madbadgalaxyman
15-03-2014, 11:09 AM
In a recent post in the Visual Deep Sky forum, our eagle-eyed Paddy saw quite a lot of detail in this dwarf galaxy, on account of its nearness and its large angular diameter.
Here is Mad Galaxy Man's "quick and easy" description of this interesting, easy to observe, but much neglected galaxy.
_____________
NGC 625 is a nearby and very extended galaxy (in angular terms) and the surface brightness of its inner part is not low, which explains Paddy's success in teasing out significant details within NGC 625.
This dwarf galaxy is not well studied by visual observers or amateur imagers, but it has a very interesting structure that is worthy of prolonged observation.
This is a big galaxy, in angular terms, and it is associated with one of the nearest galaxy groups. The ESO-LV catalog of surface photometry of southern galaxies gives a total blue magnitude of 11.5 and a D25 standard Major Axis length of 6.5 arcminutes.
The diameter to the very faint D27 isophote is measured in this catalog as 11.66 arcminutes!!(though I wouldn't place much reliance on the D27 value)
NGC 625 is sometimes - but not always - included as a member of the Sculptor (south polar) group of galaxies. However, the Sculptor group is not actually a gravitationally bound group, because it is freely expanding with the expansion of the universe, so it is in reality a "cloud" or "association" of galaxies (that is, it is a noticeable overdensity of galaxies which is not grav. bound). Thus, the idea of this galaxy's membership in the Sculptor Cloud of galaxies must include membership in this overdensity, rather than the idea that it is gravitationally bound to N55, N300, N253 etc.
,
Here is a sky plot showing the Sculptor group and associated galaxies:
158658
Considering its redshift and and its angular proximity to the other galaxies, Sculptor Group membership makes sense for NGC 625. Certainly, it does not deserve its undue neglect by amateur astronomers!
This is a small galaxy in actual physical size, probably comparable in luminosity to the Small Magellanic Cloud...... for instance the (Karachentsev et al.) Updated Nearby Galaxy Catalog gives blue absolute magnitude Minus 16.4 for this galaxy.
I have recorded a published measurement of 3.89 Megaparsecs for the distance of this galaxy , but I don't know how accurate this estimate of 12.7 million light years is. Aha!!!! .....this is actually a TRGB distance estimate; so it should be reasonably accurate ( that is, distance should be accurate to within + or minus 10-15 percent.)
(astro-ph/ 0309542 published this distance estimate)
NED quotes another independent distance measurement for NGC 625, also using the TRGB method, and this distance determination is 4.21 Megaparsecs (= 13.7 million light years)
NGC 625 is sometimes called a 'dwarf starburst galaxy', indeed it has Wolf-Rayet features in its integrated optical spectrum, as its Star Formation Rate is high , at least compared to that of a typical Dwarf Irregular galaxy;
its total Star Fornation Rate of 0.05 solar masses per annum is good going for a small galaxy
(a table of Star Formation Rates, as calculated from UV emission and also from Ha emission, for 300 galaxies within 11 Mpc of the Milky Way, can be found in: 2009, ApJ, 706, 599 )
There is strong patchy H-alpha and Radio Continuum emission coming from NGC 625, as you would expect if there is a significant population of supermassive (OB) stars.......and the inner distribution of OB stars will elevate the surface brightness, making this an easier galaxy for visual observation than a lot of the more quiescent dwarf systems.
The Hubble class of this galaxy is indeterminate (a single Hubble Type is not uniquely assignable), as I have recently determined from my perusal of available imaging material;
its very-extended Old stellar distribution looks smooth and is oval in shape, resembling that found in a dwarf S0 galaxy. However, superposed on this smooth stellar spheroid is some relatively high-surface-brightness Knotty and Patchy blue material which is the most obvious sign of an ongoing burst of formation of hot young blue stars. Thus, the inner region of this galaxy more resembles a dwarf irregular galaxy.
If you look at Steve Crouch's image of this galaxy on his website, you get a sense of an elongated blue knotty region in the central region of this galaxy, surrounded by a smooth elliptical envelope.
To illustrate these morphological features within this galaxy, I attach here the color composite image of NGC 625 from the Carnegie-Irvine Survey of galaxies:
158659
Indeed, I read in the literature that this galaxy has had many and diverse individual Hubble Classifications assigned to it;
it has been classified as everything from a dwarf S0 to a Dwarf Irregular Galaxy to a Blue Compact Dwarf Galaxy (!), all of which strongly suggests to me that it is difficult or impossible to assign a single unique Hubble Type to this galaxy.
