Andrew Gould

Black Hole Masses and Eddington Ratios at 0.3 < z < 4

We study the distribution of Eddington luminosity ratios, Lbol/LEdd, of active galactic nuclei (AGNs) discovered in the AGN and Galaxy Evolution Survey (AGES). We combine Hβ, Mg II, and C IV line widths with continuum luminosities to estimate black hole (BH) masses in 407 AGNs, covering the redshift range z ~ 0.3-4 and the bolometric luminosity range Lbol ~ 1045-1047 ergs s-1. The sample consists of X-ray or mid-infrared (24 μm) point sources with optical magnitude R ≤ 21.5 mag and optical emission-line spectra characteristic of AGNs. For the range of luminosity and redshift probed by AGES, the distribution of estimated Eddington ratios is well described as log-normal, with a peak at Lbol/LEdd 1/4 and a dispersion of 0.3 dex. Since additional sources of scatter are minimal, this dispersion must account for contributions from the scatter between estimated and true BH mass and the scatter between estimated and true bolometric luminosity. Therefore, we conclude that (1) neither of these sources of error can contribute more than ~0.3 dex rms, and (2) the true Eddington ratios of optically luminous AGNs are even more sharply peaked. Because the mass estimation errors must be smaller than ~0.3 dex, we can also investigate the distribution of Eddington ratios at fixed BH mass. We show for the first time that the distribution of Eddington ratios at fixed BH mass is peaked, and that the dearth of AGNs at a factor of ~10 below Eddington is real and not an artifact of sample selection. These results provide strong evidence that supermassive BHs gain most of their mass while radiating close to the Eddington limit, and they suggest that the fueling rates in luminous AGNs are ultimately determined by BH self-regulation of the accretion flow rather than galactic-scale dynamical disturbances.

Discovering planetary systems through gravitational microlenses

Planetary systems of Galactic disk stars can be detected as microlenses of stars in the Galactic bulge. Planets in a solar-like system positioned half-way to the Galactic center should leave a noticeable signature (magnification larger that 5%) on the light curve of a gravitationally lensed bulge star in ∼20% of the microlensing events. This high probability results from a coincidence between Jupiters orbital radius and the solar Einstein radius at this distance. Typical planetary signals last for about 1 day, a small fraction of the approximately 1 month duration of the entire microlensing event

Extending the MACHO search to about 10 exp 6 solar masses

The search for a microlensing (changing light-curve) signature of massive compact halo objects (Machos) by the Macho Collaboration is currently believed to be sensitive in the range 10 -7 -10 2 M ⊙ . Microlensing events from higher mass objects last longer than the 4 yr duration of the planned experiments and therefore, according to current beliefs, cannot be distinguished from long-term variables. In fact the signature of Machos in the range 10 2 -10 3 M ⊙ can be distinguished from background events by the annual modulation in light magnification induced by the Earths motion

Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog

We complete construction of a catalog containing improved astrometry and new optical/infrared photometry for the vast majority of NLTT stars lying in the overlap of regions covered by POSS I and by the second incremental Two Micron All Sky Survey (2MASS) release, approximately 44% of the sky. The epoch 2000 positions are typically accurate to 130 mas, the proper motions to 5.5 mas yr-1, and the V-J colors to 0.25 mag. Relative proper motions of binary components are measured to 3 mas yr-1. The false-identification rate is ~1% for 11 V 18 and substantially less at brighter magnitudes. These improvements permit the construction of a reduced proper-motion diagram that, for the first time, allows one to classify NLTT stars into main-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We in turn use this diagram to analyze the properties of both our catalog and the NLTT catalog on which it is based. In sharp contrast to popular belief, we find that NLTT incompleteness in the plane is almost completely concentrated in MS stars, and that SDs and WDs are detected almost uniformly over the sky δ > -33°. Our catalog will therefore provide a powerful tool to probe these populations statistically, as well as to reliably identify individual SDs and WDs.

M Dwarfs from Hubble Space Telescope Star Counts. III. The Groth Strip

We analyze the disk M dwarfs found in 31 new fields observed with the Wide Field Camera 2 (WFC2) on the Hubble Space Telescope, together with the sample previously analyzed from 22 WFC2 fields and 162 prerepair Planetary Camera 1 fields. The new observations, which include the 28 high-latitude fields comprising the Large Area Multi-Color Survey (Groth Strip), increase the total sample to 337 stars, and more than double the number of late M dwarfs (MV > 13.5) from 23 to 47. The mass function changes slope at M ~ 0.6 M☉, from a near-Salpeter power-law index of α = -1.21 to α = 0.44. In both regimes, the mass function at the Galactic plane is given by The correction for secondaries in binaries changes the low-mass index from α = 0.44 to α ~ 0.1. If the Salpeter slope continued to the hydrogen-burning limit, we would expect 500 stars in the last four bins (14.5 < MV < 18.5), instead of the 25 actually detected. The explanation of the observed microlensing rate toward the Galactic bulge requires either a substantial population of bulge brown dwarfs or that the disk and bulge mass functions are very different for stars with M 0.5 M☉.

