arXiv subjects

Richard W. Pogge

Publications and source records attributed to Richard W. Pogge.

At least 235 records · Page 13Linked to original sources

arXiv2004

Chandra Observations of X-ray Weak Narrow-Line Seyfert 1 Galaxies

We present Chandra observations of 17 optically-selected, X-ray weak narrow-line Seyfert 1 (NLS1) galaxies. These objects were optically identified by Williams et al. (2002) in the Sloan Digital Sky Survey Early Data Release, but were not found in the ROSAT All-Sky Survey (RASS) despite having optical properties similar to RASS-detected NLS1s. All objects in this sample were detected by Chandra and exhibit a range of 0.5-2 keV photon indices Gamma=1.1-3.4. One object was not detected in the soft band, but has a best-fit Gamma=0.25 over the full 0.5-8 keV range. These photon indices extend to values far below what are normally observed in NLS1s. A composite X-ray spectrum of the hardest objects in this sample does not show any signs of absorption, although the data do not prohibit one or two of the objects from being highly absorbed. We also find a strong correlation between Gamma and L_1keV; this may be due to differences in L_bol/L_Edd among the NLS1s in this sample. Such variations are seemingly in conflict with the current paradigm that NLS1s accrete near the Eddington limit. Most importantly, this sample shows that strong, ultrasoft X-ray emission is not a universal characteristic of NLS1s; in fact, a substantial number may exhibit weak and/or low-Gamma X-ray emission.

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arXiv2003

Survey for Transiting Extrasolar Planets in Stellar Systems: I. Fundamental Parameters of the Open Cluster NGC 1245

We derive fundamental parameters for the old, rich open cluster NGC 1245 using BVI photometry from the MDM 1.3m and 2.4m telescopes. Based on detailed isochrone fitting, we find NGC 1245 has a slightly sub-solar metallicity, [Fe/H]=-0.05 +- 0.03 (statistical) +- 0.08 (systematic), and an age of 1.04 +- 0.02 +- 0.09 Gyr. In contrast to previous studies, we find no evidence for significant differential reddening. We determine an extinction of A_V=0.68 +- 0.02 +- 0.09 mag and a distance modulus of (m-M)o=12.27 +- 0.02 +- 0.12 mag, which corresponds to a distance of 2.8 +- 0.2 Kpc. We derive a logarithmic mass-function slope for the cluster of a=-3.12 +- 0.27, where a Salpeter slope is a=-1.35. Fits to the radial surface-density profile yield a core radius of r_c=3.10 +- 0.52 arcmin (2.57 +- 0.47 pc). NGC 1245 is highly relaxed and contains a strongly mass segregated population. The mass function for the inner cluster has a very shallow slope, b=-0.56 +- 0.28. In contrast, the outer periphery of the cluster is enriched with low mass members and devoid of high mass members out to the tidal radius, r_t=20 arcmin (16.5 pc). Based on the observed surface-density profile and an extrapolated mass function, we derive a total cluster mass, M=1300 +- 90 +- 170 M_(solar).

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arXiv2003

Stellar Populations in NGC 4038/39 (The Antennae): Exploring A Galaxy Merger Pixel-by-Pixel

We present deep, photometrically calibrated BVRJHK images of the nearby interacting galaxy pair NGC 4038/39 (``The Antennae''). Color maps of the images are derived, and those using the B, V, and K-bands are analyzed with techniques developed for examining the colors of stars. From these data we derive pixel-by-pixel maps of the distributions of stellar populations and dust extinction for the galaxies. Analysis of the stellar population map reveals two distinct episodes of recent star formation: one currently in progress and a second that occurred ~600 Myr ago. A roughly 15 Gyr-old population is found which traces the old disks of the galaxies and the bulge of NGC 4038. The models used successfully reproduce the locations of clusters, and the ages we derive are consistent with those found from previous Hubble Space Telescope observations of individual star clusters. We also find 5 luminous ``super star clusters'' in our K-band images that do not appear in the B or V-band images. These clusters are located in the overlap region between the two galaxies, and are hidden by dust with visual extinctions of A_V ~> 3 mag. The techniques we describe in this paper should be generally applicable to the study of stellar populations in galaxies for which detailed spatial resolution with Hubble is not possible.

