Dead or Alive? How Bursty Star Formation and Patchy Dust Can Cause Temporary Quiescence in High Redshift Galaxies

Abstract

The recent discovery of a galaxy at z=7.3 with undetected optical emission lines and a blue UV to optical continuum ratio in JWST spectroscopy is surprising and needs to be explained physically. Here, we explore two possibilities that could cause such a seemingly quiescent ~5e8 ${\mathrm{M}}_{\odot }$ galaxy in the early Universe: (i) stochastic variations in the star formation history (SFH) and (ii) the effect of spatially varying dust attenuation on the measured line and continuum emission properties. Both scenarios can play at the same time to amplify the effect. A stochastic star formation model (similar to realistic SFHs from hydrodynamical simulations of similar-mass galaxies) can create such observed properties if star formation is fast-varying with a correlation time of <150 Myrs given a reasonable burst amplitude of ~0.6 dex. The total time spent in this state is less than 20 Myrs and the likelihood of such a state to occur over 500 Myrs at z=7 is ~50% (consistent with current observations). On the other hand, we show that a spectrum with blue UV continuum and lack of emission lines can be reproduced by a blue+red composite spectrum. The UV continuum is emitted from dust-free density bounded HII regions (blue component), while the red component is a dust-obscured starburst with weakened emission lines due to strong differential dust attenuation between stellar and nebular emission. Future resolving far-infrared observations with ALMA will shed light on the latter scenario.

8 pages, 5 figures. Accepted for publication in ApJ

References in corpus (31)

• Cosmic Star Formation History
• Mass and environment as drivers of galaxy evolution in SDSS and zCOSMOS and the origin of the Schechter function
• Star Formation in AEGIS Field Galaxies since z=1.1 : The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-Forming Galaxies
• The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies
• What do we learn from IRAC observations of galaxies at 2 < z < 3.5?
• The Main Sequence of star-forming galaxies across cosmic times
• The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era
• A massive quiescent galaxy at redshift 4.658
• Reionization Era Bright Emission Line Survey: Selection and Characterization of Luminous Interstellar Medium Reservoirs in the z>6.5 Universe
• An Atlas of Color-selected Quiescent Galaxies at $z>3$ in Public $JWST$ Fields
• The evolution of star formation histories of quiescent galaxies
• A surprising abundance of massive quiescent galaxies at 3 < z < 5 in the first data from JWST CEERS
• Normal, Dust-Obscured Galaxies in the Epoch of Reionization
• Downsizing by Shutdown in Red Galaxies
• The ALMA REBELS Survey: Dust Continuum Detections at z > 6.5
• The equilibrium view on dust and metals in galaxies: Galactic outflows drive low dust-to-metal ratios in dwarf galaxies
• Searching for Extremely Blue UV Continuum Slopes at $z=7-11$ in JWST/NIRCam Imaging: Implications for Stellar Metallicity and Ionizing Photon Escape in Early Galaxies
• Stochastic star formation in early galaxies: JWST implications
• Stochastic modeling of star-formation histories I: the scatter of the star-forming main sequence
• An Extremely Massive Quiescent Galaxy at $z=3.493$: Evidence of Insufficiently Rapid Quenching Mechanisms in Theoretical Models
• LyC escape from SPHINX galaxies in the Epoch of Reionization
• ALMA Characterises the Dust Temperature of z ~ 5.5 Star-Forming Galaxies
• The Gemini Deep Deep Survey: VIII. When Did Early-type Galaxies Form?
• Mini-quenching of $z=4-8$ galaxies by bursty star formation
• EPOCHS VII: Discovery of high redshift ($6.5<z<12$) AGN candidates in JWST ERO and PEARLS data
• Paschen-line Constraints on Dust Attenuation and Star Formation at z~1-3 with JWST/NIRSpec
• HST grism spectroscopy of z~3 massive quiescent galaxies: Approaching the metamorphosis
• Quiescent low-mass galaxies observed by JWST in the Epoch of Reionization
• JWST catches the assembly of a $z\sim 5$ ultra-low-mass galaxy
• A spatially resolved analysis of star-formation burstiness by comparing UV and H$\alpha$ in galaxies at z$\sim$1 with UVCANDELS
• The ALMA-ALPINE [CII] survey: sub-kpc morphology of 3 main-sequence galaxy systems at z~4.5 revealed by ALMA

Cited by in corpus (5)

• Zapped then Napped? A rapidly quenched remnant leaker candidate with a steep spectroscopic ${\beta }_{UV}$ slope at z=8.5
• Anatomy of a z=6 Lyman-α emitter down to parsec scales: extreme UV slopes, metal-poor regions and possibly leaking star clusters
• The THESAN-ZOOM project: Burst, quench, repeat -- unveiling the evolution of high-redshift galaxies along the star-forming main sequence
• The density-bounded twilight of starbursts in the early Universe
• COSMOS-Web: The emergence of the Hubble Sequence