# ![HELP LOGO](https://avatars1.githubusercontent.com/u/7880370?s=75&v=4) DMU 23 Spectroscopic Redshift Repository

This repository contains all the HELP spectroscopic redshifts (spec-z's) across the various HELP fields. These catalogues contain any spectroscopic estimates measured for the fields covered by HELP. The column descriptions for the catalogue is as follows:

- **field**: The HELP designation for the field (e.g., ELAIS-N1)
- **specz_id**: The unique HELP spectroscopic ID (e.g., HELP_SPECZ_J160834.495+550234.328)
- **ra**: The Right Ascension (J2000) of the redshift in degrees (e.g., 242.14373 degrees)
- **dec**: The Declination (J2000) of the redshift in degrees (e.g., 55.042869 degrees)
- **z_spec**: The spectroscopic redshift measurement (e.g., 0.7145746)
- **z_source**: The integer source code of the survey or surveys which produced the spectroscopic redshift (e.g., 4). See the paragraph below for a detailed explaination.
- **z_qual**: The quality flag (Q) is designated using the method from Colless et al. (2001), and also used for the GAMA database. The quality flag is ranked on a five-point scale, with the interpretation:
	- Q = 1 - no redshift could be estimated
	- Q = 2 - a possible but doubtful redshift estimate
	- Q = 3 - nmeans a probable redshift (notionally 90% confidence)
	- Q = 4 - means a reliable redshift (notionally 99% confidence)
	- Q = 5 - means a reliable redshift and high quality spectrum
  Note that sometimes information is unavailable to make an exact designation, in these cases a cautious and reasonable estimate is chosen.
- **z_rel**: For the surveys where we know the exact reliability measurements are available. The value is given as a probability between 0 and 1 (i.e., 60% reliability would be 0.6). This can be useful as some surveys have reliability estimates which would put it below Q=3 but still have a good success rate (for example 85%).
- **agn_flag** - If the redshift survey provides information identifing the spectrum as a QSO or AGN object this flag is set to 1.
For COSMOS and extra column is present (Notes), please see the readme in the COSMOS folder.

Each field contains a merged redshift catalogue where different sources of redshift information have been merged into a single catalogue. The source of each redsift can be identified form the z_source column. The integer number is a binary identifier; for each field a file lists the surveys and their integer identifier, and so for any particular redshift the surveys that produced a redshift can be identified. For example in the SGP field 2dF = 1, 6dF = 2, and 2MRS = 4, therefore a redshift with z_source = 5 has matching values in both the 2dF and the 2MRS survey.
Therefore, to identify any individual spec-z in the main spec-z table (specz.main), the field and specz_id are required.


The full files which are not stored in the Git repository can be downloaded from HeDaM:

- [http://hedam.lam.fr/HELP/dataproducts/](http://hedam.lam.fr/HELP/dataproducts/)

or an easier way is to use the HELP VO server:

- [https://herschel-vos.phys.sussex.ac.uk/](https://herschel-vos.phys.sussex.ac.uk/)

## HELP Spectroscopic Redshift Methodology

To create the spectroscopic redshift catalogues first each source of information is coverted into a standard format, and 
any duplicates objects are removed. The individual catalogues are matched typically on a scale of 2", however matches from 1" 
to 5" are investigated to assess which are real associations. The matching radius is then adjusted, with indidual cases either split or matched if required. The redshifts for any matched object are then merged with reliable redshifts (Q >= 3) used instead of un-reliable measurements. For the cases with two or more reliable measurements (Q >= 3) if they agree to within 0.01 and 5% the redshift is attributed to all sources (with the exact value taken from the source deemed most accurate). For a very small minority of cases a conflict in reliable redshifts will be reported in which case a decision is made on which redshift is considered the most reliable.

## Fields

The number of redshifts in each field is given in the table below.

Coming Soon!
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*This is a default HELP Readme markdown footer which should be identical across all fields*

Matthew Smith (http://http://www.cardiff.ac.uk/people/view/913880-smith-matthew)

 ![HELP LOGO](https://avatars1.githubusercontent.com/u/7880370?s=75&v=4)
 
For a full description of the database and how it is organised in to `dmu_products` please see the top level [readme](../readme.md).
 
The Herschel Extragalactic Legacy Project, ([HELP](http://herschel.sussex.ac.uk/)), is a [European Commission Research Executive Agency](https://ec.europa.eu/info/departments/research-executive-agency_en)
funded project under the SP1-Cooperation, Collaborative project, Small or medium-scale focused research project, FP7-SPACE-2013-1 scheme, Grant Agreement
Number 607254.

[Acknowledgements](http://herschel.sussex.ac.uk/acknowledgements)