arXiv subjects

Kambiz Fathi

Publications and source records attributed to Kambiz Fathi.

At least 19 recordsLinked to original sources

arXiv2016

What powers the starburst activity of NGC 1068? Star-driven gravitational instabilities caught in the act

We explore the role that gravitational instability plays in NGC 1068, a nearby Seyfert galaxy that exhibits unusually vigorous starburst activity. For this purpose, we use the Romeo-Falstad disc instability diagnostics and data from BIMA SONG, SDSS and SAURON. Our analysis illustrates that NGC 1068 is a gravitationally unstable "monster". Its starburst disc is subject to unusually powerful instabilities. Several processes, including AGN/stellar feedback, try to quench such instabilities from inside out by depressing the surface density of molecular gas across the central kpc, but they do not succeed. Gravitational instability "wins" because it is driven by the stars via their much higher surface density. In this process, stars and molecular gas are strongly coupled, and it is such a coupling that ultimately triggers local gravitational collapse/fragmentation in the molecular gas.

astro-ph.GA↗

arXiv2015

Submillimeter-HCN Diagram for an Energy Diagnostics in the Centers of Galaxies

Compiling data from literature and the ALMA 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 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 $>$ 10 is a plausible explanation for the submillimeter HCN-enhancement. However, a counter argument of a systematically higher gas density in AGNs than in SB galaxies can also be a plausible scenario. Although we could not fully discriminate these two scenarios at this moment due to insufficient amount of multi-transition, multi-species data, the former scenario equivalently claims for abnormal chemical composition in AGNs. Regarding the actual mechanism to realize the composition, we suggest it is difficult with conventional gas phase X-ray dominated region (XDR) 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 at high temperature environments and/or mechanically heated regions to further understand the high line ratios in AGNs.

astro-ph.GA↗

arXiv2015

ALMA Observations of the Submillimeter Dense Molecular Gas Tracers in the Luminous Type-1 Active Nucleus of NGC 7469

We present ALMA Cycle 1 observations of the central kpc region of the luminous type-1 Seyfert galaxy NGC 7469 with unprecedented high resolution (0.5$"$ $\times$ 0.4$"$ = 165 pc $\times$ 132 pc) at submillimeter wavelengths. Utilizing the wide-bandwidth of ALMA, we simultaneously obtained HCN(4-3), HCO$^+$(4-3), CS(7-6), and partially CO(3-2) line maps, as well as the 860 $μ$m continuum. The region consists of the central $\sim$ 1$"$ component and the surrounding starburst ring with a radius of $\sim$ 1.5$"$-2.5$"$. Several structures connect these components. Except for CO(3-2), these dense gas tracers are significantly concentrated towards the central $\sim$ 1$"$, suggesting their suitability to probe the nuclear regions of galaxies. Their spatial distribution resembles well those of centimeter and mid-infrared continuum emissions, but it is anti-correlated with the optical one, indicating the existence of dust obscured star formation. The integrated intensity ratios of HCN(4-3)/HCO$^+$(4-3) and HCN(4-3)/CS(7-6) are higher at the AGN position than at the starburst ring, which is consistent to our previous findings (submm-HCN enhancement). However, the HCN(4-3)/HCO$^+$(4-3) ratio at the AGN position of NGC 7469 (1.11$\pm$0.06) is almost half of the corresponding value of the low-luminosity type-1 Seyfert galaxy NGC 1097 (2.0$\pm$0.2), despite the more than two orders of magnitude higher X-ray luminosity of NGC 7469. But the ratio is comparable to that of the close vicinity of the AGN of NGC 1068 ($\sim$ 1.5). Based on these results, we speculate that some other heating mechanisms than X-ray (e.g., mechanical heating due to AGN jet) can contribute significantly for shaping the chemical composition in NGC 1097.

astro-ph.GA↗

arXiv2015

The OH-streamer in Sgr A revisited: analysis of hydroxyl absorption within 10 pc from the Galactic centre

