arXiv subjects

M. Wold

Publications and source records attributed to M. Wold.

At least 19 recordsLinked to original sources

arXiv2021

The planar two-body problem for spheroids and disks

We outline a new method suggested by Conway (2016) for solving the two-body problem for solid bodies of spheroidal or ellipsoidal shape. The method is based on integrating the gravitational potential of one body over the surface of the other body. When the gravitational potential can be analytically expressed (as for spheroids or ellipsoids), the gravitational force and mutual gravitational potential can be formulated as a surface integral instead of a volume integral, and solved numerically. If the two bodies are infinitely thin disks, the surface integral has an analytical solution. The method is exact as the force and mutual potential appear in closed-form expressions, and does not involve series expansions with subsequent truncation errors. In order to test the method, we solve the equations of motion in an inertial frame, and run simulations with two spheroids and two infinitely thin disks, restricted to torque-free planar motion. The resulting trajectories display precession patterns typical for non-Keplerian potentials. We follow the conservation of energy and orbital angular momentum, and also investigate how the spheroid model approaches the two cases where the surface integral can be solved analytically, i.e. for point masses and infinitely thin disks.

astro-ph.EP↗

arXiv2012

Deep Absorption Line Studies of Quiescent Galaxies at z~2: The Dynamical Mass-Size Relation, and First Constraints on the Fundamental plane

We present dynamical and structural scaling relations of quiescent galaxies at z=2, including the dynamical mass-size relation and the first constraints on the fundamental plane (FP). The backbone of the analysis is a new, very deep VLT/X-shooter spectrum of a massive, compact, quiescent galaxy at z=2.0389. We detect the continuum between 3700-22000A and several strong absorption features (Balmer series, Ca H+K, G-band), from which we derive a stellar velocity dispersion of 318 +/- 53 km/s. We perform detailed modeling of the continuum emission and line indices and derive strong simultaneous constraints on the age, metallicity, and stellar mass. The galaxy is a dusty (A_V=0.77 (+0.36,-0.32)) solar metallicity (log(Z/Zsun) = 0.02 (+0.20,-0.41)) post starburst galaxy, with a mean luminosity weighted log(age/yr) of 8.9 +/- 0.1. The galaxy formed the majority of its stars at z>3 and currently has little or no ongoing star formation. We compile a sample of three other z~2 quiescent galaxies with measured velocity dispersions, two of which are also post starburst like. Their dynamical mass-size relation is offset significantly less than the stellar mass-size relation from the local early type relations, which we attribute to a lower central dark matter fraction. Recent cosmological merger simulations qualitatively agree with the data, but can not fully account for the evolution in the dark matter fraction. The z~2 FP requires additional evolution beyond passive stellar aging, to be in agreement with the local FP. The structural evolution predicted by the cosmological simulations is insufficient, suggesting that additional, possibly non-homologous structural evolution is needed.

astro-ph.CO↗

arXiv2008

Black hole mass and variability in quasars

We report on a study that finds a positive correlation between black hole mass and variability amplitude in quasars. Roughly 100 quasars at z<0.75 were selected by matching objects from the QUEST1 Variability Survey with broad-lined objects from the Sloan Digital Sky Survey. Black hole masses were estimated with the virial method using the broad Hbeta line, and variability was characterized from the QUEST1 light curves. The correlation between black hole mass and variability amplitude is significant at the 99% level or better and does not appear to be caused by obvious selection effects inherent to flux-limited samples. It is most evident for rest frame time lags of the order a few months up to the QUEST1 maximum temporal resolution of about 2 years. The correlation between black hole mass and variability amplitude means that the more massive black holes have larger percentage flux variations. Over 2-3 orders of magnitude in black hole mass, the amplitude increases by approximately 0.2 mag. A likely explanation for the correlation is that the more massive black holes are starving and produce larger flux variations because they do not have a steady inflow of gaseous fuel. Assuming that the variability arises from changes in the accretion rate Li & Cao [8] show that flux variations similar to those observed are expected as a consequence of the more massive black holes having cooler accretion disks.

