Exploring the line-of-sight environment to a quasar with Gemini/GMOS.

Matthew Whiting (UNSW)Rachel Webster (U. Melbourne)

Paul Francis (ANU)

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PKS 2126-158 Very luminous flat-spectrum radio quasar at redshift

of 3.2663. MV=-29.8, L5GHz= 2.5x1028 W/Hz, L1keV=1039 W

Optical SED dominated by blue power law up to the Ly- emission line.

Spectrum shows many Ly- and metal-line absorption systems at redshifts from 2.3941 – 3.2165, as well as one at z~0.6631: zabs = 0.66247, 0.66282, 0.66344, 0.66374, with MgII,

MgI and CaII components.

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Surrounding field of 2126-158 Quasar is surrounded by a

collection of objects, typical colours of R-K>3.5 Colours from short exposure

images of Francis et al. (2000), taken with ANU 2.3m telescope

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

z = 0.21

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GMOS observations 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. 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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Slit positions near quasar

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Sample targets and spectra

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? 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? 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 ie. are most high-z quasars surrounded by groups of galaxies?

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Association with absorbing system 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.

Q?

?

?

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Origin of absorbing gas 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. 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).

Could originate in stripped gas, that may result from tidal stripping. 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 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 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. 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.