arXiv subjects

T. Storchi-Bergmann

Publications and source records attributed to T. Storchi-Bergmann.

At least 73 records · Page 4Linked to original sources

arXiv2005

An atlas of Calcium triplet spectra of active galaxies

We present a spectroscopic atlas of active galactic nuclei covering the region around the 8498, 8542, 8662 Calcium triplet (CaT) lines. The sample comprises 78 objects, divided into 43 Seyfert 2s, 26 Seyfert 1s, 3 Starburst and 6 normal galaxies. The spectra pertain to the inner ~300 pc in radius, and thus sample the central kinematics and stellar populations of active galaxies. The data are used to measure stellar velocity dispersions (sigma_star) both with cross-correlation and direct fitting methods. These measurements are found to be in good agreement with each-other and with those in previous studies for objects in common. The CaT equivalent width is also measured. We find average values and sample dispersions of W_CaT of 4.6+/-2.0, 7.0 and 7.7+/-1.0 angstrons for Seyfert 1s, Seyfert 2s and normal galaxies, respectively. We further present an atlas of [SIII]λ9069 emission line profiles for a subset of 40 galaxies. These data are analyzed in a companion paper which addresses the connection between stellar and Narrow Line Region kinematics, the behaviour of the CaT equivalent width as a function of sigma_star, activity type and stellar population properties.

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arXiv2004

The Stellar Populations of Low Luminosity Active Galactic Nuclei. III: Spatially Resolved Spectral Properties

In a recent survey of the stellar populations of LINERS and LINER/HII Transition Objects (TOs) we identified a numerous class of nuclei which stand out because of their conspicuous 10^8-9 yr populations. These objects were called ``Young-TOs'', since they all have TO-like emission line ratios. In this paper we investigate the radial variations of spectral properties in Low Luminosity AGN. Our analysis is based on high S/N, 3500-5500 A, long-slit spectra for 47 galaxies. The data probe distances of typically up to 850 pc from the nucleus with a resolution of ~ 100 pc and S/N ~ 30. Stellar population gradients are mapped by the radial profiles of absorption line equivalent widths and colours along the slit. These variations are further analyzed by means of a decomposition of each spectrum in terms of template galaxies representative of very young (<= 10^7 yr), intermediate age (10^8-9 yr) and old (10^10 yr) populations. Our main findings are: (1) Significant stellar population gradients are found almost exclusively in Young-TOs. (2) The intermediate age population of Young-TOs, although heavily concentrated in the nucleus, reaches distances of up to a few hundred pc. (3) Objects with predominantly old populations present spatially homogeneous spectra, be they LINERs or TOs. (4) Young-TOs have much more dust in their central regions than other LLAGN. (5) The B-band luminosities of the central <~ 1 Gyr population in Young-TOs are within an order of magnitude of M_B = -15, implying masses of order ~ 10^7-10^8 Mo. This population was 10-100 times more luminous in its formation epoch, at which time young massive stars would have completely outshone any active nucleus, unless the AGN too was brighter in the past. (abridged)

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arXiv2004

The Ionizing Source of the Nucleus of NGC1097

We present new observations in X-ray and optical/ultraviolet of the nucleus of NGC1097, known for the abrupt appearance of broad, double-peaked Balmer lines in its spectrum in 1991. These new observations are used to construct the spectral energy distribution (SED) of the central engine. From the SED we infer that this AGN is radio-loud and has a bolometric luminosity L_Bol ~ 10^42 erg/s, implying a low Eddington ratio of L_Bol/L_Edd ~ 10^{-4}. These results suggest that the central ionizing source is an advection-dominated accretion flow (ADAF) in the form of an ellevated structure which photoionizes an outer thin disk. We fit a simplified ADAF model to the SED and obtain limits on the values of the mass accretion rate Mdot and accretion efficiency η, namely Mdot/Mdot_Edd >= 10^{-3} and η<= 10^{-2}. We identify an energy budget problem: if the central photoionizing source is isotropic, the covering factor of the line-emitting portion of the thin accretion disk is ~ 6, i. e. the central source accounts for only 20% of the energy emitted in the double-peaked Balmer lines.

