The growth of the central region by acquisition of counterrotating gas in star-forming galaxies

Abstract

Galaxies grow through both internal and external processes. In about 10% of nearby red galaxies with little star formation, gas and stars are counter-rotating, demonstrating the importance of external gas acquisition in these galaxies. However, systematic studies of such phenomena in blue, star-forming galaxies are rare, leaving uncertain the role of external gas acquisition in driving evolution of blue galaxies. Here, based on new measurements with integral field spectroscopy of a large representative galaxy sample, we find an appreciable fraction of counter-rotators among blue galaxies (9 out of 489 galaxies). The central regions of blue counter-rotators show younger stellar populations and more intense, ongoing star formation than their outer parts, indicating ongoing growth of the central regions. The result offers observational evidence that the acquisition of external gas in blue galaxies is possible; the interaction with pre-existing gas funnels the gas into nuclear regions (<1 kpc) to form new stars.

ARTICLE

Received 10 Mar 2016 | Accepted 16 Sep 2016 | Published 19 Oct 2016

DOI: 10.1038/ncomms13269 OPEN

The growth of the central region by acquisition of counterrotating gas in star-forming galaxies

Yan-Mei Chen1,2,3, Yong Shi1,2,3, Christy A. Tremonti4, Matt Bershady4, Michael Merrield5, Eric Emsellem6,7, Yi-Fei Jin1,2,3, Song Huang8, Hai Fu9, David A. Wake10, Kevin Bundy8, David Stark8, Lihwai Lin11,

Maria Argudo-Fernandez12,13, Thaisa Storchi Bergmann14,15, Dmitry Bizyaev16,17, Joel Brownstein18,Martin Bureau19, John Chisholm4, Niv Drory20, Qi Guo21, Lei Hao12, Jian Hu22,23, Cheng Li22,23, Ran Li21, Alexandre Roman Lopes24, Kai-Ke Pan16, Rogemar A. Riffel15,25, Daniel Thomas26, Lan Wang21, Kyle Westfall26 & Ren-Bin Yan27

Galaxies grow through both internal and external processes. In about 10% of nearby red galaxies with little star formation, gas and stars are counter-rotating, demonstrating the importance of external gas acquisition in these galaxies. However, systematic studies of such phenomena in blue, star-forming galaxies are rare, leaving uncertain the role of external gas acquisition in driving evolution of blue galaxies. Here, based on new measurements with integral eld spectroscopy of a large representative galaxy sample, we nd an appreciable fraction of counter-rotators among blue galaxies (9 out of 489 galaxies). The central regions of blue counter-rotators show younger stellar populations and more intense, ongoing star formation than their outer parts, indicating ongoing growth of the central regions. The result offers observational evidence that the acquisition of external gas in blue galaxies is possible; the interaction with pre-existing gas funnels the gas into nuclear regions (o1 kpc) to form new stars.

1 School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China. 2 Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China. 3 Collaborative Innovation Center of Modern Astronomy and Space Exploration, Nanjing 210093, China. 4 Department of Astronomy, University of Wisconsin-Madison, 1150 University Ave, Madison,Wisconsin 53706, USA. 5 School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 6 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany. 7 Universit Lyon 1, Observatoire de Lyon, Centre de Recherche Astrophysique de Lyon and Ecole Normale Suprieure de Lyon, 9 avenue Charles Andr, F-69230 Saint-Genis Laval, France. 8 Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), Todai Institutes for Advanced Study, the University of Tokyo, Kashiwa 277-8583, Japan. 9 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, USA. 10 Department of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK. 11 Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 106, Taiwan. 12 Shanghai Astronomical Observatory, Nandan Road 80, Shanghai 200030, China. 13 Universidad de Antofagasta, Unidad de Astronoma, Facultad Cs. Bsicas, Av. U. de Antofagasta, 02800 Antofagasta, Chile. 14 Departamento de Astronomia, Instituto de Fsica, Universidade Federal do Rio Grande do Sul, CP 15051, 91501-970, Porto Alegre, RS, Brazil. 15 Laboratrio Interinstitucional de e-AstronomiaLIneA, Rua Gal. Jos Cristino 77, Rio de Janeiro, RJ 20921-400, Brazil. 16 Apache Point Observatory and New Mexico State University, P.O. Box 59, Sunspot, New Mexico 88349-0059, USA.

