# 20.5.1 galaxy_candidates

This table contains parameters derived from various modules dedicated to the classification and characterisation of sources considered as galaxy candidates. This table has been constructed with the intention to be complete rather than pure and, as such, it will contain a large fraction of non-genuine extragalactic sources. Purer samples can be drawn using dedicated flags or queries. Please refer to Chapter 12 of the on-line documentation for details about how this table was built, its content, and for recommendations regarding its exploitation.

Columns description:

solution_id : Solution Identifier (long)

All Gaia data processed by the Data Processing and Analysis Consortium comes tagged with a solution identifier. This is a numeric field attached to each table row that can be used to unequivocally identify the version of all the subsystems that were used in the generation of the data as well as the input data used. It is mainly for internal DPAC use but is included in the published data releases to enable end users to examine the provenance of processed data products. To decode a given solution ID visit https://gaia.esac.esa.int/decoder/solnDecoder.jsp

source_id : Unique source identifier (unique within a particular Data Release) (long)

A unique single numerical identifier of the source obtained from gaia_source (for a detailed description see gaia_source.source_id).

vari_best_class_name : Name of best class, see table vari_classifier_class_definition for details of the class (string)

Best class name with corresponding classification score in best_class_score of table vari_classifier_result. For the ‘n_transits:5+’ classifier in DR3, the following classes are published: ‘ACV|CP|MCP|ROAM|ROAP|SXARI’, ‘ACYG’, ‘AGN’, ‘BCEP’, ‘BE|GCAS|SDOR|WR’, ‘CEP’, ‘CV’, ‘DSCT|GDOR|SXPHE’, ‘ECL’, ‘ELL’, ‘EP’, ‘LPV’, ‘MICROLENSING’, ‘RCB’, ‘RR’, ‘RS’, ‘S’, ‘SDB’, ‘SN’, ‘SOLAR_LIKE’, ‘SPB’, ‘SYST’, ‘WD’, and ‘YSO’. The ‘GALAXY’ classifications are largely due to artificial variability (see Section 10.3.3 of the release documentation for details) and their probabilities are published exclusively in column vari_best_class_score of table galaxy_candidates.

See vari_classifier_class_definition for a detailed description of this classifier and its published classes.

vari_best_class_score : Score of the best class (float)

It describes a quantity between 0 and 1 which is related to the (median) normalised rank of the confidence of the classifier(s) in the identification of the best class (vari_best_class_name). In the special case of class ‘EP’, all scores are set to 1. See Section 10.3.4 of the release documentation for details.

classprob_dsc_combmod_galaxy : Probability from DSC-Combmod of being a galaxy (data used: BP/RP spectrum, photometry, astrometry) (float)

Probability that the object is of the named class. This is the overall probability for this class, computed by combining the class probabilities from DSC-Specmod (which classifies objects using BP/RP spectra) and DSC-Allosmod (which classifies objects using several astrometric and photometric features). It is important to realise that the DSC classes are defined by the training data used, and that this may produce a narrower definition of the class than may be expected given the class name. This is a posterior probability that includes the global class prior, given in the documentation.

classprob_dsc_combmod_quasar : Probability from DSC-Combmod of being a quasar (data used: BP/RP spectrum, photometry, astrometry) (float)

Probability that the object is of the named class. This is the overall probability for this class, computed by combining the class probabilities from DSC-Specmod (which classifies objects using BP/RP spectra) and DSC-Allosmod (which classifies objects using several astrometric and photometric features). It is important to realise that the DSC classes are defined by the training data used, and that this may produce a narrower definition of the class than may be expected given the class name. This is a posterior probability that includes the global class prior, given in the documentation.

classlabel_dsc : Class assigned by DSC based on the probability from its Combmod classifier (string)

Class assigned by DSC based on the probability from its Combmod classifier. DSC-Combmod provides a normalized posterior probability vector across several classes. Note that this incorporates the global class prior, as explained in the documentation. This class label is set to that class with the largest probability above 0.5. If no probability is above 0.5, this class label is ‘unclassified’. If users want to perform classification using a different threshold, or by adopting a different prior, they should use the DSC probability vectors.

classlabel_dsc_joint : Class assigned by DSC based on the probability from its Specmod and Allosmod classifiers (string)

