arXiv subjects

Alexei V. Filippenko

Publications and source records attributed to Alexei V. Filippenko.

At least 577 records · Page 32Linked to original sources

arXiv2002

A Hubble Space Telescope Snapshot Survey of Nearby Supernovae

We present photometry of 12 recent supernovae (SNe) recovered in a {\it Hubble Space Telescope} Snapshot program, and tie the measurements to earlier ground-based observations, in order to study the late-time evolution of the SNe. Many of the ground-based measurements are previously unpublished, and were made primarily with a robotic telescope, the Katzman Automatic Imaging Telescope. Evidence for circumstellar interaction is common among the core-collapse SNe. Late-time decline rates for Type IIn SNe are found to span a wide range, perhaps due to differences in circumstellar interaction. An extreme case, SN IIn 1995N, declined by only 1.2 mag in $V$ over about 4 years following discovery. Template images of some SNe must therefore be obtained many years after the explosion, if contamination from the SN itself is to be minimized. Evidence is found against a previous hypothesis that the Type IIn SN 1997bs was actually a superoutburst of a luminous blue variable star. The peculiar SN Ic 1997ef, a "hypernova," declined very slowly at late times. The decline rate of the SN Ia 2000cx decreased at late times, but this is unlikely to have been caused by a light echo.

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arXiv2001

Einstein's Biggest Blunder? High-Redshift Supernovae and the Accelerating Universe

Nearly 4 years ago, two teams of observational astronomers reported that high-redshift Type Ia supernovae are fainter than expected in a decelerating or freely coasting universe. The radical conclusion that the universe has been accelerating in the past few billion years, possibly because of a nonzero value for Einstein's cosmological constant, has gripped the worlds of astronomy and physics, causing a flurry of new research. Having participated on both teams (but much more closely with one than the other), here I provide a personal, historical account of the story.

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arXiv2001

BVRI Photometry of Supernovae

We present optical photometry of one Type IIn supernova (1994Y) and nine Type Ia supernovae (1993Y, 1993Z, 1993ae, 1994B, 1994C, 1994M, 1994Q, 1994ae, and 1995D). SN 1993Y and SN 1993Z appear to be normal SN Ia events with similar rates of decline, but we do not have data near maximum brightness. The colors of SN 1994C suggest that it suffers from significant reddening or is intrinsically red. The light curves of SN 1994Y are complicated; they show a slow rise and gradual decline near maximum brightness in $VRI$ and numerous changes in the decline rates at later times. SN 1994Y also demonstrates color evolution similar to that of the SN IIn 1988Z, but it is slightly more luminous and declines more rapidly than SN 1988Z. The behavior of SN1994Y indicates a small ejecta mass and a gradual strengthening of the H$α$ emission relative to the continuum.

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arXiv2001

Measuring Black Hole Masses Using Ionized Gas Kinematics

We describe techniques for measuring the central masses of galaxies using emission-line kinematics observed with the Hubble Space Telescope. For accurate results, it is necessary to model various instrumental effects, particularly the blurring due to the telescope PSF and the width of the spectroscopic aperture. Observations of nuclear gas disks often reveal substantial internal velocity dispersions in the gas, suggesting that the disks may be partially pressure-supported. We also describe a technique for fitting 2-dimensional spectroscopic data directly in pixel space. This method may be useful for objects such as M84 that show highly complex and asymmetric line profiles.

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arXiv2001

Limits on the Mass of the Central Black Hole in 16 Nearby Bulges

We report upper limits on the masses of black holes that can be present in the centers of 16 nearby galaxy bulges. These limits for our statistically complete sample were derived from the modeling of the central emission-line widths ([N II] or [S II]), observed over a 0.25" X 0.2" (R < 9 pc) aperture. The experiment has a mean detection sensitivity of 3.9 X 10^6 solar masses. For three sample members with direct determinations of black hole masses our upper limits agree within the uncertainties, while in general our upper limits are found to be close to the masses measured in other bulges with global properties similar to ours. Remarkably, our limits lie quite closely to the recently derived relation between black hole mass and stellar velocity dispersion. These results support a picture wherein the black hole mass and overall galaxy structure are closely linked, as galaxies with exceptionally high black hole masses at a given velocity dispersion apparently are rare.

