ChapterPDF Available

# THE MOLECULAR GAS, DUST AND STELLAR POPULATION ACROSS THE DISK OF SPIRAL GALAXIES

Authors:

## Abstract and Figures

In this research article, I targeted multiple positions across the disk of the spiral galaxies NGC 5248 and NGC 3938 using CO, 3.6micron and 4.5micron data at sub-kpc resolution. Overall, in NGC 5248, the integrated CO intensity, molecular gas mass and gas surface density decrease while the stellar mass and the value of the [3.6]-[4.5] colour increase up to about 25 arcsec from the centre. However, after 25 arcsec from the centre 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 gas mass and gas surface density) and the galactocentric distance. However, the correlations between the stellar mass and distance (Spearman correlation coefficient r_s = -0.87), and between the [3.6]-[4.5] colour and distance (r_s = 0.82) are strong in NGC 3938. Additionally, there is a strong positive correlation between the molecular gas mass and stellar mass in NGC 5248 (r_s = 0.83, excluding the central three positions), whereas there is no such correlation seen in NGC 3938 (i.e. r_s = -0.24). The correlation between the [3.6]-[4.5] colour and the stellar mass is negative and strong in both galaxies, i.e. the stellar mass increases as the [3.6]-[4.5] colour gets bluer from the outskirts to the centre.
Content may be subject to copyright.
43
CHAPTER 2
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.
selcuktopal@yyu.edu.tr ORCID ID: 0000-0003-2132-5632
44
Physics Studies
45
INTRODUCTION
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
formation better.
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-
46
Physics Studies
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
47
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.
Property
NGC 3938
Ref.
Hubble type
SA(s)c
a
RA (J2000)
11h52m49.4s
a
Dec (J2000)
+44°07'14.6''
a
Major axis diameter
(arcmin)
5.0
a
Minor axis diameter
(arcmin)
5.0
a
V
hel
(km/s)
807
a
Distance (Mpc)
15.0
b
Inclination (degree)
17.6
b
aNASA/IPAC Extragalactic Database (NED); bHyperLEDA (http://leda.univ-
lyon1.fr)
48
Physics Studies
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.
49
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
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
50
Physics Studies
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
51
lower (Bolatto et al. 2013; Sandstrom et al. 2013). The expression
used to estimate the is shown below (Bolatto et al. 2013).
M= 
 , (1)
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.
52
Physics Studies
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).
53
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. × .
 × .
 ×
., (2)
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.
54
Physics Studies
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 3).
55
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 (),
respectively).
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]
56
Physics Studies
[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.
57
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. )
58
Physics Studies
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 .
CONCLUSION
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.
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
59
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
effective.
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
60
Physics Studies
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.
ACKNOWLEDGEMENTS
This work is based [in part] on archival data obtained with the Spitzer
Space Telescope, which was operated by the Jet Propulsion
Laboratory, California Institute of Technology under a contract with
NASA. We acknowledge the usage of the HyperLEDA database
(http://leda.univ-lyon1.fr). This research has made use of the
NASA/IPAC Extragalactic Database (NED), which is operated by the
Jet Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administration.
61
REFERENCES
Abdo, A. A., Ackermann, M., Ajello, M., Baldini, L., Ballet, J., Barbiellini, G.,
Bastieri, D., Baughman, B. M., Bechtol, K., Bellazzini, R. ve ark., (2010),
Fermi Observations of Cassiopeia and Cepheus: Diffuse Gamma-ray
Emission in the Outer Galaxy, The Astrophysical Journal, Vol. 710 (1), pp
133-149.
Baldry I. K., Glazebrook K., Brinkmann J., Ivezic Z., Lupton R. H., Nichol R. C.,
Szalay A. S., (2004), Quantifying the Bimodal Color-Magnitude
Distribution of Galaxies, The Astrophysical Journal, Vol. 600 (2), pp 681-
694.
Bolatto A. D., Leroy A. K., Rosolowsky E., Walter F., Blitz L., (2008), The
Resolved Properties of Extragalactic Giant Molecular Clouds, The
Astrophysical Journal, Vol. 686 (2), pp 948-965.