This confusion is simply a product of the fact that there are many specific dwarf galaxies that cannot readily be classified as one of the normal Hubble types, and that none of the the various variants of the Hubble system can be used to characterize dwarf galaxy morphology in any significant level of detail
The "amorphous" galaxy type (= galaxy class), as was described and used by Alan Sandage in the Carnegie Atlas of Galaxies and in the Revised Shapley-Ames Catalog of Bright Galaxies, was a good try at coming up with a Hubble-style class for these distinctive dwarf galaxies;
the "amorphous" classification captured these dwarf galaxies which have a lot of star formation but which also have the somewhat contradictory characteristic of a rather smooth stellar envelope.( a diffuse envelope and a blue knotty central region is characteristic of a good fraction of the dwarf galaxy population)
Indeed, it is not unusual for a dwarf galaxy to have a "mixed Hubble type" ;;
for instance NGC5102 is essentially an S0 galaxy but it has a blue central region where vigorous star formation occurred. Even the "familiar old M82" looks to be an old-stellar-population galaxy with a recent very-strong burst of star formation superposed upon it. Also within the set of dwarf starburst galaxies is the intriguing and arguably S0/Irr galaxy NGC 5253 (note the assignment of a dual and seemingly contradictory Hubble Type!)
It deserves to be noted here that the distinction between the Dwarf Irregular and dwarf Elliptical/S0 galaxy types is not always very significant for Low Luminosity galaxies;
for instance, some specific Dwarf Irregular galaxies are gradually fading into a Dwarf Elliptical galaxy (as an example, the nearby Phoenix Dwarf Galaxy is a transition type between Irregular and dwarf elliptical.)
The HI (= neutral atomic hydrogen gas) content of NGC 625 is quite significant in amount , at least for a small galaxy like this one, indicating that this galaxy might be best thought of as a dwarf irregular galaxy rather than as a dwarf S0 galaxy; but could this interstellar gas have been captured by NGC 625 from an external source?!?
Cannon et al ( 2004, ApJ, 607, 274 ) found a total HI (cold atomic hydrogen) mass of 110 million solar masses of gas that is associated with this little galaxy, so there is plenty of raw material for ongoing star formation in this galaxy. However, its rotating disk of HI is disturbed and the HI gas disk is very small in diameter (comparable to the optical diameter), as was found in radio observations by Lelli & Verheijen & Fraternali in 2003, which suggests to me that this is not one of those dwarf galaxies with a vast and massive HI halo that is able to provide raw materials for star formation to continue for billions of years into the future.
(see the recent PhD thesis of Federico Lelli, for more about the current state, and dynamics, and the future evolution, of the gas content and star formation activity in dwarf galaxes; www.astro.rug.nl/~lelli/ (http://www.astro.rug.nl/~lelli/) )
[[ madbadgalaxyman's further commentary:
the presence of low column density HI gas extending a long way outwards from the optically bright portion of NGC 625 cannot be excluded as a possibility, because HI observations, till the last few years, have been hard pressed to detect even large amounts of very-low-density gas in the outermost parts of galaxies ]]
Here is Mad Galaxy Man's "quick and easy" description of this interesting, easy to observe, but much neglected galaxy.
_____________
NGC 625 is a nearby and very extended galaxy (in angular terms) and the surface brightness of its inner part is not low, which explains Paddy's success in teasing out significant details within NGC 625.
This dwarf galaxy is not well studied by visual observers or amateur imagers, but it has a very interesting structure that is worthy of prolonged observation.
This is a big galaxy, in angular terms, and it is associated with one of the nearest galaxy groups. The ESO-LV catalog of surface photometry of southern galaxies gives a total blue magnitude of 11.5 and a D25 standard Major Axis length of 6.5 arcminutes.
The diameter to the very faint D27 isophote is measured in this catalog as 11.66 arcminutes!!(though I wouldn't place much reliance on the D27 value)
NGC 625 is sometimes - but not always - included as a member of the Sculptor (south polar) group of galaxies. However, the Sculptor group is not actually a gravitationally bound group, because it is freely expanding with the expansion of the universe, so it is in reality a "cloud" or "association" of galaxies (that is, it is a noticeable overdensity of galaxies which is not grav. bound). Thus, the idea of this galaxy's membership in the Sculptor Cloud of galaxies must include membership in this overdensity, rather than the idea that it is gravitationally bound to N55, N300, N253 etc.
,
Here is a sky plot showing the Sculptor group and associated galaxies:
158658
Considering its redshift and and its angular proximity to the other galaxies, Sculptor Group membership makes sense for NGC 625. Certainly, it does not deserve its undue neglect by amateur astronomers!
This is a small galaxy in actual physical size, probably comparable in luminosity to the Small Magellanic Cloud...... for instance the (Karachentsev et al.) Updated Nearby Galaxy Catalog gives blue absolute magnitude Minus 16.4 for this galaxy.
I have recorded a published measurement of 3.89 Megaparsecs for the distance of this galaxy , but I don't know how accurate this estimate of 12.7 million light years is. Aha!!!! .....this is actually a TRGB distance estimate; so it should be reasonably accurate ( that is, distance should be accurate to within + or minus 10-15 percent.)
