arXiv subjects

Dan Maoz

Publications and source records attributed to Dan Maoz.

At least 127 records · Page 7Linked to original sources

arXiv2005

The Relationship Between Luminosity and Broad-Line Region Size in Active Galactic Nuclei

We reinvestigate the relationship between the characteristic broad-line region size (R_blr) and the Balmer emission-line, X-ray, UV, and optical continuum luminosities. Our study makes use of the best available determinations of R_blr for a large number of active galactic nuclei (AGNs) from Peterson et al. Using their determinations of R_blr for a large sample of AGNs and two different regression methods, we investigate the robustness of our correlation results as a function of data sub-sample and regression technique. Though small systematic differences were found depending on the method of analysis, our results are generally consistent. Assuming a power-law relation R_blr \propto L^α, we find the mean best-fitting αis about 0.67+/-0.05 for the optical continuum and the broad Hβluminosity, about 0.56+/-0.05 for the UV continuum luminosity, and about 0.70+/-0.14 for the X-ray luminosity. We also find an intrinsic scatter of about 40% in these relations. The disagreement of our results with the theoretical expected slope of 0.5 indicates that the simple assumption of all AGNs having on average same ionization parameter, BLR density, column density, and ionizing spectral energy distribution, is not valid and there is likely some evolution of a few of these characteristics along the luminosity scale.

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arXiv2005

The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies

LINER nuclei, which are present in many nearby galactic bulges, may be the manifestation of low-rate or low-radiative-efficiency accretion onto supermassive central black holes. However, it has been unclear whether the compact UV nuclear sources present in many LINERs are clusters of massive stars, rather than being directly related to the accretion process. We have used HST to monitor the UV variability of a sample of 17 galaxies with LINER nuclei and compact nuclear UV sources. Fifteen of the 17 galaxies were observed more than once, with two to five epochs per galaxy, spanning up to a year. We detect significant variability in most of the sample, with peak-to-peak amplitudes from a few percent to 50%. In most cases, correlated variations are seen in two independent bands (F250W and F330W). Comparison to previous UV measurements indicates, for many objects, long-term variations by factors of a few over decade timescales. Variability is detected in LINERs with and without detected compact radio cores, in LINERs that have broad H-alpha wings detected in their optical spectra (``LINER 1's''), and in those that do not (``LINER 2s''). This variability demonstrates the existence of a non-stellar component in the UV continuum of all types, and sets a lower limit to the luminosity of this component. We note a trend in the UV color (F250W/F330W) with spectral type - LINER 1s tend to be bluer than LINER 2s. This trend may indicate a link between the shape of the nonstellar continuum and the presence or the visibility of a broad-line region. In one target, the post-starburst galaxy NGC 4736, we detect variability in a previously noted UV source that is offset by 2.5" (60 pc in projection) from the nucleus. This may be the nearest example of a binary active nucleus, and of the process leading to black hole merging.

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arXiv2005

Supernova Rates in Galaxy Clusters

Measurements of SN rates in different environments and redshifts can shed light on the nature of SN-Ia progenitors, star formation history, and chemical enrichment history. I summarize some recent work by our group in this area, and discuss the implications. The current evidence favors production of most of the iron in the ICM (and perhaps everywhere) by core-collapse SNe, rather than SNe-Ia. These SNe may have been produced by the first, top-heavy-IMF, generation of stars that reionized the Universe. Improved rate measurements can sharpen the picture, and I describe our recent efforts in this direction.

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arXiv2005

Quasar Lensing Statistics and Omega_Lambda: What Went Wrong?

In the pre-WMAP, pre-Supernova-Ia-Hubble-diagram era, quasar lensing statistics stubbornly indicated low values of Omega_Lambda. In contrast, a number of recent lensing statistics studies either find the data support the standard Lambda-CDM picture, or simply take the standard cosmological parameters as a given. Have the data or the analyses changed or improved, and how? I review several of the ``historical'' and the more recent studies, and show that there is no particular measurement, assumption, or model parameter in the old studies that was grossly wrong. Instead, at least several effects, operating together, are likely required in order to achieve agreement between the observations and the currently standard cosmology. Most likely among these effects are: a somewhat lower lensing cross section for elliptical galaxies than assumed in the past; some loss of lensed quasars in optical samples due to extinction by the lenses; and a somewhat lower-than-standard value of Omega_Lambda ~ 0.6. The agreement between recent model calculations and the results of radio lens surveys may be fortuitous, and due to a cancellation between the errors in the input parameters for the lens population and the cosmology, on the one hand, and for the source population, on the other hand.

