Anomalously Narrow Linewidths 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 $z\sim 2.5$ 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 H$\alpha$ line widths than expected. A possible interpretation is that H$\alpha$ 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 H$\alpha$ line widths of FWHM(H$\alpha$)$\sim$125-260 km s${}^{-1}$ to test this hypothesis. Surprisingly, in all three galaxies, the CO line width is similar to the H$\alpha$ line width: we find FWHM(CO)$\sim$165 km s${}^{-1}$ for all three galaxies whereas FWHM(CO)$\sim$450-700 km s${}^{-1}$ was expected from a simple virial estimator. These results show that the narrow H$\alpha$ 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 the size and axis ratio of massive galaxies.

Accepted in ApJL. Main results are in Figure 3 and 4

References in corpus (15)

• 3D-HST+CANDELS: The Evolution of the Galaxy Size-Mass Distribution since $z=3$
• An ultra-deep near-infrared spectrum of a compact quiescent galaxy at z=2.2
• 3D-HST WFC3-selected Photometric Catalogs in the Five CANDELS/3D-HST Fields: Photometry, Photometric Redshifts and Stellar Masses
• Constraining the Low-Mass Slope of the Star Formation Sequence at 0.5<z<2.5
• The KMOS^3D Survey: design, first results, and the evolution of galaxy kinematics from 0.7<z<2.7
• Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago
• The Constant Average Relationship between Dust-obscured Star Formation and Stellar Mass from $z=0$ to $z=2.5$
• Sub-kpc ALMA imaging of compact star-forming galaxies at z~2.5: revealing the formation of dense galactic cores in the progenitors of compact quiescent galaxies
• Galaxy And Mass Assembly (GAMA): ${\mathcal{M}}_{\star }-R_{\mathrm{e}}$ relations of $z=0$ bulges, discs and spheroids
• Bulge-forming galaxies with an extended rotating disk at z~2
• The stellar velocity dispersion of a compact massive galaxy at z=1.80 using X-Shooter: confirmation of the evolution in the mass-size and mass-dispersion relations
• A Massive Galaxy in its Core Formation Phase Three Billion Years After the Big Bang
• Spatially resolved kinematics in the central 1 kpc of a compact star-forming galaxy at z=2.3 from ALMA CO observations
• Low Gas Fractions Connect Compact Star-Forming Galaxies to their z ~ 2 Quiescent Descendants
• Kinematic Evolution of Simulated Star-Forming Galaxies

Cited by in corpus (4)

• RUBIES: Evolved Stellar Populations with Extended Formation Histories at $z\sim 7-8$ in Candidate Massive Galaxies Identified with JWST/NIRSpec
• Dissecting the size-mass and ${\Sigma }_1$-mass relations at 1.0 < z < 2.5: galaxy mass profiles and color gradients as a function of spectral shape
• 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
• The Diverse Molecular Gas Content of Massive Galaxies Undergoing Quenching at z~1