madbadgalaxyman
30-03-2015, 04:53 PM
ESTIMATING THE DISTANCE & LUMINOSITY OF NGC 1097 & NGC 2997
by madbadgalaxyman
My previous thread on optimizing visual SN Search isolated these galaxies as excellent candidates for visual supernova search, but galaxy distance was a major unknown factor in that discussion. Unfortunately, the literature shows that the distances of these two Bright southern galaxies are not reliably known with an uncertainty as small as +/- 10 percent (to one sigma), as the available distance estimates for these galaxies use distance determination methods that are not known for their high accuracy:
- the Tully-Fisher Relation
- the redshift (velocity) distance, corrected for non-cosmological motions of galaxies
For those galaxies within a recession velocity of 2000 km/s., there could be an uncertainty as large as 25 percent for an individual distance estimate that was made using one of these methods, so attempts must be made to reduce the distance error by combining a number of (individually inaccurate) distance estimates for a single galaxy.
________________________________
THE DISTANCE OF NGC 2997
Distance Determinations in the Literature:
mu = 30.16
Distance = 10.8 Megaparsecs
Method: distance from the recession velocity, using Vlg and a Hubble Constant of 73
mu = 30.43
Distance = 12.2 Mpc
Method: Tully-Fisher
Source: Hess et al., 2009, ApJ, 699, 76
mu = 30.20
Distance = 10.96 Megaparsecs
Method: Mean of 5 Tully-Fisher estimates
Source: NED
mu = 30.26
Distance = 11.3 Megaparsecs
Source: distance of NGC 2997 galaxy group (using the average modulus for the group)
(see: Extragalactic Distance Database ; http://edd.ifa.hawaii.edu/dfirst.php )
mu = 29.96
Distance = 9.8 Mpc
Method: Tully-Fisher (mean of the determinations for the group galaxies)
Source : Extragalactic Distance Database
Conclusions:
I have decided not to use the mean distance from NED; the scatter of the data points used to find the mean is very great, so there could be some suspect data in there.
The velocity distance must be given significant weight.
The recent distance determinations in the Extragalactic Distance Database and by Hess et al. should be given some weight. I assume that they are recent and careful calibrations.
Mean of four distance determinations: 11 Megaparsecs
Mean distance modulus: 30.20
The distance of 11 Megaparsecs (36 million light years) for NGC 2997 that is derived here is similar to its distance as derived from its velocity (redshift), which increases confidence in this estimate. However, the individual uncertainty of each data point is large, and an incorrect extinction value for the NGC 2997 field could introduce significant uncertainties in this distance estimate, as foreground extinction (from our own Galaxy) should ideally be carefully measured for the field. Also, there is a desperate need for a cross-check of this distance with more accurate distance determination methods; such as TRGB or Cepheids.
The apparent magnitude and the Luminosity of NGC 2997 :
I have extracted 5 apparent magnitude measurements of the total magnitude of NGC 2997, from various catalogs of galaxies, all of them in the range B (blue) magnitude 9.79 to 10.06, so at least the apparent brightness of this galaxy is quite well constrained. I am not aware of any individual surface photometry of this galaxy, apart from the Good Quality photographic surface photometry in the ESOLV catalog. The mean of these 5 published magnitude values is B = 9.94 magnitude, which indicates that this galaxy could well be a little brighter than 10th magnitude. The careful UBVRI aperture photometry of Prugniel and Heraudeau (A&AS, 128, 299) gives B = 9.87 magnitude for NGC 2997, which is very plausible.
Assuming that this galaxy has an apparent B (Blue) magnitude of 9.9 and then applying a distance modulus of 30.2 yields an absolute Blue magnitude of minus 20.3
This luminosity estimate has significant uncertainties...... due to poorly constrained values of distance & apparent magnitude & extinction.
___________________________________ __
___________________________________ __
THE DISTANCE OF NGC 1097
Distance Determinations in the literature:
mu = 31.07
distance = 16.4 Megaparsecs
Source: Redshift distance, using Velocity (Local Group frame) and a Hubble Constant of 73
mu = 30.81
distance = 14.5 Megaparses
Method: Velocity distance, based on a Hubble Constant of 75 km/s/Megaparsec and a model that describes velocity perturbations in the Vicinity of the Virgo Cluster.
Source: Nearby Galaxies Catalog
mu =30.76
distance = 14.2 Megaparsecs
Method: Tully-Fisher
Source: Extragalactic Distance Database ; http://edd.ifa.hawaii.edu/dfirst.php )
mu = 31.02
distance = 16.0 Megaparsecs
Source: from Cosmic Flows 2
(see: Extragalactic Distance Database ; http://edd.ifa.hawaii.edu/dfirst.php )
mu = 31.12
distance = 16.7 Megaparsecs
Method: Tully-Fisher
Source: Mark III Catalog of Galaxy Peculiar Velocities (Willick et al.)
mu = 31.15
distance = 17.0 Megaparsecs
Source: Y.Sofue (1991, PASJ, 43, 671), from CO Ring diameter.