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arXiv2004

Photometric Identification of Young Stripped-Core Supernovae

We present a method designed to identify the spectral type of young (less than ~30 days after explosion) and nearby (z < ~0.05) supernovae (SNe) using their broad-band colors. In particular, we show that stripped-core SNe (i.e., hydrogen deficient core-collapse events, spectroscopically defined as SNe Ib and SNe Ic, including broad-lined SN 1998bw-like events) can be clearly distinguished from other types of SNe. Using the full census of nearby SNe discovered during the year 2002, we estimate the impact that prompt multi-band photometry, obtained by 1m class telescopes, would have on the early identification of stripped-core events. Combining this new approach with ongoing spectroscopic follow-up programs, one can expect ~20 nearby, stripped-core events to be identified, each year, around, or before, maximum light. Follow-up studies, including prompt, multi-epoch optical spectroscopy and spectropolarimetry, as well as radio and X-ray observations, could greatly increase our understanding of these events, and shed new light on their association with cosmological gamma-ray bursts.

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arXiv2003

Constraints on SN Ia Progenitors and ICM enrichment from field and cluster SN rates

The iron mass in galaxy clusters is about 6 times larger than could have been produced by core-collapse SNe, assuming the stars in cluster galaxies formed with a standard IMF. Type-Ia SNe have been proposed as the alternative dominant iron source. We use our HST measurements of the cluster SN-Ia rate at high redshift to study the cluster iron enrichment scenario. The measurements can constrain the star-formation epoch and the SN-Ia progenitor models via the mean delay time between the formation of a stellar population and the explosion of some of its members as SNe-Ia. The low observed rate of cluster SNe-Ia at z~1 pushes back the star-formation epoch in clusters to z>2, and implies a short delay time. We also show a related analysis for high-z field SNe which implies, under some conditions, a long SN-Ia delay time. Thus, cluster enrichment by core-collapse SNe from a top-heavy IMF may remain the only viable option.

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arXiv2003

The Type-Ia Supernova Rate in z < 1 Galaxy Clusters: Implications for Progenitors and the Source of Cluster Iron

The iron mass in galaxy clusters is about 6 times larger than could have been produced by core-collapse supernovae (SNe), assuming the stars in the cluster formed with a standard initial mass function (IMF). SNe Ia have been proposed as the alternative dominant iron source. Different SN Ia progenitor models predict different ``delay functions'' between the formation of a stellar population and the explosion of some of its members as SNe Ia. We use our previous measurements of the cluster SN Ia rate at high redshift to constrain SN Ia progenitor models and the star-formation epoch in clusters. The low observed rate of cluster SNe Ia at z ~ 0 - 1 means that, if SNe Ia produced the observed amount of iron, they must have exploded at even higher z. This puts a >95% upper limit on the mean SN Ia delay time of tau <2 Gyr (<5 Gyr) if the stars in clusters formed at redshift z<2 (z<3), assuming Ho=70 km/s/Mpc. In a companion paper (astro-ph/0309796), we show that, for some current versions of cosmic (field) star formation history (SFH), observations of field SNe Ia place a lower bound on the delay time, tau>3 Gyr. If these SFHs are confirmed, the entire range of tau will be ruled out. Cluster enrichment by core-collapse SNe from a top-heavy IMF will then remain the only viable option.

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arXiv2003

The Redshift Distribution of Type-Ia Supernovae: Constraints on Progenitors and Cosmic Star Formation History