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arXiv2002

Circumnuclear Dust in Nearby Active and Inactive Galaxies. I. Data

The detailed morphology of the interstellar medium (ISM) in the central kiloparsec of galaxies is controlled by pressure and gravitation. The combination of these forces shapes both circumnuclear star formation and the growth of the central, supermassive black hole. We present visible and near-infrared Hubble Space Telescope images and color maps of 123 nearby galaxies that show the distribution of the cold ISM, as traced by dust, with excellent spatial resolution. These observations reveal that nuclear dust spirals are found in the majority of active and inactive galaxies and they possess a wide range in coherence, symmetry, and pitch angle. We have used this large sample to develop a classification system for circumnuclear dust structures. In spite of the heterogeneous nature of the complete sample, we only find symmetric, two-arm nuclear dust spirals in galaxies with large scale bars and these dust lanes clearly connect to dust lanes along the leading edges of the large scale bars. Not all dust lanes along large scale bars form two arm spirals, however, and several instead end in nuclear rings. We find that tightly wound, or low pitch angle, nuclear dust spirals are more common in unbarred galaxies than barred galaxies. Finally, the extended narrow line region in several of the active galaxies is well-resolved. The connection between the ionized gas and circumnuclear dust lanes in four of these galaxies provides additional evidence that a significant fraction of their extended narrow line region is ambient gas photoionized in situ by the active nucleus. In a future paper, we will use our classification system for circumnuclear dust to identify differences between active and inactive galaxies, as well as barred and unbarred galaxies, in well-matched subsamples of these data.

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arXiv2002

Circumnuclear Dust in Nearby Active and Inactive Galaxies. II. Bars, Nuclear Spirals, and the Fueling of Active Galactic Nuclei

(Abridged) We present a detailed study of the relation between circumnuclear dust morphology, host galaxy properties, and nuclear activity in nearby galaxies. We use our sample of 123 nearby galaxies with visible--near-infrared colormaps from the Hubble Space Telescope to create well-matched, "paired" samples of 28 active and 28 inactive galaxies, as well as 19 barred and 19 unbarred galaxies, that have the same host galaxy properties. Comparison of the barred and unbarred galaxies shows that grand design nuclear dust spirals are only found in galaxies with a large-scale bar. Tightly wound nuclear dust spirals, in contrast, show a strong tendency to avoid galaxies with large-scale bars. Comparison of the AGN and inactive samples shows that nuclear dust spirals, which may trace shocks and angular momentum dissipation in the ISM, occur with comparable frequency in both active and inactive galaxies. The only difference between the active and inactive galaxies is that several inactive galaxies appear to completely lack dust structure in their circumnuclear region, while none of the AGN lack this structure. The comparable frequency of nuclear spirals in active and inactive galaxies, combined with previous work that finds no significant differences in the frequency of bars or interactions between well-matched active and inactive galaxies, suggests that no universal fueling mechanism for low-luminosity AGN operates at spatial scales greater than ~100 pc radius from the galactic nuclei. The similarities instead suggest that the lifetime of nuclear activity is less than the characteristic inflow time from these spatial scales. An order-of-magnitude estimate of this inflow time is the dynamical timescale. This sets an upper limit of several million years to the lifetime of an individual episode of nuclear activity.

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arXiv2002

Narrow-line Seyfert 1 Galaxies from the Sloan Digital Sky Survey Early Data Release

We present a sample of 150 narrow-line Seyfert 1s (NLS1s) found within the Sloan Digital Sky Survey Early Data Release (EDR), only two of which were previously identified as such. This substantially increases the known number of NLS1s, and provides a basic method by which to identify many more with subsequent releases of SDSS data. With its large size and homogeneous, well-defined selection criteria, this sample will help alleviate two major problems which have plauged NLS1 research in the past; namely, their relative rarity and significant differences in selection algorithms between the known samples. 45 of these SDSS-selected NLS1s are detected at energies of 0.1-2 keV in the ROSAT All-Sky Survey (RASS), and are found to have ultrasoft X-ray spectra with photon indices of Gamma >~ 2, in agreement with previous results for NLS1s. However, about 10-20 of those NLS1s that were not detected by ROSAT have optical properties very similar to the detected objects, and so should also have been detected by the RASS. This may be due to either significant intrinsic absorption in many NLS1s, or a significant sub-class of NLS1s that have uncharacteristic, intrinsically flatter (hence harder) X-ray spectral energy distributions.