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arXiv2001

An Efficient Strategy to Select Targets for Gas-Dynamical Measurements of Black Hole Masses Using the Hubble Space Telescope

Gas-dynamical studies using the Hubble Space Telescope are an integral component for future progress in the search for massive black holes in galactic nuclei. Here we present an extensive set of gas rotation curves obtained with the Space Telescope Imaging Spectrograph for the central regions of 23 disk galaxies. We find that the bulges of randomly selected, nearby spiral and S0 galaxies generally do not contain well-defined gaseous disks. Only 15%-20% of disk galaxies have regular, symmetric velocity fields useful for dynamical analysis. Through comparison of the kinematics with Hubble Space Telescope images of the nuclear regions, we show that the probability of success can be significantly boosted by preselecting objects whose central dust lanes follow a well-ordered, circularly symmetric pattern. The dust morphology can be ascertained efficiently by visual inspection of unsharp-masked images.

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arXiv2001

Nuclear Cusps and Cores in Early-type Galaxies As Relics of Binary Black Hole Mergers

We present an analysis of the central cusp slopes and core parameters of early-type galaxies using a large database of surface brightness profiles obtained from Hubble Space Telescope observations. We examine the relation between the central cusp slopes, core parameters, and black hole masses in early-type galaxies, in light of two models that attempt to explain the formation of cores and density cusps via the dynamical influence of black holes. Contrary to the expectations from adiabatic-growth models, we find that the cusp slopes do not steepen with increasing black hole mass fraction. Moreover, a comparison of kinematic black hole mass measurements with the masses predicted by the adiabatic models shows that they overpredict the masses by a factor of approximately 3. Simulations involving binary black hole mergers predict that both the size of the core and the central mass deficit correlate with the final black hole mass. These relations are qualitatively supported by the present data.

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arXiv2001

The Distance to SN 1999em in NGC 1637 from the Expanding Photosphere Method

We present 30 optical spectra and 49 photometric epochs sampling the first 517 days after discovery of supernova (SN) 1999em, and derive its distance through the expanding photosphere method (EPM). SN 1999em is shown to be a Type II-plateau (II-P) event, with a photometric plateau lasting until about 100 days after explosion. We identify the dominant ions responsible for most of the absorption features seen in the optical portion of the spectrum during the plateau phase. We find the distance to SN 1999em to be D = 8.2 +/- 0.6 Mpc, with an explosion date of 5.3 +/- 1.4 days before discovery. We examine potential sources of systematic error in EPM-derived distances, and find the most significant to result from uncertainty in the theoretical modeling of the flux distribution emitted by the SN photosphere (i.e., the ``flux dilution factor''). We compare previously derived EPM distances to 5 SNe II in galaxies for which a recently revised Cepheid distance exists from the HST Key Project and find D(Cepheids) / D(EPM) = 0.96 +/- 0.09. Finally, we investigate the possible use of SNe II-P as standard candles and find that for 8 photometrically confirmed SNe II-P with previously derived EPM distances and SN 1999em, the mean plateau absolute brightness is M_V(plateau) = -16.4 +/- 0.6 mag, implying that distances good to ~30% (1-sigma) may be possible without the need for a complete EPM analysis. At M_V(plateau) = -15.9 +/- 0.2 mag, SN 1999em is somewhat fainter than the average SN II-P. The general consistency of absolute SNe II-P brightness during the plateau suggests that the standard candle assumption may allow SNe II-P to be viable cosmological beacons at z > 2.

