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Investigating the Dark Universe through Gravitational Lensing
Stockholm University, Faculty of Science, Department of Astronomy.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A variety of precision observations suggest that the present universe is dominated by some unknown components, the so-called dark matter and dark energy. The distribution and properties of these components are the focus of modern cosmology and we are only beginning to understand them.

Gravitational lensing, the bending of light in the gravitational field of a massive object, is one of the predictions of the general theory of relativity. It has become an ever more important tool for investigating the dark universe, especially with recent and coming advances in observational data.

This thesis studies gravitational lensing effects on scales ranging over ten orders of magnitude to probe very different aspects of the dark universe. Implementing a matter distribution following the predictions of recent simulations, we show that microlensing by a large population of massive compact halo objects (MACHOs) is unlikely to be the source of the observed long-term variability in quasars. We study the feasibility of detecting the so far elusive galactic dark matter substructures, the so-called “missing satellites”, via millilensing in galaxies close to the line-of-sight to distant light sources. Finally, we utilise massive galaxy clusters, some of the largest structures known in the universe, as gravitational telescopes in order to detect distant supernovae, thereby gaining insight into the expansion history of the universe. We also show, how such observations can be used to put constraints on the dark matter component of these galaxy clusters.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2011. , 67 p.
Keyword [en]
cosmology, gravitational lensing, dark matter, galaxies, galaxy clusters
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-56515ISBN: 978-91-7447-281-3OAI: oai:DiVA.org:su-56515DiVA: diva2:411704
Public defence
2011-06-01, lecture room FD5, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:15 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Submitted. Available from: 2011-05-10 Created: 2011-04-19 Last updated: 2011-05-16Bibliographically approved
List of papers
1. High-redshift microlensing and the spatial distribution of dark matter in the form of MACHOs
Open this publication in new window or tab >>High-redshift microlensing and the spatial distribution of dark matter in the form of MACHOs
2007 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 475, no 2, 453-465 p.Article in journal (Refereed) Published
Abstract [en]

A substantial part of the dark matter of the Universe could be in the form of compact objects (MACHOs), detectable through gravitational microlensing effects as they pass through the line of sight to background light sources. So far, most attempts to model the effects of high-redshift microlensing by a cosmologically distributed population of MACHOs have assumed the compact objects to be randomly and uniformly distributed along the line of sight. Here, we present a more realistic model, in which the MACHOs are assumed to follow the spatial clustering of cold dark matter. Because of sightline-to-sightline variations in surface mass density, this scenario leads to substantial scatter in MACHO optical depths, which we quantify as a function of source redshift. We find that while optical depth estimates based on a uniform line-of-sight distribution are reasonable for the highest-redshift light sources, such estimates can be incorrect by a factor of 2 for the nearby (z 0.25) Universe. Hence, attempts to derive the cosmological density of MACHOs from microlensing observations of only a few independent sightlines can be subject to substantial uncertainties. We also apply this model to the prediction of microlensing-induced variability in quasars not subject to macrolensing, and demonstrate that relaxing the assumption of randomly and uniformly distributed MACHOs only has a modest impact on the predicted light curve amplitudes. This implies that the previously reported problems with microlensing as the dominant mechanism for the observed long-term optical variability of quasars cannot be solved by taking the large-scale clustering of dark matter into account

Keyword
cosmology: dark matter -- gravitational lensing -- galaxies: quasars: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-11707 (URN)10.1051/0004-6361:20066707 (DOI)
Available from: 2008-01-14 Created: 2008-01-14 Last updated: 2011-04-20Bibliographically approved
2. Strong Lensing by Subhalos in the Dwarf Galaxy Mass Range. I. Image Separations
Open this publication in new window or tab >>Strong Lensing by Subhalos in the Dwarf Galaxy Mass Range. I. Image Separations
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2008 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 684, no 2, 804-810 p.Article in journal (Refereed) Published
Abstract [en]

