arXiv subjects

Tomas Dahlen

Publications and source records attributed to Tomas Dahlen.

49 records · Page 3Linked to original sources

arXiv2006

Evolution of the Luminosity Function, Star Formation Rate, Morphology and Size of Star-forming Galaxies Selected at Rest-frame 1500A and 2800A

Using the multiwavelength photometric and spectroscopic data covering the CDF-S obtained within the GOODS, we investigate the rest-frame UV properties of galaxies to z~2.2, including the evolution of the luminosity function, the luminosity density, star formation rate (SFR) and galaxy morphology. We find a significant brightening (~ 1 mag) in the rest-frame 2800A characteristic magnitude (M*) over the redshift range 0.3 2.5) increases from ~10% at z~0.5 to ~30% at z~2.2. At the same time, we note that galaxies get bluer at increasing redshift. This suggests a scenario where an increased fraction of the star formation takes place in bulge-dominated systems at high redshift. This could be the evidence that the present day ellipticals are a result of assembly (i.e., mergers) of galaxies at z>1. Finally, we find that galaxy sizes for a luminosity selected sample evolves as (1+z)^{-1.1} between redshifts z=2.2 and z=1.1. This is consistent with previous measurements and suggests a similar evolution over the redshift range 0 < z < 6.

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arXiv2005

Corrections for gravitational lensing of supernovae: better than average?

We investigate the possibility of correcting for the magnification due to gravitational lensing of standard candle sources, such as Type Ia supernovae. Our method uses the observed properties of the foreground galaxies along the lines-of-sight to each source and the accuracy of the lensing correction depends on the quality and depth of these observations as well as the uncertainties in translating the observed luminosities to the matter distribution in the lensing galaxies. The current work is limited to cases where the matter density is dominated by the individual galaxy halos. However, it is straightforward to generalize the method to include also gravitational lensing from cluster scale halos. We show that the dispersion due to lensing for a standard candle source at z=1.5 can be reduced from about 7% to ~< 3%, i.e. the magnification correction is useful in reducing the scatter in the Type Ia Hubble diagram, especially at high redshifts where the required long exposure times makes it hard to reach large statistics and the dispersion due to lensing becomes comparable to the intrinsic Type Ia scatter.

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arXiv2005

Lensing magnification of supernovae in the GOODS-fields

Gravitational lensing of high-redshift supernovae is potentially an important source of uncertainty when deriving cosmological parameters from the measured brightness of Type Ia supernovae, especially in deep surveys with scarce statistics. Photometric and spectroscopic measurements of foreground galaxies along the lines-of-sight of 33 supernovae discovered with the Hubble Space Telescope, both core-collapse and Type Ia, are used to model the magnification probability distributions of the sources. Modelling galaxy halos with SIS or NFW-profiles and using M/L scaling laws provided by the Faber-Jackson and Tully-Fisher relations, we find clear evidence for supernovae with lensing (de)magnification. However, the magnification distribution of the Type Ia supernovae used to determine cosmological distances matches very well the expectations for an unbiased sample, i.e.their mean magnification factor is consistent with unity. Our results show that the lensing distortions of the supernova brightness can be well understood for the GOODS sample and that correcting for this effect has a negligible impact on the derived cosmological parameters.

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arXiv2005

The Evolution of the Optical and Near-Infrared Galaxy Luminosity Functions and Luminosity Densities to z~2

Using Hubble Space Telescope and ground-based U through K- band photometry from the Great Observatories Origins Deep Survey (GOODS), we measure the evolution of the luminosity function and luminosity density in the rest-frame optical (UBR) to z ~ 2, bridging the poorly explored ``redshift desert'' between z~1 and z~2. We also use deep near-infrared observations to measure the evolution in the rest-frame J-band to z~1. Compared to local measurements from the SDSS, we find a brightening of the characteristic magnitude, (M*), by ~2.1, \~0.8 and ~0.7 mag between z=0.1 and z=1.9, in U, B, and R bands, respectively. The evolution of M* in the J-band is in the opposite sense, showing a dimming between redshifts z=0.4 and z=0.9. This is consistent with a scenario in which the mean star formation rate in galaxies was higher in the past, while the mean stellar mass was lower, in qualitative agreement with hierarchical galaxy formation models. We find that the shape of the luminosity function is strongly dependent on spectral type and that there is strong evolution with redshift in the relative contribution from the different spectral types to the luminosity density. We find good agreement in the luminosity function derived from an R-selected and a K-selected sample at z~1, suggesting that optically selected surveys of similar depth (R < 24) are not missing a significant fraction of objects at this redshift relative to a near-infrared-selected sample. We compare the rest-frame B-band luminosity functions from z~0--2 with the predictions of a semi-analytic hierarchical model of galaxy formation, and find qualitatively good agreement. In particular, the model predicts at least as many optically luminous galaxies at z~1--2 as are implied by our observations.

