A&A 621, L4 (2019)
https://doi.org/10.1051/0004-6361/201834500 Astronomy
©c ESO 2019 A&strophysics
LETTER TO THE EDITOR
Insights on bar quenching from a multiwavelength analysis:
The case of Messier 95
K. George1, P. Joseph1,2, C. Mondal2, S. Subramanian2, A. Subramaniam2, and K. T. Paul1
1 Department of Physics, Christ University, Bangalore, India
e-mail: koshyastro@gmail.com
2 Indian Institute of Astrophysics, Koramangala II Block, Bangalore, India
Received 24 October 2018 / Accepted 3 December 2018
ABSTRACT
The physical processes related to the effect of bars in the quenching of star formation in the region between the nuclear/central sub-
kiloparsec region and the ends of the bar (bar region) of spiral galaxies is not fully understood. It is hypothesized that the bar can either
stabilize the gas against collapse, inhibiting star formation, or efficiently consume all the available gas, leaving no fuel for further star
formation. We present a multiwavelength study using the archival data of an early-type barred spiral galaxy, Messier 95, which shows
signatures of suppressed star formation in the bar region. Using optical, ultraviolet (UV), infrared, CO, and HI imaging data we study
the pattern of star formation progression and stellar/gas distribution, and try to provide insights into the process responsible for the
observed pattern. The FUV–NUV pixel colour map reveals a cavity devoid of UV flux in the bar region that matches the length of
the bar, which is ∼4.2 kpc. The central nuclear region of the galaxy shows a blue colour clump and along the major axis of the stellar
bar the colour progressively becomes redder. Based on a comparison to single stellar population models, we show that the region of
galaxy along the major axis of the bar, unlike the region outside the bar, is comprised of stellar populations with ages ≥350 Myr; there
is a star-forming clump in the centre of younger ages of ∼150 Myr. Interestingly the bar region is also devoid of neutral and molecular
hydrogen but has an abundant molecular hydrogen present at the nuclear region of the galaxy. Our results are consistent with a picture
in which the stellar bar in Messier 95 is redistributing the gas by funnelling gas inflows to nuclear region, thus making the bar region
devoid of fuel for star formation.
Key words. galaxies: star formation – galaxies: evolution – galaxies: formation – ultraviolet: galaxies – galaxies: individual:
Messier 95
1. Introduction non-star-forming phase without invoking morphological trans-
formation (Fraser-McKelvie et al. 2016). Red spiral galaxies are
Galaxies in the local Universe follow a bimodal distribution found to have a higher optical bar fraction than blue spiral galax-
in the optical broadband colours with the blue region mostly ies, which highlights the importance of stellar bars in quenching
populated by star-forming spiral galaxies and the red region star formation (Masters et al. 2010, 2011).
dominated by elliptical/S0 galaxies with little or no ongoing Stellar bars redistribute the disc content of galaxies via
star formation (Strateva et al. 2001; Baldry et al. 2004). How- torques and can drive the secular evolution in spiral galaxies
ever there exists a fraction of elliptical galaxies in the blue (Combes & Sanders 1981; Combes et al. 1990; Debattista et al.
region (Schawinski et al. 2009) and spiral galaxies in the red 2004; Kormendy & Kennicutt 2004 and references therein). This
region (Masters et al. 2010). The number density of red galax- is possible through the inflow of gas from the outer disc to
ies are observed to increase from z ∼ 1, which is now under- the central region, which results in an enhanced nuclear/central
stood to be at the expense of blue galaxies (Bell et al. 2004; star formation observed in barred spiral galaxies (Athanassoula
Faber et al. 2007). Several internal and external processes have 1992; Ho et al. 1997; Sheth et al. 2005; Coelho & Gadotti 2011;
been proposed as responsible for the suppression of star for- Ellison et al. 2011; Oh et al. 2012). However, apart from the
mation (i.e. a process known as “quenching”), which often enhancement of star formation at the central regions the stellar
involves morphological transformation of spiral galaxies (see bars can also suppress star formation (bar quenching) and is dis-
Peng et al. 2015; Man & Belli 2018 and references therein). cussed in recent literature based on simulations and observations
The internal processes are AGN/stellar feedback and stellar bar (Masters et al. 2010, 2012; Cheung et al. 2013; Gavazzi et al.
