arXiv subjects

Huub Röttgering

Publications and source records attributed to Huub Röttgering.

At least 37 records · Page 2Linked to original sources

arXiv2020

One- and Two-point Source Statistics from the LOFAR Two-metre Sky Survey First Data Release

The LOFAR Two-metre Sky Survey (LoTSS) will map the complete Northern sky and provide an excellent opportunity to study the distribution and evolution of the large-scale structure of the Universe. We study the completeness of the LoTSS first data release (DR1) and find a point-source completeness of 99 % above flux densities of 0.8 mJy and define a suite of quality cuts. We determine the count-in-cell statistics and differential source counts statistic and measure the angular two-point correlation function of the LoTSS radio sources. The counts-in-cell statistic reveals that the distribution of radio sources cannot be described by a spatial Poisson process. Instead, a good fit is provided by a compound Poisson distribution. The differential source counts are in good agreement with previous findings in deep fields at low radio frequencies and with simulated catalogues from the SKA design study sky and the Tiered Radio Extragalactic Continuum Simulation. The angular two-point correlation is $<10^{-2}$ at angular scales $> 1$ deg. Restricting the value added source catalogue to low-noise regions and a flux density threshold of 2 mJy provides our most reliable estimate of the angular two-point correlation. For smaller flux density thresholds systematic issues are identified, most likely related to the flux density calibration of the individual pointings. Based on the distribution of photometric redshifts of LoTSS sources and the Planck 2018 best-fit cosmological model, the theoretically predicted angular two-point correlation between 0.1 deg and 6 deg agrees with the measured clustering for a subsample of radio sources with redshift information. We find agreement with the expectation of large-scale statistical isotropy of the radio sky at the per cent level. The angular two-point correlation agrees well with the expectation of the cosmological standard model. (abbreviated)

astro-ph.CO↗

arXiv2020

Radio constraints on dark matter annihilation in Canes Venatici I with LOFAR

Dwarf galaxies are dark matter-dominated and therefore promising targets for the search for weakly interacting massive particles (WIMPs), which are well-known candidates for dark matter. Annihilation of WIMPs produce ultra-relativistic cosmic-ray electrons and positrons that emit synchrotron radiation in the presence of magnetic fields. For typical magnetic field strengths (few $μ$G) and $\mathcal O$(GeV--TeV) WIMP masses, this emission peaks at hundreds of MHz. Here, we use the non-detection of 150-MHz radio continuum emission from the dwarf spheroidal galaxy Canes Venatici I with the LOw-Frequency ARray (LOFAR) to derive constraints on the annihilation cross section of WIMPs into primary electron-positron and other fundamental particle-antiparticle pairs. Our main underlying assumption is that the transport of the cosmic rays can be described by the diffusion approximation, thus requiring a non-zero magnetic field strength with small-scale structure. In particular, by adopting magnetic field strengths of $\mathcal O(1\,μ$G) and diffusion coefficients $\sim 10^{27}~\rm cm^2\,s^{-1}$, we obtain limits that are comparable with those set by \emph{Fermi} Large Area Telescope using gamma-ray observations of this particular galaxy. Assuming s-wave annihilation and WIMPs making up 100 per cent of the DM density, our benchmark limits exclude several thermal WIMP realisations in the $[2,20]$-GeV mass range. We caution, however, that our limits for the cross section are subject to enormous uncertainties which we also quantitatively assess. In particular, variations on the propagation parameters or on the DM halo can shift our limits up by several orders of magnitude (in the pessimistic scenario).

astro-ph.HE↗

arXiv2019

Resolved Lyman-α properties of a luminous Lyman-break galaxy in a large ionised bubble at z = 6.53

