| 1. |
- Offringa, A. R., et al.
(author)
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The LOFAR radio environment
- 2012
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 549
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Journal article (peer-reviewed)abstract
- Aims. This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations.Methods. We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz/1 s resolution.Results. We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR's nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution.Conclusions. Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills.
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| 2. |
- Yatawatta, S., et al.
(author)
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Initial deep LOFAR observations of epoch of reionization windows I. The north celestial pole
- 2013
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 550, s. A136-
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Journal article (peer-reviewed)abstract
- Aims. The aim of the LOFAR epoch of reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21 cm signal. This signal is weaker by several orders of magnitude than the astrophysical foreground signals and hence, in order to achieve this, very long integrations, accurate calibration for stations and ionosphere and reliable foreground removal are essential. Methods. One of the prospective observing windows for the LOFAR EoR project will be centered at the north celestial pole (NCP). We present results from observations of the NCP window using the LOFAR highband antenna (HBA) array in the frequency range 115 MHz to 163 MHz. The data were obtained in April 2011 during the commissioning phase of LOFAR. We used baselines up to about 30 km. The data was processed using a dedicated processing pipeline which is an enhanced version of the standard LOFAR processing pipeline. Results. With about 3 nights, of 6 h each, effective integration we have achieved a noise level of about 100 mu Jy/PSF in the NCP window. Close to the NCP, the noise level increases to about 180 mu Jy/PSF, mainly due to additional contamination from unsubtracted nearby sources. We estimate that in our best night, we have reached a noise level only a factor of 1.4 above the thermal limit set by the noise from our Galaxy and the receivers. Our continuum images are several times deeper than have been achieved previously using the WSRT and GMRT arrays. We derive an analytical explanation for the excess noise that we believe to be mainly due to sources at large angular separation from the NCP. We present some details of the data processing challenges and how we solved them. Conclusions. Although many LOFAR stations were, at the time of the observations, in a still poorly calibrated state we have seen no artefacts in our images which would prevent us from producing deeper images in much longer integrations on the NCP window which are about to commence. The limitations present in our current results are mainly due to sidelobe noise from the large number of distant sources, as well as errors related to station beam variations and rapid ionospheric phase fluctuations acting on bright sources. We are confident that we can improve our results with refined processing.
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| 3. |
- de La Vieuville, G., et al.
(author)
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Faint end of the z similar to 3-7 luminosity function of Lyman-alpha emitters behind lensing clusters observed with MUSE
- 2019
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 628
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Journal article (peer-reviewed)abstract
- Contact. This paper presents the results obtained with the Multi-Unit Spectroscopic Explorer (MUSE) at the ESOVery Large Telescope on the faint end of the Lyman-alpha luminosity function (LF) based on deep observations of four lensing clusters. The goal of our project is to set strong constraints on the relative contribution of the Lyman-alpha emitter (LAE) population to cosmic reionization.Aims. The precise aim of the present study is to further constrain the abundance of LAEs by taking advantage of the magnification provided by lensing clusters to build a blindly selected sample of galaxies which is less biased than current blank field samples in redshift and luminosity. By construction, this sample of LAEs is complementary to those built from deep blank fields, whether observed by MUSE or by other facilities, and makes it possible to determine the shape of the LF at fainter levels, as well as its evolution with redshift.Methods. We selected a sample of 156 LAEs with redshifts between 2.9 <= z <= 6.7 and magnification-corrected luminosities in the range 39 less than or similar to log L-Ly alpha [erg s(-1)] less than or similar to 43. To properly take into account the individual differences in detection conditions between the LAEs when computing the LF, including lensing configurations, and spatial and spectral morphologies, the non-parametric 1/V-max method was adopted. The price to pay to benefit from magnification is a reduction of the effective volume of the survey, together with a more complex analysis procedure to properly determine the effective volume V-max for each galaxy. In this paper we present a complete procedure for the determination of the LF based on IFU detections in lensing clusters. This procedure, including some new methods for masking, effective volume integration and (individual) completeness determinations, has been fully automated when possible, and it can be easily generalized to the analysis of IFU observations in blank fields.Results. As a result of this analysis, the Lyman-alpha LF has been obtained in four different redshift bins: 2.9 < z < 6; 7, 2.9 < z < 4.0, 4 : 0 < z < 5.0; and 5 : 0 < z < 6.7 with constraints down to log L-Ly alpha = 40.5. From our data only, no significant evolution of LF mean slope can be found. When performing a Schechter analysis also including data from the literature to complete the present sample towards the brightest luminosities, a steep faint end slope was measured varying from alpha = -1.69(-0.08)(+0.08) to alpha = -1.87(-0 .12)(+0.12) between the lowest and the highest redshift bins.Conclusions. The contribution of the LAE population to the star formation rate density at z similar to 6 is less than or similar to 50% depending on the luminosity limit considered, which is of the same order as the Lyman-break galaxy (LBG) contribution. The evolution of the LAE contribution with redshift depends on the assumed escape fraction of Lyman-alpha photons, and appears to slightly increase with increasing redshift when this fraction is conservatively set to one. Depending on the intersection between the LAE/LBG populations, the contribution of the observed galaxies to the ionizing flux may suffice to keep the universe ionized at z similar to 6.
