Modeling Seven Years of Event Horizon Telescope Observations with Radiatively Inefficient Accretion Flow Models

An initial three-station version of the Event Horizon Telescope, a millimeter-wavelength very-long baseline interferometer, has observed Sagittarius A* (Sgr A*) repeatedly from 2007 to 2013, resulting in the measurement of a variety of interferometric quantities. Of particular importance, there is now a large set of closure phases, measured over a number of independent observing epochs. We analyze these observations within the context of a realization of semi-analytic radiatively inefficient disk models, implicated by the low luminosity of Sgr A*. We find a broad consistency among the various observing epochs and between different interferometric data types, with the latter providing significant support for this class of models of Sgr A*. The new data significantly tighten existing constraints on the spin magnitude and its orientation within this model context, finding a spin magnitude of $a=0.10^{+0.30+0.56}_{-0.10-0.10}$, an inclination with respect to the line of sight of $\theta={60^\circ}^{+5^\circ+10^\circ}_{-8^\circ-13^\circ}$, and a position angle of $\xi={156^\circ}^{+10^\circ+14^\circ}_{-17^\circ-27^\circ}$ east of north. These are in good agreement with previous analyses. Notably, the previous $180^\circ$ degeneracy in the position angle has now been conclusively broken by the inclusion of the closure phase measurements. A reflection degeneracy in the inclination remains, permitting two localizations of the spin vector orientation, one of which is in agreement with the orbital angular momentum of the infrared gas cloud G2 and the clockwise disk of young stars. This possibly supports a relationship between Sgr A*'s accretion flow and these larger-scale features.Comment: 16 pages, 11 figures, accepted to Ap

The Super-linear Slope of the Spatially-Resolved Star Formation Law in NGC 3521 and NGC 5194 (M51a)

We have conducted interferometric observations with CARMA and an OTF mapping with the 45-m telescope at NRO in the CO (J=1-0) emission line of NGC 3521. Combining these new data, together with CARMA+NRO45 data for M51a and archival SINGS H$\alpha$, 24$\mu$m, THINGS H I and GALEX FUV data for both galaxies, we investigate the empirical scaling law that connects the surface density of SFR and cold gas (the Schmidt-Kennicutt law) on a spatially-resolved basis. We argue that plausibly deriving SFR maps of nearby galaxies requires the diffuse stellar/dust background emission to be carefully subtracted. An approach to complete this task is presented and applied in our pixel-by-pixel analysis on both galaxies, showing that the controversial results whether the molecular S-K law is super-linear or basically linear is a result of removing or preserving the local background. In both galaxies, the power index of the molecular S-K law is 1.5-1.9 at the highest available resolution (230 pc), and decreases monotonically for decreasing resolution; while the scatter (mainly intrinsic) increases as the resolution becomes higher, indicating a trend for which the S-K law breaks down below some scale. Both quantities are systematically larger in M51a than in NGC 3521, but when plotted against the de-projected scale (\delta_{dp}), they become highly consistent between the two galaxies, tentatively suggesting that the sub-kpc molecular S-K law in spiral galaxies depends only on the considered scale, without varying amongst spiral galaxies. A logarithmic function \gamma_{H_2}=-1.1 log[\delta_{dp}/kpc]+1.4 and a linear relation \sigma_{H_2}=-0.2 [\delta_{dp}/kpc]+0.7 are obtained through fitting to the M51a data, which describes both galaxies impressively well on sub-kpc scales. A larger sample of galaxies with better quality data are required to test the general applicability of these relations.Comment: 21 pages, 9 figures, 4 tables, accepted for publication in Ap

Spatially Resolved Magnetic Field Structure in the Disk of a T Tauri Star

Magnetic fields in accretion disks play a dominant role during the star formation process but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars themselves and possibly in the innermost part of the accretion disk, but the strength and morphology of the field in the bulk of the disk have not been observed. Unresolved measurements of polarized emission (arising from elongated dust grains aligned perpendicular to the field) imply average fields aligned with the disks. Theoretically, the fields are expected to be largely toroidal, poloidal, or a mixture of the two, which imply different mechanisms for transporting angular momentum in the disks of actively accreting young stars such as HL Tau. Here we report resolved measurements of the polarized 1.25 mm continuum emission from HL Tau's disk. The magnetic field on a scale of 80 AU is coincident with the major axis (~210 AU diameter) of the disk. From this we conclude that the magnetic field inside the disk at this scale cannot be dominated by a vertical component, though a purely toroidal field does not fit the data well either. The unexpected morphology suggests that the magnetic field's role for the accretion of a T Tauri star is more complex than the current theoretical understanding.Comment: Accepted for publication in Natur

The Relationship Between Molecular Gas and Star Formation in Low-Mass E/S0 Galaxies

We consider the relationship between molecular-gas and star-formation surface densities in 19 morphologically defined E/S0s with stellar mass <~ 4x10^10 M_sun, paying particular attention to those found on the blue sequence in color vs. stellar mass parameter space, where spiral galaxies typically reside. While some blue-sequence E/S0s must be young major-merger remnants, many low-mass blue-sequence E/S0s appear much less disturbed, and may be experiencing the milder starbursts associated with inner-disk building as spirals (re)grow. For a sample of eight E/S0s (four blue-, two mid-, and two red-sequence) whose CARMA CO(1-0), Spitzer MIPS 24um, and GALEX FUV emission distributions are spatially resolved on a 750pc scale, we find roughly linear relationships between molecular-gas and star-formation surface densities within all galaxies, with power law indices N = 0.6-1.9 (median 1.2). Adding 11 more blue-sequence E/S0s whose CO(1-0) emission is not as well resolved, we find that most of our E/S0s have global 1-8 kpc aperture-averaged molecular-gas surface densities overlapping the range spanned by the disks and centers of spiral galaxies. While many of our E/S0s fall on the same Schmidt-Kennicutt relation as local spirals, ~80% (predominantly on the blue sequence) are offset towards apparently higher molecular-gas star formation efficiency (i.e., shorter molecular gas depletion time). Possible interpretations of the elevated efficiencies include bursty star formation similar to that in local dwarf galaxies, H2 depletion in advanced starbursts, or simply a failure of the CO(1-0) emission to trace all of the molecular gas.Comment: ApJL, accepted, 16 pages with 3 color figures, 2 table