The observed properties of the Lyman-$α$ (Ly$α$) emission line are a powerful probe of neutral gas in and around galaxies. We present spatially resolved Ly$α$ spectroscopy with VLT/MUSE targeting VR7, a UV-luminous galaxy at $z=6.532$ with moderate Ly$α$ equivalent width (EW$_0\approx38$ Å). These data are combined with deep resolved [CII]$_{\rm 158 μm}$ spectroscopy obtained with ALMA and UV imaging from {\it HST} and we also detect UV continuum with MUSE. Ly$α$ emission is clearly detected with S/N $\approx40$ and FWHM of 374 km s$^{-1}$. Ly$α$ and [CII] are similarly extended beyond the UV, with effective radius r$_{\rm eff} = 2.1\pm0.2$ kpc for a single exponential model or r$_{\rm eff, Lyα, halo} = 3.45^{+1.08}_{-0.87}$ kpc when measured jointly with the UV continuum. The Ly$α$ profile is broader and redshifted with respect to the [CII] line (by 213 km s$^{-1}$), but there are spatial variations that are qualitatively similar in both lines and coincide with resolved UV components. This suggests that the emission originates from two components with plausibly different HI column densities. We place VR7 in the context of other galaxies at similar and lower redshift. The Ly$α$ halo scale length is similar at different redshifts and velocity shifts with respect to the systemic are typically smaller. Overall, we find little indications of a more neutral vicinity at higher redshift. This means that the local ($\sim 10$ kpc) neutral gas conditions that determine the observed Ly$α$ properties in VR7 resemble the conditions in post-re-ionisation galaxies.

astro-ph.GA↗

arXiv2019

An Absence of Radio-Loud Active Galactic Nuclei in Geometrically Flat Quiescent Galaxies: Implications for Maintenance-Mode Feedback Models

Maintenance-mode feedback from low-accretion rate AGN, manifesting itself observationally through radio-loudness, is invoked in all cosmological galaxy formation models as a mechanism that prevents excessive star-formation in massive galaxies (M$_*$ $\gtrsim$ 3$\times$10$^{10}$ M$_{\odot}$). We demonstrate that at a fixed mass the incidence of radio-loud AGN (L $>$ 10$^{23}$ WHz$^{- 1}$) identified in the FIRST and NVSS radio surveys among a large sample of quiescent (non-star forming) galaxies selected from the SDSS is much higher in geometrically round galaxies than in geometrically flat, disk-like galaxies. As found previously, the RL AGN fraction increases steeply with stellar velocity dispersion $σ_*$ and stellar mass, but even at a fixed velocity dispersion of 200-250 kms$^{-1}$ this fraction increases from 0.3% for flat galaxies (projected axis ratio of q $<$ 0.4) to 5% for round galaxies (q $>$ 0.8). We rule out that this strong trend is due to projection effects in the measured velocity dispersion. The large fraction of radio-loud AGN in massive, round galaxies is consistent with the hypothesis that such AGN deposit energy into their hot gaseous halos, preventing cooling and star-formation. However, the absence of such AGN in disk-like quiescent galaxies -- most of which are not satellites in massive clusters, raises important questions: is maintenance-mode feedback a generally valid explanation for quiescence; and, if so, how does that feedback avoid manifesting at least occasionally as a radio-loud galaxy?

astro-ph.GA↗

arXiv2018

LoTSS/HETDEX: Optical quasars I. Low-frequency radio properties of optically selected quasars

The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open question. Although it is thought that accretion onto supermassive black holes in the centre the host galaxies of quasars is responsible for some radio continuum emission, there is still a debate as to whether star formation or active galactic nuclei (AGN) activity dominate the radio continuum luminosity. To date, radio emission in quasars has been investigated almost exclusively using high-frequency observations in which the Doppler boosting might have an important effect on the measured radio luminosity, whereas extended structures, best observed at low radio frequencies, are not affected by the Doppler enhancement. We used a sample of quasars selected by their optical spectra in conjunction with sensitive and high-resolution low-frequency radio data provided by the LOw Frequency ARray (LOFAR) as part of the LOFAR Two-Metre Sky Survey (LoTSS) to investigate their radio properties using the radio loudness parameter ($\mathcal{R} = \frac{L_{\mathrm{144-MHz}}}{L_{\mathrm{i\,band}}}$). The examination of the radio continuum emission and RL/RQ dichotomy in quasars exhibits that quasars show a wide continuum of radio properties (i.e. no clear bimodality in the distribution of $\mathcal{R}$). Radio continuum emission at low frequencies in low-luminosity quasars is consistent with being dominated by star formation. We see a significant albeit weak dependency of $\mathcal{R}$ on the source nuclear parameters. For the first time, we are able to resolve radio morphologies of a considerable number of quasars. All these crucial results highlight the impact of the deep and high-resolution low-frequency radio surveys that foreshadow the compelling science cases for the Square Kilometre Array (SKA).