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arXiv2004

The Ultraviolet Spectra of Active Galaxies With Double-Peaked Emission Lines

We present the results of UV spectroscopy of AGNs with double-peaked Balmer emission lines. In 2/3 of the objects, the far-UV resonance lines are strong, with single-peaked profiles resembling those of Seyfert galaxies. The Mg II line is the only UV line with a double-peaked profile. In the remaining objects, the far-UV resonance lines are relatively weak but still single-peaked. The latter group also displays prominent UV absorption lines, indicative of a low-ionization absorber. We interpret the difference in the profiles of the emission lines as resulting from two different regions: a dense, low-ionization accretion disk (the predominant source of the Balmer and Mg II lines), and a lower density, higher-ionization wind (the predominant source of the far-UV resonance lines). These results suggest a way of connecting the double-peaked emitters with the greater AGN population: in double-peaked emitters the accretion rate onto the black hole is low, making the wind feeble and allowing the lines from the underlying disk to shine through. This scenario also implies that in the majority of AGNs, the wind is the source of the broad emission lines.

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arXiv2003

Evolution of the Nuclear Accretion Disk Emission in NGC 1097: Getting Closer to the Black Hole

We study the evolution of the broad, double-peaked Halpha emission-line profile of the LINER/Seyfert 1 nucleus of NGC 1097, using 24 spectra obtained over a time span of 11 yrs - from 1991 Nov. through 2002 Oct. While in the first 5 yrs the main variation was in the relative intensity of the blue and red peaks, in the last years we have also observed an increasing separation between the two peaks, at the same time as the integrated flux in the broad line has decreased. We propose a scenario in which the emission originates in an asymmetric accretion disk around a supermassive black hole, whose source of ionization is getting dimmer, causing the region of maximum emission to come closer to the center (and thus to regions of higher projected velocity). We use the observations to constrain the evolution of the accretion disk emission and to evaluate two models: the elliptical disk model previously found to reproduce the observations from 1991 to 1996 and a model of a circular disk with a single spiral arm. We favor the latter, because the whole set of data is consistent with a monotonic precession of the spiral pattern, which has completed almost two revolutions since 1991. The precession period implies a black hole mass of approximately 5x10^7 solar masses. Finally, we have found tentative evidence of the emergence of an accretion disk wind, which we hope to explore further with future observations.

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arXiv2002

Evolution of the accretion disk in the nucleus of NGC 1097

We discuss the long-term evolution of the broad double-peaked H alpha profile of the LINER/Seyfert 1 nucleus of NGC 1097. Besides the previously known variation of the relative intensities of the blue and red peaks, the profile has recently shown an increasing separation between the two peaks, at the same time as the integrated flux has decreased. We successfully model these variations using a precessing asymmetric accretion disk with a varying emissivity law. We interpret the emissivity variation as due to the fact that the source of ionization is getting dimmer, causing the region of maximum emission to drift inwards (and thus to regions of higher velocities). In addition, in the last 3 yrs of observations, the central wavelength of the double-peaked line has shifted to bluer wavelengths, which may be due to a wind from the disk. It is the first time that such evolution is observed so clearly, giving additional support for an accretion disk as the origin of the double-peaked profile in NGC 1097.

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arXiv2002

Stellar population gradients in Seyfert 2 galaxies. Northern sample

We use high signal-to-noise ratio long-slit spectra in the 3600-4700A range of the twenty brightest northern Seyfert 2 galaxies to study the variation of the stellar population properties as a function of distance from the nucleus. In order to characterize the stellar population and other continuum sources (e.g. featureless continuum FC) we have measured equivalent widths Ws of six absorption features, four continuum colours and their radial variations, and performed spectral population synthesis as a function of distance from the nucleus. About half the sample has CaIIK and G-band W values smaller at the nucleus than at 1 kpc from it, due to a younger population and/or FC. The stellar population synthesis shows that, while at the nucleus, 75% of the galaxies present contribution > 20% of ages younger or equal than 100Myr and/or of a FC, this proportion decreases to 45% at 3 kpc. In particular, 55% of the galaxies have contribution > 10% of the 3 Myr/FC component (a degenerate component in which one cannot separate what is due to a FC or to a 3 Myr stellar population) at the nucleus, but only 25% of them have this contribution at 3 kpc. As reference, the stellar population of 10 non-Seyfert galaxies, spanning the Hubble types of the Seyfert (from S0 to Sc) was also studied. A comparison between the stellar population of the Seyferts and that of the non-Seyferts shows systematic differences: the contribution of ages younger than 1 Gyr is in most cases larger in the Seyfert galaxies than in non-Seyferts, not only at the nucleus but up to 1 kpc from it.

