Himel Ghosh

A reverberation-based mass for the central black hole in NGC 4151

We have undertaken a new ground-based monitoring campaign to improve the estimates of the mass of the central black hole in NGC 4151. We measure the lag time of the broad H? line response compared to the optical continuum at 5100 ? and find a lag of 6.6 days. We combine our data with the recent reanalysis of UV emission lines by Metzroth and coworkers to calculate a weighted mean of the black hole mass, MBH = (4.57) ? 107 M?. The absolute calibration of the black hole mass is based on normalization of the AGN black hole mass-stellar velocity dispersion (MBH-?*) relationship to that of quiescent galaxies by Onken and coworkers. The scatter in the MBH-?* relationship suggests that reverberation-mapping-based mass measurements are typically uncertain by a factor of 3-4.

The Mass of the Black Hole in the Seyfert 1 Galaxy NGC 4593 from Reverberation Mapping

We present new observations leading to an improved black hole mass estimate for the Seyfert 1 galaxy NGC 4593 as part of a reverberation-mapping campaign conducted at the MDM Observatory. Cross-correlation analysis of the Hβ emission-line light curve with the optical continuum light curve reveals an emission-line time delay of τcent = 3.73 ± 0.75 days. By combining this time delay with the Hβ line width, we derive a central black hole mass of MBH = (9.8 ± 2.1) × 106 M☉, an improvement in precision of a factor of several over past results.

NGC 5548 in a Low-Luminosity State: Implications for the Broad-Line Region

We describe results from a new ground-based monitoring campaign on NGC 5548, the best-studied reverberation-mapped AGN. We find that it was in the lowest luminosity state yet recorded during a monitoring program, namely L5100 = 4.7 × 1042 ergs s-1. We determine a rest-frame time lag between flux variations in the continuum and the Hβ line of 6.3 days. Combining our measurements with those of previous campaigns, we determine a weighted black hole mass of MBH = 6.54 × 107 M☉ based on all broad emission lines with suitable variability data. We confirm the previously discovered virial relationship between the time lag of emission lines relative to the continuum and the width of the emission lines in NGC 5548, which is the expected signature of a gravity-dominated broad-line region. Using this lowest luminosity state, we extend the range of the relationship between the luminosity and the time lag in NGC 5548 and measure a slope that is consistent with α = 0.5, the naive expectation for the broad-line region for an assumed form of r ∝ Lα. This value is also consistent with the slope recently determined by Bentz et al. for the population of reverberation-mapped AGNs as a whole.

Chandra multiwavelength project. II. First results of X-ray source properties

The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei [AGNs] and high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra Deep Field surveys and previous missions. We present the first results of X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near-on-axis X-ray-bright sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the log N- log S relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA, and Chandra Deep Field surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and noncluster fields and also search for field-to-field variation, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 σ) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few × 1020 cm2, suggesting that they are typical broadline AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude that it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio, and X-ray colors.

The X-Ray Properties of 2MASS Red Active Galactic Nuclei

The Two Micron All Sky Survey is finding previously unidentified, luminous, red, active galactic nuclei (AGNs). This new sample has a space density similar to, or greater than, previously known AGNs, suggesting that a large fraction of the overall population has been missed. Chandra observations of a well-defined subset of these objects reveal that all are X-ray-faint, with the reddest sources being the faintest in X-rays. The X-ray hardness ratios cover a wide range, generally indicating NH ~ 1021-1023 cm-2, but the softest sources show no spectral evidence for intrinsic absorption. These characteristics suggest that a mix of absorbed, direct emission and unabsorbed, scattered, and/or extended emission contributes to the X-ray flux, although we cannot rule out the possibility that they are intrinsically X-ray-weak. This population of X-ray-faint, predominantly broad-line objects could provide the missing population of X-ray-absorbed AGNs required by current models of the cosmic X-ray background. The existence of AGNs that display both broad emission lines and absorbed X-rays has important implications for unification schemes and emphasizes the need for care in assigning classifications to individual AGNs.

DISCOVERY OF NUCLEAR X-RAY SOURCES IN SINGS GALAXIES

We present the results of a search for nuclear X-ray activity in nearby galaxies using Chandra archival data in a sample of 62 galaxies from the Spitzer Infrared Nearby Galaxy Survey (SINGS). We detect 37 nuclear X-ray sources; seven of these are new detections. Most of the nuclear X-ray sources are likely to be AGNs. The fraction of galaxies hosting AGNs is thus about 60%, much higher than that found with optical searches, and demonstrates the efficacy of X-ray observations to find hidden AGNs in optically normal galaxies. We find that the nuclear X-ray sources are preferentially present in earlier type galaxies. Unlike what is observed at high redshift, we do not find a strong correlation between the AGN luminosity and the 24 micron luminosity of the host galaxy; we find a strong correlation with the 3.6 micron luminosity instead. This suggests that at the present epoch the accretion rate depends on the total mass of the galaxy, as perhaps does the black hole mass.

