THE MOLECULAR GAS, DUST AND STELLAR
POPULATION ACROSS THE DISK OF SPIRAL GALAXIES
Assist. Prof. Selcuk TOPAL1
1Yüzüncü Yıl University, Faculty of Science, Department of Physics, Van, Turkey.
email@example.com ORCID ID: 0000-0003-2132-5632
There are different types of galaxies in the universe each has its own
unique properties (Hubble 1936; de Vaucouleurs et al. 1991;
Kormendy & Bender 2012). Spiral galaxies are rich in both molecular
gas and dust and show a high level of star formation. However,
elliptical galaxies are almost always devoid of gas. Lenticular
galaxies, the type of galaxies in the middle of spiral and elliptical
galaxies, are also mostly poor in gas, but some lenticulars have a
considerable amount of molecular gas (ATLAS3D survey; Young et al.
2011). Spiral galaxies are, therefore, exemplary targets to study star
formation processes across the disk of galaxies.
The interplay between gas and dust in the interstellar medium (ISM)
of galaxies has a key role in star formation, and the evolution of
galaxies. Once hydrogen molecule (H2) is formed on the surface of
dust grains (Cazaux & Tielens 2002; Perets & Ofer 2006) the door for
multiple other molecules from simple to more complex ones opens.
Since each atomic, molecular and dust emission is a result of different
physical properties in the ISM, multi-wavelength data are necessary to
understand the structure and evolution of gas clouds, and star
Nearby galaxies appear to follow a color-magnitude relation, i.e. the
galaxies on the red sequence are generally early-type galaxies (i.e.
ellipticals and lenticulars) with much less star formation activity and
cold gas, while the galaxies on the blue sequence are generally late-
type galaxies (namely spirals) with a high level of star formation
(Baldry et al. 2004). It has been shown that the color-magnitude
relation is also valid for nearby galaxy clusters and the clusters located
up to = 1 (Ellis et al. 1997; Sanchez-Blazquez et al. 2009). It is,
therefore, important to study the gas and dust in galaxies to get better
insights into galaxy evolution.
3.6m IR emission is barely affected by the extinction and could be
produced by old stars and/or dust heated by young massive stars. The
study of a large sample of galaxies showed that up to 30% of the total
lights at 3.6m originate from the dust heated by young massive stars
(Querejeta et al. 2015). The [3.6][4.5] color gradient in early-type
galaxies (i.e. elliptical and lenticular galaxies) showed that most
galaxies become redder through the outskirts (Peletier et al. 2012).
While older stellar populations tend to have colors of 0.2 < [3.6]
[4.5]< 0 (Willner et al. 2004; Pahre et al. 2004; Peletier et al. 2012;
Meidt et al. 2014), the [3.6][4.5] color with a non-stellar origin is
mostly positive, possibly due to hot dust, non-thermal emission or the
existence of young massive stars (Querejeta et al. 2015). 3.6m and
4.5m dust emissions can be used to estimate the stellar mass, ,
(Eskew et al. 2012; Querejeta et al. 2015), and the [3.6][4.5] color
also appears to have a dependence on redshift (Smit et al. 2014;
Huang et al. 2016).
We targeted spiral galaxies NGC 5248 and NGC 3938 to study the
interplay of gas, dust and stellar population across the disk of both
galaxies. The galaxies have the multi-wavelength literature data of
12CO(1-0), 3.6m and 4.5m emissions at sub-kpc resolution. This
allow us to study aforementioned properties of both galaxies in greater
detail for the first time.
1. LITERATURE DATA
The disk of both galaxies was observed in 12CO(1-0) emission as part
of the BIMA SONG Survey (Helfer et al. 2003). We used the highest
resolution CO data available for both galaxies, i.e. the beam size is 6
arcsec or 400 pc over the galaxies. Near-infrared (NIR) data at
3.6m and 4.5m wavelengths were taken from the Spitzer Space
Telescope Survey (Werner et al. 2004) conducted using the Infrared
Array Camera (IRAC; Fazio et al. 2004). The basic parameters for
both galaxies can be found in Table 1.
Table 1. Basic properties of the spiral galaxies NGC 5248 and NGC 3938.
Major axis diameter
Minor axis diameter
aNASA/IPAC Extragalactic Database (NED); bHyperLEDA (http://leda.univ-
2. DATA REDUCTION AND ANALYSIS
2.1. Position Selection
We selected multiple positions (including the center) located over the
dusty disc of both galaxies, so the positions are bright at 3.6m and
4.5m wavelengths. The positions are located in the north-eastern
(hereafter NE) and south-western (hereafter SW) of the center of each
galaxy. There are 16 and 18 positions in the NE and SW, respectively,
leading a total of 37 and 33 positions (including the center) in NGC
5248 and NGC 3938, respectively (see the illustration in Figure 1).
