arXiv subjects

N. M. Nagar

Publications and source records attributed to N. M. Nagar.

At least 19 recordsLinked to original sources

arXiv2021

GOODS-ALMA 2.0: Source catalog, number counts, and prevailing compact sizes in 1.1 mm galaxies

Submillimeter/millimeter observations of dusty star-forming galaxies with the Atacama Large Millimeter/submillimeter Array (ALMA) have shown that dust continuum emission generally occurs in compact regions smaller than the stellar distribution. However, it remains to be understood how systematic these findings are. Studies often lack homogeneity in the sample selection, target discontinuous areas with inhomogeneous sensitivities, and suffer from modest $uv$ coverage coming from single array configurations. GOODS-ALMA is a 1.1mm galaxy survey over a continuous area of 72.42arcmin$^2$ at a homogeneous sensitivity. In this version 2.0, we present a new low resolution dataset and its combination with the previous high resolution dataset from the survey, improving the $uv$ coverage and sensitivity reaching an average of $σ= 68.4μ$Jy beam$^{-1}$. A total of 88 galaxies are detected in a blind search (compared to 35 in the high resolution dataset alone), 50% at $S/N_{peak} \geq 5$ and 50% at $3.5 \leq S/N_{peak} \leq 5$ aided by priors. Among them, 13 out of the 88 are optically dark or faint sources ($H$- or $K$-band dropouts). The sample dust continuum sizes at 1.1mm are generally compact, with a median effective radius of $R_{e} = 0"10 \pm 0"05$ (a physical size of $R_{e} = 0.73 \pm 0.29$kpc at the redshift of each source). Dust continuum sizes evolve with redshift and stellar mass resembling the trends of the stellar sizes measured at optical wavelengths, albeit a lower normalization compared to those of late-type galaxies. We conclude that for sources with flux densities $S_{1.1mm} > 1$mJy, compact dust continuum emission at 1.1mm prevails, and sizes as extended as typical star-forming stellar disks are rare. The $S_{1.1mm} < 1$mJy sources appear slightly more extended at 1.1mm, although they are still generally compact below the sizes of typical star-forming stellar disks.

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arXiv2019

Nuclear kinematics in nearby AGN. I. An ALMA perspective on the Morphology and Kinematics of the molecular CO(2-1) emission

We present the molecular gas morphology and kinematics of seven nearby Seyfert galaxies obtained from our 230~GHz ALMA observations. The CO J=2-1 kinematics within the inner $\sim30$" ($\lesssim9$~kpc) reveals rotation patterns that have been explored using the Bertola rotation model and a modified version of the Kinemetry package. The latter algorithm reveals various deviations from pure circular rotation in the inner kiloparsec of all seven galaxies, including kinematic twists, decoupled and counter-rotating cores. A comparison of the global molecular gas and stellar kinematics show overall agreement in the position angle of the major axis and the systemic velocity, but larger discrepancies in the disc inclination. The residual maps obtained with both the methods shows the presence of non-circular motions in most of the galaxies. Despite its importance, a detailed interpretation of the physics responsible for non-circular motions will be discussed in a forthcoming work.

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arXiv2018

Outflows in the inner kiloparsec of NGC 1566 as revealed by molecular (ALMA) and ionized gas (Gemini-GMOS/IFU) kinematics