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| 4. |
- Hashimoto, T., et al.
(author)
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The MUSE Hubble Ultra Deep Field Survey X. Ly alpha equivalent widths at 2.9 < z < 6.6
- 2017
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 608
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Journal article (peer-reviewed)abstract
- We present rest-frame Ly alpha equivalent widths (EW0) of 417 Ly alpha emitters (LAEs) detected with Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) at 2.9 < z < 6.6 in the Hubble Ultra Deep Field. Based on the deep MUSE spectroscopy and ancillary Hubble Space Telescope (HST) photometry data, we carefully measured EW0 values taking into account extended Ly alpha emission and UV continuum slopes (beta). Our LAEs reach unprecedented depths, both in Ly alpha luminosities and UV absolute magnitudes, from log (L-Ly alpha/erg s(-1)) similar to 41.0 to 43.0 and from M-UV similar to -16 to -21 (0.01-1.0 L-z=3(*)). The EW0 values span the range of similar to 5 to 240 angstrom or larger, and their distribution can be well fitted by an exponential law N = N-0 exp(-EW0/w(0)). Owing to the high dynamic range in M-UV, we find that the scale factor, w(0), depends on M-UV in the sense that including fainter M-UV objects increases w(0), i.e., the Ando effect. The results indicate that selection functions affect the EW0 scale factor. Taking these effects into account, we find that our w(0) values are consistent with those in the literature within 1 sigma uncertainties at 2.9 < z < 6.6 at a given threshold of M-UV and LLy alpha. Interestingly, we find 12 objects with EW0> 200 angstrom above 1 sigma uncertainties. Two of these 12 LAEs show signatures of merger or AGN activity: the weak Civ lambda 1549 emission line. For the remaining 10 very large EW0 LAEs, we find that the EW0 values can be reproduced by young stellar ages (< 100 Myr) and low metallicities (less than or similar to 0.02 Z(circle dot)). Otherwise, at least part of the Ly alpha emission in these LAEs needs to arise from anisotropic radiative transfer effects, fluorescence by hidden AGN or quasi-stellar object activity, or gravitational cooling.
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| 5. |
- Verhamme, A., et al.
(author)
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Recovering the systemic redshift of galaxies from their Lyman alpha line profile
- 2018
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966 .- 1745-3925 .- 1745-3933. ; 478:1, s. L60-L65
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Journal article (peer-reviewed)abstract
- The Lyman alpha (Ly alpha) line of Hydrogen is a prominent feature in the spectra of star-forming galaxies, usually redshifted by a few hundreds of km s(-1) compared to the systemic redshift. This large offset hampers follow-up surveys, galaxy pair statistics, and correlations with quasar absorption lines when only Ly alpha is available. We propose diagnostics that can be used to recover the systemic redshift directly from the properties of the Ly alpha line profile. We use spectroscopic observations of Ly alpha emitters for which a precise measurement of the systemic redshift is available. Our sample contains 13 sources detected between z approximate to 3 and z approximate to 6 as part of various multi-unit spectroscopic explorer guaranteed time observations. We also include a compilation of spectroscopic Ly alpha data from the literature spanning a wide redshift range (z approximate to 0-8). First, restricting our analysis to double-peaked Ly alpha spectra, we find a tight correlation between the velocity offset of the red peak with respect to the systemic redshift, V-peak(red), and the separation of the peaks. Secondly, we find a correlation between V-peak(red) and the full width at half-maximum of the Ly alpha line. Fitting formulas to estimate systemic redshifts of galaxies with an accuracy of <= 100 km s(-1), when only the Ly alpha emission line is available, are given for the two methods.
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| 6. |
- Drake, A. B., et al.