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arXiv2001

Optical and Ultraviolet Spectroscopy of SN 1995N: Evidence for Strong Circumstellar Interaction

Optical and ultraviolet observations of the Type IIn supernova 1995N at epochs between 321 and 1799 days after the explosion show three distinct velocity components. The narrow lines come from circumstellar gas and show both low and high ionization. This component has a low filling factor, and is photoionized by X-rays from the shock. The intermediate component, which is dominated by newly processed oxygen, originates in a shell with velocity of 2500-5000 km/s, and most likely comes from the ejecta. The hydrogen- and helium-dominated gas has a low ionization, a high density, and velocities that extend out to ~10,000 km/s. Strong signatures of Ly-alpha-pumped fluorescence lines of Fe II are seen in the near-infrared and ultraviolet. The He/H ratio, ~ 0.3 by number, and the nitrogen overabundance provide strong evidence for CNO burning products. The fluxes of the broad hydrogen and helium lines decrease considerably faster than the oxygen lines. The H-alpha line profile shows strong evolution, with the red wing decreasing faster than the blue. Possible scenarios, involving either a clumpy circumstellar medium, or an aspherical distribution of the surrounding gas, are discussed based on the line profiles and physical conditions. Finally, we propose that Type IIn supernovae have their origin in red supergiants in a superwind phase.

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arXiv2001

The Unique Type Ia Supernova 2000cx in NGC 524

We present extensive photometric and spectroscopic observations of the Type Ia supernova (SN Ia) 2000cx in the S0 galaxy NGC 524, which reveal it to be peculiar. Photometrically, SN 2000cx is different from all known SNe Ia, and its light curves cannot be fit well by the fitting techniques currently available. There is an apparent asymmetry in the $B$-band peak, in which the premaximum brightening is relatively fast (similar to that of the normal SN 1994D), but the postmaximum decline is relatively slow (similar to that of the overluminous SN 1991T). The color evolution of SN 2000cx is also peculiar: the $(B - V)_0$ color has a unique plateau phase and the $(V - R)_0$ and $(V - I)_0$ colors are very blue. Although the premaximum spectra of SN 2000cx are similar to those of SN 1991T-like objects (with weak Si II lines), its overall spectral evolution is quite different. The Si II lines that emerged near maximum $B$-band brightness stay strong in SN 2000cx until about three weeks past maximum. The change in the excitation stages of iron-peak elements is slow. Both the iron-peak and the intermediate-mass elements are found to be moving at very high expansion velocities in the ejecta of SN 2000cx. We discuss theoretical models for SN 2000cx. SN 2000cx may be an overluminous object like SN 1991T, but with a larger yield of $^{56}$Ni and a higher kinetic energy in the ejecta. We also briefly discuss the implications of our observations for the luminosity vs. light-curve width relation.

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arXiv2001

A Composite Seyfert 2 X-ray Spectrum: Implications for the Origin of the Cosmic X-ray Background

We present a composite 1-10 keV Seyfert 2 X-ray spectrum, derived from ASCA observations of a distance-limited sample of nearby galaxies. All 29 observed objects were detected. Above ~3 keV, the composite spectrum is inverted, confirming that Seyfert 2 galaxies as a class have the spectral properties necessary to explain the flat shape of the cosmic X-ray background spectrum. Integrating the composite spectrum over redshift, we find that the total emission from Seyfert 2 galaxies, combined with the expected contribution from unabsorbed type 1 objects, provides an excellent match to the spectrum and intensity of the hard X-ray background. The principal uncertainty in this procedure is the cosmic evolution of the Seyfert 2 X-ray luminosity function. Separate composite spectra for objects in our sample with and without polarized broad optical emission lines are also presented.

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arXiv2001

Central Structural Parameters of Early-Type Galaxies as Viewed with HST/NICMOS

We present surface photometry for the central regions of a sample of 33 early-type (E, S0, and S0/a) galaxies observed at 1.6 microns (H band) using the Hubble Space Telescope (HST). We employ a new technique of two-dimensional fitting to extract quantitative parameters for the bulge light distribution and nuclear point sources, taking into consideration the effects of the point-spread function. Parameterizing the bulge profile with a ``Nuker'' law, we confirm that the central surface-brightness distributions largely fall into two categories, each of which correlates with the global properties of the galaxies. ``Core'' galaxies tend to be luminous ellipticals with boxy or pure elliptical isophotes, whereas ``power-law'' galaxies are preferentially lower luminosity systems with disky isophotes. Unlike most previous studies, however, we do not find a clear gap in the distribution of inner cusp slopes; several objects have inner cusp slopes (0.3 < gamma < 0.5) which straddle the regimes conventionally defined for core and power-law type galaxies. The nature of these intermediate objects is unclear. We draw attention to two objects in the sample which appear to be promising cases of galaxies with isothermal cores that are not the brightest members of a cluster. Unresolved nuclear point sources are found in about 50% of the sample galaxies, roughly independent of profile type, with magnitudes in the range m^{nuc}_H = 12.8 to 17.4 mag, which correspond to M_H^{nuc} = -12.8 to -18.4 mag. (Abridged)