The angular size of each selected position is 6 arcsec, equal to the
beam size of 12CO(1-0) data, i.e. the lowest angular resolution in the
data set. Since we aim to compare molecular gas properties with that
infrared, we chose 6 arcsec as the common spatial resolution. We,
therefore, de-convolved 3.6m and 4.5m data to the common beam
size to make the beam match in the data set (see Section 2.3). Selected
positions over the disk of each galaxy are shown in Figure 1.
Figure 1: The selected positions (red and black circles) are overlaid on Spitzer
3.6 and 4.5 images (grayscale with white contours) of both galaxies. The top
panels show the images for NGC 5248 while the bottom panels show the images for
NGC 3938. Each circle has a diameter of 6 arcsec or a linear size of 400 pc at an
average distance of 15 Mpc for both galaxies (see Table 1). The three positions
within the brighter central region of size 1.2 kpc are shown by black circles. The
numbering for the positions starts from the farthest position in the north-eastern, NE,
(i.e. position 1), and it ends at the farthest position in the south-western, SW, (i.e.
position 37 for NGC 5248 and position 33 for NGC 3938), i.e. the white numbers
annotated in the images (see also Section 2.1). The corresponding position number
for the center of NGC 5248 and NGC 3938 are 19 and 17, respectively. North is up
and east to the left in all images.
2.2. Integrated CO Intensity, Mass and Surface Density
We extracted CO spectra from the selected positions in the CO data
cubes using the Multichannel Image Reconstruction Image Analysis
and Display (MIRIAD; Sault et al. 1995) task imspect. After extracting
CO spectra we calculated the integrated CO line intensity,
[K km s], by fitting a Gaussian function to the spectra. We
carried out the fitting procedure using Interactive Data Language
(IDL) code MPFIT (Markwardt 2009). MPFIT optimises the fitting
parameters by applying Levenberg–Marquardt minimization
algorithm. We defined the best-fitting Gaussian parameters by
applying the test, i.e. the parameters with the smallest value of
were defined as the best fit. The results of the
fitting procedure are shown for the central three positions in Figure 2.
Figure 2: The CO Spectra Extracted from the Central Three Positions (see the black
circles in Figure 1) in NGC 5248 (top panels) and NGC 3938 (bottom panels). The
Red Line Represents the Best Gaussian Fits to the Spectra.
We estimated the total molecular gas mass () at the positions
using the CO line intensity, and an adopted CO-to-H2 conversion
factor of = 0.2 × 10 cm (K km s) and = 2 ×
10 cm (K km s) for the center (i.e. a region of size 1.2 kpc
in diameter) and the disk of the galaxies, respectively. The reasons for
these values are twofold; (1) the value of
= 2 × 10 cm (K km s) is a widely accepted value for
the disk of nearby spiral galaxies (Rosolowsky et al. 2003; Bolatto et
al. 2008; Abdo et al. 2010; Donovan Meyer et al. 2012; Bolatto,
Wolfire, & Leroy 2013), and (2) there is a depression in in the
central region of galaxies compared to the disk, i.e. up to 10 times
lower (Bolatto et al. 2013; Sandstrom et al. 2013). The expression
used to estimate the is shown below (Bolatto et al. 2013).
where the values of are = 6.4 × 10 and = 6.4 × 10 for the
center and the disk of NGC 5248, respectively, and = 6.9 × 10
and = 6.9 × 10 for the center and disk of NGC 3938,
respectively. The gas surface density () was also estimated at
each position as =
, where [Mpc(K kms)]=
.× (Narayanan et al. 2012), and is the 12CO(1-
0) integrated intensity. The values of for the adopted are
= 0.43 (K km s) and = 4.3 (K km s) for
the center and disk of the galaxies, respectively (Narayanan et al.
2012). The integrated CO line intensities and corresponding and
values, as a function of the galactocentric distance, are shown in
Figure 3: CO line intensities (main panels), molecular gas gass () and gas
surface densities () (embedded panels) as a function of galactocentric distance.
In all images, open and filled circles represent the values in the north-eastern (NE)
and south-western (SW) arms of the galaxies, respectively, while filled black
diamond shows the value at the center. The size of the symbols was also arranged
specifically so that the smallest symbol represents the farthest position with respect
to the center while the largest symbol indicates the closest position to the center. The
name of the galaxies and the meaning of each symbol are also shown on the top of
each panel. rs values in the panels (including rs(NE) and rs(SW), i.e. the correlation
for the positions only in the NE and SW, respectively) represent the Spearman
correlation coefficient estimated for each pair of physical parameters, i.e. intensity
vs. distance, vs. distance and vs. distance. The vertical dashed red lines in
the top panels represent the angular distance of 25 arcsec from the center of NGC
5248 (see Section 3).
