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Erik Zackrisson

Publications and source records attributed to Erik Zackrisson.

At least 73 records · Page 4Linked to original sources
arXiv2019

Challenges in modelling the rest-frame ultraviolet/optical spectra of galaxies at the high-redshift frontier

New challenges in the modelling of galaxy spectra are bound to emerge as upcoming telescopes like the James Webb Space Telescope will allow us to detect galaxies at fainter flux levels and higher redshifts than ever before. Here, we highlight three modelling problems that may become relevant for upcoming observations in the rest-frame ultraviolet/optical at the high-redshift frontier: stellar initial mass function sampling effects, Population III signatures and the leakage of ionizing radiation into the intergalactic medium.

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arXiv2019

Detection of the Far-infrared [O III] and Dust Emission in a Galaxy at Redshift 8.312: Early Metal Enrichment in the Heart of the Reionization Era

We present the Atacama Large Millimeter/submillimeter Array (ALMA) detection of the [O III] 88 $μ$m line and rest-frame 90 $μ$m dust continuum emission in a Y-dropout Lyman break galaxy (LBG), MACS0416_Y1, lying behind the Frontier Field cluster MACS J0416.1-2403. This [O III] detection confirms the LBG with a spectroscopic redshift of $z = 8.3118 \pm 0.0003$, making this object one of the furthest galaxies ever identified spectroscopically. The observed 850 $μ$m flux density of $137 \pm 26$ $μ$Jy corresponds to a de-lensed total infrared (IR) luminosity of $L_{\rm IR} = (1.7 \pm 0.3) \times 10^{11} L_{\odot}$ if assuming a dust temperature of $T_{\rm dust} = 50$ K and an emissivity index of $β= 1.5$, yielding a large dust mass of $4 \times 10^6 M_{\odot}$. The ultraviolet-to-far IR spectral energy distribution modeling where the [O III] emissivity model is incorporated suggests the presence of a young ($τ_{\rm age} \approx 4$ Myr), star-forming (SFR $\approx 60 M_{\odot}$ yr$^{-1}$), moderately metal-polluted ($Z \approx 0.2 Z_{\odot}$) stellar component with a mass of $M_{\rm star} = 3 \times 10^8 M_{\odot}$. An analytic dust mass evolution model with a single episode of star-formation does not reproduce the metallicity and dust mass in $τ_{\rm age} \approx 4$ Myr, suggesting a pre-existing evolved stellar component with $M_{\rm star} \sim 3 \times 10^9 M_{\odot}$ and $τ_{\rm age} \sim 0.3$ Gyr as the origin of the dust mass.

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arXiv2018

On the Detection of Supermassive Primordial Stars

The collapse of supermassive primordial stars in hot, atomically-cooled halos may have given birth to the first quasars at $z \sim$ 15 - 20. Recent numerical simulations of these rapidly accreting stars reveal that they are cool, red hypergiants shrouded by dense envelopes of pristine atomically-cooled gas at 6,000 - 8,000 K, with luminosities $L$ $\gtrsim$ 10$^{10}$ L$_{\odot}$. Could such luminous but cool objects be detected as the first stage of quasar formation in future near infrared (NIR) surveys? We have now calculated the spectra of supermassive primordial stars in their birth envelopes with the Cloudy code. We find that some of these stars will be visible to JWST at $z \lesssim$ 20 and that with modest gravitational lensing Euclid and WFIRST could detect them out to $z \sim$ 10 - 12. Rather than obscuring the star, its accretion envelope enhances its visibility in the NIR today by reprocessing its short-wavelength flux into photons that are just redward of the Lyman limit in the rest frame of the star.

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arXiv2018

The case for gravitational millilensing in the multiply--imaged quasar B1152+199

Previous Very Long Baseline Interferometry (VLBI) observations of the quasar B1152+199 at 5GHz has revealed two images of a strongly lensed jet with seemingly discordant morphologies. Whereas the jet appears straight in one of the images, the other exhibits slight curvature on milliarcsecond scales. This is unexpected from the lensing solution and has been interpreted as possible evidence for secondary, small-scale lensing (millilensing) by a compact object with a mass of $~10^5$-$10^7\ M_\odot$ located close to the curved image. The probability for such a superposition is extremely low unless the millilens population has very high surface number density. Here, we revisit the case for millilensing in B1152+199 by combining new global-VLBI data at 8.4GHz with two datasets from the European VLBI Network (EVN) at 5GHz (archival) and at 22GHz (new dataset), and the previously published 5GHz Very Long Baseline Array (VLBA) data. We find that the new data with a more circular synthesized beam, exhibits no apparent milliarcsecond-scale curvature in image B. Various observations of the object spanning $\sim$15 years apart enable us to improve the constraints on lens system (thanks also to the improved astrometry resulting from 22GHz observations) to the point that the only plausible explanation left for the apparent curvature is the artifact due to the shape of the synthesized beam.