astro-ph.GA↗

arXiv2018

Confirmation of double peaked Lyman-alpha emission at z=6.593: Witnessing a galaxy directly contributing to the reionisation of the Universe

Distant luminous Lyman-alpha emitters (LAEs) are excellent targets for spectroscopic observations of galaxies in the epoch of reionisation (EoR). We present deep high-resolution (R=5000) VLT/X-SHOOTER observations, along with an extensive collection of photometric data of `COLA1', a proposed double peaked LAE at z=6.6 (Hu et al. 2016). We rule out that COLA1's emission line is an [OII] doublet at z=1.475 on the basis of i) the asymmetric red line-profile and flux ratio of the peaks (blue/red=$0.31\pm0.03$) and ii) an unphysical [OII]/Ha ratio ([OII]/Ha > 22). We show that COLA1's observed B-band flux is explained by a faint extended foreground LAE, for which we detect Lya and [OIII] at z=2.142. We thus conclude that COLA1 is a real double-peaked LAE at z=6.593, the first discovered at z>6, confirming the result from Hu et al. (2016). COLA1 is UV luminous (M$_{1500}=-21.6\pm0.3$), has a high equivalent width (EW$_{0}$~120 Å) and very compact Lya emission (r$_{50} = 0.3$ kpc). Relatively weak inferred Hb+[OIII] line-emission from Spitzer/IRAC indicates an extremely low metallicity of Z<1/20 Z$_{\odot}$ or reduced strength of nebular lines due to high escape of ionising photons. The small Lya peak separation of $220\pm20$ km/s implies a low HI column density and an ionising photon escape fraction of ~15-30 %, providing the first direct evidence that such galaxies contribute actively to the reionisation of the Universe at z>6. Based on simple estimates, we find that COLA1 could have provided just enough photons to reionise its own ~0.3 pMpc bubble, allowing the blue Lya line to be observed. However, we also discuss alternative scenarios explaining the detected double peaked nature of COLA1. Our results show that future high-resolution observations of statistical samples of double peaked LAEs at z>5 are a promising probe of the occurrence of ionised regions around galaxies in the EoR.

astro-ph.GA↗

arXiv2018

The nature of luminous Lyman-alpha emitters at z~2-3: maximal dust-poor starbursts and highly ionising AGN

Deep narrow-band surveys have revealed a large population of faint Lyman-alpha (Lya) emitters (LAEs) in the distant Universe, but relatively little is known about the most luminous sources ($L_{Lyα}>10^{42.7}$ erg/s; $L_{Lyα}>L^*_{Lyα}$). Here we present the spectroscopic follow-up of 21 luminous LAEs at z~2-3 found with panoramic narrow-band surveys over five independent extragalactic fields (~4x10$^6$ Mpc$^{3}$ surveyed at z~2.2 and z~3.1). We use WHT/ISIS, Keck/DEIMOS and VLT/X-SHOOTER to study these sources using high ionisation UV lines. Luminous LAEs at z~2-3 have blue UV slopes ($β=-2.0^{+0.3}_{-0.1}$), high Lya escape fractions ($50^{+20}_{-15}$%) and span five orders of magnitude in UV luminosity ($M_{UV}\approx-19$ to -24). Many (70%) show at least one high ionisation rest-frame UV line such as CIV, NV, CIII], HeII or OIII], typically blue-shifted by ~100-200 km/s relative to Lya. Their Lya profiles reveal a wide variety of shapes, including significant blue-shifted components and widths from 200 to 4000 km/s. Overall, 60+-11% appear to be AGN dominated, and at $L_{Lyα}>10^{43.3}$ erg/s and/or $M_{UV}<-21.5$ virtually all LAEs are AGN with high ionisation parameters (log U=0.6+-0.5) and with metallicities of ~0.5-1 Zsun. Those lacking signatures of AGN (40+-11%) have lower ionisation parameters ($\log U=-3.0^{+1.6}_{-0.9}$ and $\logξ_{\rm ion}=25.4\pm0.2$) and are apparently metal-poor sources likely powered by young, dust-poor "maximal" starbursts. Our results show that luminous LAEs at z~2-3 are a diverse population and that 2xL$^*_{Lyα}$ and 2xM$_{UV}^*$ mark a sharp transition in the nature of LAEs, from star formation dominated to AGN dominated.

