how to retrieve specific _source inner fields

gistfile1.txt

$ curl -XPUT 'http://localhost:9200/features/Feature/Pfalciparum_REP_45?pretty=true' -d'{
	"uniqueName":"Pfalciparum_REP_45",
	"coordinates":[{"region":"Pf3D7_01","fmin":641897,"fmax":642097,"phase":null,"strand":0}]
}'


{
  "ok" : true,
  "_index" : "features",
  "_type" : "Feature",
  "_id" : "Pfalciparum_REP_45",
  "_version" : 1
}

$ curl -XGET 'http://localhost:9200/features/Feature/Pfalciparum_REP_45?pretty=true'

{
  "_index" : "features",
  "_type" : "Feature",
  "_id" : "Pfalciparum_REP_45"
}

$ curl -XGET 'http://localhost:9200/features/Feature/Pfalciparum_REP_45?pretty=true&refresh=true&fields=_source'

{
  "_index" : "features",
  "_type" : "Feature",
  "_id" : "Pfalciparum_REP_45",
  "_version" : 1, "_source" : {
"uniqueName":"Pfalciparum_REP_45",
"coordinates":[{"region":"Pf3D7_01","fmin":641897,"fmax":642097,"phase":null,"strand":0}]
}
}

$ curl -XGET 'http://localhost:9200/features/Feature/Pfalciparum_REP_45?pretty=true&refresh=true&fields=_source.coordinates'

{
  "error" : "ElasticSearchException[No mapping for field [_source.coordinates] in type [Feature]]"
}