(author)
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The MUSE Hubble Ultra Deep Field Survey VI. The faint-end of the Lyα luminosity function at 2.91
- 2017
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 608
-
Journal article (peer-reviewed)abstract
- We present the deepest study to date of the Ly alpha luminosity function in a blank field using blind integral field spectroscopy from MUSE. We constructed a sample of 604 Ly alpha emitters (LAEs) across the redshift range 2.91 < z < 6.64 using automatic detection software in the Hubble Ultra Deep Lield. The deep data cubes allowed us to calculate accurate total Ly alpha fluxes capturing low surface-brightness extended Ly alpha emission now known to be a generic property of high-redshift star-forming galaxies. We simulated realistic extended LAEs to fully characterise the selection function of our samples, and performed flux-recovery experiments to test and correct for bias in our determination of total Ly alpha fluxes. We find that an accurate completeness correction accounting for extended emission reveals a very steep faint-end slope of the luminosity function, alpha, down to luminosities of log(10) L erg s(-1) < 41.5, applying both the 1/V-max and maximum likelihood estimators. Splitting the sample into three broad redshift bins, we see the faint-end slope increasing from -2.03(-0.07)(+1.42) at z approximate to 3.44 to -2.86(-infinity)(+0.76) Z approximate to 76 at z approximate to 5.48, however no strong evolution is seen between the 68% confidence regions in L*-alpha parameter space. Using the Ly alpha line flux as a proxy for star formation activity, and integrating the observed luminosity functions, we find that LAEs' contribution to the cosmic star formation rate density rises with redshift until it is comparable to that from continuum-selected samples by z approximate to 6. This implies that LAEs may contribute more to the star-formation activity of the early Universe than previously thought, as any additional intergalactic medium (IGM) correction would act to further boost the Ly alpha luminosities. Linally, assuming fiducial values for the escape of Ly alpha and LyC radiation, and the dumpiness of the IGM, we integrated the maximum likelihood luminosity function at 5.00 < z < 6.64 and find we require only a small extrapolation beyond the data (<1 dex in luminosity) for LAEs alone to maintain an ionised IGM at z approximate to 6.
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| 7. |
- Wisotzki, L., et al.
(author)
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Nearly all the sky is covered by Lyman-alpha emission around high-redshift galaxies
- 2018
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In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 562:7726, s. 229-232
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Journal article (peer-reviewed)abstract
- Galaxies are surrounded by large reservoirs of gas, mostly hydrogen, that are fed by inflows from the intergalactic medium and by outflows from galactic winds. Absorption-line measurements along the lines of sight to bright and rare background quasars indicate that this circumgalactic medium extends far beyond the starlight seen in galaxies, but very little is known about its spatial distribution. The Lyman-alpha transition of atomic hydrogen at a wavelength of 121.6 nanometres is an important tracer of warm (about 104 kelvin) gas in and around galaxies, especially at cosmological redshifts greater than about 1.6 at which the spectral line becomes observable from the ground. Tracing cosmic hydrogen through its Lyman-a emission has been a long-standing goal of observational astrophysics(1-3), but the extremely low surface brightness of the spatially extended emission is a formidable obstacle. A new window into circumgalactic environments was recently opened by the discovery of ubiquitous extended Lyman-alpha emission from hydrogen around high-redshift galaxies(4,5). Such measurements were previously limited to especially favourable systems(6-8) or to the use of massive statistical averaging(9,10) because of the faintness of this emission. Here we report observations of low-surface-brightness Lyman-alpha emission surrounding faint galaxies at redshifts between 3 and 6. We find that the projected sky coverage approaches 100 per cent. The corresponding rate of incidence (the mean number of Lyman-alpha emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high-column-density absorbers frequently detected in the spectra of distant quasars(11-14). This similarity suggests that most circumgalactic atomic hydrogen at these redshifts has now been detected in emission.
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| 8. |
- Ciardi, B., et al.
(author)
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Simulating the 21 cm forest detectable with LOFAR and SKA in the spectra of high-z GRBs
- 2015
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 453:1, s. 101-105
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Journal article (peer-reviewed)abstract
- We investigate the feasibility of detecting 21 cm absorption features in the afterglow spectra of high redshift long Gamma Ray Bursts (GRBs). This is done employing simulations of cosmic reionization, together with estimates of the GRB radio afterglow flux and the instrumental characteristics of the LOw Frequency ARray (LOFAR). We find that absorption features could be marginally (with a S/N larger than a few) detected by LOFAR at z greater than or similar to 7 if the GRB is a highly energetic event originating from Pop III stars, while the detection would be easier if the noise were reduced by one order of magnitude, i.e. similar to what is expected for the first phase of the Square Kilometre Array (SKA1-low). On the other hand, more standard GRBs are too dim to be detected even with ten times the sensitivity of SKA1-low, and only in the most optimistic case can a S/N larger than a few be reached at z greater than or similar to 9.