astro-ph.GA↗

arXiv2017

Studying the late evolution of a radio-loud AGN in a galaxy group with LOFAR

Feedback by radio-loud active galactic nuclei (AGN) in galaxy groups is not fully understood. Open questions include the duty cycle of the AGN, the spatial extent of the radio lobes, the effect they have on the intragroup medium, and the fate of the cosmic rays. We present the discovery of a 650 kpc-radio galaxy embedded in steep diffuse emission at $z = 0.18793 \pm 5 \times 10^{-5}$ located at the center of the galaxy group MaxBCG J199.31832+51.72503 using an observation from the LOFAR Two-meter Sky Survey (LoTSS) at the central frequency of 144 MHz. Subsequently, we performed a GMRT observation at the central frequency of 607 MHz to study the spectral properties of the source. The observations reveal a radio galaxy with a total radio power $P_{\rm tot, 1.4} \sim 2.1 \times 10^{24}$ W Hz$^{-1}$, exhibiting two asymmetrical jets and lobes. The derived spectral index map shows a steepening toward the inner regions and a steep-spectrum core region. We model the integrated radio spectrum, providing two possible interpretations: the radio source is evolved but still active or it is just at the end of its active phase. Finally, in the same field of view we have discovered Mpc-sized emission surrounding a close pair of AGN located at a redshift $z = 0.0587 \pm 2 \times 10^{-4}$ (SDSS J131544.56+521213.2 and SDSS J131543.99+521055.7) which could be a radio remnant source.

astro-ph.GA↗

arXiv2017

Spectroscopic properties of luminous Lyman-α emitters at $z \approx 6 - 7$ and comparison to the Lyman-break population

We present spectroscopic follow-up of candidate luminous Ly$α$ emitters (LAEs) at $z=5.7-6.6$ in the SA22 field with VLT/X-SHOOTER. We confirm two new luminous LAEs at $z=5.676$ (SR6) and $z=6.532$ (VR7), and also present {\it HST} follow-up of both sources. These sources have luminosities L$_{\rm Lyα} \approx 3\times10^{43}$ erg s$^{-1}$, very high rest-frame equivalent widths of EW$_0\gtrsim 200$ Å and narrow Ly$α$ lines (200-340 km s$^{-1}$). VR7 is the most UV-luminous LAE at $z>6.5$, with M$_{1500} = -22.5$, even brighter in the UV than CR7. Besides Ly$α$, we do not detect any other rest-frame UV lines in the spectra of SR6 and VR7, and argue that rest-frame UV lines are easier to observe in bright galaxies with low Ly$α$ equivalent widths. We confirm that Ly$α$ line-widths increase with Ly$α$ luminosity at $z=5.7$, while there are indications that Ly$α$ lines of faint LAEs become broader at $z=6.6$, potentially due to reionisation. We find a large spread of up to 3 dex in UV luminosity for $>L^{\star}$ LAEs, but find that the Ly$α$ luminosity of the brightest LAEs is strongly related to UV luminosity at $z=6.6$. Under basic assumptions, we find that several LAEs at $z\approx6-7$ have Ly$α$ escape fractions $\gtrsim100$ \%, indicating bursty star-formation histories, alternative Ly$α$ production mechanisms, or dust attenuating Ly$α$ emission differently than UV emission. Finally, we present a method to compute $ξ_{ion}$, the production efficiency of ionising photons, and find that LAEs at $z\approx6-7$ have high values of log$_{10}(ξ_{ion}$/Hz erg$^{-1}) \approx 25.51\pm0.09$ that may alleviate the need for high Lyman-Continuum escape fractions required for reionisation.