The Initial Mass Function of the Galactic Bulge down to ~0.15 M☉*

We present a luminosity function (LF) for lower main sequence stars in the Galactic bulge near (1, b) _ (0°, —6°) to M,, — 9.3. This LF is derived from HST+NICMOS observations of a region of 221.’5 x 221’5, with the F11OW and F160W filters. Our derived mass function extends to 0.15M ⊙with a power law slope of a = —1.33 ± 0.07. Although shallower than the Salpeter one, this IMF is steeper than that recently found for the Galactic disk (a = —0.8 and a = —0.54 from the data of Reid & Gizis,1997,and Gould et al. 1997, respectively, in the same mass interval), but is virtually identical to the disk IMF derived by Kroupa et al. (1993). The bulge IMF is also quite similar to the mass functions derived for those globular clusters which are believed to have experienced little or no dynamical evolution

KELT-1b: A STRONGLY IRRADIATED, HIGHLY INFLATED, SHORT PERIOD, 27 JUPITER-MASS COMPANION TRANSITING A MID-F STAR

We present the discovery of KELT-1b, the first transiting low-mass companion from the wide-field Kilodegree Extremely Little Telescope-North (KELT-North) transit survey. A joint analysis of the spectroscopic, radial velocity, and photometric data indicates that the V = 10.7 primary is a mildly evolved mid-F star with Teff = 6516±49 K, log g = 4.228 +0.014 −0.021, and [Fe/H] = 0.052±0.079, with an inferred mass M∗ = 1.335 ± 0.063 M� and radius R∗ = 1.471 +0.045 −0.035 R� . The companion is a low-mass brown dwarf or a super-massive planet with mass MP = 27.38 ± 0.93 MJup and radius RP = 1.116 +0.038 −0.029 RJup. The companion is on a very short (∼29 hr) period circular orbit, with an ephemeris Tc(BJDTDB) = 2455909.29280 ± 0.00023 and P = 1.217501 ± 0.000018 days. KELT-1b receives a large amount of stellar insolation, resulting in an estimated equilibrium temperature assuming zero albedo and perfect redistribution of Teq = 2423 +3427 K. Comparison with standard evolutionary models suggests that the radius of KELT-1b is likely to be significantly inflated. Adaptive optics imaging reveals a candidate stellar companion to KELT-1 with a separation of 588 ± 1 mas, which is consistent with an M dwarf if it is at the same distance as the primary. Rossiter–McLaughlin measurements during transit imply a projected spin–orbit alignment angle λ = 2 ± 16 deg, consistent with a zero obliquity for KELT-1. Finally, the v sin I∗ = 56 ± 2k m s −1 of the primary is consistent at ∼2σ with tidal synchronization. Given the extreme parameters of the KELT-1 system, we expect it to provide an important testbed for theories of the emplacement and evolution of short-period companions, as well as theories of tidal dissipation and irradiated brown dwarf atmospheres.

Stellar Contribution to the Galactic Bulge Microlensing Optical Depth

We estimate the optical depth to self-lensing by stars in the Galactic bulge using the Hubble Space Telescope star counts of Holtzman et al. and Zoccali et al. as extrapolated by Gould into the brown dwarf and remnant regimes and deprojected along the line of sight using the model of Dwek et al. We find a self-lensing optical depth τ(bulge - bulge) = 0.98 × 10-6. When combined with the lensing of bulge stars by foreground stars in the disk, this yields τ(bulge - total) = 1.63 × 10-6, in reasonable agreement with the estimates of τ = 2.13 ± 0.40 × 10-6 and τ = 1.08 ± 0.30 × 10-6 based on observations of clump giants by the MACHO and EROS collaborations.

Resolution of the MACHO-LMC-5 Puzzle: The Jerk-Parallax Microlens Degeneracy

By extending the constant-acceleration analysis of Smith, Mao, & Paczynski to include jerk, I show that microlens parallax measurements are subject to a four-fold discrete degeneracy. The new degeneracy is characterized by a projected velocity j = -(3/4) csc βec( ψ βec + ψ)3/2v⊕, where βec is the ecliptic latitude, ψ is the phase of the Earths orbit relative to opposition at the time of the event maximum, and v⊕ = 30 km s-1 is the speed of the Earth. The degeneracy becomes important when the lens projected velocity is of order j. For events toward the Large Magellanic Cloud, j (3/4)v⊕, so this degeneracy is important primarily for lenses in the Milky Way disk. In particular, it solves the puzzle of MACHO-LMC-5, whose microlens parallax measurement had yielded mass and distance determinations for the lens that were inconsistent with photometric estimates. Toward the Galactic bulge, j ranges from ~0.2 km s- 1 at the summer solstice to ~200 km s- 1 at the equinoxes , so the effect of the degeneracy depends strongly on the peak time of the event. The degeneracy applies mainly to events with Einstein timescales, tE yr/2π.

Binary Black Hole Mergers from Planet-like Migrations

If supermassive black holes (BHs) are generically present in galaxy centers, and if galaxies are built up through hierarchical merging, BH binaries are at least temporary features of most galactic bulges. Observations suggest, however, that binary BHs are rare, pointing toward a binary lifetime far shorter than the Hubble time. We show that, almost regardless of the detailed mechanism, all stellar dynamical processes are too slow in reducing the orbital separation once orbital velocities in the binary exceed the virial velocity of the system. We propose that a massive gas disk surrounding a BH binary can effect its merger rapidly, in a scenario analogous to the orbital decay of super-Jovian planets due to a proto-planetary disk. As in the case of planets, gas accretion onto the secondary (here a supermassive BH) is integrally connected with its inward migration. Such accretion would give rise to quasar activity. BH binary mergers could therefore be responsible for many or most quasars.