35 research outputs found
Chandra Detection of Highest Redshift (z~6) Quasars in X-rays
We report on Chandra observations of three quasars SDSSP J083643+005453,
SDSSP J103027+052455, and SDSSP J130608+035626 at redshifts 5.82, 6.28 and 5.99
respectively. All the three sources are clearly detected in the X-ray band, up
to rest frame energies of ~55 keV. These observations demonstrate the
unprecedented sensitivity of Chandra to detect faint sources in relatively
short exposure times (5.7--8.2 ksec). The broad band X-ray properties of these
highest redshift quasars do not appear to be any different from their lower
redshift cousins. Spectra of the sources could not be determined with only few
counts detected. Observations with XMM-Newton will be able to constrain the
spectral shapes, if they are simple. Determination of complex spectra in a
reasonable amount of time, however, will have to await next generation of X-ray
missions.Comment: Submitted to ApJ Letter
Finding Local Low-mass Supermassive Black Holes
The low-mass end of the supermassive black hole mass function is unknown and
difficult to determine. Here we discuss our successful program to find active
nuclei of late type "normal" galaxies using X-ray detections and
multiwavelength identifications. We conclude that most of the Chandra detected
nuclear X-ray sources are AGNs. We then outline methods of black hole mass
determination when broad emission lines are unobservable.Comment: 7 pages. To appear in the proceedings of the Second Kolkata
Conference on Observational Evidence for Black Holes, Kolkata, February 200
Detecting Low-Mass Supermassive Black Holes
We demonstrate the feasibility of uncovering supermassive black holes in
late-type, quiescent spiral galaxies by detecting signs of very low-level
nuclear activity. We use a combination of x-ray selection and multi-wavelength
follow-up. Here, we apply this technique to NGC 3184 and NGC 5457, both of type
Scd, and show that strong arguments can be made that both host AGNs.Comment: 4 pages. To appear in the proceedings of the Second Kolkata
Conference on Observational Evidence for Black Holes, Kolkata, February 200
Chandra Observations of Candidate "True" Seyfert 2 Nuclei
The Unification Model for active galactic nuclei posits that Seyfert 2s are
intrinsically like Seyfert 1s, 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 has a broad H-alpha emission
line, either in direct or polarized light. All four have rich, high-excitation
spectra, blue continua, and Hubble Space Telescope (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 the Chandra X-ray Observatory. 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 extra-nuclear soft emission extended on the
hundreds-of-parsecs 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.Comment: 26 pages, 12 figures, submitted to Ap
Galaxy Clusters in the IRAC Dark Field. II. Mid-Infrared Sources
We present infrared (IR) luminosities, star formation rates (SFR), colors, morphologies, locations, and active galactic nuclei (AGNs) properties of 24 μm detected sources in photometrically detected high-redshift clusters in order to understand the impact of environment on star formation (SF) and AGN evolution in cluster galaxies. We use three newly identified z = 1 clusters selected from the IRAC dark field; the deepest ever mid-IR survey with accompanying, 14 band multiwavelength data including deep Hubble Space Telescope imaging and deep wide-area Spitzer MIPS 24 μm imaging. We find 90 cluster members with MIPS detections within two virial radii of the cluster centers, of which 17 appear to have spectral energy distributions dominated by AGNs and the rest dominated by SF. We find that 43% of the star-forming sample have IR luminosities L_(IR) > 10^(11) L_☉(luminous IR galaxies). The majority of sources (81%) are spirals or irregulars. A large fraction (at least 25%) show obvious signs of interactions. The MIPS-detected member galaxies have varied spatial distributions as compared to the MIPS-undetected members with one of the three clusters showing SF galaxies being preferentially located on the cluster outskirts, while the other two clusters show no such trend. Both the AGN fraction and the summed SFR of cluster galaxies increase from redshift zero to one, at a rate that is a few times faster in clusters than over the same redshift range in the field. Cluster environment does have an effect on the evolution of both AGN fraction and SFR from redshift one to the present, but does not affect the IR luminosities or morphologies of the MIPS sample. SF happens in the same way regardless of environment making MIPS sources look the same in the cluster and field, however the cluster environment does encourage a more rapid evolution with time as compared to the field
Low-Level Nuclear Activity in Nearby Spiral Galaxies
We are conducting a search for supermassive black holes (SMBHs) with masses
below 10^7 M_sun 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: (a) X-ray selection using the Chandra X-ray Observatory, since
x-rays are a ubiquitous feature of AGNs; (b) Emphasis on late-type spiral and
dwarf galaxies, as the galaxies most likely to have low-mass SMBHs; (c) Use of
multiwavelength data to verify the source is an AGN; and (d) Use of the highest
angular resolution available for observations in x-rays and other bands, to
separate nuclear from off-nuclear sources and to minimize contamination by host
galaxy light. Here we show the feasibility of this technique to find AGNs by
applying it to six nearby, face-on spiral galaxies (NGC 3169, NGC 3184, NGC
4102, NGC 4647, NGC 4713, 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.Comment: 37 pages, 10 figures, ApJ, in press. Replaced with accepted versio
Turbulent heat transfer enhancement in tubular heat exchangers with different twisted tape inserts
Experimental and numerical analyses are carried out to investigate the influence of twisted tape inserts on the heat transfer and the flow behavior in double tube heat exchangers. First, all the performance factors, namely the Nusselt number, friction factor, and thermal performance factor, were studied for a basic heat exchanger (BHE). Afterwards, twisted tapes with three different twist ratios (7.5, 6, and 4.5) were inserted inside the inner tube of the BHE, which resulted in three different modified heat exchangers (MHEs). For the numerical study, a 3D numerical model is developed with the k-ε RNG turbulent model to visualize the flow and the heat transfer behavior inside the heat exchangers. In both studies, turbulent flow field is maintained, ranging Reynolds number from 15000 to 50000. From the experimental result, an enhanced heat transfer, characterized by the performance factors, is found for all the MHEs compared to the BHE. The most enhanced thermal performance factor is achieved for the MHE with the lowest twist ratio. Finally, a good agreement between obtained numerical and experimental results reveals that the present numerical model can reliably predict the flow and heat transfer behavior in double tube heat exchangers
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 H_beta
emission-line light curve with the optical continuum light curve reveals an
emission-line time delay of 3.73 (+-0.75) days. By combining this time delay
with the H_beta line width, we derive a central black hole mass of M_BH =
9.8(+-2.1)x10^6 M_sun, an improvement in precision of a factor of several over
past results.Comment: 22 pages, 3 tables, 5 figures, accepted for publication in Ap
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 L(5100) = 4.7
x 10^42 ergs s^-1. We determine a rest-frame time lag between flux variations
in the continuum and the Hbeta line of 6.3 (+2.6/-2.3) days. Combining our
measurements with those of previous campaigns, we determine a weighted black
hole mass of M_BH = 6.54 (+0.26/-0.25) x 10^7 M_sun 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 alpha = 0.5, the naive expectation for the broad line region for an
assumed form of r ~ L^alpha. This value is also consistent with the slope
recently determined by Bentz et al. for the population of reverberation-mapped
AGNs as a whole.Comment: 24 pages, 3 tables, 7 figures, accepted for publication in Ap