action, and the external process encompass ram pressure strip- 2015; Hakobyan et al. 2016; James & Percival 2016, 2018;
ping, major mergers, harassment, starvation, and strangulation. Spinoso et al. 2017; Khoperskov et al. 2018). The stellar bar in
The existence of a population of passive red spiral galaxies massive star-forming galaxies is understood to play a dominant
(van den Bergh 1976; Couch et al. 1998; Dressler et al. 1999; role in regulating the red-shift evolution of specific star forma-
Poggianti et al. 1999; Lee et al. 2008; Cortese & Hughes 2009; tion rates and mass dependent star formation quenching in field
Deng et al. 2009; Masters et al. 2010 and references therein) galaxies (Gavazzi et al. 2015). The likelihood for disc galax-
imply that galaxies can transform from the star-forming to ies to host a stellar bar is observed to be anti-correlated with
Article published by EDP Sciences L4, page 1 of 5
A&A 621, L4 (2019)
specific star formation rate regardless of stellar mass and the
prominence of bulge (Cheung et al. 2013). Barred galaxies are
also shown to have lower star formation activity relative to
unbarred galaxies (Consolandi et al. 2017, Kim et al. 2017).
Barred galaxies are found to be devoid of Hα flux in the radial
range covered by the bar region, suggesting no ongoing or recent
star formation (James et al. 2009).
However the physical processes responsible for bar quench-
ing are not well understood. There are primarily two mecha-
nisms suggested for the quenching of star formation because
of the effect of bars. During its formation the bar col-
lects most of the gas inside the co-rotation radius. Then the
bar-induced shocks and shear can stabilize the gas against
collapse by increasing turbulence and hence inhibit star for-
mation (Tubbs 1982; Reynaud & Downes 1998; Verley et al.
2007; Haywood et al. 2016; Khoperskov et al. 2018). An alter-
nate mechanism is that the bar-induced torque drives gas inflows
that enhance the nuclear star formation and making the region
close to the bar devoid of fuel for further star formation
(Combes & Gerin 1985; Spinoso et al. 2017). It is not certain
whether one of these processes or a different unknown mecha-
nism is responsible for star formation quenching in the region Fig. 1. Colour composite created from SDSS urz filter passband imagesof M 95. The RGB image is created by assigning red (z−band), green
between the nuclear/central sub-kiloparsec region and the ends (r−band), and blue (u−band) colours to the filter pass band images. The
of the bar of barred spiral galaxies. In the scenario of the sup- dust lane along the bar is seen in the colour-composite image.
pression of star formation by the stabilization of the disc due to
bar-induced torques, the gas from the bar region of the galaxy
does not need to be redistributed or depleted. Thus the pres- associated with star formation. These authors attribute this emis-
ence/absence of gas in the bar region can put strong constraints sion to be due to post asymptotic giant branch (p-AGB) stars
on identifying the mechanism responsible for bar quenching in (James & Percival 2015). These studies have suggested that the
this galaxy. In this context we present a multiwavelength study observed nuclear starburst and suppression of recent star forma-
based on the archival data of a barred spiral galaxy, Messier 95 tion (∼10 Myr) in the bar region is from the effect of the bar.
(M 95). However, the physical mechanism responsible for this observa-
M 951 (also known as NGC 3351) is a nearby (10± 0.4 Mpc; tion is not understood. All the above points make this galaxy an
Freedman et al. 2001) early-type barred spiral galaxy (Morphol- excellent candidate to study the effect of the bar on quenching
ogy; SBb). The angular scale of 1′′ corresponds to 48 pc at the star formation. Throughout this paper, we adopt a flat Universe
distance of the galaxy. M 95 has stellar mass, HI mass, H2 mass, cosmology with Ho = 71 km s
−1 Mpc−1, ΩM = 0.27, ΩΛ = 0.73
and integrated star formation rate of ∼1010.4 M , ∼109.2 M, (Komatsu et al. 2011).
∼109 M, and ∼0.940 M yr−1, respectively (Leroy et al. 2008).
The gas phase metallicity (12 + Log O/H) of M 95 is 8.60 2. Data and analysis
(Rémy-Ruyer et al. 2014). It is a nearly face-on galaxy (incli-
nation = 41◦, position angle = 192◦) with a prominent bar (see In this study we exploit the archival data of M 95 observed from
Fig. 1). High quality multiwavelength data of M 95, ranging UV to radio wavelengths as part of different campaigns. We
from radio to ultraviolet (UV), are available. This galaxy shows used the SDSS urz DR9 (Ahn et al. 2012) optical imaging data
nuclear star formation and hosts a star-forming circumnuclear of M 95 to construct a colour-composite image shown in Fig. 1
ring with a diameter of ∼0.7 kpc. This sub-kiloparsec-scale star and to demonstrate the presence of a prominent stellar bar. The
formation is well studied in X-rays (Swartz et al. 2006), UV choice of blue (u) and red (z) passband images helps to bet-
(Ma et al. 2018; Colina et al. 1997), Hα (Planesas et al. 1997; ter visualize the spatial variation of the relative contribution of
Bresolin & Kennicutt 2002), and near-infrared (Elmegreen et al. young and evolved population of stars in the galaxy. The u band
1997). In a multiwavelength study, from UV to mid-infrared, flux is negligible in the bar region, which instead is dominant
of the nuclear ring of M 95, Ma et al. (2018) presented the in the region outside the stellar bar and could be hosting intense
integrated properties of the ring and their correlation with star formation, which we address in detail using UV data.