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arXiv2002

Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS): The Frequency of Planets in NGC 1245

We present first results from the Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS). Our goal is to assess the frequency of close-in extrasolar planets around main-sequence stars in several open clusters. By concentrating on main-sequence stars in clusters of known (and varied) age, metallicity, and stellar density, we will gain insight into how these various properties affect planet formation, migration, and survival. We show preliminary results from our 19 night photometric campaign of the old, solar metallicity cluster NGC 1245. Taking into account the photometric precision, observational window function, transit probability, and total number of stars monitored, we estimate that we should be able to probe planetary companion fractions of <1% for separations of a<0.03 AU. If 1% of the stars in the cluster have Jupiter-sized companions evenly distributed in log(a) between 0.03 and 0.3 AU, we expect to find ~2 transits. A preliminary search of our light curve data has revealed a transit with a depth ~4%. Based on its shape, it is likely to be a grazing binary eclipse rather than a planetary transit, emphasizing the need for high temporal resolution in transit surveys.

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arXiv2002

Near-IR and Optical Morphology of Spiral Galaxies

We announce the initial release of data from the Ohio State University Bright Spiral Galaxy Survey, a BVRJHK imaging survey of a well-defined sample of 205 bright, nearby spiral galaxies. We present H-band morphological classification on the Hubble sequence for the OSU Survey sample. We compare the H-band classification to B-band classification from our own images and from standard galaxy catalogs. Our B-band classifications match well with those of the standard catalogs. On average, galaxies with optical classifications from Sa through Scd appear about one T-type earlier in the H-band than in the B-band, but with large scatter. This result does not support recent claims made in the literature that the optical and near-IR morphologies of spiral galaxies are uncorrelated. We present detailed descriptions of the H-band morphologies of our entire sample, as well as B- and H-band images for a set of 17 galaxies chosen as type examples, and BRH color-composite images of six galaxies chosen to demonstrate the range in morphological variation as a function of wavelength. Data from the survey are accessible at http://www.astronomy.ohio-state.edu/~survey/

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arXiv2002

Arm Structure in Anemic Spiral Galaxies

Anemic galaxies have less prominent star formation than normal galaxies of the same Hubble type. Previous studies showed they are deficient in total atomic hydrogen but not in molecular hydrogen. Here we compare the combined surface densities of HI and H2 at mid-disk radii with the Kennicutt threshold for star formation. The anemic galaxies are below threshold, which explains their lack of prominent star formation, but they are not much different than other early type galaxies, which also tend to be below threshold. The spiral wave amplitudes of anemic and normal galaxies were also compared, using images in B and J passbands from the OSU Bright Spiral Galaxy Survey. Anemic galaxies have normal spiral wave properties too, with the same amplitudes and radial dependencies as other galaxies of the same arm class. Because of the lack of gas, spiral waves in early type galaxies and anemics do not have a continuous supply of stars with low velocity dispersions to maintain a marginally stable disk. As a result, they are either short-lived, evolving toward lenticulars and S0 types in only a few rotations at mid-disk, or they are driven by the asymmetries associated with gas removal in the cluster environment.