astro-ph.GA↗

arXiv2017

Boötes-HiZELS: an optical to near-infrared survey of emission-line galaxies at $\bf z=0.4-4.7$

We present a sample of $\sim 1000$ emission line galaxies at $z=0.4-4.7$ from the $\sim0.7$deg$^2$ High-$z$ Emission Line Survey (HiZELS) in the Boötes field identified with a suite of six narrow-band filters at $\approx 0.4-2.1$ $μ$m. These galaxies have been selected on their Ly$α$ (73), {\sc [Oii]} (285), H$β$/{\sc [Oiii]} (387) or H$α$ (362) emission-line, and have been classified with optical to near-infrared colours. A subsample of 98 sources have reliable redshifts from multiple narrow-band (e.g. [O{\sc ii}]-H$α$) detections and/or spectroscopy. In this survey paper, we present the observations, selection and catalogs of emitters. We measure number densities of Ly$α$, [O{\sc ii}], H$β$/{\sc [Oiii]} and H$α$ and confirm strong luminosity evolution in star-forming galaxies from $z\sim0.4$ to $\sim 5$, in agreement with previous results. To demonstrate the usefulness of dual-line emitters, we use the sample of dual [O{\sc ii}]-H$α$ emitters to measure the observed [O{\sc ii}]/H$α$ ratio at $z=1.47$. The observed [O{\sc ii}]/H$α$ ratio increases significantly from 0.40$\pm0.01$ at $z=0.1$ to 0.52$\pm0.05$ at $z=1.47$, which we attribute to either decreasing dust attenuation with redshift, or due to a bias in the (typically) fiber-measurements in the local Universe which only measure the central kpc regions. At the bright end, we find that both the H$α$ and Ly$α$ number densities at $z\approx2.2$ deviate significantly from a Schechter form, following a power-law. We show that this is driven entirely by an increasing X-ray/AGN fraction with line-luminosity, which reaches $\approx 100$ \% at line-luminosities $L\gtrsim3\times10^{44}$ erg s$^{-1}$.

astro-ph.GA↗

arXiv2016

The CALYMHA survey: Lya luminosity function and global escape fraction of Lya photons at z=2.23

We present the CAlibrating LYMan-$α$ with H$α$ (CALYMHA) pilot survey and new results on Lyman-$α$ (Lya) selected galaxies at z~2. We use a custom-built Lya narrow-band filter at the Isaac Newton Telescope, designed to provide a matched volume coverage to the z=2.23 Ha HiZELS survey. Here we present the first results for the COSMOS and UDS fields. Our survey currently reaches a 3$σ$ line flux limit of ~4x10$^{-17}$ erg/s/cm$^{2}$, and a Lya luminosity limit of ~10$^{42.3}$ erg/s. We find 188 Lya emitters over 7.3x10$^5$ Mpc$^{3}$, but also find significant numbers of other line emitting sources corresponding to HeII, CIII] and CIV emission lines. These sources are important contaminants, and we carefully remove them, unlike most previous studies. We find that the Lya luminosity function at z=2.23 is very well described by a Schechter function up to L~10$^{43}$ erg/s with L$^*=10^{42.59+-0.05}$ erg/s, $ϕ^*=10^{-3.09+-0.08}$ Mpc$^{-3}$ and $α$=-1.75+-0.15. Above L~10$^{43}$ erg/s the Lya luminosity function becomes power-law like, driven by X-ray AGN. We find that Lya-selected emitters have a high escape fraction of 37+-7%, anti-correlated with Lya luminosity and correlated with Lya equivalent width. Lya emitters have ubiquitous large (~40 kpc) Lya haloes, 2x larger than their Ha extents. By directly comparing our Lya and Ha luminosity functions we find that the global/overall escape fraction of Lya photons (within a 13 kpc radius) from the full population of star-forming galaxies is 5.1+-0.2% at the peak of the star formation history. An extra 3.3+-0.3% of Lya photons likely still escape, but at larger radii.

astro-ph.GA↗

arXiv2016

The production and escape of Lyman-Continuum radiation from star-forming galaxies at z~2 and their redshift evolution