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arXiv2001

Is it Round? Spectropolarimetry of the Type II-P Supernova 1999em

We present the first multi-epoch spectropolarimetry of a type II plateau supernova (SN II-P), with optical observations of SN 1999em on days 7, 40, 49, 159, and 163 after discovery. These data are used to probe the geometry of the electron-scattering atmosphere before, during, and after the plateau phase, which ended roughly 90 days after discovery. Weak continuum polarization with an unchanging polarization angle (theta ~ 160 deg) is detected at all epochs, with p ~ 0.2% on day 7, p ~ 0.3% on days 40 and 49, and p ~ 0.5% in the final observations. Distinct polarization modulations across strong line features are present on days 40, 49, 159, and 163. Uncorrected for interstellar polarization (which is believed to be quite small), polarization peaks are associated with strong P Cygni absorption troughs and nearly complete depolarization is seen across the H-alpha emission profile. The temporal evolution of the continuum polarization and sharp changes across lines indicate polarization intrinsic to SN 1999em. When modeled in terms of the oblate, electron-scattering atmospheres of Hoeflich, the observed polarization implies anasphericity of at least 7% during the period studied. The temporal polarization increase may indicate greater asphericity deeper into the ejecta. We discuss the implications of asphericity on the use of type II-P supernovae as primary extragalactic distance indicators through the expanding photosphere method (EPM). If asphericity produces directionally dependant flux and peculiar galaxy motions are characterized by sigma_v_rec = 300 km/s, it is shown that the agreement between previous EPM measurements of SNe II and distances to the host galaxies predicted by a linear Hubble law restrict mean SN II asphericity to values less than 30% (3-sigma) during the photospheric phase.

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arXiv2001

Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi

We present single-epoch spectropolarimetry of the Type II supernovae (SNe II) 1997ds, 1998A, and 1999gi. SN 1997ds and SN 1998A were both observed during the early photospheric phase, while spectropolarimetry of SN 1999gi was obtained near the start of the transition to the nebular phase. Uncorrected for interstellar polarization (ISP), SN 1997ds is characterized by p_V = 0.85 +/- 0.02%, SN 1998A has p_V = 0.24 +/- 0.05%, and SN 1999gi is polarized at p_V = 5.72 +/- 0.01%. The low continuum polarization inferred for SN 1997ds and SN 1998A and the amplitude of polarization modulations across strong line features are consistent with those measured at similar epochs for SN 1987A and the Type II-plateau SN 1999em, and supports the growing consensus that core-collapse events with hydrogen envelopes substantially intact at the time of explosion are not significantly aspherical during the early photospheric phase. The spectral shape of the high continuum polarization of SN 1999gi closely resembles a ``Serkowski'' ISP curve, and is inconsistent with the wavelength-independent nature of electron scattering expected for an aspherical SN atmosphere. Since Galactic reddening is minimal along this line-of-sight, the majority of the observed polarization in SN 1999gi is believed to be due to ISP of the host galaxy, although significant (up to p = 2%) intrinsic polarization cannot be ruled out. The potential power of SN spectropolarimetry to study the properties of interstellar dust in external galaxies is described and applied to the SN 1999gi data, where it is shown that if the polarization is indeed predominantly interstellar in origin, then R_V = 3.0 +/- 0.2 for the dust along this line-of-sight in NGC 3184.