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arXiv2002

Hubble Space Telescope Imaging of the Circumnuclear Environments of the CfA Seyfert Galaxies: Nuclear Spirals and Fueling

We present archival Hubble Space Telescope images of the nuclear regions of 43 of the 46 Seyfert galaxies found in the volume-limited, spectroscopically-complete CfA Redshift Survey sample. Using an improved method of image contrast enhancement, we create detailed high-quality structure maps that allow us to study the distributions of dust, star clusters, and emission-line gas in the circumnuclear regions (100-1000 pc scales) and in the associated host galaxy. Essentially all of these Seyfert galaxies have circumnuclear dust structures with morphologies ranging from grand-design two-armed spirals to chaotic dusty disks. In most Seyferts there is a clear physical connection between the nuclear dust spirals on hundreds of parsec scales and large-scale bars and spiral arms in the host galaxies proper. These connections are particularly striking in the interacting and barred galaxies. Such structures are predicted by numerical simulations of gas flows in barred and interacting galaxies, and may be related to the fueling of AGN by matter inflow from the host galaxy disks. We see no significant differences in the circumnuclear dust morphologies of Seyfert 1s and 2s, and very few Seyfert 2 nuclei are obscured by large-scale dust structures in the host galaxies. If Seyfert 2s are obscured Seyfert 1s, then the obscuration must occur on smaller scales than those probed by HST.

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arXiv2001

Hubble Space Telescope Observations of the CfA Seyfert 2s: Near-infrared Surface Photometry and Nuclear Bars

We present near-infrared J and H surface photometry of 24 of the nearby Seyfert 1.8, 1.9 and 2 galaxies from the CfA Seyfert sample. The excellent angular resolution of HST probes spatial scales as small as tens of parsecs in most of these AGN and is sensitive to the presence of nuclear bars and other potential signatures of the AGN fueling process that channels host galaxy gas and dust to the nuclear region. We have used elliptical isophote techniques to search for nuclear bars in all of these galaxies and have employed a two-dimensional fitting technique to model the nuclear point source and surface brightness distribution of a bright subsample of these galaxies in an attempt to alleviate the impact of the nuclear point source on our sensitivity to nuclear bars. We find stellar nuclear bar candidates in four of these galaxies: Mrk 471, Mrk 270, Mrk 573, and NGC 5929, nearly 20% of the total sample. The percentage rises to ~30% when systems with disturbed morphologies or high inclinations are excluded. The nuclear bars in Mrk 573 and Mrk 270 exhibit some evidence for dust lanes along their leading edges, analogous to the structures seen in host galaxy bars, while the dust lanes in Mrk 471 and NGC 5929 exhibit a more complex morphology. The fact that most of these AGN do not appear to contain stellar nuclear bars suggests that they are not the fueling mechanism for most low-luminosity AGN.

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arXiv2001

Supermassive Black Holes in Active Galactic Nuclei. I. The Consistency of Black Hole Masses in Quiescent and Active Galaxies

We report the first results of a program to measure accurate stellar velocity dispersions in the bulges of the host galaxies of active galactic nuclei (AGNs) for which accurate black hole (BH) masses have been determined via reverberation mapping. We find good agreement between BH masses obtained from reverberation mapping, and from the M(BH) - sigma relation as defined by quiescent galaxies, indicating a common relationship between active and quiescent black holes and their large-scale environments.

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arXiv2000

Narrow-Line Seyfert 1s: 15 Years Later

The spectroscopic properties of the objects that came to be called Narrow-Line Seyfert 1s were first systematically described and named as such 15 years ago by Osterbrock & Pogge (1985). At the time, they were a relatively rare and peculiar subclass of Seyfert galaxies. Their discovery in large numbers in X-ray surveys, however, has elevated them to a role as important members of the AGN family, ones which may hold many keys to understanding the physics of AGN across the electromagnetic spectrum. This contribution reviews the spectral classification of the narrow-line Seyfert 1s, and describes some of the properties that make this unusual class of objects interesting.

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arXiv1999

The Frequency of Barred Spiral Galaxies in the Near-IR

We have determined the fraction of barred galaxies in the H-band for a statistically well-defined sample of 186 spirals drawn from the Ohio State University Bright Spiral Galaxy survey. We find 56% of our sample to be strongly barred at H, while another 16% is weakly barred. Only 27% of our sample is unbarred in the near-infrared. The RC3 and the Carnegie Atlas of Galaxies both classify only about 30% of our sample as strongly barred. Thus strong bars are nearly twice as prevalent in the near-infrared as in the optical. The frequency of genuine optically hidden bars is significant, but lower than many claims in the literature: 40% of the galaxies in our sample that are classified as unbarred in the RC3 show evidence for a bar in the H-band, while for the Carnegie Atlas this fraction is 66%. Our data reveal no significant trend in bar fraction as a function of morphology in either the optical or H-band. Optical surveys of high redshift galaxies may be strongly biased against finding bars, as bars are increasingly difficult to detect at bluer rest wavelengths.