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| 9. |
- Claeyssens, A., et al.
(author)
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Spectral variations of Lyman alpha emission within strongly lensed sources observed with MUSE
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 489:4, s. 5022-5029
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Journal article (peer-reviewed)abstract
- We present an analysis of HI Lyman alpha emission in deep VLT/MUSE observations of two highly magnified and extended galaxies at z = 3.5 and 4.03, including a newly discovered, almost complete Einstein ring. While these Lyman a haloes are intrinsically similar to the ones typically seen in other MUSE deep fields, the benefits of gravitational lensing allow us to construct exceptionally detailed maps of Lyman alpha line properties at sub-kpc scales. By combining all multiple images, we are able to observe complex structures in the Lyman a emission and uncover small (similar to 120 km s(-1) in Lyman alpha peak shift), but significant at >4 sigma, systematic variations in the shape of the Lyman a line profile within each halo. Indeed, we observe a global trend for the line peak shift to become redder at large radii, together with a strong correlation between the peak wavelength and line width. This systematic intrahalo variation is markedly similar to the object-to-object variations obtained from the integrated properties of recent large samples. Regions of high surface brightness correspond to relatively small line shifts, which could indicate that Lyman alpha emission escapes preferentially from regions where the line profile has been less severely affected by scattering of Lyman alpha photons.
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| 10. |
- Diener, C., et al.
(author)
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The MUSE-Wide survey : detection of a clustering signal from Lyman alpha emitters in the range 3 < z < 6
- 2017
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 471:3, s. 3186-3192
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Journal article (peer-reviewed)abstract
- We present a clustering analysis of a sample of 238 Ly alpha emitters at redshift 3 less than or similar to z less than or similar to 6 from the MUSE-Wide survey. This survey mosaics extragalactic legacy fields with 1h MUSE pointings to detect statistically relevant samples of emission line galaxies. We analysed the first year observations from MUSE-Wide making use of the clustering signal in the line-of-sight direction. This method relies on comparing pair-counts at close redshifts for a fixed transverse distance and thus exploits the full potential of the redshift range covered by our sample. A clear clustering signal with a correlation length of r(0) = 2.9(-1.1)(+1.0) Mpc (comoving) is detected. Whilst this result is based on only about a quarter of the full survey size, it already shows the immense potential of MUSE for efficiently observing and studying the clustering of Ly alpha emitters.
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| 11. |
- Paalvast, M., et al.
(author)
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Properties and redshift evolution of star-forming galaxies with high [0 III]/[O II] ratios with MUSE at 0.28 < z < 0.85
- 2018
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 618
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Journal article (peer-reviewed)abstract
- We present a study of the [O III]/[O II] ratios of star-forming galaxies drawn from Multi-Unit Spectroscopic Explorer (MUSE) data spanning a redshift range 0.28 < z < 0.85. Recently discovered Lyman continuum (LyC) emitters have extremely high oxygen line ratios: [O III]lambda 5007/[O II]lambda lambda 3726, 3729 > 4. Here we aim to understand the properties and the occurrences of galaxies with such high line ratios. Combining data from several MUSE Guaranteed Time Observing (GTO) programmes, we select a population of star-forming galaxies with bright emission lines, from which we draw 406 galaxies for our analysis based on their position in the z-dependent star formation rate (SFR) stellar mass (M*) plane. Out of this sample 15 are identified as extreme oxygen emitters based on their [O III]/[O II] ratios (3.7%) and 104 galaxies have [O III]/[O II] > 1 (26%). Our analysis shows no significant correlation between M*, SFR, and the distance from the SFR M, relation with [O III]/[O II]. We find a decrease in the fraction of galaxies with [O III]/[O II] > 1 with increasing M*, however, this is most likely a result of the relationship between [O III]/[O II] and metallicity, rather than between [O III]/[O II] and M. We draw a comparison sample of local analogues with < z > 0.03 from the Sloan Digital Sky Survey, and find similar incidence rates for this sample. In order to investigate the evolution in the fraction of high [O III]/[O II] emitters with redshift, we bin the sample into three redshift subsamples of equal number, but find no evidence for a dependence on redshift. Furthermore, we compare the observed line ratios with those predicted by nebular models with no LyC escape and find that most of the extreme oxygen emitters can be reproduced by low metallicity models. The remaining galaxies are likely LyC emitter candidates.
