Z > 7 galaxies with red Spitzer/IRAC [3.6]-[4.5] colors in the full CANDELS data set: the brightest-known galaxies at Z ~ 7-9 and a probable spectroscopic confirmation at Z=7.48 (1506.00854v3)

Abstract: We identify 4 unusually bright (H < 25.5) galaxies from HST and Spitzer CANDELS data with probable redshifts z ~ 7-9. These identifications include the brightest-known galaxies to date at z > 7.5. As Y-band observations are not available over the full CANDELS program to perform a standard Lyman-break selection of z > 7 galaxies, we employ an alternate strategy using deep Spitzer/IRAC data. We identify z ~ 7.1 - 9.1 galaxies by selecting z >~ 6 galaxies from the HST CANDELS data that show quite red IRAC [3.6]-[4.5] colors, indicating strong [OIII]+Hbeta lines in the 4.5 micron band. This selection strategy was validated using a modest sample for which we have deep Y-band coverage, and subsequently used to select the brightest z > 7 sources. Applying the IRAC criteria to all HST-selected optical-dropout galaxies over the full ~900 arcmin**2 of the CANDELS survey revealed four unusually bright z ~ 7.1, 7.6, 7.9 and 8.6 candidates. The median [3.6]-[4.5] color of our selected z ~ 7.1-9.1 sample is consistent with rest-frame [OIII]+Hbeta EWs of ~1500A, in the [4.5] band. Keck/MOSFIRE spectroscopy has been independently reported for two of our selected sources, showing Ly-alpha at redshifts of 7.7302+/-0.0006 and 8.683^+0.001_-0.004, respectively. We present similar Keck/MOSFIRE spectroscopy for a third selected galaxy with a probable 4.7sigma Ly-alpha line at z_spec=7.4770+/-0.0008. All three have H-band magnitudes of ~25 mag and are ~0.5 mag more luminous (M(UV) ~ -22.0) than any previously discovered z ~ 8 galaxy, with important implications for the UV LF. Our 3 brightest, highest redshift z > 7 galaxies all lie within the CANDELS EGS field, providing a dramatic illustration of the potential impact of field-to-field variance.

• The paper demonstrates a novel IRAC color-based selection method that effectively identifies ultra-luminous galaxies at z~7–9.
• It reports spectroscopic confirmations via Keck/MOSFIRE observations with Lyα emissions detected at z=7.48, 7.73, and 8.68.
• The findings of strong [OIII]+Hβ emissions and H160 ≈ 25 mag luminosity refine the UV luminosity function and our understanding of reionization-era star formation.

Overview of Bright Galaxies at Redshift 7-9 in CANDELS

The paper presented investigates unusually luminous galaxies during the epoch of reionization, specifically within the redshift range of approximately 7 to 9. The research exploits the extensive datasets from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) alongside Spitzer/IRAC observations to identify these high-redshift galaxies through their distinctive red [3.6]-[4.5] colors. This color signature implies the presence of strong [OIII] and H$\beta$ emission lines in the 4.5$\mu$m band, serving as an indicator of active star-forming regions at these early cosmic times.

Methodology

The identification method employed caters to the challenge of lacking Y-band observations necessary for standard Lyman-break selections. Instead, the researchers utilized deep IRAC data, selecting galaxies that drop out in optical bands and exhibit sufficiently red [3.6]-[4.5] colors. This approach was validated against regions with existing Y-band data, confirming its efficacy. The final selection covered an extensive area of approximately 900 arcmin$^2$ within the CANDELS fields, revealing four bright candidates at redshifts z ~ 7.1, 7.6, 7.9, and 8.6.

Results and Analysis

Among the key findings are:

1. Spectroscopic confirmations via Keck/MOSFIRE observations where Ly$\alpha$ emission lines were detected at redshifts 7.7302, 8.683, and a probable detection at 7.477.
2. The galaxies display [3.6]-[4.5] colors consistent with notable [OIII]+H$\beta$ equivalent widths (EW) of around 1500Å, suggesting intense star formation activity.
3. The visible magnitude for these galaxies around H$_{160}$ ~ 25 mag categorizes them as the most luminous at such high redshifts so far, with considerable implications for understanding the UV luminosity function (LF) and the evolution of early galaxies.

Implications

The discovery of these bright galaxies contributes to refining the UV LF at z > 7 and places constraints on the number density and intrinsic brightness of early galaxies. The presence of significant [OIII] and H$\beta$ emission indicates prevalent intense star-forming regions, providing insights into the physical conditions during reionization. These findings also emphasize the potential of field-to-field variance, as all key discoveries were localized within the CANDELS EGS field.

Future Directions

The paper proposes further exploration with deeper infrared observations and spectroscopy to solidify redshift identifications and better understand the nature of these galaxies. With upcoming facilities like the James Webb Space Telescope (JWST), more detailed spectroscopic studies will be feasible, potentially unveiling even fainter and more numerous galaxies at these high redshifts. Such studies will be essential for piecing together the role of galaxies in the reionization of the universe and for testing models of galaxy formation and evolution during these nascent stages.

In summary, the research outlines a novel selection technique utilizing IRAC colors, yielding significant discoveries of ultra-luminous galaxies in the early universe. This work not only contributes to our understanding of early galaxy formation but also sets a foundation for future explorations in high-redshift astrophysics.