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arXiv1999

Hubble Space Telescope Observations of the CfA Seyfert 2s: The Fueling of Active Galactic Nuclei

We present an investigation of possible fueling mechanisms operating in the inner kiloparsec of Seyfert galaxies. We analyze visible and near-infrared Hubble Space Telescope images of 24 Seyfert 2s from the CfA Redshift Survey sample. In particular, we are searching for the morphological signatures of dynamical processes reponsible for transporting gas from kiloparsec scales into the nucleus. The circumnuclear regions are very rich in gas and dust, often taking the form of nuclear spiral dust lanes on scales of a few hundred parsecs. While these nuclear spirals are found in 20 of our 24 Seyferts, we find only 5 nuclear bars among the entire sample, strongly reinforcing the conclusions of other investigators that nuclear bars are not the primary means of transporting this material into the nucleus. An estimate of the gas density in the nuclear spirals based on extinction measurements suggests that the nuclear spiral dust lanes are probably shocks in nuclear gas disks that are not strongly self-gravitating. Since shocks can dissipate energy and angular momentum, these spiral dust lanes may be the channels by which gas from the host galaxy disks is being fed into the central engines.

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arXiv1998

Emission-Line Probes of Circumnuclear Dust in AGNs

Emission lines that trace elements subject to strong depletion onto grains provide a means for probing the dust content of AGNs. Examples include infrared [FeII] and optical [CaII] lines. The excitation mechanisms underlying the [FeII] lines remain controversial, resulting in related disagreement over the gas-phase abundance of iron in Seyfert narrow-line regions. In this contribution we emphasize the utility of the [CaII] lines as a consistency test for claims of grain destruction affecting the [FeII] lines. A search for [CaII] emission in NGC 1068 at a location of strongest [FeII] emission along the radio jet yields a strong upper limit, but no detection. This result suggests that grains survive largely intact in a region that otherwise shows strong evidence of shock processing.

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arXiv1998

Star Formation in Polar-Ring Galaxies

Polar-ring galaxies are systems with two nearly orthogonal rotational axes, and are therefore clearly the result of galaxy-galaxy interactions. The host galaxies of polar rings are typically gas-poor early-type systems. The rings themselves, however, are typically gas-rich, blue, star-forming systems. They have traditionally been modelled as the remnants of accreted dwarf irregular galaxies. Published results on the molecular gas content, and nebular abundances argue against this model. We present H-alpha imaging data for three polar-ring galaxies, and derive their HII region luminosity functions. In all three galaxies, the HII region luminosity functions are much steeper than found for any dwarf galaxy. The slopes are generally closer to those of Sa galaxies. Thus the HII region LFs of polar rings join the ISM abundances in resembling the very earliest-type spirals galaxies, rather than their supposed dwarf irregular precursors.

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arXiv1998

Star Formation and Abundances in S0 Galaxies

A significant number of S0 galaxies with a detectable ISM show some level of on-going massive-star formation activity in the form of visible HII regions. A rich ISM, however, does not guarantee star formation: a similar number of gas-rich S0s have no detectable HII regions down to the level of regions ionized by single O stars. The H-alpha luminosities of star-forming S0s imply global star formation rates as much as 2-3 orders of magnitude lower than in spirals. Spectroscopy of the bright HII regions in the six star-forming S0s studied thus far reveal solar gas-phase oxygen abundances. In general, S0s follow the general trends of O/H with galaxy luminosity and Hubble Type seen for Spirals and Irregulars, and have O/H abundances typical of spirals of similar total stellar luminosity. The implications are that S0s are the low end of the Hubble Sequence of Spirals, and that their ISMs are primarily internal in origin. The surprise is that two counter-rotating S0s, which show direct evidence of a massive infall of gas and stars, also follow these trends.

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