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| 12. |
- Stott, John P., et al.
(author)
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The XMM Cluster Survey : the interplay between the brightest cluster galaxy and the intracluster medium via AGN feedback
- 2012
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 422:3, s. 2213-2229
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Research review (peer-reviewed)abstract
- Using a sample of 123 X-ray clusters and groups drawn from the XMM Cluster Survey first data release, we investigate the interplay between the brightest cluster galaxy (BCG), its black hole and the intracluster/group medium (ICM). It appears that for groups and clusters with a BCG likely to host significant active galactic nuclei (AGN) feedback, gas cooling dominates in those with TX > 2 keV while AGN feedback dominates below. This may be understood through the subunity exponent found in the scaling relation we derive between the BCG mass and cluster mass over the halo mass range 1013 < M500 < 1015 M? and the lack of correlation between radio luminosity and cluster mass, such that BCG AGN in groups can have relatively more energetic influence on the ICM. The LXTX relation for systems with the most massive BCGs, or those with BCGs co-located with the peak of the ICM emission, is steeper than that for those with the least massive and most offset, which instead follows self-similarity. This is evidence that a combination of central gas cooling and powerful, well fuelled AGN causes the departure of the ICM from pure gravitational heating, with the steepened relation crossing self-similarity at TX= 2 keV. Importantly, regardless of their black hole mass, BCGs are more likely to host radio-loud AGN if they are in a massive cluster (TX? 2 keV) and again co-located with an effective fuel supply of dense, cooling gas. This demonstrates that the most massive black holes appear to know more about their host cluster than they do about their host galaxy. The results lead us to propose a physically motivated, empirical definition of cluster and group, delineated at 2 keV.
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| 13. |
- Jensen, Hannes, et al.
(author)
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Probing reionization with LOFAR using 21-cm redshift space distortions
- 2013
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 435:1, s. 460-474
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Journal article (peer-reviewed)abstract
- One of the most promising ways to study the epoch of reionization (EoR) is through radio observations of the redshifted 21-cm line emission from neutral hydrogen. These observations are complicated by the fact that the mapping of redshifts to line-of-sight positions is distorted by the peculiar velocities of the gas. Such distortions can be a source of error if they are not properly understood, but they also encode information about cosmology and astrophysics. We study the effects of redshift space distortions on the power spectrum of 21-cm radiation from the EoR using large-scale N-body and radiative transfer simulations. We quantify the anisotropy introduced in the 21-cm power spectrum by redshift space distortions and show how it evolves as reionization progresses and how it relates to the underlying physics. We go on to study the effects of redshift space distortions on LOFAR observations, taking instrument noise and foreground subtraction into account. We find that LOFAR should be able to directly observe the power spectrum anisotropy due to redshift space distortions at spatial scales around k similar to 0.1 Mpc(-1) after greater than or similar to 1000 h of integration time. At larger scales, sample errors become a limiting factor, while at smaller scales detector noise and foregrounds make the extraction of the signal problematic. Finally, we show how the astrophysical information contained in the evolution of the anisotropy of the 21-cm power spectrum can be extracted from LOFAR observations, and how it can be used to distinguish between different reionization scenarios.
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| 14. |
- Patil, A. H., et al.
(author)
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Upper Limits on the 21cm Epoch of Reionization Power Spectrum from One Night with LOFAR
- 2017
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 838:1
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Journal article (peer-reviewed)abstract
- We present the first limits on the Epoch of Reionization 21 cm H I power spectra, in the redshift range z = 7.910.6, using the Low-Frequency Array (LOFAR) High-Band Antenna (HBA). In total, 13.0 hr of data were used from observations centered on the North Celestial Pole. After subtraction of the sky model and the noise bias, we detect a non-zero Delta(2)(I)=(56 +/- 13 mK)(2) (1-sigma) excess variance and a best 2-sigma upper limit of Delta(2)(21) < (79.6 mK)(2) at k = 0.053 h cMpc(-1) in the range z = 9.610.6. The excess variance decreases when optimizing the smoothness of the direction- and frequency-dependent gain calibration, and with increasing the completeness of the sky model. It is likely caused by (i) residual side-lobe noise on calibration baselines, (ii) leverage due to nonlinear effects, (iii) noise and ionosphere-induced gain errors, or a combination thereof. Further analyses of the excess variance will be discussed in forthcoming publications.