(astro-ph/ 0309542 published this distance estimate)
NED quotes another independent distance measurement for NGC 625, also using the TRGB method, and this distance determination is 4.21 Megaparsecs (= 13.7 million light years)
NGC 625 is sometimes called a 'dwarf starburst galaxy', indeed it has Wolf-Rayet features in its integrated optical spectrum, as its Star Formation Rate is high , at least compared to that of a typical Dwarf Irregular galaxy;
its total Star Fornation Rate of 0.05 solar masses per annum is good going for a small galaxy
(a table of Star Formation Rates, as calculated from UV emission and also from Ha emission, for 300 galaxies within 11 Mpc of the Milky Way, can be found in: 2009, ApJ, 706, 599 )
There is strong patchy H-alpha and Radio Continuum emission coming from NGC 625, as you would expect if there is a significant population of supermassive (OB) stars.......and the inner distribution of OB stars will elevate the surface brightness, making this an easier galaxy for visual observation than a lot of the more quiescent dwarf systems.
The Hubble class of this galaxy is indeterminate (a single Hubble Type is not uniquely assignable), as I have recently determined from my perusal of available imaging material;
its very-extended Old stellar distribution looks smooth and is oval in shape, resembling that found in a dwarf S0 galaxy. However, superposed on this smooth stellar spheroid is some relatively high-surface-brightness Knotty and Patchy blue material which is the most obvious sign of an ongoing burst of formation of hot young blue stars. Thus, the inner region of this galaxy more resembles a dwarf irregular galaxy.
If you look at Steve Crouch's image of this galaxy on his website, you get a sense of an elongated blue knotty region in the central region of this galaxy, surrounded by a smooth elliptical envelope.
To illustrate these morphological features within this galaxy, I attach here the color composite image of NGC 625 from the Carnegie-Irvine Survey of galaxies:
158659
Indeed, I read in the literature that this galaxy has had many and diverse individual Hubble Classifications assigned to it;
it has been classified as everything from a dwarf S0 to a Dwarf Irregular Galaxy to a Blue Compact Dwarf Galaxy (!), all of which strongly suggests to me that it is difficult or impossible to assign a single unique Hubble Type to this galaxy.
This confusion is simply a product of the fact that there are many specific dwarf galaxies that cannot readily be classified as one of the normal Hubble types, and that none of the the various variants of the Hubble system can be used to characterize dwarf galaxy morphology in any significant level of detail
The "amorphous" galaxy type (= galaxy class), as was described and used by Alan Sandage in the Carnegie Atlas of Galaxies and in the Revised Shapley-Ames Catalog of Bright Galaxies, was a good try at coming up with a Hubble-style class for these distinctive dwarf galaxies;
the "amorphous" classification captured these dwarf galaxies which have a lot of star formation but which also have the somewhat contradictory characteristic of a rather smooth stellar envelope.( a diffuse envelope and a blue knotty central region is characteristic of a good fraction of the dwarf galaxy population)
Indeed, it is not unusual for a dwarf galaxy to have a "mixed Hubble type" ;;
for instance NGC5102 is essentially an S0 galaxy but it has a blue central region where vigorous star formation occurred. Even the "familiar old M82" looks to be an old-stellar-population galaxy with a recent very-strong burst of star formation superposed upon it. Also within the set of dwarf starburst galaxies is the intriguing and arguably S0/Irr galaxy NGC 5253 (note the assignment of a dual and seemingly contradictory Hubble Type!)
It deserves to be noted here that the distinction between the Dwarf Irregular and dwarf Elliptical/S0 galaxy types is not always very significant for Low Luminosity galaxies;
for instance, some specific Dwarf Irregular galaxies are gradually fading into a Dwarf Elliptical galaxy (as an example, the nearby Phoenix Dwarf Galaxy is a transition type between Irregular and dwarf elliptical.)
The HI (= neutral atomic hydrogen gas) content of NGC 625 is quite significant in amount , at least for a small galaxy like this one, indicating that this galaxy might be best thought of as a dwarf irregular galaxy rather than as a dwarf S0 galaxy; but could this interstellar gas have been captured by NGC 625 from an external source?!?
Cannon et al ( 2004, ApJ, 607, 274 ) found a total HI (cold atomic hydrogen) mass of 110 million solar masses of gas that is associated with this little galaxy, so there is plenty of raw material for ongoing star formation in this galaxy. However, its rotating disk of HI is disturbed and the HI gas disk is very small in diameter (comparable to the optical diameter), as was found in radio observations by Lelli & Verheijen & Fraternali in 2003, which suggests to me that this is not one of those dwarf galaxies with a vast and massive HI halo that is able to provide raw materials for star formation to continue for billions of years into the future.
(see the recent PhD thesis of Federico Lelli, for more about the current state, and dynamics, and the future evolution, of the gas content and star formation activity in dwarf galaxes; www.astro.rug.nl/~lelli/ (http://www.astro.rug.nl/~lelli/) )
[[ madbadgalaxyman's further commentary:
the presence of low column density HI gas extending a long way outwards from the optically bright portion of NGC 625 cannot be excluded as a possibility, because HI observations, till the last few years, have been hard pressed to detect even large amounts of very-low-density gas in the outermost parts of galaxies ]]