Large Equivalent Width Lyα line Emission at z=4.5: Young Galaxies in a Young Universe?

By S. Malhotra, J. Rhoads. 2001

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The Large Area Lyman Alpha survey has found ≈150 Lyα emitters at z = 4.5. While stellar models predict a maximum Lyα equivalent width (EW) of 240 Å, 60% of the Lyα emitters have EWs exceeding this value. We attempt to model the observed EW distribution by combining stellar population models with an extrapolation of a Lyman break galaxy luminosity function at z = 4, incorporating observational selection effects and Malmquist bias. To reproduce the high EWs seen in the sample, we need to postulate a stellar initial mass function with an extreme slope of α = 0.5 (instead of 2.35), zero-metallicity stars, or narrow-line active galactic nuclei (AGNs). Not all the high-EW sources can be AGNs without exceeding the X-ray background or postulating a very steep redshift evolution. Only 7.5%-15% of galaxies need to show Lyα emission to explain the observed number counts. This raises the possibility either that star formation in high-redshift galaxies is episodic or that the Lyα galaxies we are seeing are the youngest 7.5%-15% and that Lyα is strongly quenched by dust at about 107 yr of age.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

• Population III stars: hidden or disappeared? (2007, background): “Thus, a non-negligible fraction of observable z > 3 objects may be powered by the radiative (Lyman-α emitters, Lyman Break Galaxies: SSF, Malhotra & Rhoads 2002; Dawson et al. 2004; Jimenez & Haiman 2006) or mechanical (pair-instability supernovae: Scannapieco et al. 2005) input of Pop III stars.”
• Hyper-Eddington accretion flows on to massive black holes (2015, background): “…(e.g. Hagen et al. 2014) and some LAEs do not have a stellar continuum detection, and/or have rest-frame equivalent width in excess of 240 Å, which cannot be explained by a normal stellar population with a Salpeter IMF (e.g. Malhotra & Rhoads 2002; Dawson et al. 2004; Shimasaku et al. 2006).”
• EVIDENCE FOR PopIII-LIKE STELLAR POPULATIONS IN THE MOST LUMINOUS Lyα EMITTERS AT THE EPOCH OF REIONIZATION: SPECTROSCOPIC CONFIRMATION (2015, background): “The estimated rest-frame EW of the Lyα line from spectroscopy is very high (> 200 Å), thus implying a likely very metal-poor stellar population (Malhotra & Rhoads 2002).”
• Lyα Emitters at z = 5.7 in the Subaru Deep Field (2006, methodology): “Malhotra & Rhoads (2002) used the first method, while Hu et al. (2004) applied both and then adopted the first method.”
• Hα Tail, Intracluster H II Regions, and Star Formation: ESO 137-001 in Abell 3627 (2007, methodology): “Thus, the expected number of emission-line objects selected in Malhotra & Rhoads (2002) is∼ 0.02 in our field (adjusted to our flux limit by assuming a power index of -1.6 for the lu- minosity function, Malhotra & Rhoads 2002).”
• Luminosity Functions of Lyα Emitters at Redshifts z = 6.5 and z = 5.7: Evidence against Reionization at z ≤ 6.5 (2004, background): “That would require intrinsic Lya to be at least 3 times stronger at , making the mean intrinsic equivalent widthz p 6.5 185 , compared to a mean of 90 found at (Dawson˚ ˚A A z p 4.5 et al. 2004; Malhotra & Rhoads 2002; Rhoads et al. 2000).”
• Evolution in the properties of Lyman-α emitters from redshifts z ~ 3 to z ~ 2 (2008, background): “At higher redshift (z & 4), emitters with EWs larger than240 Å have been predicted (Schaerer 2003; Tumlison et al. 2003) and observed (Malhotra & Rhoads 2002; Shimasaku et al. 2006; Stanway et al. 2007).”
• Lyman α radiative transfer in a multiphase medium (2005, methodology): “For instance, features in Lyα emission have been used to suggest strong galactic outflows (Kunth et al. 1998), as a signature of strong accretion shocks (Barkana & Loeb 2003), and as evidence for an unusually strong ionizing continuum, perhaps due to Pop III stars (Malhotra & Rhoads 2002).”
• The impact of The IGM on high-redshift Lyα emission lines (2007, background): “…1997, hereafter, LAEs), existing Lyα surveys have detected several hundred galaxies out to redshift z = 7.0 (e.g. Hu et al. 1998; Rhoads et al. 2000; Malhotra & Rhoads 2002; Hu et al. 2002; Kodaira et al. 2003; Dawson et al. 2004; Stanway et al. 2004; Tran et al. 2004; Taniguchi et al. 2005;…”
• THE SPECTRAL EVOLUTION OF THE FIRST GALAXIES. I. JAMES WEBB SPACE TELESCOPE DETECTION LIMITS AND COLOR CRITERIA FOR POPULATION III GALAXIES (2011, methodology): “…all searches for pop III galaxies with largely nebular SEDs (our type A) have so far focused on the fluxes of the Lyα line, the HeII λ1640, λ4686 lines, or the Lyman ‘bump’ (e.g. Tumlinson & Shull 2000; Tumlinson et al. 2001; Oh et al. 2001; Malhotra & Rhoads 2002; Schaerer 2002, 2003; Inoue 2010).”
• SILVERRUSH. IV. Lyα luminosity functions at z = 5.7 and 6.6 studied with ∼1300 Lyα emitters on the 14–21 deg2 sky (2017, background): “…range of z = 0−8 by several approaches including narrowband surveys (e.g., Cowie & Hu 1998; Hu et al. 1998; Rhoads et al. 2000; Steidel et al. 2000; Malhotra & Rhoads 2002; Ajiki et al. 2002; Ouchi et al. 2003; Hayashino et al. 2004; Matsuda et al. 2004; Taniguchi et al. 2005; Iye et al. 2006;…”
• Titans of the early Universe: The Prato statement on the origin of the first supermassive black holes (2018, background): “Initially thought to be the first definitely Pop III-like galaxy ever observed (though for earlier claims see, e.g., Malhotra & Rhoads 2002; di Serego Alighieri et al. 2008), this bright z = 6.6 Lyman-α emitter has provided a critical test of our ability to discriminate between theoretical…”
• Cosmological Aspects of Gamma-Ray Bursts: Luminosity Evolution and an Estimate of the Star Formation Rate at High Redshifts (2001, background): “Currently, both observational and theoretical arguments (see, e.g., Larson 1998 and references therein; see also Malhotra & Rhoads 2002) suggest that the stellar initial mass function (IMF) was ‘‘ top-heavy ’’ at high redshift; that is, the mass scale of the IMF was higher in the earlier stages of…”
• Lyα Emitting Galaxies as a Probe of Reionisation (2014, background): “However, the same signature can be attributed population III stars (e.g. Raiter et al. 2010), and/or galaxies forming stars with a top-heavy initial mass function (IMF, e.g. Malhotra & Rhoads 2002), or stochastic sampling of the IMF (Forero-Romero & Dijkstra 2013).”