Transcript A GMOS dissection of the line-of

Exploring the line-of-sight
environment to a quasar with
Gemini/GMOS.
Matthew Whiting (UNSW)
Rachel Webster (U. Melbourne)
Paul Francis (ANU)
PKS 2126-158
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Very luminous flat-spectrum radio quasar at redshift
of 3.2663.
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MV=-29.8, L5GHz= 2.5x1028 W/Hz, L1keV=1039 W
Optical SED dominated by blue power law up to the
Ly-a emission line.
Spectrum shows many Ly-a and metal-line
absorption systems at redshifts from 2.3941 –
3.2165, as well as one at z~0.6631:
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zabs = 0.66247, 0.66282, 0.66344, 0.66374, with MgII,
MgI and CaII components.
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A quasar's line-of-sight environment with Gemini/GMOS
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Surrounding field of 2126-158
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Quasar is surrounded by a
collection of objects, typical
colours of R-K>3.5
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Colours from short exposure
images of Francis et al. (2000),
taken with ANU 2.3m telescope
z = 0.21
Only one had a known redshift
(Veron et al. 1990), with bluer
colours than the rest of the
nearby objects.
What/Where are they?
Is lensing important?
Are they associated with any of
the absorption systems?
K band image from IRIS, AAT
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A quasar's line-of-sight environment with Gemini/GMOS
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GMOS observations
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Pre-imaging done in i’ band – 20 minute exposure.
Use GMOS on Gemini South in MOS mode. Slits
placed on objects pre-identified from near-IR images
and pre-imaging.
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Grating R150, filter GG455, 0.75 arcsec slits.
Total of 2 hours exposure time over two nights.
Not all nearby sources could have slit placed on
them, as we only had time for one mask setting. Got
around this problem by placing multiple objects in
slits.
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A quasar's line-of-sight environment with Gemini/GMOS
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Slit positions near quasar
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Sample targets and spectra
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b=170kpc
Mi’=-22
L=4.1x1010 L
b=60kpc
Mi’=-22.2
L=4.8x1010 L
b=160kpc
Mi’=-21.6
L=2.7x1010 L
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Positions and redshifts
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2126-158: is lensing important?
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2126-158 is a very luminous quasar – is this luminosity
intrinsic, or is it being magnified?
No multiple images, but single-image magnification is
possible – are we seeing it here?
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Is the amount of matter along the line of sight sufficient to greatly
magnify the quasar flux?
Our observations show that there is no large cluster in front of
the quasar, but rather several smaller groups at different
redshifts. Hard to tell if there is strong magnification present
from current data.
Statistical properties of high-z radio quasars may be more
useful in determining importance of lensing
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ie. are most high-z quasars surrounded by groups of galaxies?
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Association with absorbing system
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We find at least 5 (possibly 7)
confirmed galaxies at z~0.664
close to the quasar (within ~170
kpc), with a few further away.
The nearest galaxy is 50kpc
from the quasar line-of-sight.
Possibly an interacting system?
(Very close in redshift and
spatial directions.)
There may be closer galaxies in
the same system, based on their
colours, but these have unknown
redshifts. One may be a star,
rather than a galaxy.
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Q
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Origin of absorbing gas
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Absorber has metal-bearing gas, with column densities up to
1017 cm-2 in MgII. Where is the gas located?
Could originate in outer halo of nearby galaxy. Nearest
confirmed galaxy is 50kpc from line-of-sight, so quasar is
likely to be still in the halo.
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Could originate in stripped gas, that may result from tidal
stripping.
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This displacement is comparable to that seen with other galaxies
associated with MgII absorbers. Redshift slightly different though.
Closest possible galaxy is ~20kpc, but is fainter (hence smaller).
From formation of large ellipticals in a group environment?
Due to interaction between two large galaxies?
Other observations, such as HI absorption or emission, would
be useful to constrain these models.
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Possible interacting system
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Possible interaction? Velocity
difference of ~500 km/s, which
seems large for an interaction
within a group.
An interaction could lead to
extra gas being ejected from
galaxies, increasing the column
density towards the quasar.
No sign of strong interaction in
either spectrum, eg star
formation tracers. Unclear from
isophotes if there is any
surrounding envelope.
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Summary
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We used GMOS-South to measure redshifts of many
red galaxies around PKS 2126-158.
We find a group of galaxies close to the line-of-sight
to the quasar at the same redshift as a metal-line
absorption system in the quasar spectrum.
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Still unclear as to origin of absorbing gas.
We also find several other lower-redshift features in
redshift-space in the field surrounding the quasar.
The relatively dense line-of-sight may help magnify
the quasar flux, without multiple lensing, partly
accounting for its high apparent luminosity.
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