We study the production rate of ionizing photons of a sample of 588 H$α$ emitters (HAEs) and 160 Lyman-$α$ emitters (LAEs) at $z=2.2$ in the COSMOS field in order to assess the implied emissivity from galaxies, based on their UV luminosity. By exploring the rest-frame Lyman Continuum (LyC) with GALEX/$NUV$ data, we find f$_{\rm esc} < 2.8\, (6.4)$% through median (mean) stacking. By combining the H$α$ luminosity density with IGM emissivity measurements from absorption studies, we find a globally averaged $\langle$f$_{\rm esc}\rangle$ of $5.9^{+14.5}_{-4.2}$ % at $z=2.2$ if we assume HAEs are the only source of ionizing photons. We find similarly low values of the global $\langle$f$_{\rm esc}\rangle$ at $z\approx3-5$, also ruling out a high $\langle$f$_{\rm esc}\rangle$ at $z<5$. These low escape fractions allow us to measure $ξ_{ion}$, the number of produced ionizing photons per unit UV luminosity, and investigate how this depends on galaxy properties. We find a typical $ξ_{ion} \approx 10^{24.77\pm0.04}$ Hz erg$^{-1}$ for HAEs and $ξ_{ion} \approx 10^{25.14\pm0.09}$ Hz erg$^{-1}$ for LAEs. LAEs and low mass HAEs at $z=2.2$ show similar values of $ξ_{ion}$ as typically assumed in the reionization era, while the typical HAE is three times less ionizing. Due to an increasing $ξ_{ion}$ with increasing EW(H$α$), $ξ_{ion}$ likely increases with redshift. This evolution alone is fully in line with the observed evolution of $ξ_{ion}$ between $z\approx2-5$, indicating a typical value of $ξ_{ion} \approx 10^{25.4}$ Hz erg$^{-1}$ in the reionization era.

astro-ph.GA↗

arXiv2016

LOFAR VLBI Studies at 55 MHz of 4C 43.15, a z=2.4 Radio Galaxy

The correlation between radio spectral index and redshift has been exploited to discover high redshift radio galaxies, but its underlying cause is unclear. It is crucial to characterise the particle acceleration and loss mechanisms in high redshift radio galaxies to understand why their radio spectral indices are steeper than their local counterparts. Low frequency information on scales of $\sim$1 arcsec are necessary to determine the internal spectral index variation. In this paper we present the first spatially resolved studies at frequencies below 100 MHz of the $z = 2.4$ radio galaxy 4C 43.15 which was selected based on its ultra-steep spectral index ($α< -1$; $S_ν \sim ν^α$ ) between 365 MHz and 1.4 GHz. Using the International Low Frequency Array (LOFAR) Low Band Antenna we achieve sub-arcsecond imaging resolution at 55 MHz with VLBI techniques. Our study reveals low-frequency radio emission extended along the jet axis, which connects the two lobes. The integrated spectral index for frequencies $<$ 500 MHz is -0.83. The lobes have integrated spectral indices of -1.31$\pm$0.03 and -1.75$\pm$0.01 for frequencies $\geq$1.4 GHz, implying a break frequency between 500 MHz and 1.4 GHz. These spectral properties are similar to those of local radio galaxies. We conclude that the initially measured ultra-steep spectral index is due to a combination of the steepening spectrum at high frequencies with a break at intermediate frequencies.

astro-ph.GA↗

arXiv2016

FR II radio galaxies at low frequencies I: morphology, magnetic field strength and energetics

Due to their steep spectra, low-frequency observations of FR II radio galaxies potentially provide key insights in to the morphology, energetics and spectrum of these powerful radio sources. However, limitations imposed by the previous generation of radio interferometers at metre wavelengths has meant that this region of parameter space remains largely unexplored. In this paper, the first in a series examining FR IIs at low frequencies, we use LOFAR observations between 50 and 160 MHz, along with complementary archival radio and X-ray data, to explore the properties of two FR II sources, 3C452 and 3C223. We find that the morphology of 3C452 is that of a standard FR II rather than of a double-double radio galaxy as had previously been suggested, with no remnant emission being observed beyond the active lobes. We find that the low-frequency integrated spectra of both sources are much steeper than expected based on traditional assumptions and, using synchrotron/inverse-Compton model fitting, show that the total energy content of the lobes is greater than previous estimates by a factor of around 5 for 3C452 and 2 for 3C223. We go on to discuss possible causes of these steeper than expected spectra and provide revised estimates of the internal pressures and magnetic field strengths for the intrinsically steep case. We find that the ratio between the equipartition magnetic field strengths and those derived through synchrotron/inverse-Compton model fitting remains consistent with previous findings and show that the observed departure from equipartition may in some cases provide a solution to the spectral versus dynamical age disparity.

