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Register a new userUnderstanding Lyman-alpha Nebulae at Low Redshift I: The Sizes, Powering, and Kinematics of Green Bean Galaxies
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Moire K. M. Prescott
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A new but rare sample of spatially extended emission line nebulae, nicknamed Green Beans, was discovered at z~0.3 thanks to strong [OIII] emission, and subsequently shown to be local cousins of the Lyman-alpha (Lya) nebulae found at high redshift. Here we use follow-up APO/DIS spectroscopy to better understand how these low redshift Lya nebulae compare to other populations of strong emission line sources. Our spectroscopic data show that low-z Lya nebulae have AGN-like emission line ratios, relatively narrow line widths (FWHM<1000 km s^{-1}), and emission line kinematics resembling those of Type 2 AGN at the same redshift, confirming that they are powered by Type 2 AGN with typical ionizing continua. While low-z Lya nebulae are larger and less concentrated than compact, star-forming Green Pea galaxies, we find that they resemble typical Type 2 AGN in terms of r-band concentration and size. Based on this pilot study, low-z Lya nebulae appear to be a subset of Type 2 AGN with bluer optical continua and high [OIII] equivalent widths but with comparable sizes and similar [OIII] kinematics. These characteristics may simply reflect the fact that low-z Lya nebulae are drawn from the high luminosity end of the Type 2 AGN distribution, with higher nuclear activity driving higher [OIII] equivalent widths and more central star formation leading to bluer optical continua. Deeper spectroscopic follow-up of the full sample will shed further light on these issues and on the relationship between these low-z Lya nebulae and the Lya nebula population at high redshift.
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Exploring the origin of Ly-alpha nebulae (blobs) requires measurements of their gas kinematics that are impossible with only the resonant, optically-thick LyA line. To define gas motions relative to the systemic velocity of the blob, the LyA line must be compared with an optically-thin line like Halpha, which is not much altered by radiative transfer effects. We obtain optical and NIR spectra of the two brightest LyA blobs from Yang et al. sample using the Magellan/MagE and VLT/SINFONI. Both the LyA and Halpha lines confirm that these blobs lie at the survey redshift, z~2.3. Within each blob, we detect several Halpha sources, which roughly correspond to galaxies seen in HST images. The Halpha detections show that these galaxies have large internal velocity dispersions (130 - 190km/s) and that, in the one system (LAB01), their velocity difference is ~440 km/s. The presence of multiple galaxies within the blobs, and those galaxies large velocity dispersions and large relative motion, is consistent with our previous finding that LyA blobs inhabit massive dark matter halos that will evolve into those typical of rich clusters today. To determine whether the gas near the embedded galaxies is predominantly infalling or outflowing, we compare the LyA and Halpha line centers, finding that LyA is not offset (Delta LyA = +0km/s) in LAB01 and redshifted by only +230 km/s in LAB02. These offsets are small compared to those of Lyman break galaxies, which average +450 km/s and extend to about +700 km/s. We test and rule out the simplest infall models and those outflow models with super/hyper-winds, which require large outflow velocities. Because of the unknown geometry of the gas distribution and the possibility of multiple sources of LyA emission embedded in the blobs, a larger sample and more sophisticated models are required to test more complex or a wider range of infall and outflow scenarios.
[Abridged] With VLT/X-shooter, we obtain optical and NIR spectra of six Ly-alpha blobs at z~2.3. Using three measures --- the velocity offset between the Lya line and the non-resonant [OIII] or H-alpha line (Dv_Lya), the offset of stacked interstellar metal absorption lines, and the spectrally-resolved [OIII] line profile --- we study the kinematics of gas along the line of sight to galaxies within each blob center. These three indicators generally agree in velocity and direction, and are consistent with a simple picture in which the gas is stationary or slowly outflowing at a few hundred km/s from the embedded galaxies. The absence of stronger outflows is not a projection effect: the covering fraction for our sample is limited to <1/8 (13%). The outflow velocities exclude models in which star formation or AGN produce super or hyper winds of up to ~1000km/s. The Dv_Lya offsets here are smaller than typical of LBGs, but similar to those of compact LAEs. The latter suggests that outflow speed cannot be a dominant factor in driving extended Lya emission. For one Lya blob (CDFS-LAB14), whose Lya profile and metal absorption line offsets suggest no significant bulk motion, we use a simple radiative transfer model to make the first column density measurement of gas in an embedded galaxy, finding it consistent with a DLA system. Overall, the absence of clear inflow signatures suggests that the channeling of gravitational cooling radiation into Lya is not significant over the radii probed here. However, one peculiar system (CDFS-LAB10) has a blueshifted Lya component that is not obviously associated with any galaxy, suggesting either displaced gas arising from tidal interactions among blob galaxies or gas flowing into the blob center. The former is expected in these overdense regions, and the latter might signify the predicted but elusive cold gas accretion along filaments.