bar strength. Mazzalay et al. (2013) presented the properties M 95 was observed in far-ultraviolet (FUV; λeff = 1538.6 Å,
of molecular gas within ∼300 pc of this galaxy using near- integration time = 1692.2 s) and near-ultraviolet (NUV; λeff =
infrared integral field spectrograph, SINFONI on the Very 2315.7 Å, integration time = 1692.2 s) wavelengths using the
Large Telescope, and suggested that the nuclear region hosts NASA GALEX mission (Martin et al. 2005). The GALEX
a stellar population of a few millionyears. Hα imaging of FUV channel imaging is at ∼4.2′′ and the NUV chan-
larger area of M 95 shows that the bar region is devoid nel imaging is at 5.3′′ resolution (Morrissey et al. 2007).
of emission (James et al. 2009). The stellar population stud- The FUV image is degraded to NUV resolution by run-
ies of this region indicate that they host an old population ning a Gaussian 2D kernel of width 0.57′′. The GALEX
(James & Percival 2016). Long-slit spectroscopy of the bar GR6/GR7 data of M 95 field observed as part of Nearby Galaxy
region showed a diffused emission that is not found to be Survey (NGS) is pipeline reduced (with good photometric
quality) and astrometry calibrated. We studied the UV properties
1 α(J2000) = 10:43:57.7 and δ(J2000) = +11:42:14 according to of this galaxy to probe recent star formation (past a few 100 Myr,
Nasa/IPAC Extragalactic Database (NED). Kennicutt & Evans 2012) over scales of ∼288 pc. The HI map of
L4, page 2 of 5
K. George et al.: Bar-induced star formation quenching in M 95
M 95 from The HI Nearby Galaxy Survey (THINGS; Walter et al.
2008) and the CO map (J2−1 transition) from CO measured by
HERA CO-Line Extragalactic Survey and Berkeley-Illinois-
Maryland Association Survey of Nearby Galaxies (HERACLES;
Leroy et al. 2009) were used to understand the gas distribution.
We used the infrared image from Spitzer IRAC 3.6 µ channel
observed as part of S4G (Sheth et al. 2010) to understand the
distribution of evolved stellar population in the bar region of the
galaxy.
The foreground extinction from the Milky Way galaxy in the
direction of M 95 is AV = 0.0762 (Schlegel et al. 1998), which we
scaled to the FUV and NUV λmean values using the Cardelli et al.
(1989) extinction law; we then corrected the magnitudes The
region of the FUV and NUV images that correspond to M 95
was isolated using the threshold set by the background counts
from the whole image. We selected pixels with values above
the 3σ of the threshold to isolate the galaxy. The counts in
the selected pixels were background subtracted, integration time
weighted, and converted to magnitude units using the zero-
points of Morrissey et al. (2007). We used the magnitudes for Fig. 2. FUV-NUV colour map of the main body of M 95. The pixels
each pixel to compute the FUV–NUV colour map of the galaxy are colour-coded in units of FUV-NUV colour. The corresponding SSP
(see Fig. 2). The pixels are colour coded in units of FUV–NUV equivalent ages are also noted in the colour bar. The contour shows the
colour. The image is of size ∼8′ × 8′ and corresponds to a phys- stellar bar detected from the Spitzer IRAC 3.6 µ image of M 95 with
ical size of ∼24 kpc on each side at the rest-frame of the galaxy. a length ∼4.2 kpc. The image measures ∼8
′ × 8′ and corresponds to a
physical size of ∼24 kpc on each side.
