes giving rise to the variable emission of SgrA* from the radio to the X-ray domain. To explain the statistics of the observed variability of the (sub-)mm spectrum of SgrA*, we use a sample of simultaneous NIR/X-ray flare peaks and model the flares using a synchrotron and SSC mechanism. The observations reveal flaring activity in all wavelength bands that can be modeled as the signal from adiabatically expanding synchrotron self-Compton (SSC) components. The model parameters suggest that either the adiabatically expanding source components have a bulk motion larger than v_exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*. For the bulk of the synchrotron and SSC models, we find synchrotron turnover frequencies in the range 300-400 GHz. For the pure synchrotron models this results in densities of relativistic particles of the order of 10^6.5 cm^-3 and for the SSC models, the median densities are about one order of magnitude higher. However, to obtain a realistic description of the frequency-dependent variability amplitude of SgrA*, models with higher turnover frequencies and even higher densities are required. We discuss the results in the framework of possible deviations from equilibrium between particle and magnetic field energy. We also summarize alternative models to explain the broad-band variability of SgrA*.

astro-ph.IM↗

arXiv2012

Beating the confusion limit: The necessity of high angular resolution for probing the physics of Sagittarius A* and its environment: Opportunities for LINC-NIRVANA (LBT), GRAVITY (VLTI) and and METIS (E-ELT)

The super-massive 4 million solar mass black hole (SMBH) SgrA* shows variable emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flux density excursions are dominated by a single state power law, with the low states of SgrA* limited by confusion through the unresolved stellar background. We show that for 8-10m class telescopes blending effects along the line of sight will result in artificial compact star-like objects of 0.5-1 mJy that last for about 3-4 years. We discuss how the imaging capabilities of GRAVITY at the VLTI, LINC-NIRVANA at the LBT and METIS at the E-ELT will contribute to the investigation of the low variability states of SgrA*.

astro-ph.IM↗

arXiv2012

The Arp 220 merger on kpc scales

For the first time we study the Eastern nucleus in greater detail and search for the more extended emission in the molecular gas in different CO line transitions of the famous ULIRG Arp 220. Furthermore we present a model of the merger in Arp 220 on large scales with the help of the CO data and an optical and near-infrared composite HST image of the prototypical ULIRG. Using the Plateau de Bure Interferometer (PdBI) we obtained CO(2-1) and (1-0) data at wavelengths of 1 and 3 mm in 1994, 1996, 1997 and 2006 at different beam sizes and spatial resolutions. The simulations of the merger in Arp 220 were performed with the Identikit modeling tool. The model parameters that describe the galaxy merger best give a mass ratio of 1:2 and result in a merger of ~6x10^8 yrs. The low resolution CO(1-0) PdBI observations suggest that there are indications for emission ~10" towards the south, as well as to the north and to the west of the two nuclei.

astro-ph.CO↗