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Register a new userDissecting the Lyman Alpha Emission Halo of LAB1
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Anne-Marie Weijmans
Publication date
2009
fields
Physics
and research's language is
English
Authors
A. Weijmans
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We report observations of Lyman Alpha Blob 1 (LAB1) in the SSA 22 protocluster region (z=3.09) with the integral-field spectrograph SAURON. We increased the signal-to-noise in the spectra by more than a factor three compared to our previous observations. This allows us to probe the structure of the LAB system in detail, examining its structure in the spatial and wavelength dimensions. We find that the emission from the system comes largely from five distinct blobs. Two of the emission regions are associated with Lyman Break Galaxies, while a third appears to be associated with a heavily obscured submillimeter galaxy. The fourth and fifth components do not appear to be associated with any galaxy despite the deep imaging that is available in this field. If we interpret wavelength shifts in the line centroid as velocity structure in the underlying gas, many of these emission systems show evidence of velocity shear. It remains difficult to distinguish between an underlying rotation of the gas and an outflow driven by the central object. We have examined all of the line profiles for evidence of strong absorption features. While several systems are better fitted by the inclusion of a weak absorption component, we do not see evidence for a large-scale coherent absorption feature such as that seen in LAB2.
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We have used the SAURON panoramic integral field spectrograph to study the structure of the Ly-alpha emission-line halo, LAB1, surrounding the sub-millimeter galaxy SMM J221726+0013. This emission-line halo was discovered during a narrow-band imaging survey of the z=3.1 large-scale structure in the SSA22 region. Our observations trace the emission halo out to almost 100 kpc from the sub-millimeter source and identify two distinct Ly-alpha `mini-haloes around the nearby Lyman-break galaxies. The main emission region has a broad line profile, with variations in the line profile seeming chaotic and lacking evidence for a coherent velocity structure. The data also suggests that Ly-alpha emission is suppressed around the sub-mm source. Interpretation of the line structure needs care because Ly-alpha may be resonantly scattered, leading to complex radiative transfer effects, and we suggest that the suppression in this region arises because of such effects. We compare the structure of the central emission-line halo with local counter-parts, and find that the emission line halo around NGC 1275 in the Perseus cluster may be a good local analogue, although the high redshift halo is factor of ~100 more luminous and appears to have higher velocity broadening. Around the Lyman-break galaxy C15, the emission line is narrower, and a clear shear in the emission wavelength is seen. A plausible explanation for the line profile is that the emission gas is expelled from C15 in a bipolar outflow, similar to that seen in M82.
Ly$alpha$ photons scattered by neutral hydrogen atoms in the circumgalactic media or produced in the halos of star-forming galaxies are expected to lead to extended Ly$alpha$ emission around galaxies. Such low surface brightness Ly$alpha$ halos (LAHs) have been detected by stacking Ly$alpha$ images of high-redshift star-forming galaxies. We study the origin of LAHs by performing radiative transfer modeling of nine $z=3.1$ Lyman-Alpha Emitters (LAEs) in a high resolution hydrodynamic cosmological galaxy formation simulation. We develop a method of computing the mean Ly$alpha$ surface brightness profile of each LAE by effectively integrating over many different observing directions. Without adjusting any parameters, our model yields an average Ly$alpha$ surface brightness profile in remarkable agreement with observations. We find that observed LAHs cannot be accounted for solely by photons originating from the central LAE and scattered to large radii by hydrogen atoms in the circumgalactic gas. Instead, Ly$alpha$ emission from regions in the outer halo is primarily responsible for producing the extended LAHs seen in observations, which potentially includes both star-forming and cooling radiation. With the limit on the star formation contribution set by the ultra-violet (UV) halo measurement, we find that cooling radiation can play an important role in forming the extended LAHs. We discuss the implications and caveats of such a picture.
As a result of resonant scatterings off hydrogen atoms, Lyman-alpha (Lya) emission from star-forming galaxies provides a probe of the (hardly isotropic) neutral gas environment around them. We study the effect of the environmental anisotropy on the observed Lya emission by performing radiative transfer calculations for models of neutral hydrogen clouds with prescriptions of spatial and kinematic anisotropies. The environmental anisotropy leads to corresponding anisotropy in the Lya flux and spectral properties and induces correlations among them. The Lya flux (or observed luminosity) depends on the viewing angle and shows an approximate correlation with the initial Lya optical depth in the viewing direction relative to those in all other directions. The distribution of Lya flux from a set of randomly oriented clouds is skewed to high values, providing a natural contribution to the Lya equivalent width (EW) distribution seen in observation. A narrower EW distribution is found at a larger peak offset of the Lya line, similar to the trend suggested in observation. The peak offset appears to correlate with the line shape (full width at half maximum and asymmetry), pointing to a possibility of using Lya line features alone to determine the systemic redshifts of galaxies. The study suggests that anisotropies in the spatial and kinematic distributions of neutral hydrogen can be an important ingredient in shaping the observed properties of Lya emission from star-forming galaxies. We discuss the implications of using Lya emission to probe the circumgalactic and intergalactic environments of galaxies.
We present Spitzer observations of Lya Blobs (LAB) at z=2.38-3.09. The mid-infrared ratios (4.5/8um and 8/24um) indicate that ~60% of LAB infrared counterparts are cool, consistent with their infrared output being dominated by star formation and not active galactic nuclei (AGN). The rest have a substantial hot dust component that one would expect from an AGN or an extreme starburst. Comparing the mid-infrared to submillimeter fluxes (~850um or rest frame far infrared) also indicates a large percentage (~2/3) of the LAB counterparts have total bolometric energy output dominated by star formation, although the number of sources with sub-mm detections or meaningful upper limits remains small (~10). We obtained Infrared Spectrograph (IRS) spectra of 6 infrared-bright sources associated with LABs. Four of these sources have measurable polycyclic aromatic hydrocarbon (PAH) emission features, indicative of significant star formation, while the remaining two show a featureless continuum, indicative of infrared energy output completely dominated by an AGN. Two of the counterparts with PAHs are mixed sources, with PAH line-to-continuum ratios and PAH equivalent widths indicative of large energy contributions from both star formation and AGN. Most of the LAB infrared counterparts have large stellar masses, around 10^11 Mo. There is a weak trend of mass upper limit with the Lya luminosity of the host blob, particularly after the most likely AGN contaminants are removed. The range in likely energy sources for the LABs found in this and previous studies suggests that there is no single source of power that is producing all the known LABs.
We present the results of a high-spatial-resolution study of the line emission in a sample of z=3.1 Lyman-Alpha-Emitting Galaxies (LAEs) in the Extended Chandra Deep Field-South. Of the eight objects with coverage in our HST/WFPC2 narrow-band imaging, two have clear detections and an additional two are barely detected (~2-sigma). The clear detections are within ~0.5 kpc of the centroid of the corresponding rest-UV continuum source, suggesting that the line-emitting gas and young stars in LAEs are spatially coincident. The brightest object exhibits extended emission with a half-light radius of ~1.5 kpc, but a stack of the remaining LAE surface brightness profiles is consistent with the WFPC2 point spread function. This suggests that the Lyman Alpha emission in these objects originates from a compact (<~2 kpc) region and cannot be significantly more extended than the far-UV continuum emission (<~1 kpc). Comparing our WFPC2 photometry to previous ground-based measurements of their monochromatic fluxes, we find at 95% (99.7%) confidence that we cannot be missing more than 22% (32%) of the Lyman Alpha emission.
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