I provide an overview about star-forming galaxies at high redshift and their physical properties. Starting from the populations of Ly-$alpha$ emitters and Lyman break galaxies, I summarize their common features and distinction. Then I summarize recent insight onto their physical properties gained from SED models including nebular emission, and various implications from these studies on the properties of star-formation at high redshift. Finally, I present new results and an overview on the dust content and UV attenuation of $z>6$ galaxies obtained from IRAM and ALMA observations.
We report on the serendipitous discovery of a z=4.0, M1500=-22.20 star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (~60% escaping), a remarkable multiple peaked Lya emission, and significant Lya radiation directly emerging at the resonance frequency. This is the highest redshift confirmed LyC emitter in which the ionising and Lya radiation possibly share a common ionised cavity (with N_HI<10^17.2 cm^-2). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni NV1240 profile, and has blue ultraviolet continuum (beta = -2.5 +/- 0.25, F_lambda~ lambda^beta) with weak low-ionisation interstellar metal lines. Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6um and 4.5um imaging show a clear photometric signature of the Halpha line with equivalent width of 1000A rest-frame emerging over a flat continuum (Ks-4.5um ~ 0). From the SED fitting we derive a stellar mass of 1.5x10^9 Msun, SFR of 140 Msun/yr and age of ~10 Myr, with a low dust extinction, E(B-V)< 0.1, placing the source in the starburst region of the SFR-M^* plane. Ion3 shows similar properties of another LyC emitter previously discovered (z=3.21, Ion2, Vanzella et al. 2016). Ion3 (and Ion2) represents ideal high-redshift reference cases to guide the search for reionising sources at z>6.5 with JWST.
Lyman-alpha (Ly{alpha}) photons from ionizing sources and cooling radiation undergo a complex resonant scattering process that generates unique spectral signatures in high-redshift galaxies. We present a detailed Ly{alpha} radiative transfer study of a cosmological zoom-in simulation from the Feedback In Realistic Environments (FIRE) project. We focus on the time, spatial, and angular properties of the Ly{alpha} emission over a redshift range of z = 5-7, after escaping the galaxy and being transmitted through the intergalactic medium (IGM). Over this epoch, our target galaxy has an average stellar mass of $M_{rm star} approx 5 times 10^8 {rm M}_odot$. We find that many of the interesting features of the Ly{alpha} line can be understood in terms of the galaxys star formation history. The time variability, spatial morphology, and anisotropy of Ly{alpha} properties are consistent with current observations. For example, the rest frame equivalent width has a ${rm EW}_{{rm Ly}alpha,0} > 20 {rm AA}$ duty cycle of 62% with a non-negligible number of sightlines with $> 100 {rm AA}$, associated with outflowing regions of a starburst with greater coincident UV continuum absorption, as these conditions generate redder, narrower (or single peaked) line profiles. The lowest equivalent widths correspond to cosmological filaments, which have little impact on UV continuum photons but efficiently trap Ly{alpha} and produce bluer, broader lines with less transmission through the IGM. We also show that in dense self-shielding, low-metallicity filaments and satellites Ly{alpha} radiation pressure can be dynamically important. Finally, despite a significant reduction in surface brightness with increasing redshift, Ly{alpha} detections and spectroscopy of high-$z$ galaxies with the upcoming James Webb Space Telescope is feasible.
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