We study the structure and kinematics of the OH-streamer and the +80 km/s cloud and their interactions with the circumnuclear disk (CND) and with other molecular clouds in the vicinity of the Galactic centre (GC), and we map OH absorption at about 6" resolution at R $\le$ 10 pc from the GC, with about 9 km/s velocity resolution. The VLA was used to map OH line absorption at the 1665 and 1667 MHz lambda doublet main lines towards the Sagittarius A complex. Strong OH absorption was found in the OH-streamer, the southern streamer (SS), the +20, +50, and +80 km/s molecular clouds, the molecular belt, the CND, the expanding molecular ring (EMR), and the high negative velocity gas (HNVG). The OH-streamer was found to comprise three parts, head, middle, and tail, and to interact with the SS/+20, +80 km/s clouds and the CND. Optical depths and column densities have been calculated for the OH-streamer and the +80 km/s cloud. The OH-streamer, the SS, the +20 and +80 km/s clouds, and the CND are intimately related in position and velocity space. The OH-streamer was found to be a clumpy object stretching in projection from the inner radius of the CND at about 1.8 pc from Sgr A* towards and partly engulfing Sgr A*. As a side result of our data, a possible link between the near side of the EMR and the CND's southwest lobe was found. Additionally, we found OH absorption against all four of the previously known Compact HII Regions A-D, located east of Sgr A East, indicating their close association with the +50 km/s cloud.

astro-ph.GA↗

arXiv2015

Kinematics of Haro11 - the miniature Antennae

(abridged) Luminous blue compact galaxies are among the most active galaxies in the local universe in terms of their star formation rate per unit mass. They may be seen as the local analogs of higher redshift Lyman Break Galaxies. Studies of their kinematics is key to understanding what triggers their unusually active star formation In this work we investigate the kinematics of stars and ionised gas in Haro11, one of the most luminous blue compact galaxies in the local universe. Previous works have indicated that many such galaxies may be triggered by galaxy mergers. We have employed Fabry-Perot interferometry, long-slit spectroscopy and Integral Field Unit (IFU) spectroscopy to explore the kinematics of Haro11. We target the near infrared Calcium triplet to derive the stellar velocity field and velocity dispersion. Ionised gas is analysed through emission lines from hydrogen, [OIII] , and [SIII]. When spectral resolution and signal to noise allows we investigate the the line profile in detail and identify multiple velocity components when present. We find that to first order, the velocity field and velocity dispersions derived from stars and ionised gas agree. Hence the complexities reveal real dynamical disturbances providing further evidence for a merger in Haro11. Through decomposition of emission lines we find evidence for kinematically distinct components, for instance a tidal arm behind the galaxy. The ionised gas velocity field can be traced to large galactocentric radii, and shows significant velocity dispersion even far out in the halo. We discuss the origin of the line width, and interpreted as virial motions it indicates a mass of ~1E11 M_sun. Morphologically and kinematically Haro11 shows many resemblances with the famous Antennae galaxies, but is much denser which is the likely explanation for the higher star formation efficiency in Haro11.

astro-ph.GA↗

arXiv2015

Local instability signatures in ALMA observations of dense gas in NGC7469

We present an unprecedented measurement of the disc stability and local instability scales in the luminous infrared Seyfert 1 host, NGC7469, based on ALMA observations of dense gas tracers and with a synthesized beam of 165 x 132 pc. While we confirm that non-circular motions are not significant in redistributing the dense interstellar gas in this galaxy, we find compelling evidence that the dense gas is a suitable tracer for studying the origin of its intensely high-mass star forming ring-like structure. Our derived disc stability parameter accounts for a thick disc structure and its value falls below unity at the radii in which intense star formation is found. Furthermore, we derive the characteristic instability scale and find a striking agreement between our measured scale of ~ 180 pc, and the typical sizes of individual complexes of young and massive star clusters seen in high-resolution images.

astro-ph.GA↗

arXiv2015

A double molecular disc in the triple-barred starburst galaxy NGC 6946: structure and stability