We aim to map the distribution and kinematics of molecular and ionized gas in a sample of active galaxies, to quantify the nuclear inflows and outflows. Here, we analyze the nuclear kinematics of NGC 1566 via ALMA observations of the CO J:2-1 emission at 24 pc spatial and $\sim$2.6 km s$^{-1}$ spectral resolution, and Gemini-GMOS/IFU observations of ionized gas emission lines and stellar absorption lines at similar spatial resolution, and 123 km s$^{-1}$ of intrinsic spectral resolution. The morphology and kinematics of stellar, molecular (CO) and ionized ([N II]) emission lines are compared to the expectations from rotation, outflows, and streaming inflows. While both ionized and molecular gas show rotation signatures, there are significant non-circular motions in the innermost 200 pc and along spiral arms in the central kpc (CO). The nucleus shows a double-peaked CO profile (Full Width at Zero Intensity of 200 km s$^{-1}$), and prominent ($\sim$80 km s$^{-1}$) blue and redshifted lobes are found along the minor axis in the inner arcseconds. Perturbations by the large-scale bar can qualitatively explain all features in the observed velocity field. We thus favour the presence of a molecular outflow in the disk with true velocities of $\sim$180 km s$^{-1}$ in the nucleus and decelerating to 0 by $\sim$72 pc. The implied molecular outflow rate is $5.6~[M_{o}yr^{-1}]$, with this gas accumulating in the nuclear 2 arcsec arms. The ionized gas kinematics support an interpretation of a similar, but more spherical, outflow in the inner 100 pc, with no signs of deceleration. There is some evidence of streaming inflows of $\sim$50 km s$^{-1}$ along specific spiral arms, and the estimated molecular mass inflow rate, $\sim0.1~[M_{o}yr^{-1}]$, is significantly larger than the SMBH accretion rate ($\dot{m}=4.8\times10^{-5}~[M_{o}yr^{-1}]$).

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arXiv2018

Outflows in the Narrow Line Region of Bright Seyfert Galaxies - I: GMOS-IFU Data

We present two-dimensional maps of emission-line fluxes and kinematics, as well as of the stellar kinematics of the central few kpc of five bright nearby Seyfert galaxies -- Mrk\,6, Mrk\,79, Mrk\,348, Mrk\,607 and Mrk\,1058 -- obtained from observations with the Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (IFU) on the Gemini North Telescope. The data cover the inner 3\farcs5$\times$5\farcs0 -- corresponding to physical scales in the range 0.6$\times$0.9 to 1.5$\times$2.2\,kpc$^2$ -- at a spatial resolution ranging from 110 to 280 pc with a spectral coverage of 4300 -- 7100\,Å and velocity resolution of $\approx$ 90\,km\,s$^{-1}$. The gas excitation is Seyfert like everywhere but show excitation, but show excitation gradients that are correlated with the gas kinematics, reddening and/or the gas density. The gas kinematics show in all cases two components: a rotation one similar to that observed in the stellar velocity field, and an outflow component. In the case of Mrk607, the gas is counter-rotating relative to the stars. Enhanced gas velocity dispersion is observed in association to the outflows according to two patterns: at the locations of the highest outflow velocities along the ionization axis or perpendicularly to it in a strip centered at the nucleus that we attribute to an equatorial outflow. Bipolar outflows are observed in Mrk\,348 and Mrk\,79, while in Mrk\,1058 only the blueshifted part is clearly observed, while in the cases of Mrk\,6 and Mrk\,607 the geometry of the outflow needs further constraints from modeling to be presented in a forthcoming study, where the mass flow rate and powers will also be obtained.

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arXiv2017

The ALMA Phasing System: A Beamforming Capability for Ultra-High-Resolution Science at (Sub)Millimeter Wavelengths

The Atacama Millimeter/submillimeter Array (ALMA) Phasing Project (APP) has developed and deployed the hardware and software necessary to coherently sum the signals of individual ALMA antennas and record the aggregate sum in Very Long Baseline Interferometry (VLBI) Data Exchange Format. These beamforming capabilities allow the ALMA array to collectively function as the equivalent of a single large aperture and participate in global VLBI arrays. The inclusion of phased ALMA in current VLBI networks operating at (sub)millimeter wavelengths provides an order of magnitude improvement in sensitivity, as well as enhancements in u-v coverage and north-south angular resolution. The availability of a phased ALMA enables a wide range of new ultra-high angular resolution science applications, including the resolution of supermassive black holes on event horizon scales and studies of the launch and collimation of astrophysical jets. It also provides a high-sensitivity aperture that may be used for investigations such as pulsar searches at high frequencies. This paper provides an overview of the ALMA Phasing System design, implementation, and performance characteristics.

