I Uncertainty Over Sextans Galaxies' Group Membership

[Si_187]

Hi all. Awesome site! Just wondering if anyone can answer my question:

If the Sextans galaxies are inside the group's zero velocity surface, why is there uncertainty over whether they're part of the group?

 

[Ophiolite]

Hi all. Awesome site! Just wondering if anyone can answer my question:

If the Sextans galaxies are inside the group's zero velocity surface, why is there uncertainty over whether they're part of the group?

Could you point to an example of where such uncertainty is expressed?

 

[Si_187]

Could you point to an example of where such uncertainty is expressed?

The opening paragraphs on the Wikipedia articles for Sextans A and B both describe them as possibly being part of the local group.

Interestingly there is a similar uncertainty over the Saggitarius Dwarf Irregular Galaxy, despite that lying outside the zero-velocity surface.

Perhaps the zero-velocity surface is only a rough estimate or not a concrete theorem?

 

[Drakkith]

If the Sextans galaxies are inside the group's zero velocity surface, why is there uncertainty over whether they're part of the group?

Hmmm. One source I found states that they are significantly outside the zero-velocity surface.
From here: https://www.universetoday.com/27388/turning-the-tides-ngc-3109-by-ken-crawford/

What makes NGC 3109 and its little band of followers so interesting? Well, chances are it may not be a member of our Local Group at all, but the nearest of the outsiders. “The small Antlia-Sextans clustering of galaxies is located at a distance of only 1.36 Mpc from the Sun and 1.72 Mpc from the adopted barycenter of the Local Group. The latter value is significantly greater than the radius of the zero-velocity surface of the Local Group that, for an assumed age of 14 Gyr, has R_0=1.18+/-0.15 Mpc.” says Sidney Van den Bergh, “This, together with the observation that the members of the Ant-Sex group have a mean redshift of 114+/-12 km s^-1 relative to the centroid of the Local Group, suggests that the Antlia-Sextans group is not bound to our Local Group and that it is expanding with the Hubble flow. If this conclusion is correct, then Antlia-Sextans may be the nearest external clustering of galaxies.”

 

[Si_187]

Ah ok yeah, incorrect assumption from me to begin with that they were within the zero-velocity surface. Along with them being strongly red-shifted (at least Saggitarius Dwarf Irregular is blue-shifted) it seems kinda odd that they would ever be considered within the local group then.

This zero-velocity surface business sounds fascinating to me. Especially having just read the Three Body Problem :)

 

[Ophiolite]

The opening paragraphs on the Wikipedia articles for Sextans A and B both describe them as possibly being part of the local group.

Interestingly there is a similar uncertainty over the Saggitarius Dwarf Irregular Galaxy, despite that lying outside the zero-velocity surface.

Perhaps the zero-velocity surface is only a rough estimate or not a concrete theorem?

Thanks. Since you didn't specify Sextans A and B I mistakenly thought you were referencing the Sextans galaxy that I understand is a satellite of ours.

 

[Si_187]

Thanks. Since you didn't specify Sextans A and B I mistakenly thought you were referencing the Sextans galaxy that I understand is a satellite of ours.

Wow didn't even realize we had a satellite Sextans dwarf galaxy, there's that many dwarf galaxies knocking about.

Its funny, this topic is no doubt considered niche in the physics community, but we're talking about the very frontier that humanity can ever reach considering the cosmological horizon.

 

[CygnusX-1]

Here's the current situation regarding the Local Group. As of early 2019, the Local Group had more than 100 known members.

The following galaxies ARE members of the Local Group:

Sagittarius Dwarf Spheroidal (discovered in 1994; a satellite galaxy of the Milky Way)
Sagittarius Dwarf Irregular Galaxy (NOT a satellite of any galaxy)
Leo I (discovered in 1950; a satellite galaxy of the Milky Way)
Leo II (discovered in 1950; a satellite galaxy of the Milky Way)
Leo A (NOT a satellite of any galaxy)
Sextans Dwarf Spheroidal (discovered on February 8, 1990; a satellite galaxy of the Milky Way)

The following galaxies are NOT members of the Local Group but instead belong to the NGC 3109 Association:

Sextans A
Sextans B
Leo P (discovered in 2012)

References:

Magnificent Universe by Ken Croswell, pages 190-191.