2.3. Near-infrared Fluxes, [.][.] Color and Stellar Mass
We calculated the beam averaged 3.6m and 4.5m NIR fluxes
(hereafter . and ., respectively) at the selected positions as
explained below. We first applied a unit conversion necessary for
Spitzer data (i.e. from MJy sr-1 to Jy). We then multiplied the flux in
each pixel in each image by a normalized 2D Gaussian function. The
Gaussian function has an FWHM of 6 arcsec, i.e. the common beam
size. We finally calculated the Gaussian weighted total NIR fluxes at
each position by summing the weighted fluxes in all pixels in the
image. We also calculated the stellar mass () using the . and .
fluxes, and the expression below (Eskew et al. 2012),
=10. × .
where D is the distance to the galaxy in the unit of Mpc (see Table 1).
To calculate the [3.6][4.5] color, we first need to estimate the
apparent magnitudes at both wavelengths. We, therefore, used the
standard expression of = × 10/., where is the zero-
point-flux densities at 3.6m and 4.5m, 280.9 Jy and 179.7 Jy,
respectively (Reach et al. 2005). stands for the flux at 3.6m and
4.5m in the unit of Jy (i.e. . and ., see above), and finally, m is
the apparent magnitude at 3.6m and 4.5m. The [3.6][4.5] color
as a function of and the distance is shown in Figure 4.
3. RESULTS AND DISCUSSION
As seen from Figure 3, there is a strong negative correlation between
the CO intensity and galactocentric distance (Spearman correlation
coefficient =0.77) in NGC 5248, i.e. the CO intensity decreases
sharply up to about 25 arcsec (or equivalently 1.7 kpc over the galaxy)
from the center and then flattens (i.e. no considerable change in CO
brightness) on each side of the disk. A similar correlation exists
between the and the distance, and between the and the
distance, after excluding the central three regions where there is a
considerable decrease in and because of the assumed
depression in .
However, the situation is quite different in NGC 3938. The CO
intensity shows a very weak negative dependence on the
galactocentric distance (see Figure 3), i.e. the correlation is more
flatter compared to NGC 5248. The dependence of and on
the distance in NGC 3938 is also weak but positive, after excluding
the central positions (see the embedded panels in the bottom image of
Figure 4: The [3.6][4.5] color as a function of the stellar mass () and the
galactocentric distance are shown for NGC 5248 (top) and NGC 3938 (bottom). The
shape and size of the symbols were arranged as stated in the caption of Figure 2. The
horizontal dashed black lines and the vertical dashed red line in the top panels (i.e.
NGC 5248) represent the lowest end of the range for the [3.6][4.5] color for
diffuse dust (see Section 3), and the angular distance of 25 arcsec from the center,
respectively. The Spearman correlation coefficient, rs, is also shown in each panel
(including the correlation for the NE and SW positions, () and (),
In Figure 4, the correlation between the [3.6][4.5] color and ,
and between the color and galactocentric distance are shown. In both
galaxies, there is a strong negative correlation between the [3.6]
[4.5] color and (i.e. . =0.86 and =0.78 for NGC 5248
and NGC 3938, respectively). The correlation is stronger in the SW
arm of NGC 5248 (i.e. () = 0.95) than the NE arm (i.e.
() = 0.52), while the situation is the opposite in NGC 3938
(i.e. () = 0.80 and () = 0.69).
As Figure 4 indicates, there are two main differences between NGC
5248 and NGC 3938. Firstly, the central region and the outskirts of
each galaxy shows some differences in terms of stellar population,
although both galaxies have bluer colors in the center compared to
their disks. While the color in the center of NGC 3938 resembles the
color for old stellar populations (i.e. 0.2 < [3.6][4.5]< 0;
Willner et al. 2004; Pahre et al. 2004; Peletier et al. 2012; Meidt et al.
2014), it is the opposite for the center of NGC 5248 (i.e. [3.6]
[4.5]> 0). Secondly, although the color continuously gets redder
from the center to the outskirts in NGC 3938 it never exceeds 0.2 as
opposed to NGC 5248. In NGC 5248, after about 25 arcsec from the
center in the SW of the disk, [3.6][4.5]> 0.2 (except one position
only), and then the color rather shows a flat distribution as the
distance from the galaxy’s center increases (see the top-right panel in
Figure 4). However, in the NE arms of NGC 5248, all positions
(except three positions only) have [3.6][4.5]< 0.2. The typical
range for the [3.6][4.5] color for diffuse dust 0.2 < [3.6] − [4.5] <
0.7 (Querejeta et al. 2015). This indicates that after about 25 arcsec
from the center in the SW of NGC 5248, the diffuse dust is
dominating the ISM compared to the NE.
