arXiv subjects

Lise Christensen

Publications and source records attributed to Lise Christensen.

62 records · Page 4Linked to original sources

arXiv2012

Gravitationally Lensed Galaxies at 2<z<3.5: Direct Abundance Measurements of Lya Emitters

Strong gravitational lensing magnifies the flux from distant galaxies, allowing us to detect emission lines that would otherwise fall below the detection threshold for medium-resolution spectroscopy. Here we present the detection of temperature-sensitive oxygen emission lines from three galaxies at 2 2 for which this has been possible. The three galaxies have ~10% solar oxygen abundances in agreement with strong emission line diagnostics. Carbon and nitrogen ratios relative to oxygen are sub-solar as expected for young metal-poor galaxies. Two of the galaxies are Lya emitters with rest-frame equivalent widths of 20 A and 40 A, respectively, and their high magnification factors allow us for the first time to gain insight into the physical characteristics of high-redshift Lya emitters. Using constraints from the physical properties of the galaxies, we accurately reproduce their line profiles with radiative transfer models. The models show a relatively small outflow in agreement with the observed small velocity offsets between nebular emission and interstellar absorption lines.

astro-ph.CO↗

arXiv2012

The Low Mass End of the Fundamental Relation for Gravitationally Lensed Star Forming Galaxies at 1<z<6

We present VLT/X-shooter spectra of 13 galaxies in the redshift range 1< z < 6, which are strongly lensed by massive galaxy clusters. Spectroscopic redshifts are measured for nine galaxies, while three sources have redshifts determined from continuum breaks in their spectra. The stellar masses of the galaxies span four orders of magnitude between 10^7 and 10^11 M_sun and have luminosities at 1500 A rest-frame between 0.004 and 9 L^* after correcting for the magnification. This allows us to probe a variety of galaxy types from young, low-mass starburst galaxies to massive evolved galaxies. The lensed galaxies with stellar masses less than 10^10 M_sun have a large scatter compared to the fundamental relation between stellar mass, star formation rates and oxygen abundances. We provide a modified fit to the fundamental relation for low-mass, low-metallicity galaxies with a weaker dependence of the metallicity on either the star formation rate or stellar mass compared to low-redshift, high-mass and high-metallicity SDSS galaxies.

astro-ph.CO↗

arXiv2011

A Lyα blob and zabs {\approx} zem damped Lyα absorber in the dark matter halo of the binary quasar Q 0151+048

Q0151+048 is a physical QSO pair at z ~ 1.929 with a separation of 3.3 arcsec on the sky. In the spectrum of Q0151+048A (qA), a DLA is observed at a higher redshift. We have previously detected the host galaxies of both QSOs, as well as a Lya blob. We performed low-resolution spectroscopy with the slit aligned with the extended emission. We also observed the system using the medium-resolution VLT/X-shooter spectrograph and the slit aligned with the two QSOs. We measure systemic redshifts of zem(A)=1.92924{\pm}0.00036 and zem(B)=1.92863{\pm}0.00042 from the Hβ and Hα emission lines, respectively. We estimate the masses of the black holes of the two QSOs to be 10^9.33 M{\odot} and 10^8.38 M{\odot} for qA and qB, respectively. From this we infered the mass of the dark matter halos hosting the two QSOs: 10^13.74 M{\odot} and 10^13.13 M{\odot} for qA and qB, respectively. We observe a velocity gradient along the major axis of the Lya blob consistent with the rotation curve of a large disk galaxy, but it may also be caused by gas inflow or outflow. We detect residual continuum in the DLA trough which we interpret as emission from the host galaxy of qA. The derived H0 column density of the DLA is log NH0 = 20.34 {\pm} 0.02. Metal column densities results in an overall metallicity of 0.01 Z{\odot}. We detect CII* which allows us to make a physical model of the DLA cloud. From the systemic redshifts of the QSOs, we conclude that the Lya blob is associated with qA rather than with the DLA. The DLA must be located in front of both the Lya blob and qA at a distance larger than 30 kpc. The two QSOs accrete at normal eddington ratios. The DM halo of this double quasar will grow to the mass of our local super-cluster at z=0. We point out that those objects therefore form an ideal laboratory to study the physical interactions in a z=2 pre-cursor of our local super-cluster.