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arXiv2001

The Soft X-ray Properties of Nearby Low-Luminosity Active Galactic Nuclei and their Contribution to the Cosmic X-ray Background

We have examined ROSAT soft X-ray observations of a complete, distance-limited sample of Seyfert and LINER galaxies. X-ray data are available for 46 out of 60 such objects which lie within a hemisphere of radius 18 Mpc. We have constructed radial profiles of the nuclear sources in order to characterize their spatial extent and, in some cases, to help constrain the amount of flux associated with a nuclear point source. PSPC data from ROSAT have been used to explore the spectral characteristics of the objects with sufficient numbers of detected counts. Based on the typical spectral parameters of these sources, we have estimated the luminosities of the weaker sources in the sample. We then explore the relationship between the soft X-ray and H alpha luminosities of the observed objects; these quantities are correlated for higher-luminosity AGNs. We find a weak correlation at low luminosities as well, and have used this relationship to predict L_X for the 14 objects in our sample that lack X-ray data. Using the results of the spatial and spectral analyses, we have compared the X-ray properties of Seyferts and LINERs, finding no striking differences between the two classes of objects. However, both types of objects often exhibit significant amounts of extended emission, which could minimize the appearance of differences in their nuclear properties. The soft X-ray characteristics of the type 1 and type 2 active galaxies in the sample are also discussed. We then compute the local X-ray volume emissivity of low-luminosity Seyferts and LINERs and investigate their contribution to the cosmic X-ray background. The 0.5-2.0 keV volume emissivity of 2.2e38 ergs/s/Mpc^3 we obtain for our sample suggests that low-luminosity AGNs produce at least 9% of the soft X-ray background.

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arXiv2001

The Farthest Known Supernova: Support for an Accelerating Universe and a Glimpse of the Epoch of Deceleration

We present photometric observations of an apparent Type Ia supernova (SN Ia) at a redshift of ~1.7, the farthest SN observed to date. SN 1997ff, was discovered in a repeat observation by the HST of the HDF-), and serendipitously monitored with NICMOS on HST throughout the GTO campaign. The SN type can be determined from the host galaxy type:an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse SNe. The class- ification is further supported by diagnostics available from the observed colors and temporal behavior of the SN, both of which match a typical SN Ia. The photo- metric record of the SN includes a dozen flux measurements in the I, J, and H bands spanning 35 days in the observed frame. The redshift derived from the SN photometry, z=1.7+/-0.1, is in excellent agreement with the redshift estimate of z=1.65+/-0.15 derived from the U_300,B_450,V_606,I_814,J_110,J_125,H_160, H_165,K_s photometry of the galaxy. Optical and near-infrared spectra of the host provide a very tentative spectroscopic redshift of 1.755. Fits to observations of the SN provide constraints for the redshift-distance relation of SNe~Ia and a powerful test of the current accelerating Universe hypothesis. The apparent SN brightness is consistent with that expected in the decelerating phase of the preferred cosmological model, Omega_M~1/3, Omega_Lambda~2/3. It is inconsistent with grey dust or simple luminosity evolution, candidate astro- physical effects which could mimic past evidence for an accelerating Universe from SNe Ia at z~0.5.We consider several sources of possible systematic error including lensing, SN misclassification, selection bias, and calibration errors. Currently, none of these effects appears likely to challenge our conclusions.

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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.

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arXiv2001

The Central Regions of Early-Type Galaxies Hosting Active Galactic Nuclei as Viewed with HST/NICMOS

We present preliminary results from surface photometry of a sample of early-type galaxies observed with NICMOS on board HST in the F160W (H-band) filter. Dust is found to occur mostly in galaxies which possess unresolved nuclei, and in a few cases the presence of dust causes the isophotes to appear boxy. We performed a 2-D modeling of the host galaxy light distribution using the empirical Nuker function, which proves to be an adequate representation of the surface-brightness profile for most early-type galaxies. The accurate modeling of the bulge light allows us to obtain the magnitude of the unresolved central source without contamination from the underlying stellar population. Our classification of early-type galaxies into core-type and power-law galaxies agrees with the results from previous work based on HST optical images, with core profiles occurring in luminous galaxies and power-law profiles occurring in low-luminosity galaxies with disky isophotes. We do not find a clear dichotomy in the distribution of inner slopes. Unresolved nuclear point sources are evident in about one-third of the sample galaxies, with H magnitudes in the range 13.5 to 17.3. Most of the type 1 Seyfert and LINER nuclei (83%) reveal a distinct point source, while the detection rate is much less for type 2 nuclei (35%). The occurrence of point sources appears to be independent of whether the underlying galaxy has core-type profile or power-law profile.

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