Sunday, June 26, 2011

Two papers relevant to MgII absorption around galaxies

Will need to reference both these papers in passing in our revised paper testing the Tinker & Chen MgII halo model.

arXiv:1106.0616 Bordoloi et al. The radial and azimuthal profiles of Mg II absorption around 0.5 < z < 0.9 zCOSMOS galaxies of different colors masses and environments

They can't say anything about covering fraction using composites, but can constrain strength of absorption with radius.  Need to mention how consistent their results are with Tinker & Chen's.

Fig. 3: absorption quite weak in composites.  Some smearing will have occurred, due to velocity offsets. 

Fig. 5a: abs. detected out to 100 proper kpc at EW~ few 0.01 Ang for non-group galaxies, but need to check what EW threshold Tinker & Chen had; if TC had a threshold of EW=0.1 Ang, they would have detected absorption only out to ~70 kpc.

We should point out that quasar asterisms offer the chance to probe the variation in absorbing EW with azimuth in individual halos, whereas this work's approach can only study statistical azimuthal variations, and then only for inclined disk galaxies.

arXiv:1106.1642 Lundgren et al. The Dark Matter Haloes and Host Galaxies of MgII Absorbers at z~1

Abstract:  They find from a sample of galaxies at 0.7<z<1.45 that the halo mass of MgII absorbers is the same as at z~0.6, which is good news for us because we don't model any redshift evolution in the halo mass distribution of absorbers.  Their f_c=0.5 (based on 1 detection! see section 6) is consistent with our measurement of f_c. 

Note that they have a sample of only 21 strong MgII absorption systems detected in SDSS quasars with which to compare to a large galaxy sample, and they only usefully probe scales of >~1 h^-1 Mpc from those absorbers with those galaxies.  (They detect MgII absorption in 2 of 3 cases of separations < 60 h^-1 kpc.)

Note that their Fig. 5 shows a slight preference for radius evolution of absorbers, despite being consistent with no evolution.  But what they're cross-correlating with their galaxy sample is a random point in an MgII halo (random because the quasar position is randomly placed within the halo), and that will increase the uncertainties a bit... hopefully they've tried to account for that in their uncertainties.

Friday, June 24, 2011

BAL Lac Objects?

Sent the following to

Dr. Zhang,

I read with interest your recent paper on a variable BAL trough in a
BL Lac object (arXiv:1106.1587, Discovery of A Variable Broad Absorption Line
in the BL Lac object PKS 0138−097).  The object is definitely worth following up
when it's in a high state to see if the trough shows up.  However, I'm not quite
convinced the trough is real.

The 4200 Ang feature seen by Stickel et al. could be explained as just
narrow MgII absorption observed at low resolution.  Whether or not there
is additional (broad) absorption in the S93 spectrum seems to me
critically dependent on where the continuum is drawn.

However, the main reason I'm skeptical is that the strongest trough is seen
in a SDSS observation with fiber 73 (MJD 53729, plate 1914).  There is a
problem in the SDSS #1 blue CCD which may explain this feature as spurious.
Here's my footnote on that problem, from an unrelated paper in preparation:

The SDSS spectrum of the BL Lac object OM~280
(SDSS J115019.21+241753.8; 2510 53877 086)
appears to show a trough at $\sim$4150\,\AA.  This trough is spurious,
likely arising from a region of non-optimal flatfielding (e.g., due to a
localized variable feature in the flatfield) in the CCD in the blue arm of
SDSS spectrograph \#1.  This region is visible as a faint circular depression on
the reduced 2-D spectrograms of that CCD in that wavelength range, centered
near fiber 81.  The affected wavelength region is interpolated over for
fibers 78-84, but its effects can sometimes be seen in other nearby fibers
such as \#86 (used for the spectrum of OM 280) or \#76 (producing a similar
spurious trough in SDSS J115013.88+234602.1). 

If the broad feature in the fiber-73 (MJD 53729) spectrum of your object turns
out to be spurious, then the reality of the feature rests on the MJD 52174
spectrum and the Stickel et al. spectrum.  The features in those spectra might
not be real broad troughs, which is why I'm not convinced.  But I agree they
could be real, which is why I think followup observations are merited for
this object, and I hope you pursue them.

Patrick Hall

PS  Note that a more accurate name for the object is PKS B0138-097;
the IAU asks that the B or J always be included to indicate the epoch,
and PKS 0138-097 recovers the object in SIMBAD but not in NED.

Also, I find it helpful to include not just the MJD of the SDSS spectra
in a publication, but the plate and fiber numbers as well.

PPS  The raw SDSS 2-D spectrograms can be found at
sorted by MJD, with names corresponding to those found in the
EXPID01, EXPID02 etc. header keywords in the spSpec files.

Wednesday, June 22, 2011


Between vacations.  Since Saturday I have gone through astro-ph (though have only looked at one paper in detail) and email (responding to all the easy items) and have Skyped with Chajet, with Rogerson scheduled for tomorrow.  Also on my plate for the next 7 days, in rough priority:

* weak-lined quasar spectroscopy: Gemini Phase II
* stay on top of: NATS1740, new computer, grant proposal letter of intent
* send ngCFHT AGN SWG invites
* Polar BAL quasar paper resubmission
* Redshifted-trough BAL quasars (annoying that the most fun thing is the lowest priority)