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| 15. |
- Wiersma, R. P. C., et al.
(author)
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LOFAR insights into the epoch of reionization from the cross-power spectrum of 21 cm emission and galaxies
- 2013
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 432:3, s. 2615-2624
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Journal article (peer-reviewed)abstract
- Using a combination of N-body simulations, semi-analytic models and radiative transfer calculations, we have estimated the theoretical cross-power spectrum between galaxies and the 21 cm emission from neutral hydrogen during the epoch of reionization. In accordance with previous studies, we find that the 21 cm emission is initially correlated with haloes on large scales (greater than or similar to 30 Mpc), anticorrelated on intermediate (similar to 5 Mpc) and uncorrelated on small (less than or similar to 3 Mpc) scales. This picture quickly changes as reionization proceeds and the two fields become anticorrelated on large scales. The normalization of the cross-power spectrum can be used to set constraints on the average neutral fraction in the intergalactic medium and its shape can be a powerful tool to study the topology of reionization. When we apply a drop-out technique to select galaxies and add to the 21 cm signal the noise expected from the LOw Frequency ARray (LOFAR) telescope, we find that while the normalization of the cross-power spectrum remains a useful tool for probing reionization, its shape becomes too noisy to be informative. On the other hand, for an Ly alpha Emitter (LAE) survey both the normalization and the shape of the cross-power spectrum are suitable probes of reionization. A closer look at a specific planned LAE observing program using Subaru Hyper-Suprime Cam reveals concerns about the strength of the 21 cm signal at the planned redshifts. If the ionized fraction at z similar to 7 is lower than the one estimated here, then using the cross-power spectrum may be a useful exercise given that at higher redshifts and neutral fractions it is able to distinguish between two toy models with different topologies.
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| 16. |
- Asad, K. M. B., et al.
(author)
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Polarization leakage in epoch of reionization windows - I. Low Frequency Array observations of the 3C196 field
- 2015
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 451:4, s. 3709-3727
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Journal article (peer-reviewed)abstract
- Detection of the 21-cm signal coming from the epoch of reionization (EoR) is challenging especially because, even after removing the foregrounds, the residual Stokes I maps contain leakage from polarized emission that can mimic the signal. Here, we discuss the instrumental polarization of Low Frequency Array (LOFAR) and present realistic simulations of the leakages between Stokes parameters. From the LOFAR observations of polarized emission in the 3C196 field, we have quantified the level of polarization leakage caused by the nominal model beam of LOFAR, and compared it with the EoR signal using power spectrum analysis. We found that at 134-166 MHz, within the central 4A degrees of the field the (Q, U) -> I leakage power is lower than the EoR signal at k < 0.3 Mpc(-1). The leakage was found to be localized around a Faraday depth of 0, and the rms of the leakage as a fraction of the rms of the polarized emission was shown to vary between 0.2 and 0.3 per cent, both of which could be utilized in the removal of leakage. Moreover, we could define an 'EoR window' in terms of the polarization leakage in the cylindrical power spectrum above the point spread function (PSF)-induced wedge and below k(ayen) similar to 0.5 Mpc(-1), and the window extended up to k(ayen) similar to 1 Mpc(-1) at all k(aSyen) when 70 per cent of the leakage had been removed. These LOFAR results show that even a modest polarimetric calibration over a field of view of a parts per thousand(2) 4A degrees in the future arrays like Square Kilometre Array will ensure that the polarization leakage remains well below the expected EoR signal at the scales of 0.02-1 Mpc(-1).
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| 17. |
- Gehlot, B. K., et al.
