Searcharxiv⌕ Search

arXiv subjects

Dan Maoz

Publications and source records attributed to Dan Maoz.

At least 145 records · Page 8Linked to original sources
arXiv2001

The Super Star Cluster NGC 1569-A Resolved on Sub-Parsec Scales with Hubble Space Telescope Spectroscopy

We present 3000--10000 Ang HST/STIS long-slit spectroscopy of the bright super star cluster A (SSC-A) in the dwarf starburst galaxy NGC 1569. The 0.05" HST angular resolution allows, for the first time, to probe for spatial variations in the stellar population of a ~ 10^6 M_sun SSC. Integrated ground-based spectra of SSC-A have previously revealed young Wolf-Rayet (WR) signatures that coexist with features from supposedly older, red supergiant (RSG), populations. We find that the WR emission complexes come solely from the subcluster A2, identified in previous HST imaging, and are absent from the main cluster A1, thus resolving the question of whether the WR and RSG features arise in a single or distinct clusters. The equivalent widths of the WR features in A2 --- including the CIV 5808 complex which we detect in this object for the first time --- are larger than previously observed in other WR galaxies. Models with sub-solar metallicity, as inferred from the nebular emission lines of this galaxy, predict much lower equivalent widths. On the ``clean'' side of A1, opposite to A2, we find no evidence for radial gradients in the observed stellar population at 0.05"<R<0.40" (~0.5 to 5 pc), neither in broad-band, low-resolution, spectra nor in medium-resolution spectra of the infrared CaII triplet.

astro-ph↗
arXiv2001

An Ultraviolet through Infrared Look at Star Formation and Super Star Clusters in Two Circumnuclear Starburst Rings

We present broad-band (U, V, I,and H) and narrow-band(H-alpha+[N II] and Paschen-alpha) images of the circumnuclear starburst rings in two nearby spiral galaxies, NGC 1512 and NGC 5248, obtained with WFPC2 and NICMOS on HST. Combined with HST images at 2300 Ang, these data provide a particularly wide wavelength range with which to study the properties of the stellar populations, the gas, and the dust in the rings. Some large (50-pc scale) line emitting regions have little associated continuum emission, but a Pa equivalent width indicating a few-Myr-old embedded stellar population. The Ha/Pa intensity ratios suggest the gas is mixed with dust, making it effective at obscuring some of the young clusters. We identify about 1000 compact continuum sources (super star clusters and individual stars) and analyze their spectral energy distributions (SEDs) from 0.2 to 1.6 micron by fitting them with a grid of spectral synthesis models with a range of ages and extinctions. Most of the visible clusters are only mildly reddened, with A_V=0 to 1 mag, suggesting that the processes that clear out the gas and dust of the stellar birth clouds are efficient and fast. The patchiness of the dust distribution makes it difficult to estimate reliably the star formation rate, based on UV continuum slope or hydrogen emission-line ratios. The cluster SEDs are consistent with a range in ages, from 1 Myr to 300 Myr, but with only a minority older than a few tens of Myr. However, after accounting for an age bias, the fraction of old clusters is consistent with continuous star formation in the rings over the past ~300 Myr. Some of the brightest young clusters have excess emission in the IR that may be radiation by circumstellar dust. The cluster mass functions follow an m^-2 power-law distribution, distinct from those of old globular clusters.

astro-ph↗
arXiv2001

'Round the Clock Observations of the Q0957+561 A,B Gravitationally Lensed Quasar

An observing campaign with 10 participating observatories has undertaken to monitor the optical brightness of the Q0957 gravitationally lensed quasar for 10 consecutive nights in January 2000. The resulting A image brightness curve has significant brightness fluctuations and makes a photometric prediction for the B image light curve for a second campaign planned for 12-21 March 2001. The ultimate purpose is to determine the gravitational lens time delay to a fraction of an hour, and to seek evidence for rapid microlensing.

astro-ph↗
arXiv1999

Supernova rates in Abell galaxy clusters and implications for metallicity

Supernovae (SNe) play a critical role in the metal enrichment of the intra-cluster medium (ICM) in galaxy clusters. Not only are SNe the main source for metals, but they may also supply the energy to eject enriched gas from galaxies by winds. However, measurements of SN rates in galaxy clusters have not been published to date. We have initiated a program to find SNe in 163 medium-redshift (0.06<z<0.2) Abell clusters, using the Wise Observatory 1m telescope. We report here our on first results and describe our main scientific goals. Following the discovery of a SN in the remote periphery (78 Kpc) of the cD galaxy of Abell 403, we discuss a novel explanation for the centrally enhanced metal abundances indicated by X-ray observations of galaxy clusters.

