Galaxy Zoo Talk

Faint galaxies are Dwarf Ellipticals?

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    I came across this faint pair in DECaLS, and they both are classified as dwarf ellipticals. They have no spectrum, only PhotoZ with a huge error margin. If i assume this PhotoZ value is correct they actually look like really big, really faint galaxies.

    Now i'm curious as to how they established them as dwarfs without a spectrum redshift value. I've searched NED and ADS but can't find out on what basis their distance and size was calculated, or is there a chance they're not dwarf ellipticals?

    enter image description here

    LEDA 42186 / VCC 1735
    PhotoZ 0.111 ZErr 0.0379
    http://legacysurvey.org/viewer?ra=189.4697&dec=2.1803&zoom=14&layer=decals-dr2

    http://skyserver.sdss.org/dr12/en/tools/explore/Summary.aspx?id=1237651753477604101

    enter image description here

    LEDA 42187 VCC 1734
    PhotoZ 0.076 ZErr 0.0692
    http://legacysurvey.org/viewer?ra=189.4744&dec=2.1494&zoom=14&layer=decals-dr2

    http://skyserver.sdss.org/dr12/en/tools/explore/Summary.aspx?id=1237651753477603671

    Here are some nearby galaxies with the same zoom factor (zoom 14) and roughly the same z value for comparison in size.

    enter image description here

    enter image description here

    enter image description here

    EDIT:

    This study from 1985 seems to be the source of the classification, mainly based on luminosity. Not sure if it was possible then to reach the magnitude limit of DECaLS?

    http://adsabs.harvard.edu/abs/1985AJ.....90.1681B

    Posted

  • JeanTate by JeanTate in response to Ghost_Sheep_SWR's comment.

    My $0.02's worth:

    • the "VCC" may be a good clue, Virgo Cluster Catalog perhaps? If so, then some VCC papers probably discuss whether galaxies like this are Virgo Cluster members or not
    • where do these galaxies fit, on a color-color diagram? Where are dwarf ellipticals usually found in such diagrams?
    • ditto, for a WISE color-color diagram
    • do they have a 'mottled' appearance (rather large surface brightness variations)? If so, they're likely fairly close (z < 0.05?) There's an extensive literature on using surface brightness fluctuations as an indicator distance ...
    • how do they compare, in appearance, with known dwarf ellipticals (in DECaLS, and SDSS)?

    Hope this helps, and happy hunting! 😃

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR in response to JeanTate's comment.

    Seems just like a bit too specialistic for a simple citizen scientist, although finding similar dwarf galaxies in DECaLS seems reasonable. Unsuccesfull getting them out of NED so far, but i'm sure i should be able to manage to find some.

    Perhaps better to not meddle with the professional stuff too much without proper education 😃

    Thanks

    Posted

  • Budgieye by Budgieye moderator

    I think Zooites are good at this, because computers can't find these faint galaxies very easily.

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    They did find them indeed searching for Virgo Cluster members; the issue i am trying to adress / answer here is that as far as i can tell the latest update on these was in 2014 based on SDSS.

    By eye these two are almost too faint for SDSS, and barely visible completely in DECaLS. Just thinking out loud if the complete galaxies are taken into account when classifying them as Dwarf Ellipticals, since only DECaLS has a just low enough magnitude.

    Finding some more similar galaxies at around Virgo Cluster redshift (0.04?) seems the logical next step for comparison.

    Just unsure how to formulate such a request in NED / SDSS

    Posted

  • JeanTate by JeanTate

    @Ghost_Sheep_SWR:

    By eye these two are almost too faint for SDSS, and barely visible completely in DECaLS.

    Surprisingly, these galaxies are not all that faint! 😮

    What property they do have which makes them 'hard to see' is a low surface brightness, making them LSB galaxies.

    Finding some more similar galaxies at around Virgo Cluster redshift (0.04?) seems the logical next step for comparison.

    You might like to read the third post in the "Spiral galaxies (and why Galaxy Zoo is perfect for their study)" - new GZ blog post thread; I have links to a number of recent studies which have found surprisingly many LSBs, and I'm not sure they can all be easily classified as dwarf ellipticals.

    Just unsure how to formulate such a request in NED / SDSS

    If you do find a way, would you please let us know? Such a thing would be very cool!

    Hope this helps, and happy hunting! 😃

    Posted

  • mlpeck by mlpeck

    Finding some more similar galaxies at around Virgo Cluster redshift
    (0.04?) seems the logical next step for comparison.