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arXiv2007

Quasar optical variability and black hole mass

In order to investigate the dependence of quasar optical-UV variability on fundamental physical parameters like black hole mass, we have matched quasars from the QUEST1 variability survey with broad-lined objects from the SDSS. Black hole masses and bolometric luminosities are estimated from Sloan spectra, and variability amplitudes from the QUEST1 light curves. Long-term variability amplitudes (rest-frame time scales 0.5--2 yrs) are found to correlate with black hole mass at the 99% significance level or better. This means that quasars with larger black hole masses have larger percentage flux variations. Partial rank correlation analysis shows that the correlation cannot explained by obvious selection effects inherent to flux-limited samples. We discuss whether the correlation is a manifestation of a relation between BH mass and accretion disk thermal time scales, or if it is due to changes in the optical depth of the accretion disk with black hole mass. Perhaps the most likely explanation is that the more massive black holes are starving, and produce larger flux variations because they do not have a steady inflow of gaseous fuel.

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arXiv2007

The influence of AGN nuclear parameters on the FRI/FRII dichotomy

We have investigated the influence of nuclear parameters such as black hole (BH) mass and photoionizing luminosity on the FRI/FRII transition in a sample of nearby (z<0.2) 3CR radio galaxies. The sample was observed with medium-resolution, optical spectroscopy and contains some galaxies with unpublished velocity dispersion measurements and emission-line fluxes. Measured velocity dispersions are 130-340 km/s with a mean of 216 km/s. Converting to BH mass, we find that the BH mass distribution is identical for FRIs and FRIIs, with a mean of approximately 2.5x10^8 Msun. We convert [OII] and [OIII] emission-line luminosities to photoionizing luminosity under the assumption that the gas is ionized by the nuclear UV continuum. Most of the galaxies with FRI morphology and/or low-excitation emission-line spectra have progressively lower BH masses at lower photoionizing (and jet) luminosities. This agrees with the Ledlow-Owen relation which states that the radio luminosity at the FRI/FRII transition depends on the optical luminosity of the host, L_radio ~ L_optical^1.8, because both L_radio and L_optical relate to AGN nuclear parameters. When recasting the Ledlow-Owen relation into BH mass versus photoionizing and jet luminosity, we find that the recasted relation describes the sample quite well. The FRI/FRII transition occurs at approximately an order of magnitude lower luminosity relative to the Eddington luminosity than the soft-to-hard transition in X-ray binaries. This difference is consistent with the Ledlow-Owen relation, which predicts a weak BH mass dependence in the transition luminosity. We conclude that the FRI/FRII dichotomy is caused by a combination of external and nuclear factors, with the latter dominating.

astro-ph↗

arXiv2007

Quasar 3C298: a test-case for meteoritic nanodiamond 3.5 microns emission

We calculate the dust emission expected at 3.43 and 3.53 microns if meteoritic (i.e. hydrogenated) nanodiamonds are responsible for most of the far-UV break observed in quasars. We integrate the UV flux that hydrogenated nanodiamonds must absorb to reproduce the far-UV break. Based on laboratory spectra of H-terminated diamond surfaces, we analyse the radiative energy budget and derive theoretically the IR emission profiles expected for possible C-H surface stretch modes of the diamonds. Using as test case a spectrum of 3C298 provided by the Spitzer Observatory, we do not find evidence of these emission bands. While diamonds without surface adsorbates remain a viable candidate for explaining the far-UV break observed in quasars, hydrogenated nanodiamonds appear to be ruled out, as they would giverise to IR emission bands, which have not been observed so far.

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arXiv2006

The dependence of quasar variability on black hole mass

In order to investigate the dependence of quasar variability on fundamental physical parameters like black hole mass, we have matched quasars from the QUEST1 Variability Survey with broad-lined objects from the Sloan Digital Sky Survey. The matched sample contains approximately 100 quasars, and the Sloan spectra are used to estimate black hole masses and bolometric luminosities. Variability amplitudes are measured from the QUEST1 light curves. We find that black hole mass correlates with several measures of the variability amplitude at the 99% significance level or better. The correlation does not appear to be caused by obvious selection effects inherent to flux-limited quasar samples, host galaxy contamination or other well-known correlations between quasar variability and luminosity/redshift. We evaluate variability as a function of rest-frame time lag using structure functions, and find further support for the variability--black hole mass correlation. The correlation is strongest for time lags of the order a few months up to the QUEST1 maximum temporal resolution of approximately 2 years, and may provide important clues for understanding the long-standing problem of the origin of quasar optical variability. We discuss whether our result is a manifestation of a relation between characteristic variability timescale and black hole mass, where the variability timescale is typical for accretion disk thermal timescales, but find little support for this. Our favoured explanation is that more massive black holes have larger variability amplitudes, and we highlight the need for larger samples with more complete temporal sampling to test the robustness of this result.