astro-ph↗
arXiv1999

Supernova rates in Abell galaxy clusters and implications for metallicity

Supernovae (SNe) play a critical role in the metal enrichment of the intra-cluster medium (ICM) in galaxy clusters. Not only are SNe the main source for metals, but they may also supply the energy to eject enriched gas from galaxies by winds. However, measurements of SN rates in galaxy clusters have not been published to date. We have initiated a program to find SNe in 163 medium-redshift (0.06<z<0.2) Abell clusters, using the Wise Observatory 1m telescope. Our program has already discovered 11 spectroscopically confirmed SNe at z=0.1 - 0.24, and several unconfirmed SNe. We present the main scientific goals of this project, and discuss a novel explanation for the centrally enhanced metal abundances indicated by X-ray observations of galaxy clusters, based on the contribution of intergalactic SNe.

astro-ph↗
arXiv1999

Reverberation Measurements for 17 Quasars and the Size-Mass-Luminosity Relations in Active Galactic Nuclei

(abridged) We have spectrophotometrically monitored a well-defined sample of 28 Palomar-Green quasars in order to obtain measurements of their BLRs and to investigate the relationships between quasar luminosity, central black hole mass, and broad emission line region (BLR) size in active galactic nuclei (AGN). Spectrophotometry was obtained every 1-4 months for 7.5 years, yielding 20-70 observing epochs per object. Both the continuum and emission line fluxes of all of the quasars were observed to change during the duration of the observing program. Seventeen of the 28 objects were observed with adequate sampling to search for correlated variations between the Balmer emission lines and the continuum flux. For each of these 17 objects, a significant correlation was observed, with the Balmer line variations lagging those of the continuum by \~100 days (rest frame). Our work increases the available luminosity range for studying the size-mass-luminosity relations in AGN by two orders of magnitude and doubles the number of objects suitable for such studies. Combining our results with comparable published data available for Seyfert 1 galaxies, we find the BLR size scales with the rest-frame 5100 A luminosity as L^{0.70+-0.03}. This determination of the scaling of the size of the BLR as a function of luminosity is significantly different from those previously published, and suggests that the effective ionization parameter in AGN may be a decreasing function of luminosity. We are also able to constrain, subject to our assumption that gravity dominates the motions of the BLR gas, the scaling relationship between the mass of the central black holes and the luminosity in AGN: M \propto L^{0.5+-0.1}. This is inconsistent with all AGN having optical luminosity that is a constant fraction of the Eddington luminosity.

astro-ph↗
arXiv1999

Intensive HST, RXTE and ASCA Monitoring of NGC 3516: Evidence Against Thermal Reprocessing

During 1998 April 13-16, NGC 3516 was monitored almost continuously with HST for 10.3 hr in the UV and 2.8 d in the optical, and simultaneous RXTE and ASCA monitoring covered the same period. The X-rays were strongly variable with the soft (0.5-2 keV) showing stronger variations (~65% peak-to-peak) than the hard (2-10 keV; ~50% peak-to-peak). The optical continuum showed much smaller but highly significant variations: a slow ~2.5% rise followed by a faster ~3.5% decline. The short UV observation did not show significant variability. The soft and hard X-ray light curves were strongly correlated with no significant lag. Likewise, the optical continuum bands (3590 and 5510 A) were also strongly correlated with no measurable lag above limits of <0.15 d. However no significant correlation or simple relationship could be found for the optical and X-ray light curves. These results appear difficult to reconcile with previous reports of correlations between X-ray and optical variations and of measurable lags within the optical band for some other Seyfert 1s. These results also present serious problems for "reprocessing" models in which the X-ray source heats a stratified accretion disk which then reemits in the optical/ultraviolet: the synchronous variations within the optical would suggest that the emitting region is <0.3 lt-d across, while the lack of correlation between X-ray and optical variations would indicate, in the context of this model, that any reprocessing region must be >1 lt-d in size. It may be possible to resolve this conflict by invoking anisotropic emission or special geometry, but the most natural explanation appears to be that the bulk of the optical luminosity is generated by some other mechanism than reprocessing.

astro-ph↗
arXiv1999

A Possible 100-day X-ray-to-Optical Lag in the Variations of the Seyfert 1 Nucleus NGC 3516

