Unresolved z~8 point sources and their impact on the bright end of the galaxy luminosity function

Abstract

The distribution and properties of the first galaxies and quasars are critical pieces of the puzzle in understanding galaxy evolution and cosmic reionization. Previous studies have often excluded unresolved sources as potential low redshift interlopers. We combine broadband color and photometric redshift analysis with morphological selections to identify a robust sample of candidates consistent with unresolved point sources at redshift $z\sim 8$ using deep Hubble Space Telescope images. We also examine G141 grism spectroscopic data to identify and eliminate dwarf star contaminants. From these analyses, we identify three, bright ($M_{UV}\lesssim -22$ ABmag) dropout point sources at $7.5<z<8.1$. Spectral energy distribution analyses suggest that these sources are either quasars or compact star-forming galaxies. The flux captured by the IRAC 4.5 $\mu$m channel suggests that they have moderate $H\beta$+$[OIII]$ equivalent widths. We calculate the number density of point sources at $z\sim 7$-8, and find that a double powerlaw model well describes the point source distribution. We then extend our analysis to estimate the combined point source + galaxy luminosity function and find that the point sources have a non-negligible contribution to the bright-end excess. The fact that the point sources dominate only at $M_{UV}\lesssim -22$ suggests that their contribution to cosmic reionization is likely limited. While spectroscopic follow-up is needed to confirm the nature of these point sources, this work demonstrates that the inclusion of Lyman dropout point sources is necessary for a complete census of the early galaxies at the epoch of cosmic reionization.

21 pages. 10 figures

References in corpus (36)

• EAZY: A Fast, Public Photometric Redshift Code
• A luminous quasar at a redshift of z = 7.085
• UV Luminosity Functions at redshifts z~4 to z~10: 10000 Galaxies from HST Legacy Fields
• An Observational Determination of the Bolometric Quasar Luminosity Function
• 3D-HST WFC3-selected Photometric Catalogs in the Five CANDELS/3D-HST Fields: Photometry, Photometric Redshifts and Stellar Masses
• Cosmic Reionization and Early Star-Forming Galaxies: A Joint Analysis of New Constraints from Planck and Hubble Space Telescope
• UltraVISTA: a new ultra-deep near-infrared survey in COSMOS
• The Chandra Deep Field-South Survey: 7 Ms Source Catalogs
• A Luminous Quasar at Redshift 7.642
• New Determinations of the UV Luminosity Functions from z~9 to z~2 show a remarkable consistency with halo growth and a constant star formation efficiency
• The Cosmic Evolution Survey (COSMOS): Subaru Observations of the HST COSMOS Field
• Pōniuā'ena: A Luminous $z=7.5$ Quasar Hosting a 1.5 Billion Solar Mass Black Hole
• The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7
• New observations of z~7 galaxies: evidence for a patchy reionization
• Inferences on the Timeline of Reionization at z~8 From the KMOS Lens-Amplified Spectroscopic Survey
• A lack of evolution in the very bright-end of the galaxy luminosity function from z = 8-10
• Constraining the neutral fraction of hydrogen in the IGM at redshift 7.5
• High-precision Photometric Redshifts from Spitzer/IRAC: Extreme [3.6]-[4.5] Colors Identify Galaxies in the Redshift Range z~6.6-6.9
• High-redshift AGN in the Chandra Deep Fields: the obscured fraction and space density of the sub-$L_{\ast }$ population
• Accelerated Evolution of Ly$\alpha$ Luminosity Function at $\text{z}\gtrsim 7$ Revealed by the Subaru Ultra-Deep Survey for Ly$\alpha$ Emitters at $\text{z}=7.3$
• CANDELS Multiwavelength Catalogs: Source Identification and Photometry in the CANDELS COSMOS Survey Field
• A spectroscopic search for AGN activity in the reionization era
• Correcting the z~8 Galaxy Luminosity Function for Gravitational Lensing Magnification Bias
• A dusty compact object bridging galaxies and quasars at cosmic dawn
• Revealing the Accretion Physics of Supermassive Black Holes at Redshift z~7 with Chandra and Infrared Observations
• The UV-brightest Lyman continuum emitting star-forming galaxy
• HST imaging of the brightest z~8-9 galaxies from UltraVISTA: the extreme bright end of the UV luminosity function
• SuperBoRG: Exploration of point sources at $z\sim 8$ in HST parallel fields
• Prospects for distinguishing galaxy evolution models with surveys at redshifts $z\gtrsim 4$
• SuperBoRG: Search for The Brightest of Reionizing Galaxies and Quasars in HST Parallel Imaging Data
• The discovery of the most UV-Lya luminous star-forming galaxy: a young, dust- and metal-poor starburst with QSO-like luminosities
• Observing the host galaxies of high-redshift quasars with JWST: predictions from the BlueTides simulation
• Beyond the Local Volume. I. Surface Densities of Ultracool Dwarfs in Deep HST/WFC3 Parallel Fields
• Galaxy Morphologies Revealed with Subaru HSC and Super-Resolution Techniques I: Major Merger Fractions of L_UV~3-15 L_UV* Dropout Galaxies at z~4-7
• Stochastic Processes as the Origin of the Double-Power Law Shape of the Quasar Luminosity Function
• A Physical Model for the Quasar Luminosity Function Evolution between Cosmic Dawn and High Noon

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