astro-ph↗

arXiv2006

The nuclear regions of NGC 7582 from [NeII] spectroscopy at 12.8 microns - an estimate of the black hole mass

We present a high-resolution (R~16,000) spectrum and a narrow-band image centered on the [NeII]12.8 micron line of the central kpc region of the starburst/Seyfert2 galaxy NGC 7582. The galaxy has a rotating circum-nuclear starburst disk, shown at great detail at a diffraction-limited resolution of 0.4 arcsec. The high spatial resolution allows us to probe the dynamics of the [NeII] gas in the nuclear regions, and to estimate the mass of the central black hole. We construct models of gas disks rotating in the combined gravitational potential from the stellar bulge and a central black hole, and derive a black hole mass of 5.5 x 10^7 solar masses with a 95% confidence interval of [3.6,8.1] x 10^7 solar masses. The black hole mass combined with stellar velocity dispersion measurements from the literature shows that the galaxy is consistent with the local M-sigma relation. This is the first time that a black hole mass in a galaxy except our own Milky Way system has been estimated from gas dynamics in the mid-infrared. We show that spatially resolved mid-infrared spectroscopy may be competitive with similar techniques in the optical and near-infrared, and may prove to be important for estimating black hole masses in galaxies with strong nuclear dust obscuration. The high spectral resolution allows us to determine the heliocentric systemic velocity of the galaxy to between 1614 and 1634 km/s. The mid-infrared image reveals several dense knots of dust-embedded star formation in the circum-nuclear disk, and we briefly discuss its morphology.

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arXiv2006

Supernova 2006aj and the associated X-Ray Flash 060218

We have studied the afterglow of the gamma-ray burst (GRB) of February 18, 2006. This is a nearby long GRB, with a very low peak energy, and is therefore classified as an X-ray Flash (XRF). XRF 060218 is clearly associated with a supernova -- dubbed SN 2006aj. We present early spectra for SN 2006aj as well as optical lightcurves reaching out to 50 days past explosion. Our optical lightcurves define the rise times, the lightcurve shapes and the absolute magnitudes in the U, V and R bands, and we compare these data with data for other relevant supernovae. SN 2006aj evolved quite fast, somewhat similarly to SN 2002ap, but not as fast as SN 1994I. Our spectra show the evolution of the supernova over the peak, when the U-band portion of the spectrum rapidly fades due to extensive line blanketing. We compare to similar spectra of very energetic Type Ic supernovae. Our first spectra are earlier than spectra for any other GRB-SN. The spectrum taken 12 days after burst in the rest frame is similar to somewhat later spectra of both SN 1998bw and SN 2003dh, implying a rapid early evolution. This is consistent with the fast lightcurve. From the narrow emission lines from the host galaxy we derive a redshift of z=0.0331+-0.0007. This makes XRF 060218 the second closest gamma-ray burst detected. The flux of these emission lines indicate a high-excitation state, and a modest metallicity and star formation rate of the host galaxy.

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arXiv2006

Nuclear embedded star clusters in NGC 7582

We report on the discovery of several compact regions of mid-infrared emission in the starforming circum nuclear disk of the starburst/Seyfert2 galaxy NGC7582. The compact sources do not have counterparts in the optical and near-infrared, suggesting that they are deeply embedded in dust. We use the [NeII]12.8 micron line emission to estimate the emission measure of the ionized gas, which in turn is used to assess the number of ionizing photons. Two of the brighter sources are found to have ionizing fluxes of ~2.5x10^52, whereas the fainter ones have ~1x10^52 photons/s. Comparing with a one Myr old starburst, we derive stellar masses in the range (3-5)x10^5 Msun, and find that the number of O-stars in each compact source is typically (0.6-1.6)x10^3. We conclude that the compact mid-infrared sources are likely to be young, embedded star clusters, of which only a few are known so far. Our observation highlights the need for high resolution mid-infrared imaging to discover and study embedded star clusters in the proximity of active galactic nuclei.