Bolatto A. D., Wolfire M., Leroy A. K., (2013), The CO-to-H2 Conversion Factor,
Annual Review of Astronomy and Astrophysics, Vol. 51 (1), pp 207-268.
Cazaux, S. and A. Tielens, (2002), Molecular hydrogen formation in the interstellar
medium. The Astrophysical Journal, Vol. 575 (1), pp L29-L32.
de Vaucouleurs G., de Vaucouleurs A., Corwin, Jr. H. G., Buta R. J., Paturel G.,
Fouqu_e P., (1991), Third Reference Catalogue of Bright Galaxies. Volume
I: Explanations and references. Volume II: Data for galaxies between 0h
and 12h. Volume III: Data for galaxies between 12h and 24h.
Donovan M., J., Koda, J., Momose, R., Fukuhara, M., Mooney, T., Towers, S.,
Egusa, F., Kennicutt, R., Kuno, N., Carty, M., ve ark., (2012), Resolved
Measurements of X_CO in NGC 6946, The Astrophysical Journal, Vol. 744
(1), pp 42-52.
Ellis R. S., Smail I., Dressler A., Couch W. J., Oemler A., Jr, Butcher H., Sharples
R. M., (1997), The Homogeneity of Spheroidal Populations in Distant
Clusters, The Astrophysical Journal, Vol. 483 (2), pp 582-590.
Eskew M., Zaritsky D., Meidt S., (2012), Converting from 3.6 and 4.5 μm Fluxes to
Stellar Mass, The Astronomical Journal, Vol. 143 (6), pp 139-145.
62
Physics Studies
Fazio, G. G. ; Hora, J. L., Allen, L. E., Ashby, M. L. N., Barmby, P., Deutsch, L. K.,
Huang, J. -S., Kleiner, S., Marengo, M., Megeath, S. T. et al. (2004), The
Infrared Array Camera (IRAC) for the Spitzer Space Telescope, The
Astrophysical Journal Supplement Series, Vol. 154, pp 10-17.
Helfer, T. T., Thornley, M. D., Regan, M. W., Wong, T., Sheth, K., Vogel, S. N.,
Blitz, L., Bock, D. C. J., (2003), The BIMA Survey of Nearby Galaxies
(BIMA SONG). II. The CO Data, The Astrophysical Journal Supplement
Series, Vol. 145 (2), pp 259-327.
Huang, K-H., Bradač, M., Lemaux, B. C.,Ryan, R. E. Jr., Hoag, A., Castellano, M.,
Amorín, R., Fontana, A., Brammer, G. B., Cain, B., Lubin, L. M., Merlin,
E., Schmidt, K. B., Schrabback, T., Treu, T., Gonzalez, A. H., von der
Linden, A., Knight, R. I. (2016), Spitzer UltRa Faint SUrvey Program
(SURFS UP). II. IRAC-detected Lyman-Break Galaxies at 6 z 10
behind Strong-lensing Clusters, The Astrophysical Journal, Vol. 817 (1), pp
11-32.
Hubble, E.P., (1936), Realm of the Nebulae, by E.P. Hubble. New Haven: Yale
University Press, 1936. ISBN 9780300025002
Kormendy J., Bender R., (2012), A Revised Parallel-sequence Morphological
Classification of Galaxies: Structure and Formation of S0 and Spheroidal
Galaxies, The Astrophysical Journal Supplement Series, Vol. 198 (1), pp 2-
41.
Markwardt C. B., (2009), Non-linear Least-squares Fitting in IDL with MPFIT”,
Astronomical Society of the Pacific Conference Series, Astronomical Data
Analysis Software and Systems XVIII., Quebec City, QC, Canada, 411,
251-254.
