2,576 research outputs found
Discovery of distant high luminosity infrared galaxies
We have developed a method for selecting the most luminous galaxies detected
by IRAS based on their extreme values of R, the ratio of 60 micron and B-band
luminosity. These objects have optical counterparts that are close to or below
the limits of Schmidt surveys. We have tested our method on a 1079 deg^2 region
of sky, where we have selected a sample of IRAS sources with 60 micron flux
densities greater than 0.2 Jy, corresponding to a redshift limit z~1 for
objects with far-IR luminosities of 10^{13} L_sun. Optical identifications for
these were obtained from the UK Schmidt Telescope plates, using the likelihood
ratio method. Optical spectroscopy has been carried out to reliably identify
and measure the redshifts of six objects with very faint optical counterparts,
which are the only objects with R>100 in the sample. One object is a
hyperluminous infrared galaxy (HyLIG) at z=0.834. Of the remaining, fainter
objects, five are ultraluminous infrared galaxies (ULIGs) with a mean redshift
of 0.45, higher than the highest known redshift of any non-hyperluminous ULIG
prior to this study. High excitation lines reveal the presence of an active
nucleus in the HyLIG, just as in the other known infrared-selected HyLIGs. In
contrast, no high excitation lines are found in the non-hyperluminous ULIGs. We
discuss the implications of our results for the number density of HyLIGs at z<1
and for the evolution of the infrared galaxy population out to this redshift,
and show that substantial evolution is indicated. Our selection method is
robust against the presence of gravitational lensing if the optical and
infrared magnification factors are similar, and we suggest a way of using it to
select candidate gravitationally lensed infrared galaxies.Comment: 6 pages, accepted for publication in A&
Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies: An ALMA mm-wave spectral scan of NGC 4418
We obtained an ALMA Cycle 0 spectral scan of the dusty LIRG NGC 4418,
spanning a total of 70.7 GHz in bands 3, 6, and 7. We use a combined local
thermal equilibrium (LTE) and non-LTE (NLTE) fit of the spectrum in order to
identify the molecular species and derive column densities and excitation
temperatures. We derive molecular abundances and compare them with other
Galactic and extragalactic sources by means of a principal component analysis.
We detect 317 emission lines from a total of 45 molecular species, including 15
isotopic substitutions and six vibrationally excited variants. Our LTE/NLTE fit
find kinetic temperatures from 20 to 350 K, and densities between 10 and
10 cm. The spectrum is dominated by vibrationally excited HCN,
HCN, and HNC, with vibrational temperatures from 300 to 450 K. We find high
abundances of HCN, SiO, HS, and c-HCCCH and a low CHOH abundance. A
principal component analysis shows that NGC 4418 and Arp 220 share very similar
molecular abundances and excitation, which clearly set them apart from other
Galactic and extragalactic environments. The similar molecular abundances
observed towards NCG 4418 and Arp 220 are consistent with a hot gas-phase
chemistry, with the relative abundances of SiO and CHOH being regulated by
shocks and X-ray driven dissociation. The bright emission from vibrationally
excited species confirms the presence of a compact IR source, with an effective
diameter 350 K. The molecular abundances
and the vibrationally excited spectrum are consistent with a young
AGN/starburst system. We suggest that NGC 4418 may be a template for a new kind
of chemistry and excitation, typical of compact obscured nuclei (CON). Because
of the narrow line widths and bright molecular emission, NGC 4418 is the ideal
target for further studies of the chemistry in CONs.Comment: accepted by A&A on 29/06/201
The Double Quasar Q2138-431: Lensing by a Dark Galaxy?
We report the discovery of a new gravitational lens candidate Q2138-431AB,
comprising two quasar images at a redshift of 1.641 separated by 4.5 arcsecs.
The spectra of the two images are very similar, and the redshifts agree to
better than 115 km.sec. The two images have magnitudes and
, and in spite of a deep search and image subtraction procedure, no
lensing galaxy has been found with . Modelling of the system
configuration implies that the mass-to-light ratio of any lensing galaxy is
likely to be around , with an absolute lower limit of
for an Einstein-de Sitter universe. We conclude that
the most likely explanation of the observations is gravitational lensing by a
dark galaxy, although it is possible we are seeing a binary quasar.Comment: 17 pages (Latex), 8 postscript figures included, accepted by MNRA
Discovery of distant high luminosity infrared galaxies
We have developed a method for selecting the most luminous galaxies detected by IRAS based on their extreme values of R, the ratio of 60 micron and B-band luminosity. These objects have optical counterparts that are close to or below the limits of Schmidt surveys. We have tested our method on a 1079 deg^2 region of sky, where we have selected a sample of IRAS sources with 60 micron flux densities greater than 0.2 Jy, corresponding to a redshift limit z~1 for objects with far-IR luminosities of 10^{13} L_sun. Optical identifications for these were obtained from the UK Schmidt Telescope plates, using the likelihood ratio method. Optical spectroscopy has been carried out to reliably identify and measure the redshifts of six objects with very faint optical counterparts, which are the only objects with R>100 in the sample. One object is a hyperluminous infrared galaxy (HyLIG) at z=0.834. Of the remaining, fainter objects, five are ultraluminous infrared galaxies (ULIGs) with a mean redshift of 0.45, higher than the highest known redshift of any non-hyperluminous ULIG prior to this study. High excitation lines reveal the presence of an active nucleus in the HyLIG, just as in the other known infrared-selected HyLIGs. In contrast, no high excitation lines are found in the non-hyperluminous ULIGs. We discuss the implications of our results for the number density of HyLIGs at z<1 and for the evolution of the infrared galaxy population out to this redshift, and show that substantial evolution is indicated. Our selection method is robust against the presence of gravitational lensing if the optical and infrared magnification factors are similar, and we suggest a way of using it to select candidate gravitationally lensed infrared galaxies
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