DSC-Specmod and DSC-Allosmod each provide a normalized posterior probability vector across several classes. Both incorporate the global class prior, as explained in the documentation. If the ‘quasar’ class probability from both Specmod and Allosmod are above 0.5, this class label is set to ‘quasar’. If the ‘galaxy’ class probability from both Specmod and Allosmod are above 0.5, this class label is set to ‘galaxy’. (Note that these two cases are mutually exclusive.) Otherwise the class label is set to ‘unclassified’.

classlabel_oa : Class assigned by OA the neuron that represents the source (string)

Class label of the neuron that represents the source, as assigned by the OA module in Apsis. See Section 11.3.12 for further details.

redshift_ugc : Redshift from UGC (float)

The redshift of the source treated as a galaxy by the Apsis module UGC. The redshift is estimated from the sampled mean BP/RP spectrum applying a regression model based on Support Vector Machines (SVM), a supervised machine learning algorithm. Details are provided in Section 11.3.13 of the on-line documentation.

As an estimate of the uncertainty on redshift_ugc the value (redshift_ugc_upper $-$ redshift_ugc_lower)$/2$ may be used. We note that in the ranges redshift_ugc$<0.02$, $0.28<$redshift_ugc$<0.30$ and redshift_ugc$>0.58$ the SVM-model performance is particularly low and the derived redshifts lie outside of the achievable prediction range. A very narrow interval at $0.070<$redshift_ugc$<0.071$ includes several thousand bright sources which probably have redshifts lower than 0.04, as explained in the on-line documentation (Section 11.3.13).

redshift_ugc_lower : Redshift prediction lower limit from UGC (float)

The performance limits are based on the mean error $predictDiffMean[r_{i}]$ and its standard deviation $predictDiffStDev[r_{i}]$ ($1\sigma$-level) obtained during the SVM training with a test data set, where $r_{i}$=int(redshift_ugc/0.02). The lower limit is defined as redshift_ugc$-predictDiffMean[r_{i}]$ $-predictDiffStDev[r_{i}]$. For more details see the on-line documentation, Section 11.3.13.

redshift_ugc_upper : Redshift prediction upper limit from UGC (float)

The performance limits are based on the mean error $predictDiffMean[r_{i}]$ and its standard deviation $predictDiffStDev[r_{i}]$ ($1\sigma$-level) obtained during the SVM training with a test data set, where $r_{i}$=int(redshift_ugc/0.02). The upper limit is defined as redshift_ugc$-predictDiffMean[r_{i}]$ $+predictDiffStDev[r_{i}]$. For more details see the on-line documentation, Section 11.3.13.

n_transits : Number of transits used for the morphological analysis (int)

The number of transits used to reconstruct the image and to analyse the object morphology.

posangle_sersic : Fitted position angle of the source for the Sersic Profile (double, Angle[deg])

Position angle of the source for fitted Sersic profile with respect to Celestial North Pole.

posangle_sersic_error : Error on the fitted position angle of the source for the Sersic Profile (double, Angle[deg])

Uncertainty in the position angle of source with respect to Celestial North Pole for the Sersic profile.

intensity_sersic : Fitted intensity of the source for the Sersic Profile (double, Flux[e${}^{-}$ s${}^{-1}$])

Intensity of the source at the fitted effective radius (radius_sersic) for the Sersic profile.

intensity_sersic_error : Error on the fitted intensity of the source at effective radius radius_sersic (double, Flux[e${}^{-}$ s${}^{-1}$])

Uncertainty of the light intensity at the fitted effective radius (radius_sersic) of the source for the Sersic profile.

radius_sersic : Fitted effective radius of the source for the Sersic Profile (double, Angle[mas])

Effective radius containing half of the total luminosity of the source bulge as obtained by morphological fitting for the Sersic profile.

radius_sersic_error : Error on the fitted effective radius of the source for the Sersic Profile (double, Angle[mas])

Uncertainty in the effective radius containing half of the total luminosity of the source for the Sersic profile.

ellipticity_sersic : Fitted ellipticity of source for the Sersic Profile (double)

Ellipticity (defined as 1-($b/a$) where $b/a$ is the axis ratio) of the source for the Sersic profile.