We present optical broadband (B and R) observations of the Seyfert 1 nucleus NGC 3516, obtained at Wise Observatory from 1997 March to 1998 September, contemporaneously with the X-ray 2-10 keV measurements of RXTE. The cross correlation function shows a positive peak when the optical variations lead the X-rays by about 100 days. We show that the putative correlation signal at 100 days is entirely due to the slow ($\gtorder 30$ days) components of the light curves. During the first year or this monitoring, smoothed versions of the light curves are nearly identical copies of each other, but scaled in amplitude and shifted in time. However, for the next 200 days, the X-ray and optical variations are clearly different. During the whole period, the faster-changing components of the light curves are uncorrelated at any lag. The detection of this lag is tentative and the significance of the correlation uncertain. If the 100-day delay is real, however, one interpretation is that that the slowly-varying part of the X-ray emission is an echo of the optical emission, Compton scattered from a medium located about 50-100 lt days from the optical source. We point out that a possibly analogous phenomenon, of a lag between hard and soft X-rays for a given variability timescale, exists in Galactic stellar-mass accretors. Remarkably, in both cases the lag corresponds to a light travel distance of order $10^4$ gravitational radii. Alternatively, the lag may not represent a physical size, but for example, the timescale of an instability propagating inward in an accretion flow, appearing first in the optical and then in the X-rays. In any event, there is no strong correlation at zero, or small positive, lag, ruling out an energetically-significant X-ray reprocessed component in the optical emission.

astro-ph↗
arXiv1999

Long-term optical variability properties of the Palomar-Green quasars

We present results from a monitoring program of 42 quasars from the Palomar Green sample. The objects were observed for 7 years at the Wise Observatory, as part of a long term effort to monitor AGN of various types. This is the most extensive program of its kind carried out to date on a well-defined optically selected quasar sample. The typical sampling interval is ~40 days. One third of the quasars were observed at ~60 epochs and the rest at ~30 epochs in two bands (B&R) with photometric accuracy of ~0.01 mag. We present lightcurves for all of the sources and discuss the sample variability properties. All of the quasars in the sample varied during the campaign with intrinsic rms amplitudes of 5%<sigma_B<34% and 4%<sigma_R<26%. The rms amplitude and colour for the entire sample are sigma_B=14%, sigma_R=12%, and sigma_{B-R}=5%. On time scales of 100- 1000 days the power spectra of the sources have a power-law shape with index approximately 2.0 and a spread <0.6. At least half of the quasars, particularly those that are most variable, become bluer when they brighten, and the rest do not show this behaviour. We quantify this phenomenon, which has been observed previously mainly in Seyfert galaxies. The quasars which are most variable tend also to exhibit asymmetry in their variations, in the sense that the brightening phases last longer than the fading phases. We have searched for correlations between the measured variability properties and other parameters of the quasars, such as luminosity, redshift, radio loudness, and X-ray slope. We find several new correlations, and reproduce some of the correlations reported by previous studies.

astro-ph↗
arXiv1999

A Tully Fisher Relation for S0 Galaxies

We present an I-band Tully-Fisher relation (TFR) for 18 nearby S0 galaxies using kinematics derived from long slit spectroscopy of stellar absorption lines. Our estimates of the circular velocity, V_c, at 2-3 exponential disk scale lengths account for line-of-sight projection and for the stellar random motions through an asymmetric drift correction. Uniform and accurate distance calibration for all objects is available from surface brightness fluctuation measurements of Tonry et al. (1998). Despite the care taken in estimating both V_c and M_I, the TFR shows an intrinsic scatter, $\sim 0.7$ mag in M_I, or 0.15 in log_10(V_c). This result is surprising, as S0 galaxies appear to have both the simple kinematics of disk galaxies, and the simple stellar populations of early-type galaxies. Remarkably, in this sample of overall rotation-dominated galaxies, the central stellar velocity dispersion is a better predictor of the total I-band luminosity (through the Fundamental Plane relations) than the circular speed at several exponential scale lengths. Furthermore, the TFR zeropoint, or the mean stellar I-band luminosity at a given V_c, differs by only $\sim 0.5$ mag between our sample of S0s and Mathewson et al.'s (1992) sample of late-type spirals, once both data sets are brought onto a consistent distance scale. This offset is less than expected if S0s are former spiral galaxies with prematurely truncated star-formation ($\gtorder 4$ Gyrs ago).