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arXiv2001

Circumnuclear stellar population, morphology and environment of Seyfert 2 galaxies: an evolutionary scenario

We investigate the relation between the characteristics of the circumnuclear stellar population and both the galaxy morphology and the presence of close companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present unambiguous signatures of recent episodes of star formation within $\approx$300 pc from the nucleus. When we relate this property with the Hubble type of the host galaxy, we find that the incidence of recent circumnuclear star formation increases along the Hubble sequence, and seems to be larger than in non-Seyfert galaxies for the early Hubble types S0 and Sa, but similar to that in non-Seyfert galaxies for later Hubble types. Both in early-type and late-type Seyferts, the presence of recent star-formation is related to the galaxy morphology in the inner few kiloparsecs, as observed in HST images through the filter F606W by Malkan et al., who has assigned a late ``inner Hubble type'' to most Seyfert 2s with recent nuclear star-formation. This new classification is due to the presence of dust lanes and spiral structures in the inner region. The presence of recent star formation in Seyfert 2 nuclei is also related to interactions: among the 13 galaxies of the sample with close companions or in mergers, 9 have recent star formation in the nucleus. These correlations between the presence of companions, inner morphology and the incidence of recent star formation suggest an evolutionary scenario in which the interaction is responsible for sending gas inwards which both feeds the AGN and triggers star-formation. The starburst then fades with time and the composite Seyfert 2 + Starburst nucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellar population.

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arXiv2001

The Obscuring Starburst of NGC 6221 and Implications for the Hard X-ray Background

We present NGC 6221 as a case study of ``X-ray loud composite galaxies,'' which appear similar to starbursts at optical wavelengths and resemble traditional active galactic nuclei in X-rays. The net optical spectrum of NGC 6221 is properly characterized as a starburst galaxy, but in X-rays, NGC 6221 is similar to Seyfert 1 galaxies, exhibiting a power-law continuum spectrum, a broad Fe K alpha line, and continuum variability on timescales of days and years. High-resolution images reveal that the detected active nucleus is relatively weak, not only at optical, but also at near-infrared wavelengths. An obscuring starburst, in which the interstellar gas and dust associated with the starburst conceal the active nucleus, accounts for these peculiar features. We demonstrate quantitatively that obscuration by column density N_H = 10^{22} cm^{-2} combined with relatively weak intrinsic nuclear activity can produce an optical spectrum characteristic of the surrounding starburst alone. While optical surveys would not identify the active nuclei that make these galaxies significant X-ray sources, such galaxies may in fact be important contributors to the X-ray background.