We use the redshift distribution of type-Ia supernovae (SNe) discovered by the Supernova Cosmology Project to constrain the star formation history (SFH) of the Universe and SN Ia progenitor models. Given some of the recent determinations of the SFH, the observed SN Ia redshift distribution indicates a long (>~1 h^-1 Gyr) mean delay time between the formation of a stellar population and the explosion of some of its members as SNe Ia. For example, if the Madau et al. (1998) SFH is assumed, the delay time tau is constrained to be tau > 1.7 (tau > 0.7) h^-1 Gyr at the 95%(99%) confidence level (CL). SFHs that rise at high redshift, similar to those advocated by Lanzetta et al. (2002), are inconsistent with the data at the 95% CL unless tau > 2.5 h^-1 Gyr. Long time delays disfavor progenitor models such as edge-lit detonation of a white dwarf accreting from a giant donor, and the carbon core ignition of a white dwarf passing the Chandrasekhar mass due to accretion from a subgiant. The SN Ia delay may be shorter, thereby relaxing some of these constraints, if the field star formation rate falls, between z=1 and the present, less sharply than implied, e.g., by the original Madau plot. We show that the discovery of larger samples of high-z SNe Ia by forthcoming observational projects should yield strong constraints on the progenitor models and the SFH. In a companion paper (astro-ph/0309797), we demonstrate that if SNe Ia produce most of the iron in galaxy clusters, and the stars in clusters formed at z~2, the SN Ia delay time must be lower than 2 Gyr. If so, then the Lanzetta et al. (2002) SFH will be ruled out by the data presented here.

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arXiv2003

Using Multi-Band Photometry to Classify Supernovae

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arXiv2003

The redshift distribution of gravitational lenses revisited: Constraints on galaxy mass evolution

The redshifts of lens galaxies in known gravitational lens systems probe the volume distribution of lensing mass. Following earlier work by Kochanek, we re-derive the lens redshift probability distribution, allowing for mass and number density evolution of the lensing galaxies, and apply this test to a much enlarged sample of lens systems. From a literature survey of all known lenses, we have selected an unbiased sample of 15 lenses with complete redshift information. For a flat Universe and no lens evolution, we can only put an upper limit on the cosmological constant of Omega_lambda<0.89 at the 95% CL. Omega_lambda~0.7 and no evolution is consistent with the data. Allowing for evolution in an Omega_m=0.3, Omega_lambda=0.7 cosmology, we find that the best-fit evolution in sigma* (i.e., the characteristic velocity dispersion in a Schechter-like function) of early-type galaxies, in the redshift range z~0 to 1, is d[log sigma*(z)]/dz=-0.10+/-0.06. This is consistent with no evolution and implies that, at 95% CL, sigma* of early-type galaxies at z~1 was at least 63% of its current value. Alternatively, if there is no mass evolution, a present-day value of sigma*>175 km/s for elliptical galaxies is required (95% CL).

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arXiv2003

Time delay measurement of the lensed quasar HE1104-1805

We have measured the time delay between the two images of the gravitationally lensed quasar HE1104-1805 by combining observations made with the Wise Observatory 1m telescope and published observations of this system by Schechter et al., spanning a total of five years, from 1997 to 2002. Based on a variety of techniques, we find that the best fit time delay is -161 (+/-7)(+34 -11) days. The 68% and 95% confidence intervals include the systematic errors due to an observed component of uncorrelated variability between images. The delay is shorter than predicted by simple models and may indicate a large external shear or a large value of the Hubble parameter, h>0.75 (95% confidence). The residual light curve between the two images shows a longterm trend of ~0.04 mag/yr, presumably the result of microlensing by stars in the lens galaxy, but also short timescale (~1 month) variability with a mean amplitude of about 0.07 mag.

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arXiv2002

A Population of Intergalactic Supernovae in Galaxy Clusters

We have discovered seven type Ia cluster supernovae (SNe) in the course of the Wise Observatory Optical Transients Search in the fields of galaxy clusters with redshifts between z=0.06 and z=0.2. Two of these events, SN 1998fc in Abell 403 (z=0.10) and SN 2001al in Abell 2122/4 (z = 0.066), have no obvious hosts. Both events appear projected on the halos of the central cD galaxies, but have velocity offsets of 750-2000 km/s relative to those galaxies, suggesting they are not bound to them. We use deep Keck imaging of the locations of the two SNe to put upper limits on the luminosities of possible dwarf hosts, M_R > -14 mag for SN 1998fc and M_R > -11.8 mag for SN 2001al. The fractions of the cluster luminosities in dwarf galaxies fainter than our limits are less than 3 x 10^-3 and 3 x 10^-4, respectively. Thus, 2/7 of the SNe would be associated with less than 3 x 10^-3 of the luminosity attributed to galaxies. We argue, instead, that the progenitors of both events were probably members of a diffuse population of intergalactic stars, recently detected in local clusters via planetary nebulae and red giants. Considering the higher detectability of hostless SNe compared to normal SNe, we estimate that 20^{+12}_{-15} percent of the SN Ia parent stellar population in clusters is intergalactic. This fraction is consistent with other measurements of the intergalactic stellar population, and implies that the process that produces intergalactic stars (e.g., tidal disruption of cluster dwarfs) does not disrupt or enhance significantly the SN Ia formation mechanism. Hostless SNe are potentially powerful tracers of the formation of the intergalactic stellar population out to high redshift.

