The dwarf galaxy Triangulum (M33) presents an interesting testbed for studying stellar halo formation: it is sufficiently massive so as to have likely accreted smaller satellites, but also lies within the regime where feedback and other “in situ” formation mechanisms are expected to play a role. In this work, we analyze the line-of-sight kinematics of stars across M33 from the TREX survey, with a view to understanding the origin of its halo. We split our sample into two broad populations of varying age, comprising 2032 “old” red giant branch stars and 671 “intermediate-age” asymptotic giant branch and carbon stars. We find decisive evidence for two distinct kinematic components in both the old and intermediate-age populations: a low-dispersion (∼22 km s−1) disk-like component corotating with M33's H
Triangulum (M33) is a low-mass, relatively undisturbed spiral galaxy that offers a new regime in which to test models of dynamical heating. In spite of its proximity, M33's dynamical heating history has not yet been well-constrained. In this work, we present the TREX Survey, the largest stellar spectroscopic survey across the disk of M33. We present the stellar disk kinematics as a function of age to study the past and ongoing dynamical heating of M33. We measure line-of-sight velocities for ∼4500 disk stars. Using a subset, we divide the stars into broad age bins using Hubble Space Telescope and Canada–France–Hawaii Telescope photometric catalogs: massive main-sequence stars and helium-burning stars (∼80 Myr), intermediate-mass asymptotic branch stars (∼1 Gyr), and low-mass red giant branch stars (∼4 Gyr). We compare the stellar disk dynamics to that of the gas using existing H
- NSF-PAR ID:
- 10364083
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astronomical Journal
- Volume:
- 163
- Issue:
- 4
- ISSN:
- 0004-6256
- Format(s):
- Medium: X Size: Article No. 166
- Size(s):
- ["Article No. 166"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract i gas and a significantly higher-dispersion component (∼50–60 km s−1) that does not rotate in the same plane as the gas and is thus interpreted as M33's stellar halo. While kinematically similar, the fraction of stars associated with the halo component differs significantly between the two populations: this is consistently ∼10% for the intermediate-age population, but decreases from ∼34% to ∼10% as a function of radius for the old population. We additionally find evidence that the intermediate-age halo population is systematically offset from the systemic velocity of M33 by ∼25 km s−1, with a preferred central LOS velocity of ∼ − 155 km s−1. This is the first detection and characterization of an intermediate-age halo in M33, and suggests in situ formation mechanisms, as well as potentially tidal interactions, have helped shaped it. -
more » « less
ABSTRACT We study the kinematics of stars both at their formation and today within 14 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. We quantify the relative importance of cosmological disc settling and post-formation dynamical heating. We identify three eras: a Pre-Disc Era (typically ≳ 8 Gyr ago), when stars formed on dispersion-dominated orbits; an Early-Disc Era (≈8–4 Gyr ago), when stars started to form on rotation-dominated orbits but with high velocity dispersion, σform; and a Late-Disc Era (≲ 4 Gyr ago), when stars formed with low σform. σform increased with time during the Pre-Disc Era, peaking ≈8 Gyr ago, then decreased throughout the Early-Disc Era as the disc settled and remained low throughout the Late-Disc Era. By contrast, the dispersion measured today, σnow, increases monotonically with age because of stronger post-formation heating for Pre-Disc stars. Importantly, most of σnow was in place at formation, not added post-formation, for stars younger than ≈10 Gyr. We compare the evolution of the three velocity components: at all times, σR, form > σϕ, form > σZ, form. Post-formation heating primarily increased σR at ages ≲ 4 Gyr but acted nearly isotropically for older stars. The kinematics of young stars in FIRE-2 broadly agree with the range observed across the MW, M31, M33, and PHANGS-MUSE galaxies. The lookback time that the disc began to settle correlates with its dynamical state today: earlier-settling galaxies currently form colder discs. Including stellar cosmic-ray feedback does not significantly change disc rotational support at fixed stellar mass.