(author)
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The first power spectrum limit on the 21-cm signal of neutral hydrogen during the Cosmic Dawn at z=20-25 from LOFAR
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 488:3, s. 4271-4287
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Journal article (peer-reviewed)abstract
- Observations of the redshifted 21-cm hyperfine line of neutral hydrogen from early phases of the Universe such as Cosmic Dawn and the Epoch of Reionization promise to open a new window onto the early formation of stars and galaxies. We present the first upper limits on the power spectrum of redshifted 21-cm brightness temperature fluctuations in the redshift range z = 19.8-25.2 (54-68 MHz frequency range) using 14 h of data obtained with the LOFAR-Low Band Antenna (LBA) array. We also demonstrate the application of a multiple pointing calibration technique to calibrate the LOFAR-LBA dual-pointing observations centred on the North Celestial Pole and the radio galaxy 3C220.3, We observe an unexplained excess of similar to 30-50 per cent in Stokes / noise compared to Stokes V for the two observed fields, which decorrelates on greater than or similar to 12 s and might have a physical origin. We show that enforcing smoothness of gain errors along frequency direction during calibration reduces the additional variance in Stokes I compared Stokes V introduced by the calibration on sub-band level. After subtraction of smooth foregrounds, we achieve a 2 sigma upper limit on the 21-cm power spectrum of Delta(2)(21) < (14561 mK)(2) at k similar to 0.038 h cMpc(-1) and Delta(2)(21) < (14886 mK)(2) at k 0.038 h cMpc(-1) for the 3C220 and NCP fields respectively and both upper limits are consistent with each other. The upper limits for the two fields are still dominated by systematics on most k modes.
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| 18. |
- Ghara, Raghunath, et al.
(author)
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Constraining the intergalactic medium at z approximate to 9.1 using LOFAR Epoch of Reionization observations
- 2020
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 493:4, s. 4728-4747
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Journal article (peer-reviewed)abstract
- We derive constraints on the thermal and ionization states of the intergalactic medium (IGM) at redshift approximate to 9.1 using new upper limits on the 21-cm power spectrum measured by the LOFAR radio telescope and a prior on the ionized fraction at that redshift estimated from recent cosmic microwave background (CMB) observations. We have used results from the reionization simulation code GRIZZLY and a Bayesian inference framework to constrain the parameters which describe the physical state of the IGM. We find that, if the gas heating remains negligible, an IGM with ionized fraction greater than or similar to 0.13 and a distribution of the ionized regions with a characteristic size greater than or similar to 8 h(-1) comoving megaparsec (Mpc) and a full width at half-maximum (FWHM) greater than or similar to 16 h(-1) Mpc is ruled out. For an IGM with a uniform spin temperature T-S greater than or similar to 3 K, no constraints on the ionized component can be computed. If the large-scale fluctuations of the signal are driven by spin temperature fluctuations, an IGM with a volume fraction less than or similar to 0.34 of heated regions with a temperature larger than CMB, average gas temperature 7-160 K, and a distribution of the heated regions with characteristic size 3.5-70 h(-1) Mpc and FWHM of less than or similar to 110 h(-1) Mpc is ruled out. These constraints are within the 95 per cent credible intervals. With more stringent future upper limits from LOFAR at multiple redshifts, the constraints will become tighter and will exclude an increasingly large region of the parameter space.
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| 19. |
- Harker, Geraint J. A., et al.
(author)
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Detection and extraction of signals from the epoch of reionization using higher-order one-point statistics
- 2009
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 393, s. 1449-1458
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Journal article (peer-reviewed)abstract
- Detecting redshifted 21-cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock data cubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21-cm signal. We argue that some of these features - such as a dip at the onset of reionization, followed by a rise towards its later stages - may be generic, and give us a promising route to a statistical detection of reionization.
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| 20. |
- Mertens, F. G., et al.
(author)
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Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z approximate to 9.1 from LOFAR
- 2020
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 493:2, s. 1662-1685
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Journal article (peer-reviewed)abstract
- A new upper limit on the 21 cm signal power spectrum at a redshift of z approximate to 9.1 is presented, based on 141 h of data obtained with the Low-Frequency Array (LOFAR). The analysis includes significant improvements in spectrally smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally weighted power spectrum inference. Previously seen 'excess power' due to spectral structure in the gain solutions has markedly reduced but some excess power still remains with a spectral correlation distinct from thermal noise. This excess has a spectral coherence scale of 0.25-0.45 MHz and is partially correlated between nights, especially in the foreground wedge region. The correlation is stronger between nights covering similar local sidereal times. A best 2-sigma upper limit of Delta(2)(21) < (73)(2) mK(2) at k = 0.075 h cMpc(-1) is found, an improvement by a factor approximate to 8 in power compared to the previously reported upper limit. The remaining excess power could be due to residual foreground emission from sources or diffuse emission far away from the phase centre, polarization leakage, chromatic calibration errors, ionosphere, or low-level radiofrequency interference. We discuss future improvements to the signal processing chain that can further reduce or even eliminate these causes of excess power.
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| 21. |
- Patil, Ajinkya H., et al.