The late-type spiral galaxy NGC 6946 is a prime example of molecular gas dynamics driven by "bars within bars". Here we use data from the BIMA SONG and HERACLES surveys to analyse the structure and stability of its molecular disc. Our radial profiles exhibit a clear transition at distance R ~ 1 kpc from the galaxy centre. In particular, the surface density profile breaks at R ~ 0.8 kpc and is well fitted by a double exponential distribution with scale lengths R_1 ~ 200 pc and R_2 ~ 3 kpc, while the 1D velocity dispersion sigma decreases steeply in the central kpc and is approximately constant at larger radii. The fact that we derive and use the full radial profile of sigma rather than a constant value is perhaps the most novel feature of our stability analysis. We show that the profile of the Q stability parameter traced by CO emission is remarkably flat and well above unity, while the characteristic instability wavelength exhibits clear signatures of the nuclear starburst and inner bar within bar. We also show that CO-dark molecular gas, stars and other factors can play a significant role in the stability scenario of NGC 6946. Our results provide strong evidence that gravitational instability, radial inflow and disc heating have driven the formation of the inner structures and the dynamics of molecular gas in the central kpc.

astro-ph.GA↗

arXiv2015

Comparative internal kinematics of the HII regions in interacting and isolated galaxies: implications for massive star formation modes

We have observed 10 interacting galaxy pairs using the Fabry-Perot interferometer GH$α$FaS (Galaxy H$α$ Fabry-Perot system) on the $4.2\rm{m}$ William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos, La Palma. We present here the H$α$ surface brightness, velocity and velocity dispersion maps for the 10 systems we have not previously observed using this technique, as well as the physical properties (sizes, H$α$ luminosities and velocity dispersion) of 1259 HII regions from the full sample. We also derive the physical properties of 1054 HII regions in a sample of 28 isolated galaxies observed with the same instrument in order to compare the two populations of HII regions. We find a population of the brightest HII regions for which the scaling relations, for example the relation between the H$α$ luminosity and the radius, are clearly distinct from the relations for the regions of lower luminosity. The regions in this bright population are more frequent in the interacting galaxies. We find that the turbulence, and also the star formation rate, are enhanced in the HII regions in the interacting galaxies. We have also extracted the H$α$ equivalent widths for the HII regions of both samples, and we have found that the distribution of HII region ages coincides for the two samples of galaxies. We suggest that the SFR enhancement is brought about by gas flows induced by the interactions, which give rise to gravitationally bound gas clouds which grow further by accretion from the flowing gas, producing conditions favourable to star formation.

astro-ph.GA↗

arXiv2014

The ratio of pattern speeds in double-barred galaxies

We have obtained two-dimensional velocity fields in the ionized gas of a set of 8 double-barred galaxies, at high spatial and spectral resolution, using their H$α$ emission fields measured with a scanning Fabry-Perot spectrometer. Using the technique by which phase reversals in the non-circular motion indicate a radius of corotation, taking advantage of the high angular and velocity resolution we have obtained the corotation radii and the pattern speeds of both the major bar and the small central bar in each of the galaxies; there are few such measurements in the literature. Our results show that the inner bar rotates more rapidly than the outer bar by a factor between 3.3 and 3.6.

astro-ph.GA↗

arXiv2013

Hydroxyl, water, ammonia, carbon monoxide and neutral carbon towards the Sgr A complex

We observed OH, H$_2$O, HN$_3$, C$^{18}$O, and C$_I$ towards the +50 km/s cloud (M-0.02-0.07), the CND and the +20 km/s (M-0.13-0.08) cloud in the Sgr A complex with the VLA, Odin and SEST. Strong OH absorption, H$_2$O emission and absorption lines were seen at all three positions. Strong C$^{18}$O emissions were seen towards the +50 and +20 km/s clouds. The CND is rich in H$_2$O and OH, and these abundances are considerably higher than in the surrounding clouds, indicating that shocks, star formation and clump collisions prevail in those objects. A comparison with the literature reveals that it is likely that PDR chemistry including grain surface reactions, and perhaps also the influences of shocks has led to the observed abundances of the observed molecular species studied here. In the redward high-velocity line wings of both the +50 and +20 km/s clouds and the CND, the very high H$_2$O abundances are suggested to be caused by the combined action of shock desorption from icy grain mantles and high-temperature, gas-phase shock chemistry. Only three of the molecules are briefly discussed here. For OH and H$_2$O three of the nine observed positions are shown, while a map of the C$^{18}$O emission is provided. An extensive paper was recently published with Open Access (Karlsson et al. 2013; http://www.aanda.org/articles/aa/pdf/2013/06/aa20471-12.pdf ).

