Science with the ngVLA: H$_2$O Megamaser Cosmology
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In combination with observations of the Cosmic Microwave Background, a measurement of the Hubble Constant provides a direct test of the standard $Λ$CDM cosmological model and a powerful constraint on the equation of state of dark energy. Observations of circumnuclear water vapor megamasers at 22 GHz in nearby active galaxies can be used to measure their distances, geometrically, and thereby provide a direct, one step measurement of the Hubble Constant. The measurement is independent of distance ladders and standard candles. With present-day instrumentation, the Megamaser Cosmology Project is expected to reach a $\sim$4% measurement using the megamaser technique, based on distances to fewer than 10 megamaser systems. A goal of the observational cosmology community is to reach a percent-level measurement of H$_0$ from several independent astrophysical measurements to minimize the systematics. The ngVLA will provide the sensitivity at 22 GHz required to discover and measure the additional megamaser disks that will enable an H$_0$ measurement at the $\sim$1% level.Keywords:
Megamaser
Cosmic distance ladder
Observational cosmology
In combination with observations of the Cosmic Microwave Background, a measurement of the Hubble Constant provides a direct test of the standard $Λ$CDM cosmological model and a powerful constraint on the equation of state of dark energy. Observations of circumnuclear water vapor megamasers at 22 GHz in nearby active galaxies can be used to measure their distances, geometrically, and thereby provide a direct, one step measurement of the Hubble Constant. The measurement is independent of distance ladders and standard candles. With present-day instrumentation, the Megamaser Cosmology Project is expected to reach a $\sim$4% measurement using the megamaser technique, based on distances to fewer than 10 megamaser systems. A goal of the observational cosmology community is to reach a percent-level measurement of H$_0$ from several independent astrophysical measurements to minimize the systematics. The ngVLA will provide the sensitivity at 22 GHz required to discover and measure the additional megamaser disks that will enable an H$_0$ measurement at the $\sim$1% level.
Megamaser
Cosmic distance ladder
Observational cosmology
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We test the FLRW cosmology by reconstructing in a model-independent way both the Hubble parameter $H(z)$ and the comoving distance $D(z)$ via the most recent Hubble and Supernovae Ia data. In particular we use data binning with direct error propagation, the principal component analysis, the genetic algorithms and the Pad\'e approximation. Using our reconstructions we evaluate the Clarkson et al. test known as ${\mathrm{\ensuremath{\Omega}}}_{K}(z)$, whose value is constant in redshift for the standard cosmological model, but deviates otherwise. Using present data, we find good agreement with the expected values of the standard cosmological model within the experimental errors, which are, however, large enough to allow for alternative cosmologies. A discrimination between the models may be possible for more precise future data. Finally, we provide forecasts, exploiting the baryon acoustic oscillation measurements from the Euclid survey.
Friedmann–Lemaître–Robertson–Walker metric
Cosmic distance ladder
Observational cosmology
Baryon Acoustic Oscillations
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Recent determinations of the Hubble constant, H_0, at extremely low and very high redshifts based on the cosmic distance ladder (grounded with trigonometric parallaxes) and a cosmological model (applied to Planck 2013 data) respectively, are revealing an intriguing discrepancy (nearly 9% or 2.4sigma) that is challenging astronomers and theoretical cosmologists. In order to shed some light on this problem, here we discuss a new determination of H_0 at intermediate redshifts (z ~ 1), using the following four cosmic probes: (i) measurements of the angular diameter distances (ADD) for galaxy clusters based on the combination of Sunyaev-Zeldovich effect and X-ray data (0.14 < z < 0.89$), (ii) the inferred ages of old high redshift galaxies (OHRG) (0.62 < z < 1.70), (iii) measurements of the Hubble parameter H(z) (0.1 < z < 1.8), and (iv) the baryon acoustic oscillation (BAO) signature (z=0.35). In our analysis, assuming a flat LCDM cosmology and considering statistical plus systematic errors we obtain H_0 = 74.1^{+2.2}_{-2.2} km/s.Mpc (1sigma) which is a 3% determination of the Hubble constant at intermediate redshifts. We stress that each individual test adopted here has error bars larger than the ones appearing in the calibration of the extragalactic distance ladder. However, the remarkable complementarity among the four tests works efficiently in reducing greatly the possible degeneracy on the space parameter (Omega_m,h) ultimately providing a value of H_0 that is in excellent agreement with the determination using recessional velocities and distances to nearby objects.
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Observational cosmology
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Abstract To constrain models of dark energy, a precise measurement of the Hubble constant, H 0 , provides a powerful complement to observations of the cosmic microwave background. Recent, precise measurements of H 0 have been based on the ‘extragalactic distance ladder,’ primarily using observations of Cepheid variables and Type Ia supernovae as standard candles. In the past, these methods have been limited by systematic errors, so independent methods of measuring H 0 are of high value. Direct geometric distance measurements to circumnuclear H 2 O megamasers in the Hubble flow provide a promising new method to determine H 0 . The Megamaser Cosmology Project (MCP) is a systematic effort to discover suitable H 2 O megamasers and determine their distances, with the aim of measuring H 0 to a few percent. Based on observations of megamasers in UGC 3789 and NGC 6264, and preliminary results from Mrk 1419, the MCP has so far measured H 0 = 68.0 ± 4.8 km s −1 Mpc −1 . This measurement will improve as distances to additional galaxies are incorporated. With the Green Bank Telescope, we recently discovered three more excellent candidates for distance measurements, and we are currently acquiring data to measure their distances.