(author)
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Constraining the epoch of reionization with the variance statistic : simulations of the LOFAR case
- 2014
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 443:2, s. 1113-1124
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Journal article (peer-reviewed)abstract
- Several experiments are underway to detect the cosmic-redshifted 21-cm signal from neutral hydrogen from the Epoch of Reionization (EoR). Due to their very low signal-to-noise ratio, these observations aim for a statistical detection of the signal by measuring its power spectrum. We investigate the extraction of the variance of the signal as a first step towards detecting and constraining the global history of the EoR. Signal variance is the integral of the signal's power spectrum, and it is expected to be measured with a high significance. We demonstrate this through results from a simulation and parameter estimation pipeline developed for the Low-Frequency Array (LOFAR)-EoR experiment. We show that LOFAR should be able to detect the EoR in 600 h of integration using the variance statistic. Additionally, the redshift (z(r)) and duration (Delta z) of reionization can be constrained assuming a parametrization. We use an EoR simulation of z(r) = 7.68 and Delta(z) = 0.43 to test the pipeline. We are able to detect the simulated signal with a significance of four standard deviations and extract the EoR parameters as z(r) = 7.72(-0.18)(+0.37) and Delta z = 0.53(-0.23)(+0.12) in 600 h, assuming that systematic errors can be adequately controlled. We further show that the significance of detection and constraints on EoR parameters can be improved by measuring the cross-variance of the signal by cross-correlating consecutive redshift bins.
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| 22. |
- Thomas, Rajat M., et al.
(author)
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Fast large-scale reionization simulations
- 2009
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 393, s. 32-48
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Journal article (peer-reviewed)abstract
- We present an efficient method to generate large simulations of the epoch of reionization without the need for a full three-dimensional radiative transfer code. Large dark-matter-only simulations are post-processed to produce maps of the redshifted 21-cm emission from neutral hydrogen. Dark matter haloes are embedded with sources of radiation whose properties are either based on semi-analytical prescriptions or derived from hydrodynamical simulations. These sources could either be stars or power-law sources with varying spectral indices. Assuming spherical symmetry, ionized bubbles are created around these sources, whose radial ionized fraction and temperature profiles are derived from a catalogue of one-dimensional radiative transfer experiments. In case of overlap of these spheres, photons are conserved by redistributing them around the connected ionized regions corresponding to the spheres. The efficiency with which these maps are created allows us to span the large parameter space typically encountered in reionization simulations. We compare our results with other, more accurate, three-dimensional radiative transfer simulations and find excellent agreement for the redshifts and the spatial scales of interest to upcoming 21-cm experiments. We generate a contiguous observational cube spanning redshift 6 to 12 and use these simulations to study the differences in the reionization histories between stars and quasars. Finally, the signal is convolved with the Low Frequency Array (LOFAR) beam response and its effects are analysed and quantified. Statistics performed on this mock data set shed light on possible observational strategies for LOFAR.
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| 23. |
- Zaroubi, S., et al.
(author)
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Imaging neutral hydrogen on large scales during the Epoch of Reionization with LOFAR
- 2012
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 425:4, s. 2964-2973
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Journal article (peer-reviewed)abstract
- The first generation of redshifted 21 cm detection experiments, carried out with arrays like Low Frequency Array (LOFAR), Murchison Widefield Array (MWA) and Giant Metrewave Telescope (GMRT), will have a very low signal-to-noise ratio (S/N) per resolution element (less than or similar to 0.2). In addition, whereas the variance of the cosmological signal decreases on scales larger than the typical size of ionization bubbles, the variance of the formidable galactic foregrounds increases, making it hard to disentangle the two on such large scales. The poor sensitivity on small scales, on the one hand, and the foregrounds effect on large scales, on the other hand, make direct imaging of the Epoch of Reionization of the Universe very difficult, and detection of the signal therefore is expected to be statistical. Despite these hurdles, in this paper we argue that for many reionization scenarios low-resolution images could be obtained from the expected data. This is because at the later stages of the process one still finds very large pockets of neutral regions in the intergalactic medium, reflecting the clustering of the large-scale structure, which stays strong up to scales of approximate to 120 h(-1) comoving Mpc (approximate to 1 degrees). The coherence of the emission on those scales allows us to reach sufficient S/N (greater than or similar to 3) so as to obtain reionization 21 cm images. Such images will be extremely valuable for answering many cosmological questions but above all they will be a very powerful tool to test our control of the systematics in the data. The existence of this typical scale (approximate to 120 h(-1) comoving Mpc) also argues for designing future EoR experiments, e. g. with Square Kilometre Array, with a field of view of at least 4 degrees.
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