The cold dark matter scenario predicts that a large number of dark subhalos should be located within the halo of each Milky Way-sized galaxy. One telltale signature of such dark subhalos could be additional milliarcsecond-scale image splitting of quasars previously known to be multiply imaged on arcsecond scales. Here we estimate the image separations for the subhalo density profiles favored by recent N-body simulations and compare these to the angular resolution of both existing and upcoming observational facilities. We find that the image separations produced are very sensitive to the exact subhalo density profile assumed, but in all cases they are considerably smaller than previous estimates based on the premise that subhalos can be approximated by singular isothermal spheres. Only the most optimistic subhalo models produce image separations that would be detectable with current technology, and many models produce image separations that will remain unresolved with all telescopes expected to become available in the foreseeable future. Detections of dark subhalos through image-splitting effects will therefore be far more challenging than currently believed, albeit not necessarily impossible.

Keyword
dark matter, galaxies: dwarf, galaxies: halos, gravitational lensing, quasars: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-16165 (URN)10.1086/590541 (DOI)
Available from: 2008-12-15 Created: 2008-12-15 Last updated: 2011-04-20Bibliographically approved
3. Strong Lensing by Subhalos in the Dwarf-galaxy-mass Range. II. Detection Probabilities
Open this publication in new window or tab >>Strong Lensing by Subhalos in the Dwarf-galaxy-mass Range. II. Detection Probabilities
2009 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 700, no 2, 1552-1558 p.Article in journal (Refereed) Published
Abstract [en]

The dark halo substructures predicted by current cold dark matter simulations may in principle be detectable through strong-lensing image-splitting of quasars on small angular scales (0.01 arcsec or below). Here, we estimate the overall probabilities for lensing by substructures in a host halo closely aligned to the line of sight to a background quasar. Under the assumption that the quasar can be approximated as a point source, the optical depth for strong gravitational lensing by subhalos typically turns out to be very small (τ < 0.01), contrary to previous claims. We therefore conclude that it is currently not feasible to use this strategy to put the simulation predictions for the dark matter subhalo population to the test. However, if one assumes the source to be spatially extended, as is the case for a quasar observed at radio wavelengths, there is a reasonable probability for witnessing substructure lensing effects even at rather large projected distances from the host galaxy, provided that the angular resolution is sufficient. While multiply imaged, radio-loud quasars would be the best targets for unambiguously detecting dark matter subhalos, even singly imaged radio quasars might be useful for setting upper limits on the abundance and central surface mass density of subhalos.

Keyword
dark matter, galaxies: halos, gravitational lensing, quasars: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-35755 (URN)10.1088/0004-637X/700/2/1552 (DOI)000268098100052 ()0004-637X (ISBN)
Available from: 2010-01-20 Created: 2010-01-20 Last updated: 2011-04-20Bibliographically approved
4. Near-IR search for lensed supernovae behind galaxy clusters. I. Observations and transient detection efficiency
Open this publication in new window or tab >>Near-IR search for lensed supernovae behind galaxy clusters. I. Observations and transient detection efficiency
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2009 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 507, no 1, 61-69 p.Article in journal (Refereed) Published
Abstract [en]

Context: Massive galaxy clusters at intermediate redshift can magnify the flux of distant background sources by several magnitudes.

Aims: We exploit this effect to search for lensed distant supernovae that may otherwise be too faint to be detected.

Methods: A supernova search was conducted at near infrared wavelengths using the ISAAC instrument at the VLT. The massive galaxy clusters Abell 1689, Abell 1835, and AC114 were observed for a total of 20 h to search for supernovae in gravitationally magnified background galaxies. The observations were split into individual epochs of 2 h of exposure time, separated by approximately one month. Image-subtraction techniques were used to search for transient objects with light curve properties consistent with supernovae, both in our new and archival ISAAC/VLT data. The limiting magnitude of the individual epochs was estimated by adding artificial stars to the subtracted images. Most of the epochs reach 90% detection efficiency at SZ(J) ≃ 23.8-24.0 mag (Vega).

