Strange object

• by CeciliaB

  Just above the edgeon there is a faint , blueish 'cloud'. SDSS calls it a galaxy. But the PhotoZ puzzles me: -3.403E-3 zErr 0.085. Does the minus sign mean blueshift? Could the cloud be a very distant early galaxy or does that high z mean something else?

  Posted November 7, 2016 4:53 PM

• by Ghost_Sheep_SWR

  SDSS DR13 gives a PhotoZ of z=0.972242 zErr= 0.073225 but a lot of red flags. (photoErrorClass = -7)

  Link: http://skyserver.sdss.org/dr13/en/tools/explore/DisplayResults.aspx?&name=PhotoZ&id=1237655499736285639&spec=&apid=&fieldId=0x112d091ea0170000&ra=199.356495078475&dec=-1.76362184662329&plateId=

  Looks much more massive in GALEX though:

  http://galex.stsci.edu/GR6/?page=explore&objid=6378832288601349378&photo=False&spec=False

  Posted November 7, 2016 6:25 PM

• by CeciliaB

  Thanks! Funny how the PhotoZ could be so different in DR9 and DR13!

  By the look of it, it could perhaps be a low surface brightness dwarf galaxy. It doesn't have much of a bulge and only a small stellar content.

  Posted November 7, 2016 9:15 PM

• by Ghost_Sheep_SWR in response to CeciliaB's comment.

  Yes it is funny, I have no clue what the difference in PhotoZ method is though, someone else?

  Low surface brightness (dwarf) galaxy crossed my mind too, but probably as almost always no definitive answer without spectral redshift, more data in NED / SIMBAD or references 😕

  Posted November 7, 2016 9:56 PM

• by Budgieye moderator

  Ooops, I made an error. Here is the edited number.

  The first negative sign appears to make this a negative number. So the galaxy is possibly coming towards us.

  The second negative sign is a way of expressing small numbers. Move the decimal 3 to the left to get the actual value.

  z -3.403E-3
  zErr 0.085121

  or -0.003 error of 0.085 the error is bigger than the number. The redshift is effectively zero. So it may not be coming towards us after all.

  ---

  redshift z 0.048 0.00001

  photoz 0.028584 zErr 0.01034

  

  http://skyservice.pha.jhu.edu/DR9/ImgCutout/getjpeg.aspx?ra=199.36009908&dec=-1.7686768&scale=0.2&width=200&height=200&opt=G

  http://skyserver.sdss.org/dr9/en/tools/explore/obj.asp?ra=199.36015410321707&dec=-1.7687318013991844

  

  http://skyserver.sdss.org/dr9/en/get/specById.asp?id=382980549184088064

  ---

  

  http://skyservice.pha.jhu.edu/DR9/ImgCutout/getjpeg.aspx?ra=199.35649508&dec=-1.76362183&scale=0.2&width=200&height=200&opt=G

  z -3.403E-3
  zErr 0.085121

  or -0.003 er 0.085

  Posted November 7, 2016 10:11 PM

• by Budgieye moderator

  Photoz is an algorithm to guess what an object is by the relative amounts of each colour. We do much the same with our eyes. Photoz always has large errors attached, sometimes the error margins are as large as the distance away. Sometimes, it can be right on, and sometimes it can be wildly wrong.

  Posted November 8, 2016 4:56 AM

• by Budgieye moderator

  But I agree that the blue cloud is a nearby low surface brightness LSB object.

  Posted November 8, 2016 4:57 AM

• by CeciliaB

  Interesting! Thank you!

  Posted November 8, 2016 9:40 AM