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arXiv2001

Empirical diagnostics of the starburst-AGN connection

We examine a representative sample of 35 Seyfert 2 nuclei. Previous work has shown that nearly 1/2 of these nuclei show the direct (but difficult-to-detect) spectroscopic signatures at optical-UV wavelengths of the hot massive stars that power circum-nuclear starbursts. In this paper we examine a variety of more-easily-measured quantities for this sample, such as the equivalent widths of strong absorption features, continuum colors, emission line equivalent widths, ratios and profiles, far-IR luminosities and near-UV surface brightness. We compare the composite starburst+Seyfert 2 nuclei to ``pure'' Seyfert 2's, Starburst galaxies and normal galactic nuclei. Our goals are to verify whether these properties in composite nuclei are consistent with the expected impact of a starburst, and to investigate alternative less-demanding methods to infer the presence of starbursts in Seyfert 2's, applicable to larger or more distant samples. We show that starbursts do indeed leave clear and easily quantifiable imprints on the near-UV to optical continuum range. Composite starburst+Seyfert 2 systems can be recognized by: (1) a strong ``Featureless Continuum'' which dilutes the CaII K line from the host's bulge to W_K < 10 A; (2) emission lines whose equivalent widths are intermediate between Starburst galaxies and ``pure'' Seyfert 2's; (3) relatively low excitation line-ratios, which indicate that part of the gas ionization in these Seyfert 2's (typically \sim 50% of Hbeta) is due to photoionization by OB stars; (4) large far IR luminosities (> 10^10 Lsun); (5) High near-UV surface brightness (~10^3 Lsun/pc^2). These characteristics are all consistent with the expected impact of circum-nuclear starbursts on the observed properties of Seyfert 2's. (abridged)

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arXiv2000

Hubble Space Telescope Imaging of the Circinus Galaxy

We present an HST imaging study of the nearby (4 Mpc distant) Circinus galaxy, which contains the nearest type 2 Seyfert nucleus and prominent circumnuclear star formation. Images have been obtained in the [OIII]$λ$5007, H$α$ and H$_2$ v=1--0 S(1) emission lines, and in the green (5470 Å), red (8140 Å) and near infrared (2.04 and 2.15 $μ$m) continua. An image in the [Fe II] $λ$1.644 $μ$m line has been taken with a ground-based telescope. The [OIII] and H$α$ images reveal the detailed structure of the complex of streamers and knots of high-excitation gas, which extends out of the galaxy disk. The morphology some 250 pc from the nucleus strongly suggests that the high-excitation gas is concentrated on the surface of a hollow cone with apex close to the nucleus. Such a structure may result through entrainment of dense gas from a circumnuclear torus in the galaxy disk by a low density, outflowing wind or jet. Within 40 pc of the nucleus, the high-excitation gas takes the form of a striking, filled V-shaped structure extending in the same direction as the larger scale high-excitation emission. This `V' can be described as an ionization cone, though a matter-bounded structure is also possible. The implied collimation of the ionizing photons or gaseous outflow must occur within 2 pc of the apex of the cone, presumed to be the location of the nucleus. The H$α$ image shows a complex structure of HII regions, including the well known starburst ring of radius 150 - 270 pc. In addition, there is a more compact (40 pc radius), elliptical ring of HII regions around the `ionization cone'. We argue that this latter ring, which we call the nuclear ring, is intrinsically circular and located in the plane of the galaxy disk. Abstract truncated.

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arXiv2000

Gas properties of HII and Starburst galaxies: relation with the stellar population

We study the gas emission of galaxies with active star formation, consisting mostly of HII galaxies and Starbursts, as well as some Seyfert 2s, and determine chemical and physical parameters. The data consist of 19 high signal-to-noise ratio optical templates, a result of grouping 185 emission line galaxy spectra. Underlying stellar population models were subtracted from the templates in order to isolate the pure emission component. We analyse the distribution of these improved signal-to-noise ratio emission spectra in diagnostic diagrams and find that the HII templates show a smaller spread in log([OIII]/Hβ) values than the individual galaxies, apparently due to the population subtraction and better signal-to-noise ratio. We thus suggest the template sequence as a fiducial observational locus for HII galaxies which can be used as reference for models. The sequence of line ratios presented by the HII galaxies in the diagram log([OIII}]\lambda5007/Hβ) X log([NII]\lambda6584/Hα) is primarily due to the gas metallicity, of which the log([NII]/Hα) ratio is a direct estimator. We also study the properties of the Starburst galaxies and those intermediate between HII and Starburst galaxies, which are more metal rich and sit on more massive galaxies. We discuss the present results in the frame of a recently proposed equivalent width diagnostic diagram for emission line galaxies and conclude that the observed ranges in W([OII])/W(Hβ) and W(Hβ) are mostly due to the non-ionizing stellar population contribution. W(Hβ) can be used as an estimator of this contribution to the continuum, and briefly discuss implications to the cosmological use of HII galaxies.

