Forming Compact Massive Galaxies

Abstract: In this paper we study a key phase in the formation of massive galaxies: the transition of star forming galaxies into massive (M_stars~10^11 Msun), compact (r_e~1 kpc) quiescent galaxies, which takes place from z~3 to z~1.5. We use HST grism redshifts and extensive photometry in all five 3D-HST/CANDELS fields, more than doubling the area used previously for such studies, and combine these data with Keck MOSFIRE and NIRSPEC spectroscopy. We first confirm that a population of massive, compact, star forming galaxies exists at z~2, using K-band spectroscopy of 25 of these objects at 2.0<z<2.5. They have a median NII/Halpha ratio of 0.6, are highly obscured with SFR(tot)/SFR(Halpha)~10, and have a large range of observed line widths. We infer from the kinematics and spatial distribution of Halpha that the galaxies have rotating disks of ionized gas that are a factor of ~2 more extended than the stellar distribution. By combining measurements of individual galaxies, we find that the kinematics are consistent with a nearly Keplerian fall-off from V_rot~500 km/s at 1 kpc to V_rot~250 km/s at 7 kpc, and that the total mass out to this radius is dominated by the dense stellar component. Next, we study the size and mass evolution of the progenitors of compact massive galaxies. Even though individual galaxies may have had complex histories with periods of compaction and mergers, we show that the population of progenitors likely followed a simple inside-out growth track in the size-mass plane of d(log r_e) ~ 0.3 d(log M_stars). This mode of growth gradually increases the stellar mass within a fixed physical radius, and galaxies quench when they reach a stellar density or velocity dispersion threshold. As shown in other studies, the mode of growth changes after quenching, as dry mergers take the galaxies on a relatively steep track in the size-mass plane.

Insightful Overview of "Forming Compact Massive Galaxies"

The study by van Dokkum et al., titled "Forming Compact Massive Galaxies," offers an in-depth exploration of the transitional phase where star-forming galaxies evolve into compact, massive quiescent galaxies, specifically between redshifts (z \sim 3) and (z \sim 1.5). Utilizing advanced spectroscopic and photometric data from various instruments including HST, Keck MOSFIRE, and NIRSPEC, the research provides quantitative assessment of such galaxies’ structural, kinematic, and evolutionary characteristics.

Key Contributions and Findings

1. Confirmation of Compact Star-forming Galaxies: The research confirms the existence of a population of massive, compact star-forming galaxies at (z \gtrsim 2). Using (K)-band spectroscopy of 25 galaxies, they demonstrate diverse line widths and a median \niiha\ ratio of 0.6, indicative of heavily obscured star formation with total SFR roughly tenfold that derived from \ha{} observations.

2. Rotating Disk Kinematics: The study infers that these galaxies have ionized gas rotating in disks more extended than their stellar distributions. The observed kinematics suggest a near Keplerian rotation, with velocities decreasing from (\sim 500) km/s at 1 kpc to (\sim 250) km/s at 7 kpc, dominated by the dense stellar component.

3. Size-Mass Plane Evolution: Although individual galaxies exhibit complex growth histories, the population overall follows a simple inside-out growth trajectory in the size-mass plane, with (\Delta \log r_e \sim 0.3 \Delta \log M_{\rm stars}). Galaxy quenching aligns with reaching specific stellar density or velocity dispersion thresholds.

4. Implication for Galaxy Formation Models: The findings support the notion that substantial gas redistribution is necessary to form compact massive galaxies. This could result from processes such as mergers, efficient gas cooling, or disk compaction, each with distinct observational signatures concerning tidal features and star formation rates.

Implications and Speculations on Future Developments

The study’s insights have substantial implications for both theoretical models of galaxy evolution and observational strategies:
- Understanding Black Hole Formation and Stellar IMF: By studying progenitor star-forming galaxies, researchers can better understand how massive black holes form and determine the stellar IMF's role in early galactic evolution.
- Assessment of Gas Redistribution Mechanisms: As the research provides evidence for rotating disks, further investigations into how efficiently gas funnels into these dense regions could reveal specifics about galaxy mass growth and resultant quenching mechanisms.
- Influence on Future AI Applications: This research could inform machine learning algorithms to predict stellar dynamics based on observable parameters, enhancing our understanding of galactic assembly.

Closing Remarks

Van Dokkum et al.'s robust methodological framework and comprehensive data analysis contribute valuable insights into the formation of compact massive galaxies. Their findings align with existing simulations and extend our understanding of galaxy evolution's complex and rapid transitional periods. Future work could greatly benefit from higher spatial resolution to study these galaxies' internal dynamics and further elucidate star formation cessation mechanisms in dense galactic cores.