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arXiv2018

Reionization in Technicolor

We present the Technicolor Dawn simulations, a suite of cosmological radiation-hydrodynamic simulations of the first 1.2 billion years. By modeling a spatially-inhomogeneous UVB on-the-fly with 24 frequencies and resolving dark matter halos down to $10^8 M_\odot$ within 12 $h^{-1}$ Mpc volumes, our simulations unify observations of the intergalactic and circumgalactic media, galaxies, and reionization into a common framework. The only empirically-tuned parameter, the fraction $f_{\mathrm{esc,gal}}(z)$ of ionizing photons that escape the interstellar medium, is adjusted to match observations of the Lyman-$α$ forest and the cosmic microwave background. With this single calibration, our simulations reproduce the history of reionization; the stellar mass-star formation rate relation of galaxies; the number density and metallicity of damped Lyman-$α$ absorbers (DLAs) at $z\sim5$; the abundance of weak metal absorbers; the ultraviolet background (UVB) amplitude; and the Lyman-$α$ flux power spectrum at $z=5.4$. The galaxy stellar mass and UV luminosity functions are underproduced by $\leq2\times$, suggesting an overly vigorous feedback model. The mean transmission in the Lyman-$α$ forest is underproduced at $z<6$, indicating tension between measurements of the UVB amplitude and Lyman-$α$ transmission. The observed SiIV column density distribution is reasonably well-reproduced ($\sim 1σ$ low). By contrast, CIV remains significantly underproduced despite being boosted by an intense $>4$ Ryd UVB. Solving this problem by increasing metal yields would overproduce both weak absorbers and DLA metallicities. Instead, the observed strength of high-ionization emission from high-redshift galaxies and absorption from their environments suggest that the ionizing flux from conventional stellar population models is too soft.

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arXiv2018

SETI with Gaia: The observational signatures of nearly complete Dyson spheres

A star enshrouded in a Dyson sphere with high covering fraction may manifest itself as an optically subluminous object with a spectrophotometric distance estimate significantly in excess of its parallax distance. Using this criterion, the Gaia mission will in coming years allow for Dyson-sphere searches that are complementary to searches based on waste-heat signatures at infrared wavelengths. A limited search of this type is also possible at the current time, by combining Gaia parallax distances with spectrophotometric distances from ground-based surveys. Here, we discuss the merits and shortcomings of this technique and carry out a limited search for Dyson-sphere candidates in the sample of stars common to Gaia Data Release 1 and RAVE Data Release 5. We find that a small fraction of stars indeed display distance discrepancies of the type expected for nearly complete Dyson spheres. To shed light on the properties of objects in this outlier population, we present follow-up high-resolution spectroscopy for one of these stars, the late F-type dwarf TYC 6111-1162-1. The spectrophotometric distance of this object is about twice that derived from its Gaia parallax, and there is no detectable infrared excess. While our analysis largely confirms the stellar parameters and the spectrophotometric distance inferred by RAVE, a plausible explanation for the discrepant distance estimates of this object is that the astrometric solution has been compromised by an unseen binary companion, possibly a rather massive white dwarf ($\approx 1\ M_\odot$). This scenario can be further tested through upcoming Gaia data releases.

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arXiv2018

The onset of star formation 250 million years after the Big Bang

A fundamental quest of modern astronomy is to locate the earliest galaxies and study how they influenced the intergalactic medium a few hundred million years after the Big Bang. The abundance of star-forming galaxies is known to decline from redshifts of about 6 to 10, but a key question is the extent of star formation at even earlier times, corresponding to the period when the first galaxies might have emerged. Here we present spectroscopic observations of MACS1149-JD1, a gravitationally lensed galaxy observed when the Universe was less than four per cent of its present age. We detect an emission line of doubly ionized oxygen at a redshift of $9.1096\pm0.0006$, with an uncertainty of one standard deviation. This precisely determined redshift indicates that the red rest-frame optical colour arises from a dominant stellar component that formed about 250 million years after the Big Bang, corresponding to a redshift of about 15. Our results indicate the it may be possible to detect such early episodes of star formation in similar galaxies with future telescopes.