ellipticity_sersic_error : Error on the fitted ellipticity of the source for the Sersic Profile (double)

Uncertainty in the ellipticity (defined as 1-($b/a$) where $b/a$ is the axis ratio) of the source for the Sersic profile.

n_sersic : Fitted Sersic Index for Sersic Profile (float)

Sersic index for the Sersic profile. The Sersic profile is a mathematical function that describes how the intensity of a galaxy varies with the distance from its centre. The Sersic index describes how steep this variation is.

n_sersic_error : Error on the fitted Sersic Index for Sersic Profile (float)

Uncertainty in the Sersic index for the Sersic profile.

l2_sersic : L2 norm for the Sersic Profile (double)

This value represents the mean squared error between the integrated flux of all observed samples (from the Sky Mapper and Astrometric Field) and the integrated flux of synthetic samples produced with the fitted profile.

morph_params_corr_vec_sersic : Vector form of the upper triangle of the correlation matrix for the fitted parameters for the Sersic Profile (double[10] array)

Vector form of the correlation matrix of the fitted profile parameters, as obtained by morphological fitting for the Sersic profile, in the following order: [0]: position angle [1]: intensity [2]: radius [3]: ellipticity [4]: Sersic index

Only non-zero, non-unity, correlation coefficients from the correlation matrix M are provided here. They are served as a linear array of size $S=n(n-1)/2$, where $n$ is the number of fitted parameters covered in the correlation matrix. The ordering of the elements in the linear array follows a column-major storage scheme, i.e.:

${\bf M}=\begin{bmatrix}1&C[0]&C[1]&C[3]&C[6]&\cdots&C[S-(n-1)]\\ &1&C[2]&C[4]&C[7]&\cdots&C[S-(n-2)]\\ &&1&C[5]&C[8]&\cdots&C[S-(n-3)]\\ &&&1&C[9]&\cdots&C[S-(n-4)]\\ &&&&\ddots&\ddots&\vdots\\ &&&&&1&C[S-1]\\ &&&&&&1\\ \end{bmatrix}$

flags_sersic : Flag indicative of processing or scientific quality for the morphological parameters fitting for the Sersic Profile (byte)

This flag provides information about the processing or scientific quality of the results of the Galaxy morphology analysis chain for the source of interest for the Sersic Profile.

The flag coding is the following. Note that not all flag cases apply to a given profile.

• 1:

Elliptical profile fitted, other source exists at less that 2.5${}^{\prime\prime}$, doubtful solution

• 2:

Circular profile fitted, other source exists at less that 2.5${}^{\prime\prime}$, doubtful solution

• 3:

Elliptical profile fitted, the position angle did not converge and one parameter or more is at the bound, Poor solution.

• 4:

Elliptical profile fitted, the position angle did not converge

• 5:

Elliptical profile fitted, one parameter or more is at the bound, Poor solution

• 6:

Elliptical profile fitted

• 7:

Circular profile fitted, one parameter or more is at the bound, Poor solution

• 8:

Circular profile fitted

posangle_de_vaucouleurs : Fitted position angle of the source for the de Vaucouleurs Profile (double, Angle[deg])

Position angle of the source for fitted de Vaucouleurs profile with respect to Celestial North Pole.

posangle_de_vaucouleurs_error : Error on the fitted position angle of the source for the de Vaucouleurs Profile (double, Angle[deg])

Uncertainty in the position angle of source with respect to Celestial North Pole for the de Vaucouleurs profile.

intensity_de_vaucouleurs : Fitted intensity of the source for the de Vaucouleurs Profile (double, Flux[e${}^{-}$ s${}^{-1}$])

Intensity of the source at the fitted effective radius (radius_de_vaucouleurs) for the de Vaucouleurs profile.

intensity_de_vaucouleurs_error : Error on the fitted intensity of the bulge for the de Vaucouleurs Profile (double, Flux[e${}^{-}$ s${}^{-1}$])

Uncertainty of the light intensity at the fitted effective radius (radius_de_vaucouleurs) of the source for the de Vaucouleurs profile.

radius_de_vaucouleurs : Fitted effective radius of the source for de Vaucouleurs Profile (double, Angle[mas])

