Anomalously Narrow Line Widths of Compact Massive Star-forming Galaxies at z ∼ 2.3: A Possible Inclination Bias in the Size–Mass Plane

Abstract

Compact, massive star forming galaxies at z ∼ 2.5 are thought to be building the central regions of giant elliptical galaxies today. However, a significant fraction of these objects were previously shown to have much smaller Hα line widths than expected. A possible interpretation is that Hα emission from their central regions, where the highest velocities are expected, is typically obscured by dust.Here we present ALMA observations of the CO(3-2) emission line of three compact, massive galaxies with Hα line widths of FWHM(Hα)∼125-260 km s −1 to test this hypothesis. Surprisingly, in all three galaxies, the CO line width is similar to the Hα line width: we find FWHM(CO)∼165 km s −1 for all three galaxies whereas FWHM(CO)∼450-700 km s −1 was expected from a simple virial estimator. These results show that the narrow Hα linewidths of many compact massive star-forming galaxies are not due to preferential obscuration of the highest velocity gas. An alternative explanation for the narrow line widths is that the galaxies are disks that are viewed nearly face-on. We suggest that there may be an inclination bias in the size-mass plane, such that the apparent rest-frame optical sizes of face-on galaxies are smaller than those of edge-on galaxies. Although not conclusive, this hypothesis is supported by an observed anti-correlation between size and axis ratio of massive galaxies.

Color Gradients and Half-mass Radii of Galaxies Out to z = 2 in the CANDELS/3D-HST Fields: Further Evidence for Important Differences in the Evolution of Mass-weighted and Light-weighted Sizes

“…In Figure 14, we recreate this relationship using both the light and mass semimajor axes. We find a similar correlation to Mowla et al (2019b) between the semimajor axis and axis ratio. For mass radii, there is much less correlation.…”

“…Related to this is the issue of an inclination bias in the sizemass plane, as suggested by Mowla et al (2019b). The authors investigate a discrepancy between the observed line width and that expected by simple viral estimator for three compact starforming galaxies.…”

Dissecting the size-mass and $Σ_1$-mass relations at 1.0 < z < 2.5: galaxy mass profiles and color gradients as a function of spectral shape

“…Finally, we note that our size measurements for these dusty star-forming galaxies do not appear to be driven by observational biases. First, inclination bias does not seem to play an important role: the observed axis ratios of the compact and extended z > 2 dusty star-forming galaxies do not differ significantly (though see Mowla et al 2019). Second, our methods appear able to accurately recover the half-mass radii of dusty star-forming galaxies despite their highly dust-obscured centers.…”

The Diverse Molecular Gas Content of Massive Galaxies Undergoing Quenching at z~1

“…This is comparable to the velocity dispersion measured from the stellar absorption lines (88 ± 18 km/s, Belli et al 2014), but significantly lower than the value expected for a pressuresupported system, which is σ virial > 130 km/s (this is a lower limit obtained by neglecting the dark matter contribution). The most likely explanation for this discrepancy is that both the stellar and the molecular gas content are distributed in a nearly face-on rotating disk, which is also consistent with the circular morphology of the galaxy (see Mowla et al 2019 for similar cases). Such alignment suggests that the molecular gas reservoir was formed together with the stellar content, and is not a product of recent accretion.…”