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arXiv2018

The Lyman Alpha Reference Sample IX: Revelations from deep surface photometry

The Lyman Alpha Reference Sample (LARS) of 14 star-forming galaxies offers a wealth of insight into the workings of these local analogs to high-redshift star-forming galaxies. The sample has been well-studied in terms of LyA and other emission line properties, such as HI mass, gas kinematics, and morphology. We analyze deep surface photometry of the LARS sample in UBIK broadband imaging obtained at the Nordic Optical Telescope and the Canada-France-Hawaii Telescope, and juxtaposition their derived properties with a sample of local high-redshift galaxy analogs, namely, with blue compact galaxies (BCGs). We construct radial surface brightness and color profiles with both elliptical and isophotal integration, as well as RGB images, deep contours, color maps, a burst fraction estimate, and a radial mass-to-light ratio profile for each LARS galaxy. Standard morphological parameters like asymmetry, clumpiness, the Gini and M20 coefficients are [...] analyzed, as well as isophotal asymmetry profiles for each galaxy. [...] We compare the LARS to the properties of the BCG sample and highlight the differences. Several diagnostics indicate that the LARS galaxies have highly disturbed morphologies even at the level of the faintest isophotes [...]. The ground-based photometry [...] reveals previously unexplored isophotes [...]. The burst fraction estimate suggests a spatially more extended burst region in LARS than in the BCGs. [...] The galaxies in the LARS sample appear to be in earlier stages of a merger event compared to the BCGs. Standard morphological diagnostics like asymmetry, clumpiness, Gini and M20 coefficients cannot separate the two samples, although an isophotal asymmetry profile successfully captures the average difference in morphology. These morphological diagnostics do not show any correlation with the equivalent width or the escape fraction of Lyman Alpha. [abridged]

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arXiv2018

Aligned metal absorbers and the ultraviolet background at the end of reionization

We use observations of spatially-aligned C II, C IV, Si II, Si IV, and O I absorbers to probe the slope and intensity of the ultraviolet background (UVB) at $z \sim 6$. We accomplish this by comparing observations with predictions from a cosmological hydrodynamic simulation using three trial UVBs applied in post-processing: a spectrally soft, fluctuating UVB calculated using multi-frequency radiative transfer; a soft, spatially-uniform UVB; and a hard, spatially-uniform "quasars-only" model. When considering our paired high-ionization absorbers (C IV/Si IV), the observed statistics strongly prefer the hard, spatially-uniform UVB. This echoes recent findings that cosmological simulations generically underproduce strong C IV absorbers at $z>5$. A single low/high ionization pair (Si II/Si IV), by contrast, shows a preference for the HM12 UVB, while two more (C II/C IV and O I/C IV) show no preference for any of the three UVBs. Despite this, future observations of specific absorbers, particularly Si IV/C IV, with next-generation telescopes probing to lower column densities should yield tighter constraints on the UVB.

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arXiv2017

Probing cold dark matter subhalos with simulated ALMA observations of macrolensed sub-mm galaxies

If the dark matter halos of galaxies contain large numbers of subhalos as predicted by the $Λ$CDM model, these subhalos are expected to appear in strong galaxy--galaxy lens systems as small--scale perturbations in individual images. We simulate observations of multiply--lensed sub--mm galaxies at $z\sim2$ as a probe of the dark matter halo of a lens galaxy at $z\sim0.5$. We present detection limits for dark substructures based on a visibility plane analysis of simulated Atacama Large Millimeter/submillimeter Array (ALMA) data in bands 7, 8 and 9. We explore two effects: local surface brightness anomalies on angular scales similar to the Einstein radius and the astrometric shift of macroimages. This improves the sensitivity of our lens modeling to the mass of the lens perturber. We investigate the sensitivity of the detection of low--mass subhalos to the projected position of the subhalo on the image plane as well as the source structure and inner density profile of the lens. We demonstrate that, using the most extended ALMA configuration, pseudo-Jaffe subhalos can be detected with 99% confidence down to $M = 10^7 M_\odot$. We show how the detection threshold for the three ALMA bands depends on the projected position of the subhalo with respect to the lensed images and conclude that, despite the highest nominal angular resolution, band 9 provides the poorest sensitivity due to observational noise. All simulations use the \emph{ALMA Full ops most extended} ALMA configuration setup in CASA.