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arXiv2007

XMM-Newton Detection of a Compton-thick AGN in the 1-Jy ULIRG/LINER F04103-2838

We report on the detection of Fe Kalpha emission in F04103$-$2838, an ultraluminous infrared galaxy (ULIRG; log[L$_{\rm IR}$/L$_\odot$] $\ge$ 12) that is optically classified as a LINER. Previous {\it Chandra} observations suggested the presence of both a starburst and an AGN in this source. A deeper ($\sim$20 ksec) {\it XMM-Newton} spectrum reveals an Fe Kalpha line at rest frame energy $\sim$6.4 keV, consistent with cold neutral iron. The best-fit spectral model indicates the Fe Kalpha line has an equivalent width of $\sim$1.6 keV. The hard X-ray emission is dominated by a Compton-thick AGN with intrinsic 0.2--10 keV luminosity $\sim10^{44}$ ergs s$^{-1}$, while the soft X-ray emission is from $\sim$0.1 keV gas attributed to the starburst. The X-ray spectrum of this source bears a striking resemblance to that of the archetypal luminous infrared galaxy NGC 6240 despite differences in merger state and infrared properties.

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arXiv2005

A Chandra X-Ray Survey of Ultraluminous Infrared Galaxies

We present results from Chandra observations of 14 ultraluminous infrared galaxies (ULIRGs; log(L_IR/L_Sun) >= 12) with redshifts between 0.04 and 0.16. The goals of the observations were to investigate any correlation between infrared color or luminosity and the properties of the X-ray emission and to attempt to determine whether these objects are powered by starbursts or active galactic nuclei (AGNs). The sample contains approximately the same number of high and low luminosity objects and ``warm'' and ``cool'' ULIRGs. All 14 galaxies were detected by Chandra. Our analysis shows that the X-ray emission of the two Seyfert 1 galaxies in our sample are dominated by AGN. The remaining 12 sources are too faint for conventional spectral fitting to be applicable. Hardness ratios were used to estimate the spectral properties of these faint sources. The photon indices for our sample plus the Chandra-observed sample from Ptak et al.(2003) peak in the range of 1.0-1.5, consistent with expectations for X-ray binaries in a starburst, an absorbed AGN, or hot bremsstrahlung from a starburst or AGN. The values of photon index for the objects in our sample classified as Seyferts (type 1 or 2) are larger than 2, while those classified as HII regions or LINERs tend to be less than 2. The hard X-ray to far-infrared ratios for the 12 weak sources are similar to those of starbursts, but we cannot rule out the possibility of absorbed, possibly Compton-thick, AGNs in some of these objects. Two of these faint sources were found to have X-ray counterparts to their double optical and infrared nuclei.

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arXiv2005

New H2O masers in Seyfert and FIR bright galaxies

Extragalactic water vapor masers with 50, 1000, 1, and 230 solar (isotropic) luminosities were detected toward Mrk1066 (UGC2456), Mrk34, NGC3556 and Arp299, respectively. The interacting system Arp299 appears to show two maser hotspots separated by 20 arcsec. A statistical analysis of 53 extragalactic H2O sources indicates (1) that the correlation between IRAS Point Source and H2O luminosities, established for individual star forming regions in the galactic disk, also holds for AGN dominated megamaser galaxies, (2) that maser luminosities are not correlated with 60/100 micron color temperatures and (3) that only a small fraction of the luminous megamasers detectable with 100-m sized telescopes have so far been identified. The slope of the H2O luminosity function, -1.5, indicates that the number of detectable masers is almost independent of their luminosity. If the LF is not steepening at very high maser luminosities, H2O megamasers at significant redshifts should be detectable with present day state-of-the-art facilities.