The FUV–NUV colour map of M 95 displays a redder region at
the centre (with an embedded small blue clump), which is sepa-
rated from the rest of the galaxy by a region with negligible UV of the galaxy M 95. The FUV–NUV pixel colour maps and the
flux. The redder region in Fig. 2 coincides with the major axis of derived ages can therefore be considered as the upper limits of
the bar of M 95. It is interesting to note that the bar region has the actual values.
negligible UV flux. This region also coincides with the region The Spitzer IRAC 3.6 µ image of a galaxy can be used as a
identified to be devoid of emission in Hα (James et al. 2009). extinction-free tracer for the evolved stellar population, which
The FUV–NUV colour map can be used to understand the dominates the underlying stellar mass (Meidt et al. 2014). The
star formation history of M 95 and can, in particular, offer Spitzer IRAC 3.6 µ image of M 95 is shown in Fig. 4 with appro-
insights into the last burst of star formation. We used the priate scaling to enhance the appearance of the stellar bar. We
Starburst99 stellar synthesis code to characterize the age of note that the stellar bar is prominent in the infrared image and
the underlying stellar population in M 95 (Leitherer et al. 1999). could be hosting evolved stellar population. The length of the
We selected 19 single stellar population (SSP) models in an age stellar bar from the infrared image is ∼87′′ (∼4.2 Kpc). The HI
range of 1–900 Myr assuming a Kroupa IMF (Kroupa 2001) and contours (black colour) and CO contours (yellow colour) are
solar metallicity (Z = 0.02). The synthetic spectral energy dis- overlaid over the Spitzer image. Comparing Figs. 2 and 4, it is
tribution for a given age was then convolved with the effective interesting to see that the 4.2 kpc diameter circular region, i.e. the
area of the FUV and NUV passbands to compute the expected length covered by stellar bar, avoiding the central nuclear region,
fluxes. We then used the estimated values to calculate the SSP lacks molecular/neutral hydrogen and star formation. The cen-
ages corresponding to the observed FUV–NUV colours. We per- tral sub-kiloparsec nuclear region of the galaxy hosts significant
formed a linear interpolation for the observed colour value and molecular gas content, star formation, and some amount of neu-
estimated the corresponding ages in all pixels in the FUV–NUV tral hydrogen.
colour map. The ages for the FUV–NUV colour is shown in the
colour bar in Fig. 2. This exercise shows that the region along the
major axis of the bar hosts stellar populations of age ≥350 Myr 3. Discussion
and the nuclear/central sub-kiloparsec region shows an embed- The stellar bar can channel the gas inwards of the central regions
ded bluer, younger clump of star formation (∼150–250 Myr). of the galaxy within which star formation can happen and is
Figure 3 shows an azimuthally averaged colour profile of M 95. proposed to be responsible for the formation of pseudo-bulge
The FUV–NUV colour has been measured in concentric annuli (Sanders & Huntley 1976; Roberts et al. 1979; Athanassoula
of width 6′′ (∼0.3 kpc). We note the striking change in the colour 1992; Ho et al. 1997; Kormendy & Kennicutt 2004; Jogee et al.
profile moving outwards, where the colour changes from blue to 2005; Lin et al. 2017; Spinoso et al. 2017). On the other hand
redder values in the very central region and finally to progres- the bar can also suppress recent star formation in galaxy discs
sively bluer colours with increasing distance from the galaxy (James & Percival 2016, 2018; Spinoso et al. 2017). Recent sim-
centre. There is a slight change to redder colours from 1′ to 1.5′ ulations demonstrate that the stellar bar to be efficient in quench-
away from the centre of the galaxy. This is the region on the ing star formation with a reduction in star formation rate by
galaxy where the stellar bar meets the outer star-forming region a factor of ten in less than 1 Gyr (Khoperskov et al. 2018).
and hosts dust lanes as seen in optical colour-composite image These simulations also predict stellar bars as long-lived fea-
(Fig. 1). The FUV and NUV flux is subjected to extinction at the tures in isolated disc galaxies with lifetimes up to ∼1000 Myr
rest-frame of the galaxy. We do not have a proper extinction map (Athanassoula et al. 2013). This implies that stellar bars can
keep the galaxy quenched for at least 109 yr and could be a domi-
2 NED. nant mechanism in shutting down star formation in galaxies over
L4, page 3 of 5
A&A 621, L4 (2019)
Verley et al. 2007; Haywood et al. 2016; Khoperskov et al. 2018),
or efficiently consume all the available gas, leaving no fuel for fur-
ther star formation (Combes & Gerin 1985; Spinoso et al. 2017 ).
