Measurement of [V-R] Color-Magnitude Inversion in the Barred Spiral Galaxy NGC5850

C. David Kearsley

Kim Doberstein

Juan Cabanela(Graduate Advisor)

Experimental Methods in Astrophysics (Ast5201)

Department of Astronomy

University of Minnesota

Abstract

Analysis of long-integration CCD images indicates substantial (V-R)

color-magnitude inversion within the bar-ring structure of the galaxy

NGC5850. It was found that the (V-R) magnitude differential was [0.007

± .0006] and [-0.546 ± .044] for the bar and ring respectively. Among the

possible explanations for these observations is heterogeneous distribution

of HI and HII in the nebular disk[1].

Introduction

The galaxy NGC5850 is an SBb(sr)I-II (or SBR3) located in the constellation Virgo ( a = 15:04:35, d = +01:44:17, [1950]). Long time-exposure photographic plates show that it possesses a prominent bar and a relatively well-defined ring feature while nonetheless exhibiting significant spiral-arm structure.

 

The significance of the morphology of the ringed-barred-spiral type galaxies is considerable if one subscribes to the currently accepted models of galactic dynamics and evolution[2,3,4], since both the structure and composition of spiral galaxies in particular is believed to be highly time-dependent.

Observations

Observations of NGC5850 were performed using the 0.8 meter (Cassegranian) telescope located at the O'Brien Observatory in Marine-on-St.Croix, operated by the University of Minnesota. The instrument was equipped with a Thompson CCD, with dimensions [512 x 512] pixels or [4.56 x 4.56] arcminutes.

The CCD standard field for our observations was PG1528+062 (a = 15:30:50, d = +06:00:56, [1950]) (A.U.Landolt, 1992), with element (B) of the field serving as our magnitude standard. Two pair of integrations were performed in (V) and (R) filters at extinction: (average) airmass values of [0.422:1.785] and [0.313: 1.950] respectively. The integration times were [600, 300] seconds for (R) and [600,600] seconds for (V).

 

The observations of NGC5850 were performed as two pair of 600-second integrations at extinction: (average) airmass values of [0.422:1.61] in (V) and [0.313:1.53] in (R). This integration time was dictated primarily by marginal pointing/tracking. It should be noted that circumstances beyond the control of the author(s) required that the observations be performed during "bright time".

Data Analysis I

Image Preparation

All images were bias subtracted and, following flat-fielding, images of like-type were co-added in a normalized mode (in addition, the 300-second integration in (R) was co-added to itself in a non-normalized mode in order to maintain time-base uniformity). A CCD generated gradient was removed from the co-added images using a quadratic fit function.

Data Analysis II

Photometry-1

 

Collapsed-column (N/S) samples were taken across the CCD image in two locations. These samples took the form of [25 x 512] pixel bands intersecting the longitudinal axis of the bar through both the galactic nucleus and the northern outer extent of the bar structure, at an angle of approximately 63°. The (25) rows were then "collapsed" to form a single, normalized linear sample, with each of the (512) elements representing an average flux-per-pixel. A back-ground sample of equivalent dimensions (taken from the extreme southern end of the image-field) was subtracted from the bar/ring and nucleus samples for normalization.

Data Analysis III

Photometry-2

Flux data in counts-per-pixel was converted to magnitudes-per-square-arcsecond by first dividing each col-lapsed pixel count by [0.534"]2 (the number of square arcseconds per pixel). These flux counts were subsequently adjusted for extinction and airmass using the following the relation:

I = I0e-tlsecz [eq.1][5]

 

where (I0) is the intensity at the top of the Earth's atmosphere, (tl) is the current extinction coefficient-per-wavelength and (z) is the angular displacement from zenith. Instrumental magnitudes were then generated using the relation: 

m = -2.5log[F] [eq.2][6]

Data Analysis IV

Photometry-3

All average pixel-flux values were then summed and statistically evaluated in order to establish an average for each particular feature (nucleus, bar, and ring) of the galaxy. These flux averages were then compared with the standard, using the magnitude difference formula:

 

m2 = m1 - 2.5log[F2/F1] [eq.3][7]

 

where (m1, F1) and (m2, F2) are the (magnitude, Flux) of the standard and unknown objects, respectively. This provided the author(s) with final magnitude values in (V, R). These are also presented graphically in figures (1-4).

Data Analysis V

Error Analysis

 

Our statistical package generated mean, RMS, standard deviation and standard error (among other) information for each of the average flux values, based on the number of pixel columns per sample. Since the samples differ in angular diameter (nucleus: 30", bar: 120", ring: 60") the number of pixel-columns differs correspondingly. How-ever, since each average pixel flux value is actually the normalized sum of the elements of (25) pixel rows the standard error (s) can be divided by the square-root of the number of pixel rows, resulting in an improvement by a factor of (5) in our final experimental error [8]. The (V - R) error was established by taking the RMS sum of the individual (V) and (R) percentage errors [9].

Conclusions

It is readily apparent upon close inspection of fig.2-4, that a color magnitude inversion does in fact occur between the bar and ring of NGC5850 resulting in a negative (V-R) value for the ring structure and gradual reddening (V-R > 0) towards the central axis of the bar structure. A comprehensive (HST?) spectrometric investigation of NGC5850 would doubtless make some significant measure of progress towards revealing the underlying dynamics associated with this color-magnitude inversion behavior.

Fig.1 V-R Magnitude (Nucleus)

0 50 100 150 200 250 300

-10

-9

-8

-7

-6

-5

-4

NGC5850 V-R (Nucleus-WIDE)

Vmag/squarcsec

Rmag/squarcsec

theta(arcsec)

magnitude/squarcsec

Fig.2 V-R Magnitude (Ring)

20 30 40 50 60 70 80 90 100 110-8

-7

-6

-5

-4

-3NGC5850 V-R (Ring)

Vmag/square

Rmag/square

theta(arcsec)

magnitude/sqarcsec

Fig.3 V-R Magnitude (Bar)

75 100 125 150 175 200 225 250-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.5

-4NGC5850 V-R (Bar)

Vmag/square

Rmag/square

theta(arcsec)

magnitude/sqrarcsec

Fig.4 Ring-Bar Magnitude Inversion

50 75 100 125 150 175 20010

11

12

13

14

15NGC5850 V-R Magnitude Inversion

Vmag

Rmag

scan-centroid(arcsec)

magnitude/squarcsec

(Bar)

(Nucleus)

(Ring)

References I

[1] Introductory Astronomy and Astrophysics.

Michael Zeilik, Stephan A Gregory, Elske v.P.Smith, 1992,

Harcourt Brace Jovanovich, Publishers.

 

[2] Morphology of Spiral Galaxies - General Properties

P.J.Grosbol, Astronomy and Astrophysics Supplement Series,

Vol.60, May 1985, pp.261-276.

 

[3] Arm Classifications for Spiral Galaxies

Debra Meloy Elmgreen, Bruce G. Elmgreen, Astrophysical Journal, Part 1, Vol.314, March 1, 1987, pp.3-9.

References II

[4] "The Structure and Dynamics of Ringed Galaxies",

R.Buta, Univ. of Texas Publications in Astronomy, No.23.

 

[5] Introduction to Stellar Astrophysics,Vol.1

Erika Bohm-Vitense, Cambridge University Press, 1989.

[6] ibid.

[7] ibid.

 

[8] An Introduction to Error Analysis

John R.Taylor, University Science Books, 1982.

[9] ibid.