We use a sample of 97 galaxies selected from the Arecibo legacy fast ALFA ( ALFALFA ) 21cm survey to make an accurate measurement of the baryonic Tully-Fisher relation ( BTFR ) .
These galaxies are specifically selected to be heavily gas-dominated ( M _ { \mathrm { gas } } / M _ { \ast } \gtrsim 2.7 ) and to be oriented edge-on .
The former property ensures that the error on the galactic baryonic mass is small , despite the large systematic uncertainty involved in galactic stellar mass estimates .
The latter property means that rotational velocities can be derived directly from the width of the 21cm emission line , without any need for inclination corrections .
We measure a slope for the linewidth-based BTFR of \alpha = 3.75 \pm 0.11 , a value that is somewhat steeper than ( but in broad agreement with ) previous literature results .
The relation is remarkably tight , with almost all galaxies being located within a perpendicular distance of \pm 0.1 dex from the best fit line .
The low observational error budget for our sample enables us to establish that , despite its tightness , the measured linewidth-based BTFR has some small ( i.e. , non-zero ) intrinsic scatter .
We furthermore find a systematic difference in the BTFR of galaxies with “ double-horned ” 21cm line profiles –suggestive of flat outer galactic rotation curves– and those with “ peaked ” profiles –suggestive of rising rotation curves .
When we restrict our sample of galaxies to objects in the former category , we measure a slightly steeper slope of \alpha = 4.13 \pm 0.15 .
Overall , the high-accuracy measurement of the BTFR presented in this article is intended as a reliable observational benchmark against which to test theoretical expectations .
Here we consider a representative set of semi-analytic models and hydrodynamic simulations in the lambda cold dark matter ( \Lambda CDM ) context , as well as modified Newtonian dynamics ( MOND ) .
In the near future , interferometric follow-up observations of several sample members will enable us to further refine the BTFR measurement , and make sharper comparisons with theoretical models .