"The Faintest Dwarf Galaxies" by Joshua Simon, Annual Reviews, 2019, in press. Table 1 lists 54 of the 57 known satellite galaxies of the Milky Way (it omits the two most massive, the Large and Small Magellanic Clouds, and also the most recent discovery, Antlia 2).

"The Observed Properties of Dwarf Galaxies in and Around the Local Group" by Alan McConnachie, The Astronomical Journal, 144, 4 (2012). Table 1 gives the boundary between Local Group members and those beyond.

Sky and Telescope, October 2018, cover story.

The Alchemy of the Heavens by Ken Croswell. Pages vii-ix tell the story of the discovery of the Sextans dwarf that orbits the Milky Way.

 

[stefan r]

..., but we're talking about the very frontier that humanity can ever reach considering the cosmological horizon.

Humanity has no such limits. People have not left Earth's Hill sphere yet but that is not a barrier that is impossible to cross. Maffei and the Virgo cluster require higher delta-v and much longer flight times but it is just more difficult not impossible.

 

[Si_187]

Humanity has no such limits. People have not left Earth's Hill sphere yet but that is not a barrier that is impossible to cross. Maffei and the Virgo cluster require higher delta-v and much longer flight times but it is just more difficult not impossible.

People haven't but man-made objects have gone way beyond Earth's hill sphere/zero-velocity limit.

The argument regarding humans never escaping the local group is based around objects outside it receeding from us faster than the speed of light by the time we could get to the edge of the group from the present day.

So if we can't travel faster than light (a very hard limit by most reckoning), then we'll never get to anything that's currently outside the local group. That's my understanding anyway, I may be wrong.

 

[Si_187]

Here's the current situation regarding the Local Group. As of early 2019, the Local Group had more than 100 known members.

The following galaxies ARE members of the Local Group:

Sagittarius Dwarf Spheroidal (discovered in 1994; a satellite galaxy of the Milky Way)
Sagittarius Dwarf Irregular Galaxy (NOT a satellite of any galaxy)
Leo I (discovered in 1950; a satellite galaxy of the Milky Way)
Leo II (discovered in 1950; a satellite galaxy of the Milky Way)
Leo A (NOT a satellite of any galaxy)
Sextans Dwarf Spheroidal (discovered on February 8, 1990; a satellite galaxy of the Milky Way)

The following galaxies are NOT members of the Local Group but instead belong to the NGC 3109 Association:

Sextans A
Sextans B
Leo P (discovered in 2012)

References:

Magnificent Universe by Ken Croswell, pages 190-191.

"The Faintest Dwarf Galaxies" by Joshua Simon, Annual Reviews, 2019, in press. Table 1 lists 54 of the 57 known satellite galaxies of the Milky Way (it omits the two most massive, the Large and Small Magellanic Clouds, and also the most recent discovery, Antlia 2).

"The Observed Properties of Dwarf Galaxies in and Around the Local Group" by Alan McConnachie, The Astronomical Journal, 144, 4 (2012). Table 1 gives the boundary between Local Group members and those beyond.

Sky and Telescope, October 2018, cover story.

The Alchemy of the Heavens by Ken Croswell. Pages vii-ix tell the story of the discovery of the Sextans dwarf that orbits the Milky Way.

That's awesome, thankyou!

 

[stefan r]

People haven't but man-made objects have gone way beyond Earth's hill sphere/zero-velocity limit.

The argument regarding humans never escaping the local group is based around objects outside it receeding from us faster than the speed of light by the time we could get to the edge of the group from the present day.

So if we can't travel faster than light (a very hard limit by most reckoning), then we'll never get to anything that's currently outside the local group. That's my understanding anyway, I may be wrong.

The Virgo Cluster is 16.5 Mparsec. The Hubble constant is around 70 km/s/Mparsec. So space expands at 1,155 km/s. That is 0.385% of light speed. If a star leaves the milky way at 2% of light speed it will eventually get there.
The Virgo Cluster will eventually cross the event horizon and then we will not hear any more messages from our colonies there.

 

[davenn]

So if we can't travel faster than light (a very hard limit by most reckoning), then we'll never get to anything that's currently outside the local group. That's my understanding anyway, I may be wrong.

We won't get to anywhere significant within our own galaxy, let alone out of our galaxy to the nearest neighbour