astro-ph.CO↗

arXiv2010

X-shooter Observations of the Gravitational Lens System CASSOWARY 5

We confirm an eighth gravitational lens system in the CASSOWARY catalogue. Exploratory observations with the X-shooter spectrograph on the VLT show the system CSWA5 to consist of at least three images of a blue star-forming galaxy at z = 1.0686, lensed by an apparent foreground group of red galaxies one of which is at z = 0.3877. The lensed galaxy exhibits a rich spectrum with broad interstellar absorption lines and a wealth of nebular emission lines. Preliminary analysis of these features shows the galaxy to be young, with an age of 25-50 Myr. With a star-formation rate of approximately 20 solar masses/yr, the galaxy has already assembled a stellar mass of 3 x 10^9 solar masses and reached half-solar metallicity. Its blue spectral energy distribution and Balmer line ratios suggest negligible internal dust extinction. A more in-depth analysis of the properties of this system is currently hampered by the lack of a viable lensing model. However, it is already clear that CSWA5 shares many of its physical characteristics with the general population of UV-selected galaxies at redshifts z = 1-3, motivating further study of both the source and the foreground mass concentration responsible for the gravitational lensing.

astro-ph.CO↗

arXiv2009

CASSOWARY 20: a Wide Separation Einstein Cross Identified with the X-shooter Spectrograph

We have used spectra obtained with X-shooter, the triple arm optical-infrared spectrograph recently commissioned on the Very Large Telescope of the European Southern Observatory, to confirm the gravitational lens nature of the CASSOWARY candidate CSWA 20. This system consists of a luminous red galaxy at redshift z(abs) = 0.741, with a very high velocity dispersion sigma(lens) = 500 km/s, which lenses a blue star-forming galaxy at z(em) = 1.433 into four images with mean separation of about 6 arcseconds. The source shares many of its properties with those of UV-selected galaxies z = 2-3: it is forming stars at a rate of 25 solar masses per year, has a metallicity of about 1/4 solar, and shows nebular emission from two components separated by 0.4 arcseconds in the image plane, possibly indicating a merger. It appears that foreground interstellar material within the galaxy has been evacuated from the sight-line along which we observe the starburst, giving an unextinguished view of its stars and H II regions. CSWA 20, with its massive lensing galaxy producing a high magnification of an intrinsically luminous background galaxy, is a promising target for future studies at a variety of wavelengths.

astro-ph.CO↗

arXiv2008

Unraveling the dynamics and kinematics of GRB hosts with high resolution spectroscopy

In the last years the research on GRB host galaxies has proceeded to more detailed studies both using high resolution afterglow spectroscopy and spatially resolved spectra of nearby hosts. High resolution spectra give a detailed picture of the kinematic properties of matter along the line-of-sight in the host. Two afterglow spectra show clear indications for outflows from their host, namely GRB 030329 and GRB 060206, derived from the position of absorption and host emission lines, nonvariability of Mg I and the radiation field calculated from fine-structure line detections. In nearby GRB hosts it is possible to resolve the actual GRB site. GRB 060505, a SN-less GRB, originated in a relatively metal poor, star forming region with similar properties as other long-duration hosts. A similar conclusion is reached for the site of GRB 980425/SN 1998bw.

astro-ph↗

arXiv2003

The Host Galaxy of GRB 990712

We present a comprehensive study of the z=0.43 host galaxy of GRB 990712, involving ground-based photometry, spectroscopy, and HST imaging. The broad-band UBVRIJHKs photometry is used to determine the global spectral energy distribution (SED) of the host galaxy. Comparison with that of known galaxy types shows that the host is similar to a moderately reddened starburst galaxy with a young stellar population. The estimated internal extinction in the host is Av=0.15+/-0.1 and the star-formation rate (SFR) from the UV continuum is 1.3+/-0.3 M_sun/yr, (not corrected for the effects of extinction). Other galaxy template spectra than starbursts failed to reproduce the observed SED. We also present VLT spectra leading to the detection of Halpha from the GRB host galaxy. A SFR of 2.8+/-0.7 M_sun/yr, is inferred from the Halpha line flux, and the presence of a young stellar population is supported by a large equivalent width. Images from HST/STIS show that the host has two separate knots, which could be two distinct star-forming regions.

astro-ph↗

arXiv2001

Optical observations of the dark Gamma-Ray Burst GRB 000210

We report on optical observations on GRB 000210 obtained with the 2.56-m Nordic Optical Telescope and the 1.54-m Danish Telescope starting 12.4 hours after the gamma-ray event. The content of the X-ray error box determined by the Chandra satellite is discussed.

astro-ph↗