CARMA Survey Toward Infrared-bright Nearby Galaxies (STING): Molecular Gas Star Formation Law in NGC4254

This study explores the effects of different assumptions and systematics on the determination of the local, spatially resolved star formation law. Using four star formation rate (SFR) tracers (H\alpha with azimuthally averaged extinction correction, mid-infrared 24 micron, combined H\alpha and mid-infrared 24 micron, and combined far-ultraviolet and mid-infrared 24 micron), several fitting procedures, and different sampling strategies we probe the relation between SFR and molecular gas at various spatial resolutions and surface densities within the central 6.5 kpc in the disk of NGC4254. We find that in the high surface brightness regions of NGC4254 the form of the molecular gas star formation law is robustly determined and approximately linear and independent of the assumed fraction of diffuse emission and the SFR tracer employed. When the low surface brightness regions are included, the slope of the star formation law depends primarily on the assumed fraction of diffuse emission. In such case, results range from linear when the fraction of diffuse emission in the SFR tracer is ~30% or less (or when diffuse emission is removed in both the star formation and the molecular gas tracer), to super-linear when the diffuse fraction is ~50% and above. We find that the tightness of the correlation between gas and star formation varies with the choice of star formation tracer. The 24 micron SFR tracer by itself shows the tightest correlation with the molecular gas surface density, whereas the H\alpha corrected for extinction using an azimuthally-averaged correction shows the highest dispersion. We find that for R<0.5R_25 the local star formation efficiency is constant and similar to that observed in other large spirals, with a molecular gas depletion time ~2 Gyr.Comment: accepted for publication in ApJ, vol 729, March 10 2011 issue; 30 pages; 14 figures; revised version includes referee's comments; results unchange

Confirming the Primarily Smooth Structure of the Vega Debris Disk at Millimeter Wavelengths

Clumpy structure in the debris disk around Vega has been previously reported at millimeter wavelengths and attributed to concentrations of dust grains trapped in resonances with an unseen planet. However, recent imaging at similar wavelengths with higher sensitivity has disputed the observed structure. We present three new millimeter wavelength observations that help to resolve the puzzling and contradictory observations. We have observed the Vega system with the Submillimeter Array (SMA) at a wavelength of 880 μm and an angular resolution of 5"; with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at a wavelength of 1.3 mm and an angular resolution of 5"; and with the Green Bank Telescope (GBT) at a wavelength of 3.3 mm and angular resolution of 10". Despite high sensitivity and short baselines, we do not detect the Vega debris disk in either of the interferometric data sets (SMA and CARMA), which should be sensitive at high significance to clumpy structure based on previously reported observations. We obtain a marginal (3σ) detection of disk emission in the GBT data; the spatial distribution of the emission is not well constrained.We analyze the observations in the context of several different models, demonstrating that the observations are consistent with a smooth, broad, axisymmetric disk with inner radius 20–100 AU and width ≾50 AU. The interferometric data require that at least half of the 860 μm emission detected by previous single-dish observations with the James Clerk Maxwell Telescope be distributed axisymmetrically, ruling out strong contributions from flux concentrations on spatial scales of ≾100 AU. These observations support recent results from the Plateau de Bure Interferometer indicating that previous detections of clumpy structure in the Vega debris disk were spurious

Interferometric Follow-Up of WISE Hyper-Luminous Hot, Dust-Obscured Galaxies

WISE has discovered an extraordinary population of hyper-luminous dusty galaxies which are faint in the two bluer passbands ($3.4\, \mu$m and $4.6\, \mu$m) but are bright in the two redder passbands of WISE ($12\, \mu$m and $22\, \mu$m). We report on initial follow-up observations of three of these hot, dust-obscured galaxies, or Hot DOGs, using the CARMA and SMA interferometer arrays at submm/mm wavelengths. We report continuum detections at $\sim$ 1.3 mm of two sources (WISE J014946.17+235014.5 and WISE J223810.20+265319.7, hereafter W0149+2350 and W2238+2653, respectively), and upper limits to CO line emission at 3 mm in the observed frame for two sources (W0149+2350 and WISE J181417.29+341224.8, hereafter W1814+3412). The 1.3 mm continuum images have a resolution of 1-2 arcsec and are consistent with single point sources. We estimate the masses of cold dust are 2.0$\times 10^{8} M_{\odot}$ for W0149+2350 and 3.9$\times 10^{8} M_{\odot}$ for W2238+2653, comparable to cold dust masses of luminous quasars. We obtain 2$\sigma$ upper limits to the molecular gas masses traced by CO, which are 3.3$\times 10^{10} M_{\odot}$ and 2.3$\times 10^{10} M_{\odot}$ for W0149+2350 and W1814+3412, respectively. We also present high-resolution, near-IR imaging with WFC3 on the Hubble Space Telescope for W0149+2653 and with NIRC2 on Keck for W2238+2653. The near-IR images show morphological structure dominated by a single, centrally condensed source with effective radius less than 4 kpc. No signs of gravitational lensing are evident.Comment: 13 pages, 3 figures. ApJ in pres