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Megamaser
Green Bank Telescope
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A sensitive search for 6.7--GHz methanol maser emission has been made towards 10 galaxies that have yielded detectable microwave molecular--line transitions. These include several which show OH megamaser or superluminous \water\/ maser emission. Within the Galaxy, \methanol\/ and OH masers often occur in the same star formation regions and, in most cases, the \methanol\/ masers have a greater peak flux density than their OH counterparts. Thus we might expect \methanol\/ masers to be associated with extragalactic OH maser sources. We failed to detect any emission or absorption above our 60--mJy detection limit. We conclude that if the physical conditions exist to produce \methanol\/ megamaser emission, they are incompatible with the conditions which produce OH megamaser emission.
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To constrain models of dark energy, the best complement to observations of the Cosmic Microwave Background is a precise measurement of the Hubble constant. The H2O megamaser method can measure direct angular-diameter distances to galaxies in the Hubble flow, and thereby provides an opportunity to determine the Hubble constant independent of the Extragalactic Distance Ladder. In this thesis we present sensitive VLBI and single-dish observations of the megamasers in NGC 6264 and NGC 6323 and measure their distances using the megamaser method. This is the first time the method has been applied to galaxies beyond 100 Mpc. For NGC 6264 we developed an ensemble approach that fits the systemic masers with a multi-ring model, and we determine a distance of 150.5±33.6 Mpc (22% accuracy). We also apply a Bayesian technique that models the maser distribution as a warped disk and allows for eccentric orbits, and obtain a distance of 152.3±16.2 Mpc (10.6%). The corresponding H0 is 65.8±7.2 km s Mpc. The best fit model from the Bayesian technique has a slight warp and a small eccentricity (e ∼ 0.06), but this substructure has only a minor effect on the distance determination. For NGC 6323, although we made the most sensitive maser map ever observed, we do not obtain a comparably precise distance measurement because of the extremely low flux densities of the systemic masers. Nonetheless, the work on this galaxy helped develop a new self-calibration technique that enables efficient imaging of distant megamaser disks. In addition to the observations of these two galaxies, we also present sensitive VLBI images of four other megamaser galaxies, plus a seventh previously published, from which we determine accurate masses of the supermassive BHs at their nuclei. The BH masses are all within a factor of 3 of 2.2 × 10M⊙ and the accuracy of each is primarily limited by the uncertainty in the Hubble constant. These accurate BH iii masses contribute to the observational basis for testing the M − σ⋆ relation at the low-mass end.
Megamaser
Cosmic distance ladder
Angular diameter
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Megamaser
Line (geometry)
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Abstract The Megamaser Cosmology Project (MCP) measures the Hubble Constant by determining geometric distances to circumnuclear 22 GHz H 2 O megamasers in galaxies at low redshift (z < 0.05) but well into the Hubble flow. In combination with the recent, exquisite observations of the Cosmic Microwave Background by WMAP and Planck, these measurements provide a direct test of the standard cosmological model and constrain the equation of state of dark energy. The MCP is a multi-year project that has recently completed observations and is currently working on final analysis. Based on distance measurements to the first four published megamasers in the sample, the MCP currently determines H 0 = 69.3 ± 4.2 km s −1 Mpc −1 . The project is finalizing analysis for five additional galaxies. When complete, we expect to achieve a ~4% measurement. Given the tension between the Planck prediction of H 0 in the context of the standard cosmological model and astrophysical measurements based on standard candles, the MCP provides a critical and independent geometric measurement that does not rely on external calibrations or a distance ladder.
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CMB cold spot
Megamaser
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Abstract. Here we discuss the results of VLBI observations of OH megamasers in ultra-luminous infrared galaxies. To date, in all OH megamaser sources observed with VLBI a large part of the OH maser emission is associated with circumnuclear disks or tori. The most striking result is that of III Zw 35, where the maser emission appears to be in a ring-like structure. This ring geometry can explain the two compact clumps of emission previously observed, and theoretical modeling using a torus can successfully reproduce the observed emission. In some OH megamaser galaxies new data further suggests that in addition to an OH maser disk, some maser emission may also be associated with outflows.
Megamaser
Very-long-baseline interferometry
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We present a measurement of the Hubble constant made using geometric distance measurements to megamaser-hosting galaxies. We have applied an improved approach for fitting maser data and obtained better distance estimates for four galaxies previously published by the Megamaser Cosmology Project: UGC 3789, NGC 6264, NGC 6323, and NGC 5765b. Combining these updated distance measurements with those for the maser galaxies CGCG 074-064 and NGC 4258, and assuming a fixed velocity uncertainty of 250 km s$^{-1}$ associated with peculiar motions, we constrain the Hubble constant to be $H_0 = 73.9 \pm 3.0$ km s$^{-1}$ Mpc$^{-1}$ independent of distance ladders and the cosmic microwave background. This best value relies solely on maser-based distance and velocity measurements, and it does not use any peculiar velocity corrections. Different approaches for correcting peculiar velocities do not modify $H_0$ by more than ${\pm}1{\sigma}$, with the full range of best-fit Hubble constant values spanning 71.8-76.9 km s$^{-1}$ Mpc$^{-1}$. We corroborate prior indications that the local value of $H_0$ exceeds the early-Universe value, with a confidence level varying from 95-99% for different treatments of the peculiar velocities.
Constant (computer programming)
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