As seen in Figure 5, there is a strong positive correlation between
and in NGC 5248 (i.e. = 0.83), while the correlation is much
weaker and negative in NGC 3938 (=0.24). The has a strong
negative correlation with the galactocentric distance in both galaxies
(i.e. =0.76 and =0.87 for NGC 5248 and NGC 3938,
respectively). Similar to the CO intensity, , , and the [3.6] −
[4.5] color, after about 25 arcsec from the center also shows a flat
distribution as a function of the distance in the outskirts of NGC 5248
(see the top-right panel in Figure 5). However, the continuously
decreases from the center to the outskirts in NGC 3938.
Figure 5: The stellar mass () as a function of and the galactocentric distance
is shown for NGC 5248 (top panels) and NGC 3938 (bottom panels). The vertical
dashed red line in the top-right panel represents the angular distance of 25 arcsec
from the center of NGC 5248. The size of the symbols was defined as stated in the
caption of Figure 2. The Spearman correlation coefficient for all positions (i.e. )
and the positions in the NE (i.e. () ) and SW (i.e. ()) is also shown in
each panel. The correlations involving does not include the data for the central
three positions (i.e. the positions 18, 19 and 20 for NGC 5248, and the positions
16,17 and 18 for NGC 3938, see Figure 1), because of the assumed depression in
causing the substantial decrease in .
Multiple positions across the spiral arms of NGC 5248 and NGC 3938
were studied using multi-wavelength data including 12CO(1-0)
transition and 3.6m and 4.5m NIR emissions. Molecular gas mass
(), gas surface density (), [3.6][4.5] color and stellar mass
() were obtained for each position studied, including the central
region. Our main conclusions are as follows.
1- The integrated CO intensity shows a radial gradient with a
strong correlation with the galactocentric distance in NGC 5248,
i.e. the intensity decreases sharply up to about 25 arcsec from
the center and then it flattens. The same behavior is seen for the
and , after excluding the central three positions where
there is an assumed depression in . On the contrary, there is
no strong (or even mild) correlation between the CO intensity
and the galactocentric distance across the disk of NGC 3938
(and this is also true for and ). The spiral arms of NGC
3938 seem to have an ISM with a different level of star
formation processes compared to NGC 5248.
2- Each galaxy shows a color gradient, i.e. the galaxies get redder
from the center to the outskirts. The central region of NGC 3938
has negative (the bluest) colors indicating the old stellar
populations dominating the ISM there. However, the [3.6]
[4.5] color is always positive across the disk of NGC 5248.
3- The galaxies also show different behavior in their outskirts. In
the outskirts of NGC 5248 (after 25 arcsec from the center in the
SW arm), the [3.6][4.5]> 0.2. However the [3.6][4.5] is
always lower than 0.2 across the disk of NGC 3938. This
indicates that the diffuse dust could be dominating the outskirts
of NGC 5248 compared to the rest of its disk and the disk of
NGC 3938. As NGC 5248 is a member of the galaxy group, this
could explain the diffuse dust at the outskirts of the galaxy
where any interactions with nearby galaxies could be more
Overall, in NGC 5248, the integrated CO intensity, and
decrease while the stellar mass and the value of the [3.6][4.5] color
increase up to about 25 arcsec from the center. However, after 25
arcsec from the center in NGC 5248, the distribution of all parameters
as a function of the galactocentric distance flattens. In NGC 3938,
there is no statistically significant correlation between the CO
intensity (also and ) and the galactocentric distance.
However, the correlations between the and distance (=0.87)
and between the [3.6][4.5] color and distance (= 0.82) are
strong in NGC 3938. Additionally, there is a strong positive
correlation between the and in NGC 5248 (= 0.83,
excluding the central three positions), whereas there is no such
correlation seen in NGC 3938 (=0.24). The correlation between
the [3.6][4.5] color and is negative and strong in both galaxies,
i.e. the increases as the [3.6][4.5] color gets bluer from the
outskirts to the center. However, the strength of the correlation
between the and [3.6][4.5] color in the NE and SW arms of
each galaxy is different.
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Space Telescope, which was operated by the Jet Propulsion
Laboratory, California Institute of Technology under a contract with
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NASA/IPAC Extragalactic Database (NED), which is operated by the
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