Meidt, Sharon E.; Schinnerer, Eva; van de Ven, Glenn; Zaritsky, Dennis; Peletier,
Reynier; Knapen, Johan H.; Sheth, Kartik; Regan, Michael; Querejeta,
Miguel; Muñoz-Mateos, Juan-Carlos, (2014), Reconstructing the Stellar
Mass Distributions of Galaxies Using S4G IRAC 3.6 and 4.5 μm Images.
II. The Conversion from Light to Mass, The Astrophysical Journal, Vol.
788 (2), pp 144-155.
63
Narayanan, D., Krumholz, M., Ostriker, E. C., Hernquist, L., (2012), A general
model for the CO–H2 conversion factor in galaxies with applications to the
star formation law, Monthly Notices of the Royal Astronomical Society,
Vol. 421, pp 31273146.
Pahre, Michael A., Ashby, M. L. N., Fazio, G. G., Willner, S. P., (2004), Mid-
Infrared Galaxy Morphology along the Hubble Sequence, Astrophysical
Journal Supplement Series, Vol. 154, pp 235-241.
Peletier, R. F., Kutdemir, E., van der Wolk, G., Falcón-Barroso, J., Bacon, R.,
Bureau, M., Cappellari, M., Davies, R. L, de Zeeuw, P. T., Emsellem, E.,
(2012), The SAURON project - XX. The Spitzer [3.6] - [4.5] colour in
early-type galaxies: colours, colour gradients and inverted scaling relations,
Monthly Notices of the Royal Astronomical Society. Vol. 419 (3), pp 2031-
2053.
Perets, H. B., Ofer, B., (2006), Molecular hydrogen formation on porous dust grains,
Monthly Notices of the Royal Astronomical Society, Vol. 365 (3), pp 801-
806.
Querejeta, M., Meidt, S. E., Schinnerer, E., Cisternas, M., Muñoz-Mateos, J. C.,
Sheth, K., Knapen, J., van de Ven, G., Norris, M. A., Reynier, P., (2015),
The Spitzer Survey of Stellar Structure in Galaxies (S4G): Precise Stellar
Mass Distributions from Automated Dust Correction at 3.6 μm,
Astrophysical Journal Supplement Series, Vol. 219, pp 5-23.
Reach, William T., Megeath, S. T., Cohen, M., Hora, J., Carey, S., Surace, J.,
Willner, S. P., Barmby, P., Wilson, G., Glaccum, W., et al., (2005),
Absolute Calibration of the Infrared Array Camera on the Spitzer Space
Telescope, The Publications of the Astronomical Society of the Pacific,
Vol. 117 (835), pp 978-990.
Rosolowsky E., Engargiola G., Plambeck R., Blitz L., (2003), Giant Molecular
Clouds in M33. II. High-Resolution Observations, The Astrophysical
Journal, Vol. 599 (1), pp 258-274.
Sánchez-Blázquez, P., Jablonka, P., Noll, S., Poggianti, B. M., Moustakas, J.,
Milvang-Jensen, B., Halliday, C., Aragón-Salamanca, A., Saglia, R. P.,
64
Physics Studies
Desai, V., (2009), Evolution of red-sequence cluster galaxies from redshift
0.8 to 0.4: ages, metallicities, and morphologies, Astronomy and
Astrophysics, Vol. 499 (1), pp 47-68.
Sault R. J., Teuben P. J., Wright M. C. H., (1995), A Retrospective View of
MIRIAD, Astronomical Society of the Pacific Conference Series,
Astronomical Data Analysis Software and Systems IV., Baltimore,
Maryland, USA, 77, 433-436,
Sandstrom, K. M., Leroy, A. K., Walter, F., Bolatto, A. D., Croxall, K. V., Draine,
B. T., Wilson, C. D., Wolfire, M., Calzetti, D., Kennicutt, R. C. ve ark.,
(2013), The CO-to-H2 Conversion Factor and Dust-to-gas Ratio on
Kiloparsec Scales in Nearby Galaxies, The Astrophysical Journal, Vol. 777
(1), pp 5-37.
