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Photometric Profiles of Nearby Early-Type Galaxies Using SDSS
Beth Johnson, Aaron J. Romanowsky
San José State University
Introduction
The mechanisms for the formation of large early-type
galaxies are not well-known. Using isophote profiles may
help determine any differences between the inner and
outer region stellar populations. Differences could then
be analyzed further to determine whether the stars were
formed in situ or accreted from extra-galactic sources.
The SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) survey has attempted to analyze a collection of
twenty-five early-type galaxies (Fig. 1) for various
characteristics, including their metallicity [1], stellar
kinematics [2], and mass profiles [3].
Methods
For this project, the ten brightest galaxies included in
Kormendy [4] were analyzed. Acquiring the image data
from the SDSS required downloading the FITS files for a
mosaic version of each image, including only the g and r
wavelengths, with a size of 0.5" by 0.5" and a 0.396
pixel/arcsec ratio. The images were then processed using
the ellipse package included in stsdas.analysis.isophote of
IRAF. The resulting tables were exported to .txt files.
The g- and r-band values were converted to the V-
band value using equation
𝑉 = −0.565 ∗ (𝑔 − 𝑟) − 0.016 + 𝑔
and then plotted against the V-band dataset included for
each galaxy in Kormendy . The ellipticity and position
angle of the r-bands were also plotted against the
Kormendy data set. All plots were created in MATLAB.
Analysis and Results
The purpose of this initial attempt at isophote analysis
was to compare our data analysis against previous data
sets. For example, our profiles for NGC 4261 show little
deviation from the Kormendy results (Fig. 2), and the
other nine galaxies are similar in quality.
Fig. 3 and 4 include ellipticity profiles for all ten of the
brightest galaxies from the Kormendy paper and position
angle profiles for eight of those ten (NGC 4374 and 4552
had anomalies that need further analysis). An overall
upward trend in ellipticity is shown while the position
angles tend to hover around zero with little evidence of
decoupling. Zero was determined by the central region of
each galaxy. Note that the profile for NGC 4374 appears
to become circular around 200 arcsec before following
the trend. This difference is likely due to nearby galaxy
NGC 4406 and needs to be accounted for in future
analysis. Additionally, NGC 4382 (Fig. 5) shows two
distinct periods of increasing ellipticity and a widely
varying position angle. This galaxy is known to be a
merger remnant, which could explain the unusual
profiles.
Abstract
The outer regions of galaxies provide information about their assembly histories. We are carrying out a detailed analysis of the Sloan Digital Sky
Survey (SDSS) photometric profiles of a large sample of nearby early-type galaxies (ETGs), in order to look for transitions between their inner and
outer properties. Here we present initial results from this survey.
Next steps
Next we will attempt to decrease our deviations from the
Kormendy research by running the analysis using a
process to mask nearby foreground objects. The final
objective is to utilize these methods on the twenty-five
galaxies of the SLUGGS survey.
Fig 1: The 25 galaxies included in the SLUGGS survey. All images
taken from SDSS via Wikisky.
References
[1] Pastorello, et al. “The SLUGGS survey: Exploring
the metallicity gradients of nearby early-type galaxies to
large radii”. arXiv preprint. arXiv:1405.2338, 2014.
[2] Arnold, et al. “The SLUGGS survey: Wide-field stellar
kinematics of early-type galaxies”. arXiv preprint
arXiv:1310.2607, 2013.
[3] Romanowsky, et al. “The mass profiles of early-type
galaxies from x-ray and optical constraints”. In AAS/High
Energy Astrophysics Division, volume 13, 2013.
[4] Kormendy, et al. “Structure and formation of elliptical
and spheroidal galaxies”. The Astrophysical Journal
Supplement Series, 182(1):216, 2009.
Fig 2: Stellar Surface Brightness Profile of NGC 4261.
Fig 3: Isophote Ellipticity Profiles of Ten Brightest Virgo ETGs.
Fig 4: Position Angle Profiles of Eight Bright Virgo ETGs.
Fig 5: NGC 4382, still
showing signs of a
merger. Image from SDSS
via Wikisky.