astro-ph↗
arXiv1998

The Role of Stars in the Energetics of LINERs

Imaging studies have shown that about 25% of LINER galaxies display a compact nuclear UV source. I compare the HST ultraviolet spectra now available for seven such ``UV-bright'' LINERs. The spectra of NGC 404, NGC 4569, and NGC 5055 show clear absorption-line signatures of massive stars, indicating a stellar origin for the UV continuum. Similar features are probably present in NGC 6500. The same stellar signatures may be present but undetectable in NGC 4594, due to the low signal-to-noise ratio of the spectrum, and in M81 and NGC 4579, due to superposed strong, broad emission lines. The compact central UV continuum source that is observed in these galaxies is a nuclear star cluster rather than a low-luminosity active galactic nucleus (AGN), at least in some cases. At least four of the LINERs suffer from an ionizing photon deficit, in the sense that the ionizing photon flux inferred from the observed far-UV continuum is insufficient to drive the optical H I recombination lines. Examination of the nuclear X-ray flux of each galaxy shows a high X-ray/UV ratio in the four ``UV-photon starved'' LINERs. In these four objects, a separate component, emitting predominantly in the extreme-UV, is the likely ionizing agent, and is perhaps unrelated to the observed nuclear UV emission. Future observations can determine whether the UV continuum in LINERs is always dominated by a starburst or, alternatively, that there are two types of UV-bright LINERs: starburst-dominated and AGN-dominated. Interestingly, recent results show that starbursts dominate the nuclear energetics in many Seyfert 2s as well.

astro-ph↗
arXiv1998

The Ultraviolet Spectra of LINERs: A Comparative Study

Imaging studies have shown that about 25% of LINER galaxies display a compact nuclear UV source. As part of a program to study the nature of LINERs and their connection to the active galaxy phenomenon, we compare the HST ultraviolet (1150-3200 Ang) spectra of seven such UV-bright LINERs. Data for three of the galaxies (NGC 404, NGC 4569, and NGC 5055) are presented for the first time, while data for four others (M81, NGC 4594, NGC 4579, and NGC 6500) have been recently published. The spectra of NGC 404, NGC 4569, and NGC 5055 show clear absorption-line signatures of massive stars, indicating a stellar origin for the UV continuum. Similar features are probably present in NGC 6500. The same stellar signatures may be present but undetectable in NGC 4594, due to the low signal-to-noise ratio of the spectrum, and in M81 and NGC 4579, due to superposed strong, broad emission lines. The compact central UV continuum source that is observed in these galaxies is a nuclear star cluster rather than a low-luminosity active galactic nucleus (AGN), at least in some cases. Except for the two LINERs with broad emission lines (M81 and NGC 4579), the LINERs have weak or no detectable UV emission lines. At least four of the LINERs suffer from an ionizing photon deficit, in the sense that the ionizing photon flux inferred from the observed far-UV continuum is insufficient to drive the optical H I recombination lines. Examination of the nuclear X-ray flux of each galaxy shows a high X-ray/UV ratio in the four ``UV-photon starved'' LINERs. Future observations can determine whether the UV continuum in LINERs is always dominated by a starburst or, alternatively, that there are two types of UV-bright LINERs: starburst-dominated and AGN-dominated.

astro-ph↗
arXiv1998

On Uncertainties in Cross-Correlation Lags and the Reality of Wavelength-Dependent Continuum Lags in Active Galactic Nuclei

We describe a model-independent method of assessing the uncertainties in cross-correlation lags determined from AGN light curves, and use this method to investigate the reality of lags between UV and optical continuum variations in well-studied AGNs. Our results confirm the existence of such lags in NGC 7469. We find that the continuum variations at 1825 A, 4845 A, and 6962 A follow those at 1315A by 0.22^{+0.12}_{-0.13} days, 1.25^{+0.48}_{-0.35} days, and 1.84^{+0.93}_{-0.94} days, respectively, based on the centroids of the cross-correlation functions; the error intervals quoted correspond to 68% confidence levels, and each of these lags is greater than zero at no less than 97% confidence. We do not find statistically significant interband continuum lags in NGC 5548, NGC 3783, or Fairall 9. Wavelength-dependent continuum lags may be marginally detected in the case of NGC 4151. However, on the basis of theoretical considerations, wavelength-dependent continuum lags in sources other than NGC 7469 are not expected to have been detectable in previous experiments. We also confirm the existence of a statistically significant lag between X-ray and UV continuum variations in the blazar PKS 2155-304.