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arXiv2004

High Redshift Supernova Rates

We use a sample of 42 supernovae detected with the Advanced Camera for Surveys on-board the Hubble Space Telescope as part of the Great Observatories Origins Deep Survey to measure the rate of core collapse supernovae to z~0.7 and type Ia supernovae to z~1.6. This significantly increases the redshift range where supernova rates have been estimated from observations. The rate of core collapse supernovae can be used as an independent probe of the cosmic star formation rate. Based on the observations of 17 core collapse supernovae, we measure an increase in the core collapse supernova rate by a factor of 1.6 in the range 0.3 1 compared to low redshift. At higher redshift (z>1), we find a suggested decrease in the type Ia rate with redshift. This evolution of the Ia rate with redshift is consistent with a type Ia progenitor model where there is a substantial delay between the formation of the progenitor star and the explosion of the supernova. Assuming that the type Ia progenitor stars have initial main sequence masses 3-8 M_Sun, we find that 5-7% of the available progenitors explode as type Ia supernovae.

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arXiv2004

The galaxy population of intermediate-redshift clusters

Using photometric redshifts we determine the galaxy population of the clusters of galaxies Cl0016+16 at z=0.55, Cl1600+41 at z=0.54, Cl1601+42 at z=0.54 and MS1008-1224 at z=0.31. Comparing the clusters, we find no evidence for a universal shape of the total luminosity function (LF) at these redshifts. When dividing the LFs into spectral types, we find that the LF of the early-type galaxies alone can be described by a Gaussian, while the LF of the late-type galaxies is well fitted by a Schechter function, suggesting that the separate LFs for different populations may be universal. The difference in the total LFs can mainly be attributed to the varying relative normalisation of these populations, implying that clusters with an abundant population of late-type galaxies also have steeper faint-end slopes. In MS1008-1224 we detect a faint blue population that dominates over a population with colours consistent with dwarf ellipticals, opposite to clusters at lower redshift. Compared to low redshift clusters, we find that a general fading of the late-type population by ~2 mag and the early-type population by ~1 mag describes the evolution from z=0.55 to z=0 well. As a consequence of the different early-type and late-type LFs and their dependence on cluster radius, the fraction of blue cluster galaxies, as measured by the Butcher-Oemler effect, differs between the clusters and depends on limiting magnitude and radius. We find a correlation between the dwarf-to-giant ratio and the surface density, indicating that the high density environment in the cluster cores is hostile to dwarf galaxies.

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arXiv2003

Structure and Evolution of Starburst and Normal Galaxies

We study the rest-frame morphology and structural properties of optically selected starburst galaxies at redshift z < 1, using multi-waveband (BViz) high resolution images taken by the Advanced Camera for Surveys (ACS) as part of the Great Observatories Origins Deep Survey (GOODS). We classify galaxies into starburst, early and late types by comparing their observed spectral energy distributions (SEDs) with local templates. We find that early-type systems have significantly higher rest-frame B -band concentration indices and AGN fraction (> 25%) than late-type spirals and optically-selected starbursts. These results are consistent with the scenario in which early-epoch (z > 1) gas-rich dissipative processes (e.g., major mergers) have played an important role in developing large central concentrations in early-type E/Sa galaxies, leading to concurrent growth of central black holes and bulge formation in some of these early merger events. The starbursts have, on average, larger asymmetries than our control sample of normal galaxies, suggesting that a significant fraction of the starburst activity is tidally triggered.

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arXiv2003

Observing the Formation of the Hubble Sequence in the Great Observatories Origins Deep Survey

Understanding the physical formation of the Hubble sequence remains one of the most important unsolved astrophysical problems. Searches for proto-disks and proto-ellipticals can now be effectively done using deep wide-field Hubble Space Telescope images taken with the new Advanced Camera for Surveys. Through an analysis of concentrations (C), asymmetries (A) and clumpiness values (S) (CAS) of galaxies found in the GOODS Field South, we are able to identify objects possibly forming onto the Hubble sequence. Using this approach, we detect a sizeable population of star forming luminous diffuse objects and star forming luminous asymmetric objects between redshifts 0.5 < z < 2. These galaxies have extremely low light concentrations, or high asymmetries, with absolute magnitudes M_B < -19. The luminous diffuse objects are found in abundance between z = 1 - 2, with fewer objects at z > 2 and z < 1. The luminous asymmetric objects are found at a similar abundance, with a peak at z~1. We argue that these galaxies are a subset of modern disks and ellipticals in formation. The co-moving volume density of the luminous diffuse objects between z = 1 - 2 is similar to the local density of bright disk galaxies, with values ~5*10^{5} Gpc^{-3}. The SEDs of these objects are mostly consistent with starbursts, or star-forming normal galaxies, with average uncorrected for extinction star formation rates of ~4 M_0 yr^{-1}. These galaxies also host 35-40% of the star formation activity at 1 < z < 2. We briefly discuss the implications of these objects for understanding the origin of the Hubble sequence.