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arXiv2017

The Lyman Continuum Escape and ISM properties in Tololo 1247-232 - New Insights from HST and VLA

Low- and intermediate mass galaxies are widely discussed as cause of reionization at redshift $z\sim10-6$. However, observational proof of galaxies that are leaking ionizing radiation (Lyman continuum; LyC) is a currently ongoing challenge and the list of LyC emitting candidates is still short. Tololo 1247-232 is among those very few galaxies with recently reported leakage. We performed intermediate resolution ultraviolet (UV) spectroscopy with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope and confirm ionizing radiation emerging from Tololo 1247-232. Adopting an improved data reduction procedure, we find that LyC escapes from the central stellar clusters, with an escape fraction of 1.5$\pm$0.5% only, i.e. the lowest value reported for the galaxy so far. We further make use of FUV absorption lines of Si II and Si IV as a probe of the neutral and ionized interstellar medium. We find that most of the ISM gas is ionized, likely facilitating LyC escape from density bounded regions. Neutral gas covering as a function of line-of-sight velocity is derived using the apparent optical depth method. The ISM is found to be sufficiently clumpy, supporting the direct escape of LyC photons. We further report on broadband UV and optical continuum imaging as well as narrowband imaging of Ly$α$, H$α$ and H$β$. Using stellar population synthesis, a Ly$α$ escape fraction of 8% was derived. We also performed VLA 21cm imaging. The hydrogen hyperfine transition was not detected, but a deep upper limit atomic gas mass of $\lesssim10^9 M_{\odot}$ could be derived. The upper limit gas fraction defined as $\frac{M_{H I}}{M_*}$ is only 20%. Evidence is found that the H I gas halo is relatively small compared to other Lyman Alpha emitters.

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arXiv2017

An analytic resolution to the competition between Lyman-Werner radiation and metal winds in direct collapse black hole hosts

A near pristine atomic cooling halo close to a star forming galaxy offers a natural pathway for forming massive direct collapse black hole (DCBH) seeds which could be the progenitors of the $z>6$ redshift quasars. The close proximity of the haloes enables a sufficient Lyman-Werner flux to effectively dissociate H$_2$ in the core of the atomic cooling halo. A mild background may also be required to delay star formation in the atomic cooling halo, often attributed to distant background galaxies. In this letter we investigate the impact of metal enrichment from both the background galaxies and the close star forming galaxy under extremely unfavourable conditions such as instantaneous metal mixing. We find that within the time window of DCBH formation, the level of enrichment never exceeds the critical threshold (Z$_{cr} \sim 1 \times 10^{-5} \ \rm Z_{\odot})$, and attains a maximum metallicity of Z $\sim 2 \times 10^{-6} \ \rm Z_{\odot}$. As the system evolves, the metallicity eventually exceeds the critical threshold, long after the DCBH has formed.

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arXiv2017

Unveiling the first black holes with JWST: multi-wavelength spectral predictions

Growing the supermassive black holes (~10^9 Msun) that power the detected luminous, highest redshift quasars (z > 6) from light seeds - the remnants of the first stars - within ~ 1 Gyr of the Big Bang poses a timing challenge for growth models. The formation of massive black hole seeds via direct collapse with initial masses ~ 10^4 - 10^5 Msun alleviates this problem. Physical conditions required to form these massive direct collapse black hole (DCBH) seeds are available in the early universe. These viable DCBH formation sites, satellite halos of star-forming galaxies, merge and acquire a stellar component. These produce a new, transient class of objects at high redshift, Obese Black hole Galaxies (OBGs), where the luminosity produced by accretion onto the black hole outshines the stellar component. Therefore, the OBG stage offers a unique way to discriminate between light and massive initial seeds. We predict the multi-wavelength energy output of OBGs and growing Pop III remnants at a fiducial redshift (z = 9), exploring both standard and slim disk accretion onto the growing central black hole for high and low metallicities of the associated stellar population. With our computed templates, we derive the selection criteria for OBGs, that comprise a pre-selection that eliminates blue sources; followed by color-color cuts ([F_{070W} - F_{220W}] > 0; -0.3 < [F_{200W} - F_{444W}] < 0.3) and when available, a high ratio of X-ray flux to rest-frame optical flux (F_X/F_{444W} >> 1) (Abridged).

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arXiv2017

Lyman-Werner Escape Fractions from the First Galaxies

Direct collapse black holes forming in pristine, atomically-cooling haloes at $z \approx 10-20$ may act as the seeds of supermassive black holes (BH) at high redshifts. In order to create a massive BH seed, the host halo needs to be prevented from forming stars. H$_2$ therefore needs to be irradiated by a large flux of Lyman-Werner (LW) UV photons in order to suppress H$_2$ cooling. A key uncertainty in this scenario is the escape fraction of LW radiation from first galaxies, the dominant source of UV photons at this epoch. To better constrain this escape fraction, we have performed radiation-hydrodynamical simulations of the growth of HII regions and their associated photodissociation regions in the first galaxies using the ZEUS-MP code. We find that the LW escape fraction crucially depends on the propagation of the ionisation front (I-front). For an R-type I-front overrunning the halo, the LW escape fraction is always larger than 95%. If the halo recombines later from the outside--in, due to a softened and weakened spectrum, the LW escape fraction in the rest-frame of the halo (the near-field) drops to zero. A detailed and careful analysis is required to analyse slowly moving, D-type I-fronts, where the escape fraction depends on the microphysics and can be as small as 3% in the near-field and 61% in the far-field or as large as 100% in both the near-field and the far-field.