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arXiv2000

Population synthesis of HII galaxies

We study the stellar population of galaxies with active star formation, determining ages of the stellar components by means of spectral population synthesis of their absorption spectra. The data consist of optical spectra of 185 nearby ($z \leq 0.075$) emission line galaxies. They are mostly HII galaxies, but we also include some Starbursts and Seyfert 2s, for comparison purposes. They were grouped into 19 high signal-to-noise ratio template spectra, according to their continuum distribution, absorption and emission line characteristics. The templates were then synthesized with a star cluster spectral base. The synthesis results indicate that HII galaxies are typically age-composite stellar systems, presenting important contribution from generations up to as old as 500 Myr. We detect a significant contribution of populations with ages older than 1 Gyr in two groups of HII galaxies. The age distributions of stellar populations among Starbursts can vary considerably despite similarities in the emission line spectra. In the case of Seyfert 2 groups we obtain important contributions of old population, consistent with a bulge. From the diversity of star formation histories, we conclude that typical HII galaxies in the local universe are not systems presently forming their first stellar generation.

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arXiv1999

The Dust Content and Opacity of Actively Star-Forming Galaxies

(Abridged) We present far-infrared (FIR) photometry at 150 micron and 205 micron of eight low-redshift starburst galaxies obtained with the ISO Photometer. Five of the eight galaxies are detected in both wavebands and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda>40 micron. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T~40-55 K (warm dust) and T~20-23 K (cool dust), and with a dust emissivity index epsilon=2. The cool dust can be a major contributor to the FIR emission of starburst galaxies, representing up to 60% of the total flux. This component is heated not only by the general interstellar radiation field, but also by the starburst itself. The cool dust mass is up to ~150 times larger than the warm dust mass, bringing the gas-to-dust ratios of the starbursts in our sample close to Milky Way values, once rescaled for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 micron and the IRAS FIR emission in the range 40-120 micron is ~1.75, with small variations from galaxy to galaxy. The FIR emission predicted by the dust reddening of the UV-to-nearIR stellar emission is within a factor ~2 of the observed value in individual galaxies and within 20% when averaged over a large sample. If our sample of local starbursts is representative of high-redshift (z>1), UV-bright, star-forming galaxies, these galaxies' FIR emission will be generally undetected in sub-mm surveys, unless (1) their bolometric luminosity is comparable to or larger than that of ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.

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arXiv1998

Extended Gas in Seyfert Galaxies: Near Infrared Observations of NGC 2110 and Circinus

We present results of near--IR long-slit spectroscopy in the J and K bands of the Seyfert 2 galaxies NGC 2110 and Circinus, investigating the gaseous distribution, excitation, reddening and kinematics. In NGC 2110, the emission line ratio [FeII]/Pa beta increases towards the nucleus (to ~ 7). The nuclear [Fe II]1.257 (microns) and Pa beta lines are broader (FWHM ~ 500 km/s) than the H2 (2.121) line (FWHM ~ 300 km/s). Both these results suggest that shocks, driven by the radio jet, are an important source of excitation of [Fe II]. The H2 excitation appears to be dominated by X-rays from the nucleus. In Circinus, both [FeII]/Pa beta and H2/Br gamma decrease from ~ 2 at 4 arcsec from the nucleus to nuclear values of ~ 0.6 and ~ 1, respectively, suggesting that the starburst dominates the nuclear excitation, while the AGN dominates the excitation further out (r > 2 arcsec). For both galaxies, the gaseous kinematics are consistent with circular rotation in the plane of the disk. Our rotation curves suggest that the nucleus (identified with the peak of the IR continuum) is displaced from the kinematic centre of the galaxies. This effect has been observed previously in NGC 2110 based on the kinematics of optical emission lines, but the displacement is smaller in the infrared, suggesting the effect is related to obscuration. The continuum J-K colours of the nuclear region indicate a red stellar population in NGC 2110 and a reddened young stellar population in Circinus. Right at the nucleus of both galaxies, the colours are redder, apparently a result of hot dust emission from the inner edge of a circumnuclear torus.