Smit, R., Bouwens, R. J., Labbé, I., Zheng, W., Bradley, L., Donahue, M. ; Lemze,
D., Moustakas, J., Umetsu, K., Zitrin, A. et al. (2014), Evidence for
Ubiquitous High-equivalent-width Nebular Emission in z ~ 7 Galaxies:
Toward a Clean Measurement of the Specific Star-formation Rate Using a
Sample of Bright, Magnified Galaxies, The Astrophysical Journal, Vol.
784, pp 58-65.
Werner, M. W., Roellig, T. L., Low, F. J., Rieke, G. H., Rieke, M., Hoffmann, W.
F., Young, E., Houck, J. R., Brandl, B., Fazio, G. G. et al. (2004), The
Spitzer Space Telescope Mission, The Astrophysical Journal Supplement
Series, Vol. 154, pp 1-9.
Willner, S. P., Ashby, M. L. N., Barmby, P., Fazio, G. G., Pahre, M., Smith, H. A.,
Kennicutt, R. C. Jr., Calzetti, D., Dale, D. A., Draine, B. T., (2004),
Infrared Array Camera (IRAC) Observations of M81, Astrophysical Journal
Supplement Series, Vol. 154 (1), pp 222-228.
Young, Lisa M., Bureau, M., Davis, Timothy A., Combes, F., McDermid, R. M.,
Alatalo, K., Blitz, L., Bois, M., Bournaud, F., Cappellari, M., Davies, R. L.,
de Zeeuw, P. T., Emsellem, E., Khochfar, S., Krajnović, D., Kuntschner,
H., Lablanche, P., Morganti, R., Naab, T., Oosterloo, T., Sarzi M., Scott,
N., Serra, P., Weijmans, A., (2011), The ATLAS3D project - IV. The
65
molecular gas content of early-type galaxies, Monthly Notices of the Royal
Astronomical Society, Vol. 414, pp 940-967.
ResearchGate has not been able to resolve any citations for this publication.
Article
Full-text available
We present the largest sample to date of giant molecular clouds (GMCs) in a substantial spiral galaxy other than the Milky Way. We map the distribution of molecular gas with high resolution and image fidelity within the central 5 kpc of the spiral galaxy NGC 6946 in the 12CO (J = 1-0) transition. By combining observations from the Nobeyama Radio Observatory 45 m single dish telescope and the Combined Array for Research in Millimeter Astronomy interferometer, we are able to obtain high image fidelity and accurate measurements of L CO compared with previous purely interferometric studies. We resolve individual GMCs, measure their luminosities and virial masses, and derive X CO—the conversion factor from CO measurements to H2 masses—within individual clouds. On average, we find that X CO = 1.2 × 1020 cm–2 (K km s–1)–1, which is consistent within our uncertainties with previously derived Galactic values as well as the value we derive for Galactic GMCs above our mass sensitivity limit. The properties of our GMCs are largely consistent with the trends observed for molecular clouds detected in the Milky Way disk, with the exception of six clouds detected within ~400 pc of the center of NGC 6946, which exhibit larger velocity dispersions for a given size and luminosity, as has also been observed at the Galactic center.
Article
Full-text available
The Infrared Array Camera (IRAC) is one of three focal plane instruments in the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 µm. Two nearly adjacent 5.2×5.2 arcmin fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 µm; 4.5 and 8 µm). All four detector arrays in the camera are 256×256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.