Effective radius containing half of the total luminosity of the source bulge as obtained by morphological fitting for the de Vaucouleurs profile.

radius_de_vaucouleurs_error : Error on the fitted effective radius of the source for the de Vaucouleurs Profile (double, Angle[mas])

Uncertainty in the effective radius containing half of the total luminosity of the source for the de Vaucouleurs profile.

ellipticity_de_vaucouleurs : Fitted ellipticity of source for the de Vaucouleurs Profile (double)

Ellipticity (defined as 1-($b/a$) where $b/a$ is the axis ratio) of the source for the de Vaucouleurs profile.

ellipticity_de_vaucouleurs_error : Error on the fitted ellipticity of the source for the de Vaucouleurs Profile (double)

Uncertainty in the ellipticity (defined as 1-(b/a) where b/a is the axis ratio) of the source for the de Vaucouleurs profile.

l2_de_vaucouleurs : L2 norm for the de Vaucouleurs Profile (double)

This value represents the mean squared error between the integrated flux of all observed samples (from the Sky Mapper and Astrometric Field) and the integrated flux of synthetic samples produced with the fitted profile.

morph_params_corr_vec_de_vaucouleurs : Vector form of the upper triangle of the correlation matrix for the fitted parameters for the de Vaucouleurs Profile (double[21] array)

Vector form of the correlation matrix of the fitted profile parameters, as obtained by morphological fitting for the de Vaucouleurs profile, in the following order: [0]: position angle [1]: intensity [2]: radius [3]: ellipticity

Only non-zero, non-unity, correlation coefficients from the correlation matrix M are provided here. They are served as a linear array of size $S=n(n-1)/2$, where $n$ is the number of fitted parameters covered in the correlation matrix. The ordering of the elements in the linear array follows a column-major storage scheme, i.e.:

${\bf M}=\begin{bmatrix}1&C[0]&C[1]&C[3]&C[6]&\cdots&C[S-(n-1)]\\ &1&C[2]&C[4]&C[7]&\cdots&C[S-(n-2)]\\ &&1&C[5]&C[8]&\cdots&C[S-(n-3)]\\ &&&1&C[9]&\cdots&C[S-(n-4)]\\ &&&&\ddots&\ddots&\vdots\\ &&&&&1&C[S-1]\\ &&&&&&1\\ \end{bmatrix}$

flags_de_vaucouleurs : Flag indicative of processing or scientific quality for the morphological parameters fitting for the de Vaucouleurs Profile (byte)

This flag provides information about the processing or scientific quality of the results of the Galaxy morphology analysis chain for the source of interest for the de Vaucouleurs Profile.

The flag coding is the following. Note that not all flag cases apply to a given profile.

• 1:

Elliptical profile fitted, other source exists at less that 2.5${}^{\prime\prime}$, doubtful solution

• 2:

Circular profile fitted, other source exists at less that 2.5${}^{\prime\prime}$, doubtful solution

• 3:

Elliptical profile fitted, the position angle did not converge and one parameter or more is at the bound, Poor solution.

• 4:

Elliptical profile fitted, the position angle did not converge

• 5:

Elliptical profile fitted, one parameter or more is at the bound, Poor solution

• 6:

Elliptical profile fitted

• 7:

Circular profile fitted, one parameter or more is at the bound, Poor solution

• 8:

Circular profile fitted

source_selection_flags : Bit indicative of whether the input data from a given module met the source list eligibility criteria for the source of interest (int)

Bit indicative of whether the input data from a given module met the source list eligibility criteria for the source of interest.

The bit is coded as follows:

• bit 0: The source meets the eligibility criteria for the output of the Surface brightness analysis module (see Chapter 9).

• bit 1: The source belongs to the galaxy_catalogue_name table.

• bit 2: The source meets the eligibility criteria for the output of the classification module based on photometric lightcurves (see Chapter 10.3).

• bit 3: The source meets the eligibility criteria for the output of the DSC module (see Chapter 11.3.2).

• bit 4: The source meets the eligibility criteria for the redshift determined by the UGC module (see Chapter 11.3.13).

• bit 5: The source meets the eligibility criteria for the classification output of the UGC module (not yet applicable to DR3).