astro-ph.GA↗

arXiv2013

The Schwarzian Curvature

We start with introducing one of the most fundamental notions of differential geometry, Manifolds. We present some properties and constructions such as submanifolds, tangent spaces and the tangent map. Then we continue with introducing the real and complex projective space, and describe them from some different points of view. This part is finished by showing that CP^n is a Grassmannian manifold. At this stage we are ready to present the main subject of this thesis. The Schwarzian curvature, usually seems to be an accidental by-product of the calculations, can be seen as a geometric interpretation of the Schwarzian derivative. Flanders interpreted the Schwarzian derivative of a C''' function as a curvature for curves in the projective line by using the moving frame method of Cartan. The same argumentation was extended by Gao to obtain the Schwarzian curvatures for curves in higher dimensional projective spaces. I give detailed presentation of Gao's work, where he presented the general formulas for the Schwarzian curvatures for curves in CP^n and gives some properties for the behaviour of the formulas, for example the transformation rules under change of coordinates. The Schwarzian curvatures for curves in CP, CP^2 and CP^3 are calculated, and some examples are given.

math.DG↗

arXiv2013

Submillimeter ALMA Observations of the Dense Gas in the Low-Luminosity Type-1 Active Nucleus of NGC 1097

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.

astro-ph.GA↗

arXiv2013

ALMA follows streaming of dense gas down to 40 pc from the supermassive black hole in NGC1097

We present a kinematic analysis of the dense molecular gas in the central 200 parsecs of the nearby galaxy NGC1097, based on Cycle 0 observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA). We use the HCN(4-3) line to trace the densest interstellar molecular gas, and quantify its kinematics, and estimate an inflow rate for the molecular gas. We find a striking similarity between the ALMA kinematic data and the analytic spiral inflow model that we have previously constructed based on ionized gas velocity fields on larger scales. We are able to follow dense gas streaming down to 40 pc distance from the supermassive black hole in this Seyfert 1 galaxy. In order to fulfill marginal stability, we deduce that the dense gas is confined to a very thin disc, and we derive a dense gas inflow rate of 0.09 Msun/yr at 40 pc radius. Combined with previous values from the Ha and CO gas, we calculate a combined molecular and ionized gas inflow rate of 0.2 Msun/yr at 40 pc distance from the central supermassive black hole of NGC1097.

astro-ph.CO↗

arXiv2012

Disc scalelengths out to redshift 5.8

We compute the exponential disc scalelength for 686 disc galaxies with spectroscopic redshifts out to redshift 5.8 based on Hubble Space Telescope archival data. We compare the results with our previous measurements based on 30000 nearby galaxies from the Sloan Digital Sky Survey. Our results confirm the presence of a dominating exponential component in galaxies out to this redshift. At the highest redshifts, the disc scalelength for the brightest galaxies with absolute magnitude between -24 and -22 is up to a factor 8 smaller compared to that in the local Universe. This observed scalelength decrease is significantly greater than the value predicted by a cosmological picture in which baryonic disc scalelength scales with the virial radius of the dark matter halo.

astro-ph.CO↗

arXiv2011

Resonant structure in the disks of spiral galaxies, using phase-reversals in streaming motions from 2D Hα Fabry-Perot spectroscopy

In this article we introduce a technique for finding resonance radii in a disk galaxy. We use a two-dimensional velocity field in Hα emission obtained with Fabry-Perot interferometry, derive the classical rotation curve, and subtract it off, leaving a residual velocity map. As the streaming motions should reverse sign at corotation, we detect these reversals, and plot them in a histogram against galactocentric radius, excluding points where the amplitude of the reversal is smaller than the measurement uncertainty. The histograms show well-defined peaks which we assume to occur at resonance radii, identifying corotations as the most prominent peaks corresponding to the relevant morphological features of the galaxy (notably bars and spiral arm systems). We compare our results with published measurements on the same galaxies using other methods and different types of data.

astro-ph.GA↗

arXiv2011

Detection of infalling hydrogen in transfer between the interacting galaxies NGC 5426 and NGC 5427