astro-ph↗
arXiv1997

Detectability of High Redshift Ellipticals in the Hubble Deep Field

Relatively few intensively star-forming galaxies at redshifts z>2.5 have been found in the Hubble Deep Field (HDF). This has been interpreted to imply a low space density of elliptical galaxies at high z, possibly due to a late (z<2.5) epoch of formation, or to dust obscuration of the ellipticals that are forming at z~3. I use HST UV (2300 Ang) images of 25 local early-type galaxies to investigate a third option, that ellipticals formed at z>4.5, and were fading passively by 2 4.5, and have faded passively to their present brightnesses at UV wavelengths, they would generally be below the HDF detection limits in any of its bands at z>2.5. Quiescent z ~ 3 ellipticals, if they exist, should turn up in sufficiently deep IR images.

astro-ph↗
arXiv1996

A survey for large image-separation lensed quasars

The statistics of gravitationally lensed quasars with multiple images in the 0.1''-7'' range have been measured in various surveys. Little is known, however, about lensed-quasar statistics at larger image separations, which probe masses on the scale of galaxy clusters. We extend the results of the HST Snapshot Survey for Lensed Quasars to the 7''-50'' range for a sub-sample of 76 quasars that is free of known selection effects. Using a combination of multicolor photometry and spectroscopy, we show that none of the point sources in the entire field of view of the HST observations of these quasars are lensed images. Large-separation quasar lensing is therefore not common. We carry out a detailed calculation of the expected statistics of large-separation lensing for this quasar sample, incorporating realistic input for the mass profiles and mass function of galaxy clusters. We find that the observational null results are consistent with the expected effect of galaxy clusters, even if these have existed in their present form and number since z of about 2. The rarity of large-separation lensed quasars can rule out some extreme scenarios, e.g. that the mass-function of clusters has been severely underestimated, or that large mass concentrations that are not associated with galaxies (i.e. ``failed'' clusters) are common. The rareness of wide lensing also sets limits on the cosmological constant that are independent of limits derived from galaxy lensing. The lensing statistics of larger quasar samples can probe the structure, number, and evolution of clusters, as well as the geometry of space.

astro-ph↗
arXiv1996

Evidence for a New Class of Extreme UV Sources

Most of the sources detected in the extreme ultraviolet (EUV; 100 Ang to 600 Ang) by the Rosat WFC and EUVE all-sky surveys have been identified with active late-type stars and hot white dwarfs that are near enough to escape absorption by interstellar gas. However, about 15% of EUV sources are as of yet unidentified with any optical counterparts. We examine whether the unidentified EUV sources may consist of the same population of late-type stars and white dwarfs. We present B and R photometry of stars in the fields of seven of the unidentified EUV sources. We detect in the optical the entire main-sequence and white-dwarf population out to the greatest distances where they could still avoid absorption. We use colour-magnitude diagrams to demonstrate that, in most of the fields, none of the observed stars have the colours and magnitudes of late-type dwarfs at distances less than 100 pc. Similarly, none are white dwarfs within 500 pc that are hot enough to be EUV-emitters. The unidentified EUV sources we study are not detected in X-rays, while cataclysmic variables, X-ray binaries, and active galactic nuclei generally are. We conclude that some of the EUV sources may be a new class of nearby objects, that are either very faint at optical bands or which mimic the colours and magnitudes of distant late-type stars or cool white dwarfs. One candidate for optically faint objects is isolated old neutron stars, slowly accreting interstellar matter. Such neutron stars are expected to be abundant in the Galaxy, and have not been unambiguously detected.

astro-ph↗
arXiv1996

Measurement of the Broad Line Region Size in Two Bright Quasars

We present 4 years of spectrophotometric monitoring data for two radio-quiet quasars, PG 0804+762 and PG 0953+414, with typical sampling intervals of several months. Both sources show continuum and emission line variations. The variations of the H$β$ line follow those of the continuum with a time lag, as derived from a cross-correlation analysis, of 93$\pm$30 days for PG 0804+762 and 111$\pm$55 days for PG 0953+414. This is the first reliable measurement of such a lag in active galactic nuclei with luminosity $L>10^{45}$ erg s$^{-1}$. The broad line region (BLR) size that is implied is almost an order of magnitude larger than that measured in several Seyfert 1 galaxies and is consistent with the hypothesis that the BLR size grows as $L^{0.5}$.

astro-ph↗