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arXiv2004

The Influence of Black Hole Mass and Accretion Rate on the FRI/FRII Radio Galaxy Dichotomy

We use medium resolution optical spectra of 3CR radio galaxies to estimate their black hole masses and accretion rates. Black hole masses are found from central stellar velocity dispersions and accretion rates are derived from narrow emission-line luminosities. The sample covers both Fanaroff-Riley (FR) classes; the more powerful FRIIs and the less powerful FRIs. We find that FRIs and FRIIs separate in diagrams of black hole mass versus radio luminosity and narrow-line luminosity. This suggests that, at a given black hole mass, the FRIIs accrete more efficiently, or accrete more matter, than FRIs.

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arXiv2003

Overdensities of Extremely Red Objects in the Fields of High-Redshift Radio-Loud Quasars

We have examined the occurrence of Extremely Red Objects (EROs) in the fields of 13 luminous quasars (11 radio-loud and two radio-quiet) at 1.8 < z < 3.0. The average surface density of K_s<=19 mag EROs is two-three times higher than in large, random-field surveys, and the excess is significant at the $\approx 3$ sigma level even after taking into account that the ERO distribution is highly inhomogeneous. This is the first systematic investigation of the surface density of EROs in the fields of radio-loud quasars above z=2, and shows that a large number of the fields contain clumps of EROs, similar to what is seen only in the densest areas in random-field surveys. The high surface densities and angular distribution of EROs suggest that the excess originates in high-z galaxy concentrations, possibly young clusters of galaxies. The fainter EROs at K_s>19 mag show some evidence of being more clustered in the immediate 20 arcsec region around the quasars, suggesting an association with the quasars.Comparing with predictions from spectral synthesis models, we find that if the $K_s\approx19$ mag ERO excess is associated with the quasars at $z\approx2$, their magnitudes are typical of >~ L* passively evolving galaxies formed at z~3.5 (Omega_m=0.3, Omega_l=0.7, and H0=70 km/s/Mpc). Another interpretation of our results is that the excess originates in concentrations of galaxies at $z\approx1$ lying along the line of sight to the quasars. If this is the case, the EROs may be tracing massive structures responsible for a magnification bias of the quasars.

astro-ph↗

arXiv2003

The surface density of Extremely Red Objects in high-z quasar fields

We report on a study of the surface density of Extremely Red Objects (EROs) in the fields of 13 radio-loud quasars at 1.8 < z < 3.0 covering a total area of 61.7 sqr arcmin. There is a large variation in the ERO surface density from field to field, and as many as 30--40 % of the fields have roughly 4--5 times more EROs than what is expected from a random distribution. The average surface density exceeds the value found in large random-field surveys by a factor of 2--3, a result which is significant at the >3 sigma level. Hence, it appears that the quasar lines of sight are biassed towards regions of high ERO density. This might be caused by clusters or groups of galaxies physically associated with the quasars. However, an equally likely possibility is that the observed ERO excess is part of overdensities in the ERO population along the line of sight to the quasars. In this case, the non-randomness of quasar fields with respect to EROs may be explained in terms of gravitational lensing.

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arXiv2002

AGN-selected clusters as revealed by weak lensing

As a pilot study to investigate the distribution of matter in the fields of powerful AGN, we present weak lensing observations of deep fields centered on the radio-quiet quasar E1821 + 643, the radio galaxy 3C295, and the two radio-loud quasars, 3C334 and 3C254. The host clusters of E1821 + 643 and 3C295 are comfortably detected via their weak lensing signal, and we report the detection of a cluster-sized mass concentration in the field of the z=0.734 quasar 3C254. The data for the 3C334 field are so far inconclusive. We find that the clusters are massive and have smooth mass distributions, although one shows some evidence of past merger activity. The mass-to-light ratios are found to be moderately high. We discuss the results in light of the cooling flow and the merger/interaction scenarios for triggering and fuelling AGN in clusters, but find that the data do not point unambiguously to neither of the two. Instead, we speculate that sub-cluster mergers may be responsible for generating close encounters or mergers of galaxies with the central cluster member, which could trigger the AGN.