The turbulence set by the bar prevents the fragmentation of molec-
ular gas within the co-rotation radius and thus suppresses star for-
mation in the bar region of the galaxy. In such a scenario the gas
in the bar region of the galaxy need not be depleted nor redis-
tributed to quench star formation. The presence, or alternatively,
the absence of the neutral and molecular hydrogen in the quenched
barred galaxies can provide insight into the mechanisms respon-
sible for bar quenching. We note that in the scenario where the
suppression of star formation is due to bar-induced turbulence, it
is not clear whether all the gas will be shock heated. Signatures
of shock heating should be seen in Hα observations.
The multiwavelength study of M 95 based on the archival
data ranging from UV, optical, infrared, neutral hydrogen, and
molecular hydrogen, as traced by CO, paint a picture of star for-
mation quenching happening in the bar region. There is no star
formation in the last 100–200 Myr as evident from the FUV–
Fig. 3. Azimuthally averaged FUV–NUV colour profile of M 95. The NUV colour map. The lack of molecular and neutral hydrogen
2′ (∼6 kpc) region of the galaxy been averaged in colour in concen- in this region implies that the stellar bar might have redistributed
tric annuli of width 6′′. The profile shows an inner blue region gradu- the gas. The stellar bar can funnel the gas to the centre and can be
ally changing to redder colours, followed by a change to blue colours. the reason for significant molecular gas content and recent star
The FUV–NUV colour values and the corresponding age estimates are formation observed in the central sub-kiloparsec nucelar/central
shown on the left and right axes, respectively. region. This can lead to nuclear starbursts and formation of sub-
structures (such as circumnuclear rings). M 95 is known to have
such features (Colina et al. 1997; Ma et al. 2018). We note that
the barred galaxies are demonstrated to have an enhanced star
formation at the centre (Ellison et al. 2011) and in the case of
M 95 also it is observed to have younger age clumps (∼150–
250 Myr). This funneling of gas to the central sub-kiloparsec
region would have depleted gas in the bar region and hence sup-
pressed star formation due to lack of fuel. On the other hand,
there is significant neutral hydrogen present outside the length
of the bar along with the presence of a young stellar population.
The absence of CO and HI in the bar region of M 95 can be
considered as a support to the scenario of gas redistribution. The
scenario of gas heating due to the stabilization of the disc by
bar-induced torques can prevent gas cooling, which in turn can
inhibit star formation. However we expect to see the signature of
such a gas heating in the form of significant Hα emission, which
is lacking along the stellar bar (within the detection limits) in the
case of M 95 as demonstrated by the Hα imaging observations of
James et al. (2009); see also James & Percival (2015) in which a
diffuse emission in Hα and [NII] 6584 Å is attributed to p-AGB
stars but shocks are not completely ruled out.
−1 We present evidence for gas redistribution due to the stel-Fig. 4. Spitzer IRAC 3.6 µ image (flux unit (MJy sr )) of Messier lar bar and subsequent star formation quenching within the bar
95 with the black contour demarcating stellar bar detected with a
length ∼4.2 kpc. The HI contour (black colour) from THINGS (levels co-rotation radius in M 95. The main result of our analysis is
−0.66,20.22,41.10,61.99,82.87 in flux unit (Jy beam−1 s−1)) and the CO a region, between the nuclear region and the ends of the bar,
contour (yellow colour) from HERACLES (levels −1.65, 2.27 in flux devoid of gas and star formation in the past a few 100 Myr. Star
unit (K Km s−1)) are overlaid. There is an offset between the HI and CO formation is quenched in this region and the absence of molec-
emission peak at the centre of the galaxy. The colour scale is adjusted ular/neutral hydrogen gas implies no further star formation is
such that the stellar bar feature is prominently seen in the IRAC 3.6 µ possible or, in other words, bar quenching is a dominant star for-
image. mation suppression mechanism in M 95. In the absence of an
external supply of gas, the star formation in the centre depletes
the molecular hydrogen completely and the galaxy is eventually
all redshift. It is therefore necessary to have a detailed under- devoid of star formation in the bar and central nuclear region.
standing of the processes operating during the bar quenching in It is not clear whether bar quenching is the dominant process
galaxies. responsible for star formation suppression in barred spiral galax-
There are primarily two mechanisms suggested for the ies in general and the redistribution of the gas due to stellar bar
quenching of star formation due to the effect of bars. The stel- is the main governing process. The pilot study reported in this
lar bar in the galaxy can either stabilize the disc against collapse, work demonstrates the capability of multiwavelength analysis in
inhibiting star formation (Tubbs 1982; Reynaud & Downes 1998; understanding the role of stellar bar in star formation progression
L4, page 4 of 5
K. George et al.: Bar-induced star formation quenching in M 95
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