Article
Full-text available
The small scatter observed for the (U-V) colors of spheroidal galaxies in nearby clusters of galaxies provides a powerful constraint on the history of star formation in dense environments. However, with local data alone, it is not possible to separate models where galaxies assembled synchronously over redshifts 0 < z < 1 from ones where galaxies formed stochastically at much earlier times. Here we attempt to resolve this ambiguity via high-precision rest-frame UV-optical photometry of a large sample of morphologically selected spheroidal galaxies in three z ~ 0.54 clusters that have been observed with the Hubble Space Telescope (HST). We demonstrate the robustness of using the HST to conduct the morphological separation of spheroidal and disk galaxies at this redshift and use our new data to repeat the analysis conducted locally at a significant look-back time. We find a small scatter (<0.1 mag rms) for galaxies classed as E's and E/S0's, both internally within each of the three clusters and externally from cluster to cluster. We do not find any trend for the scatter to increase with decreasing luminosity down to L~L*V+3, other than can be accounted for by observational error. Neither is there evidence for a distinction between the scatter observed for galaxies classified as ellipticals and S0. Our result provides a new constraint on the star formation history of cluster spheroidals prior to z 0.5 confirming and considerably strengthening the earlier conclusions. Most of the star formation in the elliptical galaxies in dense clusters was completed before z 3 in conventional cosmologies. Although we cannot rule out the continued production of some ellipticals, our results do indicate an era of initial star formation consistent with the population of star-forming galaxies recently detected beyond z 3.
Article
Full-text available
Infrared Array Camera (IRAC) images of M81 show three distinct morphological constituents: a smooth distribution of evolved stars with bulge, disk, and spiral arm components; a clumpy distribution of dust emission tracing the spiral arms; and a pointlike nuclear source. The bulge stellar colors are consistent with M-type giants, and the disk colors are consistent with a slightly younger population. The dust emission generally follows the blue and ultraviolet emission, but there are large areas that have dust emission without ultraviolet and smaller areas with ultraviolet but little dust emission. The former are presumably caused by extinction, and the latter may be due to cavities in the gas and dust created by supernova explosions. The nucleus appears fainter at 8 μm than expected from ground-based 10 μm observations made 4 years ago.
Article
Full-text available
The BIMA Survey of Nearby Galaxies is a systematic imaging study of the 3 mm CO J = 1--0 molecular emission within the centers and disks of 44 nearby spiral galaxies. The typical spatial resolution of the survey is 6", or 360 pc at the average distance (12 Mpc) of the sample, over a field of view of 10kpc. The velocity resolution of the CO observations is 4 km/s. The sample was not chosen based on CO or infrared brightness; instead, all spirals were included that met the selection criteria of vsun <= 2000 km/s, dec >= -20deg, inc <= 70deg, D25 < 70', and BT < 11.0. The detection rate was 41/44 sources or 93%. Fully-sampled single-dish CO data were incorporated into the maps for 24 galaxies; these single-dish data comprise the most extensive collection of fully-sampled, two-dimensional single-dish CO maps of external galaxies to date. We also tabulate direct measurements of the global CO flux densities for these 24 sources. We demonstrate that the measured ratios of flux density recovered are a function of the signal-to-noise of the interferometric data. We examine the degree of central peakedness of the molecular surface density distributions and show that the distributions exhibit their brightest CO emission within the central 6" in only 20/44 or 45% of the sample. We show that all three Local Group spiral galaxies have CO morphologies that are represented in SONG, though the Milky Way CO luminosity is somewhat below the SONG average, and M31 and M33 are well below average. This survey provides a unique public database of integrated intensity maps, channel maps, spectra, and velocity fields of molecular emission in nearby galaxies.
Article
Full-text available
We present a comprehensive analysis of the stellar population properties and morphologies of red-sequence galaxies in 24 clusters and groups from $z\sim0.75$ to $z\sim0.45$. The dataset, consisting of 215 spectra drawn from the ESO Distant Cluster Survey, constitutes the largest spectroscopic sample at these redshifts for which such an analysis has been conducted. Analysis reveals that the evolution of the stellar population properties of red-sequence galaxies depend on their mass: while the properties of the most massive are well described by passive evolution and high-redshift formation, those of the less massive galaxies are consistent with a more extended star-formation history. We show that these scenarios reproduce the index-$\sigma$ relations and the galaxy colours. The two main results of this work are: (1) the evolution of the line-strength indices for the red-sequence galaxies can be reproduced if 40% of the galaxies with $\sigma< 175$ km s$^{-1}$ entered the red-sequence between $z=0.75$ to $z=0.45$, in agreement with the fraction derived in studies of the luminosity functions; and (2) the percentage of red-sequence galaxies exhibiting early-type morphologies (E and S0) decreases by 20% from $z=0.75$ to $z=0.45$. This can be understood if the red-sequence becomes more populated at later times with disc galaxies whose star formation has been quenched. We conclude that the processes quenching star formation do not necessarily produce a simultaneous morphological transformation of the galaxies entering the red-sequence.
