Publication about 4 new galaxies

Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: Publication about 4 new galaxies

Post by Robin Leadbeater »

Robin Leadbeater wrote:
Yes I agree. I made this clear in my post. I agree with you that the quoted absolute accuracy in the paper is not supported by evidence presented there.

Cheers
Robin
To be clear, the quoted precision is supported by the individual line measurements. For example
"Ra 69 (Tab.4), we derive a mean redshift of z = 0.04360 ± 0.00051" (0.0436 +-0.0005 would be better)
This is reasonable (0.0005 = 150km/s) but there is no evidence presented that this is the absolute accuracy

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Benjamin Mauclaire
Posts: 258
Joined: Thu Sep 29, 2011 10:14 am

Re: Publication about 4 new galaxies

Post by Benjamin Mauclaire »

Your citation is weel chosen for telling that the accuracy is realistic.
But you forgot the two others objects:
- App 1: z = 0.00597 ± 0.00016 -> DeltaV~48 kms
- Pre 24: z = 0.01454 ± 0.00029 -> DeltaV~87 kms
Line fitting may give 0.5 to 0.25 pixel (if S/N is OK) accuracy leading to 300 or 150 kms.
But no S/N is described in the paper and lines profils seem having low S/N as targets are very weak.

Benji
Spcaudace spectroscopy software: saving you hundred hours of frustration.
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: Publication about 4 new galaxies

Post by Robin Leadbeater »

Hi Benji,

The uncertainties quoted in the paper are actual measurements based on the combined values from several individual lines so I do not think we can dispute those.
Also I know from practical experience that is possible to measure the centroid of a single line to a precision better than 1/10 line width (50km/s) provided the SNR is good. Each z measurement here was the mean of 8 lines so I think the quoted precision of 48km/s is reasonable.

The question for me is what is the the absolute accuracy due to potential systematic errors, for example offsets between the calibration lamp and measurement on the sky? There is no mention of any RV standards being measured to check that.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Post Reply