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arXiv2004

Radio Luminosity Function, Importance of Jet Power, and Radio Properties of Nearby Low-Luminosity Active Galactic Nuclei

We present the completed results of a high resolution radio imaging survey of all (around 200) low-luminosity active galactic nuclei (LLAGNs) in the Palomar spectroscopic Sample of all (around 470) nearby bright northern galaxies. The high incidence of pc-scale radio nuclei, with implied brightness temperatures of the order of 10E7 K, and sub-parsec jets argue for accreting black holes in 50% of all LLAGNs; there is no evidence against all LLAGNs being mini-AGNs.The detected parsec-scale radio nuclei are preferentially found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Halpha emission). The radio luminosity function (RLF) of Palomar Sample LLAGNs extends three orders of magnitude below, and is continuous with, that of "classical" AGNs.We find marginal evidence for a low-power turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of accretion in the local universe. The accretion energy output in LLAGNs is dominated by the energy in the observed jets rather than the radiated bolometric luminosity. The Palomar LLAGNs follow the same scaling between jet power and narrow line region (NLR) luminosity as the parsec to kilo-parsec jets in powerful radio galaxies. Low accretion rates (of 10E-2 to 10E-6 of the Eddington rate) are impliedin both advection- and jet-type models, with evidence for increasing "radio-loudness" with decreasing Eddington fraction. The jets are energetically more significant than supernovae in the LLAGN host galaxies, and are potentially able to dump sufficient energy into the innermost parsecs to significantly slow the accretion inflow. Detailed results can be found in Nagar et al. (2002a) and Nagar et al. (2004, to appear in Astronomy & Astrophysics).

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arXiv2004

Compact radio cores: from the first black holes to the last

One of the clearest signs of black hole activity is the presence of a compact radio core in the nuclei of galaxies. With the Square Kilometer Array (SKA) these cores can be used to study the evolution of black holes throughout the universe and even to detect the very first generation of supermassive black holes. We start by introducing some of the basic properties of compact radio cores and how they scale with accretion power. The relative contribution of jets and radio cores to the Spectral Energy Distribution (SED) is strongest in sub-Eddington black holes but also present in the most luminous objects. Radio and X-rays are correlated as a function of black hole mass such that the most massive black holes are most suited for radio detections. We present a radio core luminosity function for the present universe down to the least luminous AGN. The SKA will essentially detect all dormant black holes in the local universe, such as that in our Milky Way, out to several tens of Megaparsecs. It will also be able to see black holes in the making at redshifts z>10 for black hole masses larger than 10^7 M_sun. Finally, we suggest that the first generation of black holes may have jets that are frustrated in their dense environment and thus appear as Gigahertz-Peaked-Spectrum (GPS) sources. Since their intrinsic size and peak frequency are related and angular size and frequency scale differently with redshift, there is a unique region in parameter space that should be occupied by emerging black holes in the epoch of reionization. This can be well probed by radio-only methods with the SKA. (abridged)

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arXiv2004

H2O megamasers: Accretion disks, jet interaction, outflows or massive star formation?

The 25 years following the serendipitous discovery of megamasers have seen tremendous progress in the study of luminous extragalactic H$_2$O emission. Single-dish monitoring and high resolution interferometry have been used to identify sites of massive star formation, to study the interaction of nuclear jets with dense molecular gas and to investigate the circumnuclear environment of active galactic nuclei (AGN). Accretion disks with radii of 0.1--3 pc were mapped and masses of nuclear engines of order 10$^{6}$--10$^{8}$ M$_{\odot}$ were determined. So far, $\sim$50 extragalactic H$_2$O maser sources have been detected, but few have been studied in detail.