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arXiv2002

Reverberation Mapping of High-Redshift, High-Luminosity Quasars

We present preliminary results from a reverberation mapping program to measure the Broad Line Region size in high-redshift, high-luminosity quasars. The observations are carried out at the Hobby-Eberly Telescope and at the Wise Observatory. The data cover 8 yr of photometric monitoring of 11 quasars, and 2.5 yr of spectrophotometric monitoring of 7 of these sources. Thus far we detected continuum variations but no line variations. We find that the continua of the high-luminosity quasars have smaller variability amplitudes and longer variability timescales compared with low-luminosity AGNs.

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arXiv2002

A Survey for Large Separation Lensed FIRST Quasars, II. Magnification Bias and Redshift Distribution

The statistics of large-separation gravitational lensing are a powerful tool to probe mass distributions on the scale of galaxy clusters. In this paper we refine the analysis of our survey for large-separation (>5'') lensed FIRST quasars (Ofek et al. 2001) by estimating the magnification bias and the source redshift distribution. Finding no large separation lens among 8000 likely quasars in that sample, implies an upper bound on the lensed fraction of 3.7 x 10^-4 at 95% CL. From a published deep 1.4 GHz radio survey of the Hubble Deep Field, and corresponding optical searches for faint quasars, we calculate a lower limit to the `double flux magnification bias' affecting our radio-optically selected sample, of B>1.1. From the four-colour information in the SDSS Early Data Release, we calculate the photometric redshift distribution of a sample of FIRST quasar candidates and compare it with the redshift distribution from the FIRST Bright Quasar Survey. We find that the median redshift of the quasars in our sample is about 1.4. With these new results, we find that for all plausible cosmologies, the absence of lensed quasars in our survey is consistent with a model based on an empirical, non-evolving, cluster mass function, where clusters are represented by singular isothermal spheres. On the other hand, comparison of our results to the lensing predictions of published N-body-ray-tracing simulations (Wambsganss et al. 1995, 1998) rejects the COBE normalised Ω_{0}=1 CDM model at 99.9% confidence.

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arXiv2002

X-ray vs. Optical Variations in the Seyfert 1 Nucleus NGC 3516: A Puzzling Disconnectedness

We present optical broadband (B and R) observations of the Seyfert 1 nucleus NGC 3516, obtained at Wise Observatory from March 1997 to March 2002, contemporaneously with X-ray 2-10 keV measurements with RXTE. With these data we increase the temporal baseline of this dataset to 5 years, more than triple to the coverage we have previously presented for this object. Analysis of the new data does not confirm the 100-day lag of X-ray behind optical variations, tentatively reported in our previous work. Indeed, excluding the first year's data, which drive the previous result, there is no significant correlation at any lag between the X-ray and optical bands. We also find no correlation at any lag between optical flux and various X-ray hardness ratios. We conclude that the close relation observed between the bands during the first year of our program was either a fluke, or perhaps the result of the exceptionally bright state of NGC 3516 in 1997, to which it has yet to return. Reviewing the results of published joint X-ray and UV/optical Seyfert monitoring programs, we speculate that there are at least two components or mechanisms contributing to the X-ray continuum emission up to 10 keV: a soft component that is correlated with UV/optical variations on timescales >1 day, and whose presence can be detected when the source is observed at low enough energies (about 1 keV), is unabsorbed, or is in a sufficiently bright phase; and a hard component whose variations are uncorrelated with the UV/optical.