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arXiv1998

EUVE J0425.6-5714: A Newly Discovered AM Herculis Star

We detected a new AM Her star serendipitously in a 25 day observation with the EUVE satellite. A coherent period of 85.82 min is present in the EUVE Deep Survey imager light curve of this source. A spectroscopic identification is made with a 19th magnitude blue star that has H and He emission lines, and broad cyclotron humps typical of a magnetic cataclysmic variable. A lower limit to the polar magnetic field of 46 MG is estimated from the spacing of the cyclotron harmonics. EUVE J0425.6-5714 is also detected in archival ROSAT HRI observations spanning two months, and its stable and highly structured light curve permits us to fit a coherent ephemeris linking the ROSAT and EUVE data over a 1.3 yr gap. The derived period is 85.82107 +/- 0.00020 min, and the ephemeris should be accurate to 0.1 cycles until the year 2005. A narrow but partial X-ray eclipse suggests that this object belongs to the group of Am Her stars whose viewing geometry is such that the accretion stream periodically occults the soft X-ray emitting accretion spot on the surface of the white dwarf. A non-detection of hard X-rays from ASCA observations that are contemporaneous with the ROSAT HRI shows that the soft X-rays must dominate by at least an order of magnitude, which is consistent with a known trend among AM Her stars with large magnetic field. This object should not be confused with the Seyfert galaxy 1H 0419-577 (= LB 1727), another X-ray/EUV source which lies only 4' away, and was the principal target of these monitoring observations.

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arXiv1997

Dust and Recent Star Formation in the Core of NGC5253

(Abridged) Ultraviolet and optical narrow and broad band images of NGC5253 obtained with the HST WFPC2 are used to derive the properties of the dust distribution and the recent star formation history of this metal-poor dwarf galaxy. Corrections for the effects of dust are important in the center of NGC5253: dust reddening is markedly inhomogeneous across the galaxy's central 20" region. One of the most obscured regions coincides with the region of highest star formation activity in the galaxy: clouds of more than 9~magnitudes of optical depth at V enshroud a young (2.5~Myr old) stellar cluster in the region. Star formation has been active at least over the past 100 Myr in the core of the galaxy, as indicated by the age distribution of both the blue diffuse stellar population and the bright stellar clusters. The star formation is currently concentrated in a 6" region, about 5 Myr old and with a star formation intensity 10-100 times higher than the average within the central 20". The 2.5 Myr old cluster coincides with the peak intensity of the star formation; its mass may be as large as 10^6 solar masses, making this one a Super-Star-Cluster candidate.

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arXiv1997

Evidence For A Precessing Accretion Disk in the Nucleus of NGC 1097

We present new spectroscopic observations of the LINER (and now Seyfert 1) nucleus of NGC 1097, and discuss the evolution of its broad, double-peaked Balmer lines. When originally discovered in 1991, the red peak of the double-peaked H-alpha line was stronger than the blue, while by 1994 the H-alpha profile had become almost symmetric and the integrated line flux had decreased to half its original value. Our new spectrum, taken in 1996, shows that the broad, double-peaked lines have returned to almost their original strengths, the profiles of H-beta and H-alpha are identical to within errors, and the broad-line emitting region is unreddened. However, the profile of the Balmer lines is now such that the blue peak is stronger than the red, opposite to the asymmetry observed in 1991. Various models are considered for the observed behavior, all assuming that the emission lines originate in an accretion disk. We present a refined version of the precessing, planar, elliptical accretion ring model proposed by Storchi-Bergmann et al. and Eracleous et al. This model provides an acceptable fit to the line profiles. We also consider the possibility that the line profile evolution results from a precessing warp in the disk, induced by irradiation from the center, and show that the range of radii and precession time scales expected in this model are consistent with the observations. The sudden appearance of the "disk-like" broad line profiles in NGC 1097 could have resulted from the formation of a new accretion disk due to, for example, the tidal disruption of a star, or the illumination of a pre-existing disk by a transient ionizing source at the center of the disk.

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