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arXiv2003

The Redshift Distribution of Near-IR Selected Galaxies in the Great Observatories Origins Deep Survey as a Test of Galaxy Formation Scenarios

The redshift distribution of near-IR selected galaxies is often used to attempt to discriminate between the classical view of galaxy formation, in which present-day luminous galaxies were assembled at early times and evolve due to the passive aging of their stellar populations, and that of hierarchical structure formation, in which galaxies were assembled more recently via the merging of smaller objects. We carry out such a test here, by computing the distribution of photometric redshifts of K_AB<22 galaxies in the Great Observatories Origins Deep Field Survey GOODS Southern Field, and comparing the results with predictions from a semi-analytic model based on hierarchical structure formation, and a classical `passive evolution' model. We find that the redshift distributions at z <~ 1.5 of both the hierarchical and passive models are very similar to the observed one. At z>~1.5, the hierarchical model shows a deficit of galaxies, while the passive model predicts an excess. We investigate the nature of the observed galaxies in the redshift range where the models diverge, and find that the majority have highly disturbed morphologies, suggesting that they may be merger-induced starbursts. While the hierarchical model used here does not produce these objects in great enough numbers, the appearance of this population is clearly in better qualitative agreement with the hierarchical picture than with the classical passive evolution scenario. We conclude that the observations support the general framework of hierarchical formation, but suggest the need for new or modified physics in the models.

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arXiv2002

Selection of high-z supernovae candidates

Deep, ground based, optical wide-field supernova searches are capable of detecting a large number of supernovae over a broad redshift range up to z~1.5. While it is practically unfeasible to obtain spectroscopic redshifts of all the supernova candidates right after the discovery, we show that the magnitudes and colors of the host galaxies, as well as the supernovae, can be used to select high-z supernova candidates, for subsequent spectroscopic and photometric follow-up. Using Monte-Carlo simulations we construct criteria for selecting galaxies in well-defined redshift bands. For example, with a selection criteria using B-R and R-I colors we are able to pick out potential host galaxies for which z>0.85 with 80% confidence level and with a selection efficiency of 64-86%. The method was successfully tested using real observations from the HDF. Similarly, we show that that the magnitude and colors of the supernova discovery data can be used to constrain the redshift. With a set of cuts based on V-R and R-I in a search to m_I~25, supernovae at z~1 can be selected in a redshift interval sigma_z <0.15.

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arXiv2001

The Galaxy Population of Cl1601+42 at z=0.54

Photometric redshifts are used to determine the rest frame luminosity function (LF) of both early-type and late-type galaxies to M_B ~ -17.6 for the cluster Cl1601+42 at z = 0.54. The total LF shows a steep faint-end slope alpha \~ -1.4, indicating the existence of a numerous population of dwarf galaxies. Luminous galaxies, with M_B < -19.5 are mostly red, early-type galaxies, with a LF best described by a Gaussian. Faint galaxies are predominantly blue, late-type galaxies, well fitted by a Schechter function with alpha ~ -1.7. Compared to clusters at lower redshift, the steepening of the faint end starts at brighter magnitudes for Cl1601+42, which may indicate a brightening of todays dwarf population relative to the giant population with increasing redshift. Early-type galaxies are centrally concentrated, and dominate the core region, implying that the radial gradient of early-type galaxies seen in local clusters is already established at z ~ 0.5. Bright, late-type galaxies are rare, consistent with a decrease in star formation in field galaxies as they are accreted on to the cluster, while faint, blue galaxies are evenly distributed across the cluster, except for a depletion in the core region. The blue fraction is f_B ~ 0.15, which is somewhat lower than the Butcher-Oemler average at z ~ 0.5. The value of f_B is found to increase with limiting magnitude and with radius from the centre.

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arXiv1999

Rates and redshift distribution of high-z supernovae

Using observed star formation rates at redshifts up to z ~ 5, we calculate cosmic supernova rates for core collapse and Type Ia supernovae. Together with supernova statistics and detailed light curves, we estimate the number of supernovae, and their distribution in redshift, that should be detectable in different filters with various instruments, including both existing and future telescopes, in particular the NGST. We find that the NGST should detect several tens of core collapse supernovae in a single frame. Most of these will be core collapse supernovae with 1 z 2, but about one third will have z > 2. Rates at z > 5 are highly uncertain. For ground based 8-10 m class telescopes we predict ~ 0.1 supernova per square arcmin to I_AB = 27, with about twice as many core collapse SNe as Type Ia's. The typical redshift will be z ~ 1, with an extended tail up to z ~ 2. Detectability of high redshift supernovae from ground is highly sensitive to the rest frame UV flux of the supernova, where line blanketing may decrease the rates severely in filters below 1 micron.

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arXiv1998

High z supernovae with the NGST

We use different assumptions for the combination of dust extinction and star formation up to redshifts z >= 5, as well as detailed modeling of supernova properties, to estimate the number of supernovae that should be observable with various instruments, in particular the NGST. In the model we use realistic light curves and spectral shapes that evolve with time for the different types supernovae. We find that the NGST should be able to detect several tens of core collapse SNe in a single detection if the observational limit in the range 1-5 microns is 1 nJy and the field is 16 sq. arcmin. We also estimate the observable number of Type Ia supernovae. Due to the time delay between the formation of the progenitor star and the explosion of the supernova, there are additional aspects that have to be considered when interpreting observational rates of these supernovae.

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