astro-ph.GA↗

arXiv2016

The CALYMHA survey: Ly$α$ escape fraction and its dependence on galaxy properties at $z=2.23$

We present the first results from our CAlibrating LYMan-$α$ with H$α$ (CALYMHA) pilot survey at the Isaac Newton Telescope. We measure Ly$α$ emission for 488 H$α$ selected galaxies at $z=2.23$ from HiZELS in the COSMOS and UDS fields with a specially designed narrow-band filter ($λ_c$ = 3918 Å, $Δλ$= 52 Å). We find 17 dual H$α$-Ly$α$ emitters ($f_{\rm Lyα} >5\times10^{-17}$ erg s$^{-1}$ cm$^{-2}$, of which 5 are X-ray AGN). For star-forming galaxies, we find a range of Ly$α$ escape fractions (f$_{\rm esc}$, measured with 3$"$ apertures) from $2$\%$-30$\%. These galaxies have masses from $3\times10^8$ M$_{\odot}$ to 10$^{11}$ M$_{\odot}$ and dust attenuations E$(B-V)=0-0.5$. Using stacking, we measure a median escape fraction of $1.6\pm0.5$\% ($4.0\pm1.0$\% without correcting H$α$ for dust), but show that this depends on galaxy properties. The stacked f$_{\rm esc}$ tends to decrease with increasing SFR and dust attenuation. However, at the highest masses and dust attenuations, we detect individual galaxies with f$_{\rm esc}$ much higher than the typical values from stacking, indicating significant scatter in the values of f$_{\rm esc}$. Relations between f$_{\rm esc}$ and UV slope are bimodal, with high f$_{\rm esc}$ for either the bluest or reddest galaxies. We speculate that this bimodality and large scatter in the values of f$_{\rm esc}$ is due to additional physical mechanisms such as outflows facilitating f$_{\rm esc}$ for dusty/massive systems. Ly$α$ is significantly more extended than H$α$ and the UV. f$_{\rm esc}$ continues to increase up to at least 20 kpc (3$σ$, 40 kpc [2$σ$]) for typical SFGs and thus the aperture is the most important predictor of f$_{\rm esc}$.

astro-ph.GA↗

arXiv2015

Evidence for PopIII-like stellar populations in the most luminous Lyman-$α$ emitters at the epoch of re-ionisation: spectroscopic confirmation

Faint Lyman-$α$ (Ly$α$) emitters become increasingly rarer towards the re-ionisation epoch (z~6-7). However, observations from a very large (~5deg$^2$) Ly$α$ survey at z=6.6 (Matthee et al. 2015) show that this is not the case for the most luminous emitters. Here we present follow-up observations of the two most luminous z~6.6 Ly$α$ candidates in the COSMOS field: `MASOSA' and `CR7'. We used X-SHOOTER, SINFONI and FORS2 (VLT), and DEIMOS (Keck), to confirm both candidates beyond any doubt. We find redshifts of z=6.541 and z=6.604 for MASOSA and CR7, respectively. MASOSA has a strong detection in Ly$α$ with a line width of $386\pm30$ km/s (FWHM) and with high EW$_0$ (>200 Å), but it is undetected in the continuum. CR7, with an observed Ly$α$ luminosity of $10^{43.93\pm0.05}$erg/s is the most luminous Ly$α$ emitter ever found at z>6. CR7 reveals a narrow Ly$α$ line with $266\pm15$ km/s FWHM, being detected in the NIR (rest-frame UV, with $β=-2.3\pm0.1$) with an excess in $J$, and also strongly detected in IRAC/Spitzer. We detect a narrow HeII1640$Å$ emission line ($6σ$) which explains the excess seen in the $J$ band photometry (EW$_0$~80 Å). We find no other emission lines from the UV to the NIR in our X-SHOOTER spectra (HeII/OIII]1663A>3 and HeII/CIII]1908A>2.5). We find that CR7 is best explained by a combination of a PopIII-like population which dominates the rest-frame UV and the nebular emission, and a more normal stellar population which dominates the mass. HST/WFC3 observations show that the light is indeed spatially separated between a very blue component, coincident with Ly$α$ and HeII emission, and two red components (~5 kpc away), which dominate the mass. Our findings are consistent with theoretical predictions of a PopIII wave, with PopIII star formation migrating away from the original sites of star formation.