Using velocity tagging we have detected hydrogen from NGC 5426 falling onto its interacting partner NGC 5427. Our observations, with the GHaFaS Fabry-Perot spectrometer, produced maps of the two galaxies in Halpha surface brightness and radial velocity. We found emission with the range of velocities associated with NGC 5426 along lines of sight apparently emanating from NGC 5427, superposed on the velocity map of the latter. After excluding instrumental effects we assign the anomalous emission to gas pulled from NGC 5426 during its passage close to NGC 5427. Its distribution, more intense between the arms and just outside the disk of NGC 5427, and weak, or absent, in the arms, suggests that the infalling gas is behind the disk., ionized by Lyman continuum photons escaping from NGC 5427. Modeling this, we estimate the distances of these gas clouds- behind the plane: a few hundred pc to a few kpc. We also estimate the mass of the infalling (ionized plus neutral) gas, finding an infall rate of 10 solar masses per year, consistent with the high measured SFR across the disk of NGC 5427 and with the detected circumnuclear galactic wind.

astro-ph.GA↗

arXiv2011

An H-alpha nuclear spiral structure in the E0 active galaxy Arp102B

We report the discovery of a two-armed mini-spiral structure within the inner kiloparsec of the E0 LINER/Seyfert 1 galaxy Arp102B. The arms are observed in H-alpha emission and located East and West of the nucleus, extending up to about 1 kpc from it. We use narrow-band imaging from the Hubble Space Telescope Advanced Camera for Surveys, in combination with archival VLA radio images at 3.6 and 6 cm to investigate the origin of the nuclear spiral. From the H-alpha luminosity of the spiral, we obtain an ionized gas mass of the order of one million solar masses. One possibility is that the nuclear spiral represents a gas inflow triggered by a recent accretion event which has replenished the accretion disk, giving rise to the double-peaked emission-line profiles characteristic of Arp102B. However, the radio images show a one-sided curved jet which correlates with the eastern spiral arm observed in the H-alpha image. A published milliarcsecond radio image also shows one-sided structure at position angle about 40 degrees, approximately aligned with the inner part of the eastern spiral arm. The absence of a radio counter-part to the western spiral arm is tentatively interpreted as indicating that the jet is relativistic, with an estimated speed of 0.45c. Estimates of the jet kinetic energy and the ionizing luminosity of the active nucleus indicate that both are capable of ionizing the gas along the spiral arms. We conclude that, although the gas in the nuclear region may have originated in an accretion event, the mini-spiral is most likely the result of a jet-cloud interaction rather than an inflowing stream.

astro-ph.CO↗

arXiv2011

Physical Condition of Molecular Gas at the Centre of NGC 1097

We have used the Xco conversion factor, Local Thermal Equilibrium and Large Velocity Gradient approximation to parametrize the cold and warm phase of the interstellar medium from five different low transitions of CO in the central 21"(kpc) of NGC 1097. We have applied a one-component model and derived a typical kinetic temperature of ~33K, and its molecular Hydrogen density and a CO column density. A two-component model results in 85% cold-to-total gas fraction with a 90K warm counterpart. Furthermore, we "resolve" the spatially unresolved single dish observations by selecting velocity channels that in an interferometric velocity map correspond to specific regions. We have selected five such regions and found that the physical properties in these regions are comparable to those derived from the full line profile. This implies that the central kpc of NGC 1097 is rather homogeneous in nature, and, although the regions are not uniquely located within the ring, the star formation along the ring is homogeneously distributed (in agreement with recent Herschel observations). We have further revised the mass inflow rate onto the Supermassive Black Hole in this prototype LINER/Sy1 galaxy and found that, accounting for the total interstellar medium and applying a careful contribution of the disc thickness and corresponding stability criterion, increases the previous estimations by a factor 10. Finally we have calculated the Xco for the centre of NGC 1097 using an independent estimation of the surface density to the CO emission, and obtained Xco=(2.8+-0.5)x10^20 cm^-2(K km s^-1)^-1 at radius 10.5" and Xco=(5.0+-0.5)x10^20 cm^-2(K km s^-1)^-1 at 7.5". With the approach and analysis described, we have demonstrated that important physical properties can be derived to a resolution beyond the single dish resolution element, however, caution is necessary for interpreting the results.(Abriged)

astro-ph.CO↗