astro-ph↗

arXiv2001

QSO environments at intermediate redshifts

We have made a survey of quasar environments at 0.5 < z < 0.8, using a sample of both radio-loud and radio-quiet quasars matched in B-band luminosity. Our observations include images of background control fields to provide a good determination of the field galaxy counts. About 10 per cent of the quasars appear to live in rich clusters, whereas approximately 45 per cent live in environments similar to that of field galaxies. The richness of galaxies within a 0.5 Mpc radius around the radio-quiet quasars is found to be indistinguishable from the richness around the radio-loud quasars, corresponding on average to groups or poorer clusters of galaxies. Comparing the galaxy richness in the radio-loud quasar fields with quasar fields in the literature, we find no evidence of an evolution in the environment with epoch. Instead, a weak, but significant correlation between quasar radio luminosity and environmental richness is present. It is thus possible that the environments of quasars, at least the powerful ones, do not evolve much between the present epoch and z \approx 0.8.

astro-ph↗

arXiv2000

Radio-quiet quasar environments at 0.5 < z < 0.8

We have quantified the galaxy environments around a sample of 0.5 < z < 0.8 radio-quiet quasars using the amplitude of the spatial galaxy--quasar correlation function, B_gq. The quasars exist in a wide variety of environments, some sources are located in clusters as rich as Abell class 1--2 clusters, whereas others exist in environments comparable to the field. We find that on average, the quasars prefer poorer clusters of approximately Abell class 0, which suggests that quasars are biased tracers of mass compared to galaxies. The mean B_gq for the sample is found to be indistinguishable from the mean amplitude for a sample of radio-loud quasars matched in redshift and optical luminosity. These observations are consistent with recent studies of the hosts of radio-quiet quasars at low to intermediate redshifts, and suggest that the mechanism for the production of powerful radio jets in radio-loud quasars is controlled by processes deep within the active galactic nucleus itself, and is unrelated to the nature of the hosts or their environments.

astro-ph↗

arXiv2000

Quasar environments at 0.5 < z < 0.8

Over the past few years, we have been collecting data with the Nordic Optical Telescope (NOT) on the galaxy environments around active galactic nuclei (AGN). Here we present some results from a sample of 21 radio-loud and 20 radio-quiet quasars in the redshift range 0.5 < z < 0.8. We find a few quasars in very rich environments, perhaps as rich as Abell class 1-2 clusters, but more often the quasars seem to prefer groups and poorer clusters. We also find that on average the galaxy environments around radio-loud and radio-quiet quasars are indistinguishable, consistent with the findings that both powerful radio-loud and radio-quiet quasars appear to be hosted by luminous galaxies with luminosities higher than the break in the luminosity function (Dunlop et al. 1993; Taylor et al. 1996). Comparing the galaxy richnesses in the radio-loud quasar fields with quasar fields in the literature, we find a weak, but significant, correlation between quasar radio luminosity and environmental richness.

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arXiv1999

Clustering of galaxies around radio quasars at 0.5 < z < 0.8

We have observed the galaxy environments around a sample of 21 radio-loud, steep-spectrum quasars at 0.5<z<0.82, spanning several orders of magnitude in radio luminosity. The observations also include background control fields used to obtain the excess number of galaxies in each quasar field. The galaxy excess was quantified using the spatial galaxy-quasar correlation amplitude, B_gq, and an Abell-type measurement, N_0.5 (Hill & Lilly 1991). A few quasars are found in relatively rich clusters, but on average, they seem to prefer galaxy groups or clusters of approximately Abell class 0. We have combined our sample with literature samples extending down to z=0.2 and covering the same range in radio luminosity. By using Spearman statistic to disentangle redshift and luminosity dependences, we detect a weak, but significant, positive correlation between the richness of the quasar environment and the quasar's radio luminosity. However, we do not find any epoch dependence in B_gq, as has previously been reported for radio quasars and galaxies. We discuss the radio luminosity-cluster richness link and possible explanations for the weak correlation that is seen.

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