Conclusions:
The several separate Tully-Fisher Method determinations of this galaxy's distance are based on very similar data on the maximum rotation velocity of NGC 1097, and the data is manipulated in much the same way in all of these determinations, moreover with only minor differences in calibration. So there is not much in the way of external checking of these T-F distance determinations by means of other methods of distance estimation. The T-F determinations do, at least, derive similar distances to those which are derived from the recession velocity of NGC 1097. Needed are one or more distance determinations using methods of higher accuracy, such as TRGB and Cepheids.
Mean of 6 distance determinations : 15.8 Megaparsecs (51 million light years)
Mean distance modulus: 30.99
The apparent magnitude and the Luminosity of NGC 1097 :
There are substantial discrepancies between the various measurements of the total B (blue) apparent magnitude of NGC 1097 that are found in the literature. When the uncertainty in apparent brightness is compounded with the distance uncertainty, any consequent estimate of the total luminosity of NGC 1097 will be more in the nature of a ballpark figure. This is a very extended galaxy in angular terms (it is over 10 arcminutes along its major axis), so large-format CCD measurements could measure the magnitude with more accuracy.
I am going to exclude the total apparent magnitude of 10.39 as given in the UBVRI aperture photometry of Prugniel and Heraudeau (A&AS, 128, 299) from the mean magnitude, as this is a statistical outlier.
Some B magnitude measurements of NGC 1097 in the literature:
B = 10.23 (from NED) (looks like NED is quoting the magnitude value in the RC3)
B = 10.14 (LEDA)
B = 10.25 (Nearby Galaxies Catalog)
B = 10.07 (from EDD)(Tully & Pierce, 2000, ApJ, 533, 744)
B = 9.95 (from ESOLV)
So here is the mean (of 5 measurements) apparent B magnitude of NGC 1097:
10.13
Then, applying the derived distance modulus of 30.99 to this B magnitude yields a Blue absolute magnitude of minus -20.86 for NGC 1097.
This galaxy appears to be substantially reddened by internal dust, so it is probable that the luminosity of this galaxy will actually be somewhat higher than this, if the absolute magnitude and luminosity is corrected to the traditional "magnitude as seen in a face-on orientation"
In reality, given the magnitude and distance uncertainties, what I am really saying about NGC 1097 is something like this:
"it looks like she's about -21 absolute magnitude....."
Cheers,
Robert
by madbadgalaxyman
My previous thread on optimizing visual SN Search isolated these galaxies as excellent candidates for visual supernova search, but galaxy distance was a major unknown factor in that discussion. Unfortunately, the literature shows that the distances of these two Bright southern galaxies are not reliably known with an uncertainty as small as +/- 10 percent (to one sigma), as the available distance estimates for these galaxies use distance determination methods that are not known for their high accuracy:
- the Tully-Fisher Relation
- the redshift (velocity) distance, corrected for non-cosmological motions of galaxies
For those galaxies within a recession velocity of 2000 km/s., there could be an uncertainty as large as 25 percent for an individual distance estimate that was made using one of these methods, so attempts must be made to reduce the distance error by combining a number of (individually inaccurate) distance estimates for a single galaxy.
________________________________
THE DISTANCE OF NGC 2997
Distance Determinations in the Literature:
mu = 30.16
Distance = 10.8 Megaparsecs
Method: distance from the recession velocity, using Vlg and a Hubble Constant of 73
mu = 30.43
Distance = 12.2 Mpc
Method: Tully-Fisher
Source: Hess et al., 2009, ApJ, 699, 76
mu = 30.20
Distance = 10.96 Megaparsecs
Method: Mean of 5 Tully-Fisher estimates
Source: NED
mu = 30.26
Distance = 11.3 Megaparsecs
Source: distance of NGC 2997 galaxy group (using the average modulus for the group)
(see: Extragalactic Distance Database ; http://edd.ifa.hawaii.edu/dfirst.php )
mu = 29.96
Distance = 9.8 Mpc
Method: Tully-Fisher (mean of the determinations for the group galaxies)
Source : Extragalactic Distance Database
Conclusions:
I have decided not to use the mean distance from NED; the scatter of the data points used to find the mean is very great, so there could be some suspect data in there.
The velocity distance must be given significant weight.
The recent distance determinations in the Extragalactic Distance Database and by Hess et al. should be given some weight. I assume that they are recent and careful calibrations.
Mean of four distance determinations: 11 Megaparsecs
Mean distance modulus: 30.20
The distance of 11 Megaparsecs (36 million light years) for NGC 2997 that is derived here is similar to its distance as derived from its velocity (redshift), which increases confidence in this estimate. However, the individual uncertainty of each data point is large, and an incorrect extinction value for the NGC 2997 field could introduce significant uncertainties in this distance estimate, as foreground extinction (from our own Galaxy) should ideally be carefully measured for the field. Also, there is a desperate need for a cross-check of this distance with more accurate distance determination methods; such as TRGB or Cepheids.