Chandra Observations of Candidate “True” Seyfert 2 Nuclei

The unification model for AGNs posits that Seyfert 2 galaxies are intrinsically like Seyfert 1 galaxies, but that their broad-line regions (BLRs) are hidden from our view. A Seyfert 2 nucleus that truly lacked a BLR, instead of simply having it hidden, would be a so-called true Seyfert 2. No object has as yet been conclusively proven to be one. We present a detailed analysis of four of the best true Seyfert 2 candidates discovered to date: IC 3639, NGC 3982, NGC 5283, and NGC 5427. None of the four have a broad Hα emission line, in either direct or polarized light. All four have rich, high-excitation spectra, blue continua, and HST images showing them to be unresolved sources with no host-galaxy obscuration. To check for possible obscuration on scales smaller than that resolvable by HST, we obtained X-ray observations using Chandra. All four objects show evidence of obscuration and therefore could have hidden BLRs. The picture that emerges is of moderate to high, but not necessarily Compton-thick, obscuration of the nucleus, with extranuclear soft emission extended on the hundreds of parsec scale that may originate in the narrow-line region. Since the extended soft emission compensates, in part, for the nuclear soft emission lost to absorption, both absorption and luminosity are likely to be severely underestimated unless the X-ray spectrum is of sufficient quality to distinguish the two components. This is of special concern where the source is too faint to produce a large number of counts, or where the source is too far away to resolve the extended soft X-ray-emitting region.

LOW-LEVEL NUCLEAR ACTIVITY IN NEARBY SPIRAL GALAXIES

We are conducting a search for supermassive black holes (SMBHs) with masses below ~107 M☉ by looking for signs of extremely low-level nuclear activity in nearby galaxies that are not known to be AGNs. Our survey has the following characteristics: (1) X-ray selection using the Chandra X-Ray Observatory, since X-rays are a ubiquitous feature of AGNs; (2) emphasis on late-type spiral and dwarf galaxies as the galaxies most likely to have low-mass SMBHs; (3) use of multiwavelength data to verify that the source is an AGN; and (4) use of the highest angular resolution available for observations in X-rays and other bands, in order to separate nuclear from off-nuclear sources and to minimize contamination by host galaxy light. Here we show the feasibility of this technique of finding AGNs by applying it to six nearby, face-on spiral galaxies (NGC 3169, NGC 3184, NGC 4102, NGC 4647, NGC 4713, and NGC 5457) for which data already exist in the Chandra archive. All six show nuclear X-ray sources. The data as they exist at present are ambiguous regarding the nature of the nuclear X-ray sources in NGC 4713 and NGC 4647. We conclude, in accord with previous studies, that NGC 3169 and NGC 4102 are almost certainly AGNs. Most interestingly, a strong argument can be made that NGC 3184 and NGC 5457, both of type Scd, host AGNs.

XMM-Newton Observations of Red AGNs

XMM-Newton spectra of five red 2MASS active galactic nuclei (AGNs), selected from a sample observed by Chandra to be relatively X-ray bright and to cover a range of hardness ratios, confirm the presence of substantial absorbing material in three sources with optical classifications ranging from type 1 to type 2. A flat (hard) power-law continuum is observed in the other two. The combination of X-ray absorption and broad optical emission lines suggests either a small (nuclear) absorber or a favored viewing angle so as to cover the X-ray source but not the broad emission-line region (BELR). A soft excess is detected in all three type 1 sources. We speculate that this may arise in an extended region of ionized gas, perhaps linked to the polarized (scattered) optical light present in these sources. The spectral complexity revealed by XMM-Newton emphasizes the limitations of the low-S/N Chandra data. The new results strengthen our earlier conclusions that the observed X-ray continua of red AGNs are unusually hard at energies 2 keV. Their observed spectra are consistent with contributing significantly to the missing hard or absorbed population of the cosmic X-ray background (CXRB), although their intrinsic power-law slopes are typical of broad-line (type 1) AGNs (Γ ~ 1.7-1.9). This suggests that the missing X-ray-absorbed CXRB population may include type 1 AGNs or QSOs in addition to the type 2 AGNs generally assumed.

POTENTIAL DIRECT SINGLE-STAR MASS MEASUREMENT

We analyze the light curve of the microlensing event OGLE-2003-BLG-175/MOA-2003-BLG-45 and show that it has two properties that, when combined with future high-resolution astrometry, could lead to a direct, accurate measurement of the lens mass. First, the light curve shows clear signs of distortion due to the Earths accelerated motion, which yields a measurement of the projected Einstein radius E. Second, from precise astrometric measurements, we show that the blended light in the event is coincident with the microlensed source to within about 15 mas. This argues strongly that this blended light is the lens and hence opens the possibility of directly measuring the lens-source relative proper motion μrel and so the mass M = (c2/4G)μreltEE, where tE is the measured Einstein timescale. While the light-curve-based measurement of E is, by itself, severely degenerate, we show that this degeneracy can be completely resolved by measuring the direction of proper motion μrel.