Results: Two transient objects, both in archival images of Abell 1689 and AC114, were detected. The transient in AC114 coincides - within the position uncertainty - with an X-ray source and is likely to be a variable AGN at the cluster redshift. The transient in Abell 1689 was found at SZ = 23.24 mag, ~0.5´´away from a galaxy with photometric redshift z_gal = 0.6 ± 0.15. The light curves and the colors of the transient are consistent with a reddened type IIP supernova at redshift z = 0.59 ± 0.05. The lensing model of Abell 1689 predicts ~1.4 mag of magnification at the position of the transient, making it the most magnified supernova ever found and only the second supernova found behind a galaxy cluster. Conclusions: Our pilot survey has demonstrated the feasibility to find distant gravitationally magnified supernovae behind massive galaxy clusters. One likely supernova was found behind Abell 1689, in accordance with the expectations for this survey, as shown in an accompanying analysis paper. Based on observations made with ESO telescopes at the Paranal Observatory under program IDs 079.A-0192 and 081.A-0734.

Keyword
supernovae: general -- gravitational lensing -- methods: observational -- techniques: photometric
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-35757 (URN)10.1051/0004-6361/200911982 (DOI)000271782900012 ()0004-6361 (ISBN)
Available from: 2010-01-20 Created: 2010-01-20 Last updated: 2011-04-20Bibliographically approved
5. Near-IR search for lensed supernovae behind galaxy clusters. II. First detection and future prospects
Open this publication in new window or tab >>Near-IR search for lensed supernovae behind galaxy clusters. II. First detection and future prospects
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2009 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 507, no 1, 71-83 p.Article in journal (Refereed) Published
Abstract [en]

Aims: Powerful gravitational telescopes in the form of massive galaxy clusters can be used to enhance the light collecting power over a limited field of view by about an order of magnitude in flux. This effect is exploited here to increase the depth of a survey for lensed supernovae at near-IR wavelengths.

Methods: We present a pilot supernova search programme conducted with the ISAAC camera at VLT. Lensed galaxies behind the massive clusters A1689, A1835, and AC114 were observed for a total of 20 h divided into 2, 3, and 4 epochs respectively, separated by approximately one month to a limiting magnitude J ≲ 24 (Vega). Image subtractions including another 20 h worth of archival ISAAC/VLT data were used to search for transients with lightcurve properties consistent with redshifted supernovae, both in the new and reference data.

Results: The feasibility of finding lensed supernovae in our survey was investigated using synthetic lightcurves of supernovae and several models of the volumetric type Ia and core-collapse supernova rates as a function of redshift. We also estimate the number of supernova discoveries expected from the inferred star-formation rate in the observed galaxies. The methods consistently predict a Poisson mean value for the expected number of supernovae in the survey of between N_SN = 0.8 and 1.6 for all supernova types, evenly distributed between core collapse and type Ia supernovae. One transient object was found behind A1689, 0.5 arcsec from a galaxy with photometric redshift z_gal = 0.6 ± 0.15. The lightcurve and colors of the transient are consistent with being a reddened type IIP supernova at z_SN = 0.59. The lensing model predicts 1.4 mag of magnification at the location of the transient, without which this object would not have been detected in the near-IR ground-based search described in this paper (unlensed magnitude J ~ 25). We perform a feasibility study of the potential for lensed supernovae discoveries with larger and deeper surveys and conclude that the use of gravitational telescopes is a very exciting path for new discoveries. For example, a monthly rolling supernova search of a single very massive cluster with the HAWK-I camera at VLT would yield ≳ 10 lensed supernova lightcurves per year, where type Ia supernovae would constitute about half of the expected sample. Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme ID 079.A-0192 and ID 081.A-0734.

Keyword
cosmology: observations -- stars: supernovae: general -- galaxies: clusters: general -- gravitational lensing
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-35756 (URN)10.1051/0004-6361/200811254 (DOI)000271782900013 ()0004-6361 (ISBN)
Available from: 2010-01-20 Created: 2010-01-20 Last updated: 2011-04-20Bibliographically approved
6. Near-IR search for lensed supernovae behind galaxy clusters: III. Implications for cluster modeling and cosmology
Open this publication in new window or tab >>Near-IR search for lensed supernovae behind galaxy clusters: III. Implications for cluster modeling and cosmology
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(English)Article in journal (Refereed) Submitted
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-56513 (URN)
Available from: 2011-04-19 Created: 2011-04-19 Last updated: 2011-04-20Bibliographically approved

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