-
more » « less
Abstract We present initial results from a large spectroscopic survey of stars throughout M33's stellar disk. We analyze a sample of 1667 red giant branch (RGB) stars extending to projected distances of ∼11 kpc from M33's center (∼18 kpc, or ∼10 scale lengths, in the plane of the disk). The line-of-sight velocities of RGB stars show the presence of two kinematical components. One component is consistent with rotation in the plane of M33's H
i disk and has a velocity dispersion (∼19 km s−1), consistent with that observed in a comparison sample of younger stars, while the second component has a significantly higher velocity dispersion. A two-component fit to the RGB velocity distribution finds that the high-dispersion component has a velocity dispersion ofσ level), which favors interpreting it as a stellar halo rather than a thick disk population. A spatial analysis indicates that the fraction of RGB stars in the high-velocity-dispersion component decreases with increasing radius over the range covered by the spectroscopic sample. Our spectroscopic sample establishes that a significant high-velocity-dispersion component is present in M33's RGB population from near M33's center to at least the radius where M33's Hi disk begins to warp at 30′ (∼7.5 kpc) in the plane of the disk. This is the first detection and spatial characterization of a kinematically hot stellar component throughout M33's inner regions. -
more » « less
Abstract Stellar kinematics and metallicity are key to exploring formation scenarios for galactic disks and halos. In this work, we characterized the relationship between kinematics and photometric metallicity along the line of sight to M31's disk. We combined optical Hubble Space Telescope/Advanced Camera for Surveys photometry, from the Panchromatic Hubble Andromeda Treasury survey, with Keck/DEIMOS spectra, from the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo survey. The resulting sample of 3512 individual red giant branch stars spans 4–19 projected kpc, making it a useful probe of both the disk and inner halo. We separated these stars into disk and halo populations, by modeling the line-of-sight velocity distributions as a function of position across the disk region, where ∼73% stars have a high likelihood of belonging to the disk and ∼14% to the halo. Although stellar halos are typically thought to be metal-poor, the kinematically identified halo contains a significant population of stars (∼29%) with disk-like metallicity ([Fe/H]phot∼ −0.10). This metal-rich halo population lags the gaseous disk to a similar extent as the rest of the halo, indicating that it does not correspond to a canonical thick disk. Its properties are inconsistent with those of tidal debris originating from the Giant Stellar Stream merger event. Moreover, the halo is chemically distinct from the phase-mixed component previously identified along the minor axis (i.e., away from the disk), implying contributions from different formation channels. These metal-rich halo stars provide direct chemodynamical evidence in favor of the previously suggested “kicked-up” disk population in M31's inner stellar halo.
-
more » « less
Abstract We present kinematics of six local extremely metal-poor galaxies (EMPGs) with low metallicities (0.016–0.098
Z ⊙) and low stellar masses (104.7–107.6M ⊙). Taking deep medium/high-resolution (R ∼ 7500) integral-field spectra with 8.2 m Subaru, we resolve the small inner velocity gradients and dispersions of the EMPGs with Hα emission. Carefully masking out substructures originating by inflow and/or outflow, we fit three-dimensional disk models to the observed Hα flux, velocity, and velocity dispersion maps. All the EMPGs show rotational velocities (v rot) of 5–23 km s−1smaller than the velocity dispersions (σ 0) of 17–31 km s−1, indicating dispersion-dominated (v rot/σ 0= 0.29–0.80 < 1) systems affected by inflow and/or outflow. Except for two EMPGs with large uncertainties, we find that the EMPGs have very large gas-mass fractions off gas≃ 0.9–1.0. Comparing our results with other Hα kinematics studies, we find thatv rot/σ 0decreases andf gasincreases with decreasing metallicity, decreasing stellar mass, and increasing specific star formation rate. We also find that simulated high-z (z ∼ 7) forming galaxies have gas fractions and dynamics similar to the observed EMPGs. Our EMPG observations and the simulations suggest that primordial galaxies are gas-rich dispersion-dominated systems, which would be identified by the forthcoming James Webb Space Telescope observations atz ∼ 7.