The apparent magnitude and the Luminosity of NGC 2997 :
I have extracted 5 apparent magnitude measurements of the total magnitude of NGC 2997, from various catalogs of galaxies, all of them in the range B (blue) magnitude 9.79 to 10.06, so at least the apparent brightness of this galaxy is quite well constrained. I am not aware of any individual surface photometry of this galaxy, apart from the Good Quality photographic surface photometry in the ESOLV catalog. The mean of these 5 published magnitude values is B = 9.94 magnitude, which indicates that this galaxy could well be a little brighter than 10th magnitude. The careful UBVRI aperture photometry of Prugniel and Heraudeau (A&AS, 128, 299) gives B = 9.87 magnitude for NGC 2997, which is very plausible.
Assuming that this galaxy has an apparent B (Blue) magnitude of 9.9 and then applying a distance modulus of 30.2 yields an absolute Blue magnitude of minus 20.3
This luminosity estimate has significant uncertainties...... due to poorly constrained values of distance & apparent magnitude & extinction.
___________________________________ __
___________________________________ __
THE DISTANCE OF NGC 1097
Distance Determinations in the literature:
mu = 31.07
distance = 16.4 Megaparsecs
Source: Redshift distance, using Velocity (Local Group frame) and a Hubble Constant of 73
mu = 30.81
distance = 14.5 Megaparses
Method: Velocity distance, based on a Hubble Constant of 75 km/s/Megaparsec and a model that describes velocity perturbations in the Vicinity of the Virgo Cluster.
Source: Nearby Galaxies Catalog
mu =30.76
distance = 14.2 Megaparsecs
Method: Tully-Fisher
Source: Extragalactic Distance Database ; http://edd.ifa.hawaii.edu/dfirst.php )
mu = 31.02
distance = 16.0 Megaparsecs
Source: from Cosmic Flows 2
(see: Extragalactic Distance Database ; http://edd.ifa.hawaii.edu/dfirst.php )
mu = 31.12
distance = 16.7 Megaparsecs
Method: Tully-Fisher
Source: Mark III Catalog of Galaxy Peculiar Velocities (Willick et al.)
mu = 31.15
distance = 17.0 Megaparsecs
Source: Y.Sofue (1991, PASJ, 43, 671), from CO Ring diameter.
Conclusions:
The several separate Tully-Fisher Method determinations of this galaxy's distance are based on very similar data on the maximum rotation velocity of NGC 1097, and the data is manipulated in much the same way in all of these determinations, moreover with only minor differences in calibration. So there is not much in the way of external checking of these T-F distance determinations by means of other methods of distance estimation. The T-F determinations do, at least, derive similar distances to those which are derived from the recession velocity of NGC 1097. Needed are one or more distance determinations using methods of higher accuracy, such as TRGB and Cepheids.
Mean of 6 distance determinations : 15.8 Megaparsecs (51 million light years)
Mean distance modulus: 30.99
The apparent magnitude and the Luminosity of NGC 1097 :
There are substantial discrepancies between the various measurements of the total B (blue) apparent magnitude of NGC 1097 that are found in the literature. When the uncertainty in apparent brightness is compounded with the distance uncertainty, any consequent estimate of the total luminosity of NGC 1097 will be more in the nature of a ballpark figure. This is a very extended galaxy in angular terms (it is over 10 arcminutes along its major axis), so large-format CCD measurements could measure the magnitude with more accuracy.
I am going to exclude the total apparent magnitude of 10.39 as given in the UBVRI aperture photometry of Prugniel and Heraudeau (A&AS, 128, 299) from the mean magnitude, as this is a statistical outlier.
Some B magnitude measurements of NGC 1097 in the literature:
B = 10.23 (from NED) (looks like NED is quoting the magnitude value in the RC3)
B = 10.14 (LEDA)
B = 10.25 (Nearby Galaxies Catalog)
B = 10.07 (from EDD)(Tully & Pierce, 2000, ApJ, 533, 744)
B = 9.95 (from ESOLV)
So here is the mean (of 5 measurements) apparent B magnitude of NGC 1097:
10.13
Then, applying the derived distance modulus of 30.99 to this B magnitude yields a Blue absolute magnitude of minus -20.86 for NGC 1097.
This galaxy appears to be substantially reddened by internal dust, so it is probable that the luminosity of this galaxy will actually be somewhat higher than this, if the absolute magnitude and luminosity is corrected to the traditional "magnitude as seen in a face-on orientation"
In reality, given the magnitude and distance uncertainties, what I am really saying about NGC 1097 is something like this:
"it looks like she's about -21 absolute magnitude....."
Cheers,
Robert