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arXiv2002

Comment on "Measuring the black hole masses of high redshift quasars"

In a recent paper, McLure & Jarvis (2002, astro-ph/0204473, v.1) reanalyze AGN broad-line reverberation measurements presented in Kaspi et al. (2000). They find the broad-line region size and the AGN luminosity at 5100 Ang can be related by R_{BLR} ~ L_{5100}^{0.5}. This differs from the result of Kaspi et al., who found R_{BLR} ~ L_{5100}^{0.7}, a departure from expectations for a constant ionization parameter. In this note I investigate the source of the discrepancy. McLure & Jarvis use quasar luminosities based on the single-epoch, multichannel-spectrophotometer measurements of Neugebauer et al. (1987), obtained in 1980. I show that the Neugebauer et al. fluxes are systematically higher, by a constant flux offset, compared with the multi-epoch CCD measurements of Kaspi et al., obtained concurrently with the echo-mapping radii. In addition, McLure & Jarvis erred when converting these fluxes to luminosities. The two effects compound to a typical factor 2 (0.3 dex) overestimate of the luminosities of the PG quasars in the sample. Since McLure & Jarvis did adopt the Seyfert luminosities in Kaspi et al. (2000), they obtained a slope that is flatter by 0.2.

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arXiv2002

Not Color Blind: Using Multiband Photometry to Classify Supernovae

Large numbers of supernovae (SNe) have been discovered in recent years, and many more will be found in the near future. Once discovered, further study of a SN and its possible use as an astronomical tool (e.g., as a distance estimator) require knowledge of the SN type. Current classification methods rely almost solely on the analysis of SN spectra to determine their type. However, spectroscopy may not be possible or practical when SNe are faint, numerous, or discovered in archival studies. We present a classification method for SNe based on the comparison of their observed colors with synthetic ones, calculated from a large database of multi-epoch optical spectra of nearby events. We discuss the capabilities and limitations of this method. For example, type Ia SNe at redshifts z < 0.1 can be distinguished from most other SN types during the first few weeks of their evolution, based on V-R vs. R-I colors. Type II-P SNe have distinct (very red) colors at late (t > 100 days) stages. Broadband photometry through standard Johnson-Cousins $UBVRI$ filters can be useful to classify SNe out to z ~ 0.6. The use of Sloan Digital Sky Survey (SDSS) ugriz filters allows the extension of our classification method to even higher redshifts (z = 0.75), and the use of infrared bands, to z = 2.5. We demonstrate the application of this method to a recently discovered SN from the SDSS. Finally, we outline the observational data required to further improve the sensitivity of the method, and discuss prospects for its use on future SN samples. Community access to the tools developed is provided by a dedicated website (http://wise-obs.tau.ac.il/~dovip/typing) .

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arXiv2001

Supernovae in Deep Hubble Space Telescope Galaxy Cluster Fields: Cluster Rates and Field Counts

We have searched for high-redshift supernova (SN) candidates in multiple deep Hubble Space Telescope (HST) archival images of nine galaxy-cluster fields. We detect six apparent SNe, with I814 between 21.6 and 28.4 mag. There is roughly 1 SN per deep (flux limit I814 > 26 mag), doubly-imaged, WFPC2 cluster field. Two SNe are associated with cluster galaxies (at redshifts z=0.18 and z=0.83), three are probably in galaxies not in the clusters (at z=0.49, z=0.60, and z=0.98), and one is at unknown z. After accounting for observational efficiencies and uncertainties (statistical and systematic) we derive the rate of type-Ia SNe within the projected central 500 kpc of rich clusters: R=0.20(+0.84)(-0.19) SNu in clusters at z=0.18 to 0.37, and R=0.41(+1.23)(-0.39) SNu in clusters at z=0.83 to 1.27 (95 per cent confidence interval; H_0=50; 1 SNu = 1 SN per century per 10^10 L_B_sun). Combining the two redshift bins, the SN rate at a mean redshift of z=0.41 is R(z=0.41) = 0.30(+0.58)(-0.28) SNu. The upper bounds argue against SNe-Ia being the dominant source of the large iron mass observed in the intra-cluster medium. We also compare our observed counts of field SNe (i.e., non-cluster SNe of all types) to recent model predictions. The observed field count is zero or one SN with I814 < 26 mag, and 1 to 3 SNe with I814 < 27 mag. These counts are about two times lower than some of the predictions. Since the counts at these magnitudes are likely dominated by type-II SNe, our observations may suggest obscuration of distant SNe-II, or a SN-II luminosity distribution devoid of a large high-luminosity tail.

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