    Just unsure how to formulate such a request in NED / SDSS

    There are actually two dE's (just) visible in the vicinity at SDSS Navigate's default zoom level, and they're somewhat more easily seen in Decals:

    enter image description here

    M87 has, from memory, a redshift of z ≈ 0.004, but redshifts aren't that helpful because the Local Group is part of the greater Virgo supercluster and therefore is falling into the same potential well.

    http://adsabs.harvard.edu/abs/1985AJ.....90.1681B

    Once you have a relevant ADS abstract clicking on links is a good way to learn more. Clicking the online data link for example will take you to the Vizier catalog (a hint for this particular one: since it was published in 1985 coordinates are for 1950 and in hex. Using the buttons on the left you can calculate epoch 2000 coordinates in decimal degrees for easier cross-referencing with SDSS or Decals).

    You can also read the paper, which will tell you how they collected the data (plates!), how they analyzed it (by eye!), and how they determined morphology and potential membership in Virgo.

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Argh yes redshift ~0.004 8)

    I did read a large part of it and the Du Pont plates being scrutinized visually, the two dE's you mention are the ones from the first comment.

    I've now found a newer update study (EVCC) based mainly on SDSS ( still visually determined) to see if i can find out more. The fact that they are somewhat more easily seen in DECaLS is what i find interesting here.

    Seems they did't publish the entire catalogue in the paper (online), but their size estimates in arcseconds should be of use.

    If nothing else it's educational 😃 thanks

    Posted

  • KWillett by KWillett scientist, admin, translator

    Finding smaller and dimmer galaxies like these is intended to be one of the strengths of DECaLS; it really does go deep. It'll be quite useful to see how many dEs can be recovered from our classifications!

    Posted

  • mlpeck by mlpeck in response to Ghost_Sheep_SWR's comment.

    I didn't read your OP carefully enough and missed that you flagged both of those galaxies. Sorry!

    For the EVCC try http://vizier.cfa.harvard.edu/viz-bin/VizieR-3?-source=J/ApJS/215/22/.

    And for a very recent effort at automated source detection there's http://arxiv.org/abs/1507.01723.

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR in response to mlpeck's comment.

    That's great mlpeck!

    'Probing the Low Surface Brightness Dwarf Galaxy Population of the Virgo Cluster 2015' is even beyond the EVCC 2014, and especially aimed at Virgo Cluster membership of Dwarf galaxy population 😃

    I'll dive into it when i find quality (free) time

    Thanks!

    Posted

  • JeanTate by JeanTate in response to Ghost_Sheep_SWR's comment.

    You may also find Buta (2011) of interest: Section 14 is entitled "Giant Low Surface Brightness Galaxies"

    Hope this helps, and happy hunting! 😃

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Yes i have that in my little digital library 😃 , unaware of section 14 though

    Off-topic comment; also usefull for clarifying ( of sorts) X-shaped edge-on bulges, and ( i didn't know) a clear distinction between polar rings and dust-lane ellipticals.

    Just started with first steps SDSS querying too, mighty interesting!

    Thanks!

    EDIT

    Hypothesis : Galaxies are not Dwarf Ellipticals

    Questions to be answered to formulate a conclusion, taking one of the two (LEDA 42186 / VCC 1735 / dE0) since they look equal in size and morphology

    • how big are dE's (max)?
    • does their given size in arcsecs equal their size in DECaLS in arcsecs?
    • what is their size when considered Virgo Cluster members, at z=0.004?
    • calculate maximum z distance for maximum Dwarf size
    • compare visually with other dE's with different z values for comparison of morphology and size (NED)
    • how big are dE's (max)?

    Dwarf Elliptical galaxies appear to have many of the same global properties observed in normal elliptical galaxies, just on a smaller scale. They are elliptical in shape, contain very little or no gas, and have no evidence of recent star formation. They contain 107 to 109 solar masses of material, have diameters of 1 – 10 kiloparsecs and luminosities that range from about 105 to 107 solar.

    http://astronomy.swin.edu.au/cosmos/D/dwarf+galaxy

    • does their given size in arcsecs equal their size in DECaLS in arcsecs?

    Major Diameter (arcmin) : 0.22 (13.2 arcsec)
    Minor Diameter (arcmin). : 0.22. (13.2 arcsec)

    http://ned.ipac.caltech.edu/cgi-bin/objsearch?search_type=Obj_id&objid=66186&objname=1&img_stamp=YES&hconst=73.0&omegam=0.27&omegav=0.73&corr_z=1

    Posted