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arXiv2004

Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI

We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galaxies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their structure and proper motions on pc scales and to add some constraints on the many possible causes of the radio-quietness of Seyferts. The component configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas the configurations in NGC 5506 and Mrk 1210 have multiple components with no clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact symmetric object. Comparing the images at different epochs, we find a proper motion in NGC 7674 of (0.92+-0.07) c between the two central components separated by 282 pc and, in NGC 5506, we find a 3 sigma upper limit of 0.50 c for the components separated by 3.8 pc. Our results confirm and extend earlier work showing that the outward motion of radio components in Seyfert galaxies is non-relativistic on pc scales. We briefly discuss whether this non-relativistic motion is intrinsic to the jet-formation process or results from deceleration of an initially relativistic jet by interaction with the pc or sub-pc scale interstellar medium. We combined our sample with a list compiled from the literature of VLBI observations made of Seyfert galaxies, and found that most Seyfert nuclei have at least one flat-spectrum component on the VLBI scale, which was not seen in the spectral indices measured at arcsec resolution. We found also that the bimodal alignment of pc and kpc radio structures displayed by radio galaxies and quasars is not displayed by this sample of Seyferts, which shows a uniform distribution of misalignment between 0 deg and 90 deg. The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium.

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arXiv2004

Further Clues to the Nature of Composite LINER/HII Galaxies

We have analyzed new, archival and published high resolution radio and X-ray observations of a sample of composite LINER/HII galaxies known to exhibit AGN-like properties. Five of the 16 AGN candidates have milliarcsecond-scale detections and are found to display a compact, flat spectrum, high brightness temperature radio core, four of which also exhibit extended radio emission. Five of the eight AGN candidates with available high resolution X-ray observations were found to possess a hard X-ray nuclear source, two of which have no milliarcsecond radio detection. The combined high resolution radio and X-ray data yield a 50% detection rate of low luminosity AGN among the AGN candidates, which translates into a 12% detection rate for the entire composite LINER/HII sample. In the sources where the AGN has been unambiguously detected, the ionizing power of the AGN is not sufficient to generate the observed emission lines, unless the hard X-rays are heavily obscured. We attempt to apply a canonical advection-dominated accretion flow (ADAF) and jet model to the sample sources in order to explain the observed radio and X-ray emission. While ADAFs may be responsible for the observed emission in submillijansky radio cores like NGC 7331, they do not appear consistent with the radio emission observed in the milliarcsecond-scale radio detected cores; the latter sources are more likely to have an energetically important contribution from a radio-emitting jet.

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arXiv2003

Elusive Active Galactic Nuclei

A fraction of active galactic nuclei do not show the classical Seyfert-type signatures in their optical spectra, i.e. they are optically "elusive". X-ray observations are an optimal tool to identify this class of objects. We combine new Chandra observations with archival X-ray data in order to obtain a first estimate of the fraction of elusive AGN in local galaxies and to constrain their nature. Our results suggest that elusive AGN have a local density comparable to or even higher than optically classified Seyfert nuclei. Most elusive AGN are heavily absorbed in the X-rays, with gas column densities exceeding 10^24 cm^-2, suggesting that their peculiar nature is associated with obscuration. It is likely that in elusive AGN, the nuclear UV source is completely embedded and the ionizing photons cannot escape, which prevents the formation of a classical Narrow Line Region. Elusive AGN may contribute significantly to the 30 keV bump of the X-ray background.

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arXiv2003

The infrared supernova rate in starburst galaxies

We report the results of our ongoing search for extincted supernovae (SNe) at near-infrared wavelengths. We have monitored at 2.2 micron a sample of 46 Luminous Infrared Galaxies and detected 4 SNe. The number of detections is still small but sufficient to provide the first estimate of supernova rate at near-infrared wavelengths. We measure a SN rate ofv 7.6+/-3.8 SNu which is an order of magnitude larger than observed in quiescent galaxies. On the other hand, the observed near-infrared rate is still a factor 3-10 smaller than that estimated from the far-infrared luminosity of the galaxies. Among various possibilities, the most likely scenario is that dust extinction is so high (Av>30) to obscure most SNe even in the near-IR. The role of type Ia SNe is also discussed within this context. We derive the type Ia SN rate as a function of the stellar mass of the galaxy and find a sharp increase toward galaxies with higher activity of star formation. This suggests that a significant fraction of type Ia SNe are associated with young stellar populations. Finally, as a by-product, we give the average K-band light curve of core-collapse SNe based on all the existing data, and review the relation between SN rate and far-infrared luminosity.