Article
This book (first published in 1936) consists of the Silliman Lectures, delivered at Yale University in the autumn of 1935, with the addition of an introductory chapter. Topics discussed include the family traits, the distribution, and the distances of nebulae; the velocity-distance relation; the local group; and the general field. The work takes advantage of observations made with the Hooker 100-inch telescope of the Mount Wilson Observatory; a number of reproductions of photographs made with this telescope are presented.
Article
CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H2 "conversion factor," Xco, in different environments. In the Milky Way disk, we recommend a conversion factor Xco = 2x10^{20} cm^-2/(K km/s)^-1 with +/-30% uncertainty. Studies of other "normal galaxies" return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that Xco increases with decreasing metallicity, turning up sharply below metallicity ~1/3-1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations, Xco appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower Xco is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments.
Article
We analyze the bivariate distribution, in color versus absolute magnitude (u-r vs. Mr), of a low-redshift sample of galaxies from the Sloan Digital Sky Survey (2400 deg2, 0.004 < z < 0.08, -23.5 < Mr < -15.5). We trace the bimodality of the distribution from luminous to faint galaxies by fitting double Gaussians to the color functions separated in absolute magnitude bins. Color-magnitude (CM) relations are obtained for red and blue distributions (early- and late-type, predominantly field, galaxies) without using any cut in morphology. Instead, the analysis is based on the assumption of normal Gaussian distributions in color. We find that the CM relations are well fitted by a straight line plus a tanh function. Both relations can be described by a shallow CM trend (slopes of about -0.04, -0.05) plus a steeper transition in the average galaxy properties over about 2 mag. The midpoints of the transitions (Mr = -19.8 and -20.8 for the red and blue distributions, respectively) occur around 2 × 1010 ☉ after converting luminosities to stellar mass. Separate luminosity functions are obtained for the two distributions. The red distribution has a more luminous characteristic magnitude and a shallower faint-end slope (M* = -21.5, α = -0.8) compared to the blue distribution (α ≈ -1.3, depending on the parameterization). These are approximately converted to galaxy stellar mass functions. The red distribution galaxies have a higher number density per magnitude for masses greater than about 3 × 1010 ☉. Using a simple merger model, we show that the differences between the two functions are consistent with the red distribution being formed from major galaxy mergers.
Article
The most common means of converting an observed CO line intensity into a molecular gas mass requires the use of a conversion factor (Xco). While in the Milky Way this quantity does not appear to vary significantly, there is good reason to believe that Xco will depend on the larger-scale galactic environment. Utilising numerical models, we investigate how varying metallicities, gas temperatures and velocity dispersions in galaxies impact the way CO line emission traces the underlying H2 gas mass, and under what circumstances Xco may differ from the Galactic mean value. We find that, due to the combined effects of increased gas temperature and velocity dispersion, Xco is depressed below the Galactic mean in high surface density environments such as ULIRGs. In contrast, in low metallicity environments, Xco tends to be higher than in the Milky Way, due to photodissociation of CO in metal-poor clouds. At higher redshifts, gas-rich discs may have gravitationally unstable clumps which are warm (due to increased star formation) and have elevated velocity dispersions. These discs tend to have Xco values ranging between present-epoch gas-rich mergers and quiescent discs at low-z. This model shows that on average, mergers do have lower Xco values than disc galaxies, though there is significant overlap. Xco varies smoothly with the local conditions within a galaxy, and is not a function of global galaxy morphology. We combine our results to provide a general fitting formula for Xco as a function of CO line intensity and metallicity. We show that replacing the traditional approach of using one constant Xco for starbursts and another for discs with our best-fit function produces star formation laws that are continuous rather than bimodal, and that have significantly reduced scatter.