astro-ph.GA↗

arXiv2015

Identification of the brightest Lyα emitters at z=6.6: implications for the evolution of the luminosity function in the re-ionisation era

Using wide field narrow-band surveys, we provide a new measurement of the $z=6.6$ Lyman-$α$ Emitter (LAE) luminosity function (LF), which constraints the bright end for the first time. We use a combination of archival narrow-band NB921 data in UDS and new NB921 measurements in SA22 and COSMOS/UltraVISTA, all observed with the Subaru telescope, with a total area of $\sim 5$ deg$^2$. We exclude lower redshift interlopers by using broad-band optical and near-infrared photometry and also exclude three supernovae with data split over multiple epochs. Combining the UDS and COSMOS samples we find no evolution of the bright end of the Ly$α$ LF between $z=5.7$ and $6.6$, which is supported by spectroscopic follow-up, and conclude that sources with \emph{Himiko}-like luminosity are not as rare as previously thought, with number densities of $\sim 1.5\times10^{-5}$ Mpc$^{-3}$. Combined with our wide-field SA22 measurements, our results indicate a non-Schechter-like bright end of the LF at $z=6.6$ and a different evolution of \emph{observed} faint and bright LAEs. This differential evolution is also seen in the spectroscopic follow-up of UV selected galaxies and is now also confirmed for Ly$α$ emitters, and we argue that it may be an effect of re-ionisation. Using a toy-model, we show that such differential evolution of the LF is expected, since brighter sources are able to ionise their surroundings earlier, such that Ly$α$ photons are able to escape. Our targets are excellent candidates for detailed follow-up studies and provide the possibility to give a unique view on the earliest stages in the formation of galaxies and re-ionisation process.

astro-ph.GA↗

arXiv2015

MC$^2$: Boosted AGN and star-formation activity in CIZA J2242.8+5301, a massive post-merger cluster at z=0.19

Cluster mergers may play a fundamental role in the formation and evolution of cluster galaxies. Stroe et al. (2014) revealed unexpected over-densities of candidate H$α$ emitters near the ~1 Mpc-wide shock fronts of the massive (~2x10$^{15}$M$_{\odot}$) "Sausage" merging cluster, CIZA J2242.8+5301. We used Keck/DEIMOS and WHT/AF2 to confirm 83 H$α$ emitters in and around the merging cluster. We find that cluster star-forming galaxies in the hottest X-ray gas and/or in the cluster sub-cores (away from the shock fronts) show high [SII]6716/[SII]6761 and high [SII]6716/H$α$, implying very low electron densities (<30x lower than all other star-forming galaxies outside the cluster) and significant contribution from supernovae, respectively. All cluster star-forming galaxies near the cluster centre show evidence of significant outflows (blueshifted Na D~200-300km/s), likely driven by supernovae. Strong outflows are also found for the cluster H$α$ AGN. H$α$ star-forming galaxies in the merging cluster follow the z~0 mass-metallicity relation, showing systematically higher metallicity (~0.15-0.2 dex) than H$α$ emitters outside the cluster (projected R>2.5 Mpc). This suggests that the shock front may have triggered remaining metal-rich gas which galaxies were able to retain into forming stars. Our observations show that the merger of impressively massive (~10$^{15}$M$_\odot$) clusters can provide the conditions for significant star-formation and AGN activity, but, as we witness strong feedback by star-forming galaxies and AGN (and given how massive the merging cluster is), such sources will likely quench in a few 100 Myrs.

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