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arXiv2002

Warm-hot intergalactic baryons revealed

Several popular cosmological models predict that most of the baryonic mass in the local universe is located in filamentary and sheet-like structures associated with galaxy overdensities. This gas is expected to be gravitationally heated to ~10^6 K and therefore emitting in the soft X-rays. We have detected diffuse soft X-ray structures in a high Galactic latitude ROSAT field after point source subtraction and correction for Galactic absorption. These diffuse structures have an X-ray energy distribution that is much softer than expected from clusters, groups or unresolved emission from AGNs, but are consistent with that expected from a diffuse warm intergalactic medium. To discriminate between a Galactic or extragalactic nature of the diffuse gas we have correlated the soft X--map with multiband optical images in this field. We have found a significant overdensity of galaxies in correspondence with the strongest diffuse X-ray structure. The photometric redshift distribution of the galaxies over the X-ray peak has an excess over field galaxies at z~0.45. This result strongly suggests that the diffuse X-ray flux is due to extragalactic emission by warm gas associated with an overdense galaxy region at z~0.45.

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arXiv2002

NGC 5506 Unmasked as a Narrow Line Seyfert 1: A Direct View of the Broad Line Region using Near-IR Spectroscopy

This letter presents incontrovertible evidence that NGC5506 is a Narrow Line Seyfert 1 (NLSy1). Our new 0.9-1.4 micron spectrum of its nucleus clearly shows the permitted OI 1.1287 micron line (with full width at half maximum <2000 km/s) and the `1 micron FeII lines'. These lines can only originate in the optically-thick broad line region (BLR) and, among Seyfert nuclei the latter series of lines are seen only in NLSy1s. The obscuration to the BLR, derived from a rough estimate of the OI 1.1287 micron / OI 8446 Angstrom ratio and from the reddening of the near-IR Paschen lines, is A_v > 5. Together, these results make NGC5506 the first identified case of an optically-obscured NLSy1. This new classification helps explain its radio to X-ray properties, which until now were considered highly anomalous. However, interesting new concerns are raised: e.g., NGC5506 is unusual in hosting both a `type 1' AGN and a nuclear water vapor megamaser. As the brightest known NLSy1, NGC5506 is highly suitable for study at wavebands less affected by obscuration.

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arXiv2002

Radio Sources in Low-Luminosity Active Galactic Nuclei. III. "AGNs" in a Distance-Limited Sample of "LLAGNs"

(abbreviated): This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at D<19Mpc. We find that almost half of all LINERs and low-luminosity Seyferts have flat-spectrum radio cores when observed at 150mas resolution. Higher (2mas) resolution observations of a flux-limited subsample have provided a 100% (16 of 16) detection rate of pc-scale radio cores, with implied brightness temperatures > 10^8 K. The five LLAGNs with the highest core radio fluxes also have pc-scale `jets.' Compact radio cores are almost exclusively found in massive ellipticals and in type1 nuclei. The core radio power is correlated with the nuclear optical `broad' Halpha luminosity, the nuclear optical `narrow' emission line luminosity and width, and with the galaxy luminosity. In these correlations LLAGNs fall close to the low-luminosity extrapolations of more powerful AGNs. About half of all LLAGNs with multiple epoch data show significant inter-year radio variability. Investigation of a sample of ~150 nearby bright galaxies, most of them LLAGNs, shows that the nuclear (<150mas size) radio power is strongly correlated with both the black hole mass and the galaxy bulge luminosity; linear regression fits to all ~150 galaxies give: log P(2cm) = 1.31 log M_blackhole + 8.77 and log P(2cm) = 1.89 log L_B(bulge) - 0.17. Low accretion rates are implied in both advection- and jet-type models. In brief, all evidence points towards the presence of accreting massive black holes in a large fraction, perhaps all, of LLAGNs.

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