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arXiv2017

AGN luminosity and stellar age -- two missing ingredients for AGN unification as seen with iPTF supernovae

Active Galactic Nuclei (AGN) are extremely powerful cosmic objects, driven by accretion of hot gas upon super-massive black holes. The zoo of AGN classes are divided into two major groups, with Type-1 AGN displaying broad Balmer emission lines and Type-2 narrow ones. For a long time it was believed that a Type-2 AGN is a Type-1 AGN viewed through a dusty kiloparsec-size torus, but an emerging body of observations suggests more than just the viewing angle matters. Here we report significant differences in supernova counts and classes in the first study to date of supernovae near Type-1 and Type-2 AGN host galaxies, using data from the intermediate Palomar Transient Factory, the Sloan Digital Sky Survey Data Release 7 and Galaxy Zoo. We detect many more supernovae in Type-2 AGN hosts (size of effect $\sim$ 5.1$σ$) compared to Type-1 hosts, which shows that the two classes of AGN are located inside host galaxies with different properties. In addition, Type-1 and Type-2 AGN that are dominated by star formation according to WISE colours $m_{W1} - m_{W2} < 0.5$ and are matched in 22 $μ$m absolute magnitude differ by a factor of ten in $L$[OIII]$λ$5007 luminosity, suggesting that when residing in similar type of host galaxies Type-1 AGN are much more luminous. Our results demonstrate two more factors that play an important role in completing the current picture: the age of stellar populations and the AGN luminosity. This has immediate consequences for understanding the many AGN classes and galaxy evolution.

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arXiv2016

A Multiply-Imaged z $\sim$ 6.3 Lyman Alpha Emitter candidate behind Abell 2261

While the Lyman-$α$ ($\mathrm{Ly}α$) emission line serves as an important tool in the study of galaxies at $z\lesssim 6$, finding Ly$α$ emitters (LAE) at significantly higher redshifts has been more challenging, probably because of the increasing neutrality of the intergalactic medium above $z\sim6$. Galaxies with extremely high rest-frame Ly$α$ equivalent widths, EW(Ly$α$) $\gtrsim 150$ Å, at $z>6$ are good candidates for Ly$α$ follow-up observations, and can stand out in multiband imaging surveys because of their unusual colors. We have conducted a photometric search for such objects in the Cluster Lensing And Supernova survey with Hubble (CLASH), and report here the identification of three likely gravitationally-lensed images of a single LAE candidate at $z\sim6.3$, behind the galaxy cluster Abell 2261($z = 0.225$). In the process, we also measured with Keck/MOSFIRE the first spectroscopic redshift of a multiply-imaged galaxy behind Abell 2261, at $z = 3.337$. This allows us to calibrate the lensing model, which in turn is used to study the properties of the candidate LAE. Population III galaxy spectral energy distribution (SED) model fits to the CLASH broadband photometry of the possible LAE provide a slightly better fit than Population I/II models. The best fitted model suggests intrinsic EW(Ly$α$) $\approx 160$ Å after absorption in the interstellar and intergalactic medium. Future spectroscopic observations will examine this prediction as well as shed more light on the morphology of this object, which indicates it may be a merger of two smaller galaxies.

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arXiv2016

Detecting Direct Collapse Black Holes: making the case for CR7

We propose that one of the sources in the recently detected system CR7 by Sobral et al. (2015) through spectro-photometric measurements at $z = 6.6$ harbors a direct collapse blackhole (DCBH). We argue that the LW radiation field required for direct collapse in source A is provided by sources B and C. By tracing the LW production history and star formation rate over cosmic time for the halo hosting CR7 in a $Λ$CDM universe, we demonstrate that a DCBH could have formed at $z\sim 20$. The spectrum of source A is well fit by nebular emission from primordial gas around a BH with MBH $\sim 4.4 \times 10^6 \ \rm M_{\odot}$ accreting at a 40 % of the Eddington rate, which strongly supports our interpretation of the data. Combining these lines of evidence, we argue that CR7 might well be the first DCBH candidate.

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