CosmoPhys

New developments since this post was originally created:

January 2020: In new paper arXiv: 2001.09213, the MCP reports an updated $H_0$ value of 73.9 ± 3.0 based on improved distance measurements for 4 of the 6 systems they previously reported on. MCP researchers Braatz, Pesce, Condon, Reid, et al teamed up with SH0ES team members Scolnic and Riess for this paper. Here is a clip of Figure 1 from the paper. Additional details in this reddit thread.

July 2019: video and slides of a presentation by Mark Reid at the KITP-UCSB conference Tensions between the Early and the Late Universe on July 16, 2019: H0: NGC 4258 and the Megamaser Cosmology Project -—

This post is about the paper Science with the ngVLA: H2O Megamaser Cosmology, which was posted on the arXiv preprint server in Oct. 2018.

What's This About? A recent comment by one of the people I regularly follow (who writes excellent posts on astronomy/cosmology topics) mentioned this NRAO key project as providing direct geometric measurements of the Hubble Constant (H_0) independent of the Cosmic Microwave Background (CMB) and standard candle methods. The megamaser method gives angular diameter distances to water megamasers¹ in the nuclear regions of host active galaxies within ~0.1 pc of the SMBH. The galaxies are typically well into the Hubble flow at between 50 – 200 Mpc distance. This slide compares the distances to various studied megamasers to those for Cepheids. As indicated in the slide, these megamaser distances can be used to calibrate other distance methods.

Not being familiar with this MCP project but finding it of interest, I've tried to integrate and summarize some reading on it into this mini-overview, non-expert's report.

What Are They? These H₂O megamasers being studied are a special type of astrophysical maser originating in a flat accretion disk, with warm (~400 K) dense gas clouds, in Keplerian motion orbiting the SMBHs of host active galaxies. They produce stimulated coherent emission at 22 GHz from collisionally excited transitions between rotational energy levels of ortho-states of water². Typical rotation velocities are ~500 km s^-1. Megamasers have a large isotropic luminosity, typically 10³ solar luminosities. A radio continuum background source is needed to provide the radiation that's amplified by the maser.

What facilities are currently involved? “The MCP uses the most sensitive suite of telescopes working today at 22 GHz, including the GBT for surveys and spectral monitoring observations, and the High Sensitivity Array (the VLBA, GBT, VLA, and 100-m Effelsberg telescope) to map maser disk systems.” [quote source]

What's Involved in Measuring the Distances? Multiple steps, e.g.: (1) survey with the GBT to identify the rare, edge-on disk megamasers needed for this method (this maximizes the gain length as described in footnote¹ references), (2) image the sub-parsec disks with the High Sensitivity Array, (3) with the GBT, monitor spectral drifts in maser lines (red vs blue shifting) from centripetal accelerations of the clouds as they orbit the central BH, (4) VLBI observations for rotation curve mapping, and (5) modeling the disk dynamics. Basically, the distance to the galaxy is measured by comparing the observed angular radius of the maser's orbit to its measured linear distance from the black hole (see this visual slide and further informational details in footnote¹).

Current Status: The most complete set of H_0 results I've seen the MCP report is 67.6 ±4.0, a 6% measurement. The individual 6 measurements are shown in this slide, which also shows the distances in Mpc. A comment indicates they expect to improve the measurement to < 4%. (I've also seen a ±3% goal in several of their papers). In the slide, two of the results are shown in prep, which may explain the difference from the 69.3 ±4.2 result from their latest paper 1810.06686 which reports only 4 results.

In Extragalactic maser surveys [1802.04727] they report: “With respect to 22 GHz H₂O masers, we note that the MCP is close to completion. We can expect a final Hubble constant deduced from this survey with an uncertainty of only a few percent during the next one or two years.” (page 9) Their latest paper says their final results will be based on distances to 9 megamasers. Surveys of 3000 galaxies were necessary to identify those 9 (AGNs are required with a maser disk suitable for measurement and with an edge-on view).

What's Their Future Plan? A recent 5-page paper details their plans involving the next-generation Very Large Array: H2O Megamaser Cosmology with the ngVLA [1810.06686]. Their ultimate goal is a 1% measurement, which will (1) require a survey to identify additional megamasers – their forecast is that ngVLA can discover ~30 times more sources than the GBT, and (2) the ngVLA must have certain design features as outlined in section 6 of the paper.

An overview document on the ngVLA shows it's timeline as initiating early science in 2028 and full operations in 2034.

Are There Other Study Results from MCP? Besides its primary mission for H_0 measurements, MCP results to date include the “gold standard” most accurate method for extragalactic BH masses, (e.g., see 1801.06332), and also precise BH masses in relatively low-mass systems where the BH masses have been difficult to measure – this has important implications for galaxy evolution (e.g., see 1007.2851).

What are some other MCP references? An interesting set of slides from an NRAO presentation is Water Megamasers in Galaxies, which I recommend. The Megamaser Cosmology Project has a wiki page here.

Footnotes: ¹ For a good reference on the distance measurement techniques, see Braatz et al, Measuring the Hubble constant with observations of water-vapor megamasers. For a nice summary graphic of this geometric distance technique, see this. Also the wikipedia page has a short summary here.

² See this for a graphic showing various rotational energy level transitions of water. Also see this graphic. The particular transition of interest for megamaser study is 6₁,₆ –> 5₂,₃. For more complete data on these transitions, see Table 6 in 1510.06182. Other transitions of ortho-H20 such as at 183 GHz are also candidates, but 22 GHz is the most studied currently, and is directed at ortho-H20 rather than para-H2O: “we consider only transitions between ortho-states because these are the states for which the most extensive collisional data have been calculated, and because in a hot plasma the abundance of ortho-water will exceed that of para-water by a factor 3.” Note: these two water states “are not connected by radiative or collisional transitions.” Neufeld and Melnick 1987 ApJ 322,266. Two other good sources on the transition details is Astrophysical masers and The Physics of Water Masers observable with ALMA and SOFIA [1510.06182].

Tags: #cosmology #H0 #physics