17 Sternberg Astronomical Institute, Moscow State University, Moscow 119899, Russia. 18 Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA. 19 Sub-Department of Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK. 20 Department of Astronomy and Astrophysics, University of California, Santa Cruz, California 95064, USA. 21 National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang, Beijing 10012, China. 22 Department of Physics, Tsinghua University, Beijing 100084, China. 23 Center for Astrophysics, Tsinghua University, Beijing 100084, China. 24 Departamento de Fsica, Facultad de Ciencias, Universidad de La Serena, Cisternas 1200, La Serena, Chile.

25 Departamento de Fsica, Centro de Cincias Naturais e Exatas, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil. 26 Institute for Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Burnaby Road, Portsmouth PO1 3FX, UK. 27 Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, Kentucky 40506-0055, USA. Correspondence and requests for materials should be addressed to Y.-M.C. (email: mailto:[email protected]

Web End [email protected] ).

NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 1

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13269

In the framework of hierarchical structure formation, a galaxy grows from primordial density uctuations and its subsequent evolution is shaped by a series of external and internal

processes. Galaxies with gas and stars counter-rotating are the key demonstrations for the regulation by external processes1,2. External processes, for example major mergers, minor mergers or gas accretion, could bring gas which is counter-rotating with pre-existing stars into the galaxies. On the other hand, the gas produced by internal processes such as stellar evolution would conserve the angular momentum of stars and be co-rotating with pre-existing stars.

Phenomenon of gas and star counter-rotating is now known to be ubiquitous in elliptical and lenticular galaxies. Still, the incidence of gas-star counter-rotators in blue star forming galaxies is largely unknown. Since the early discoveries of individual cases3, systematic studies with long-slit spectroscopy have reported a fraction as high as 25% (refs 46) in early type galaxies, which decreased to a value of 1015% with integral-eld spectroscopy79. While a few individual cases of blue counter-rotators are found1013, existing statistical studies of blue galaxies failed to identify any blue counter-rotators due to limited sample size6,14 and instrumentation (for example, the limited ability of long-slit spectroscopy to effectively identify the pattern of the star-gas counter-rotating out of complicated kinematics, particularly in barred spirals15).

To place much stronger constraints on the incidence of blue counter-rotators, and to understand the inuence of gas accretion on the evolution of blue star forming galaxies, in this work we study a sample of galaxies observed with bre-optic integral-eld units (IFU) in the rst year of the survey: Mapping Nearby Galaxies at Apache Point Observatory (MaNGA)16, nding B2%

blue star-forming galaxies have counter-rotating gas. The central regions of blue counter-rotators show younger stellar populations and more intense, ongoing star formation than their outer-skirts, indicating that these galaxies accrete abundant external gas, the interaction with pre-existing gas triggers the gas into central regions and form new stars.

ResultsSample selection. We analyse gas and stellar kinematic maps of a representative sample of 1,351 nearby galaxies with stellar masses above 109 solar mass from MaNGA. Figure 1 shows an example of a counter-rotating blue star forming galaxy. The Sloan Digital Sky Survey (SDSS) false-colour image is at left, while the kinematics based on spectroscopic IFU data for stars and gas are

mapped in the second and third columns (velocities and velocity dispersions, respectively). To quantify the kinematic misalignment between stars and gas, we measured the difference in the kinematic position angle (PA) between ionized gas and stars as DPA |PA* PAgas|, where PA* is the PA of stars and PAgas is the

PA of ionized gas. The kinematic PA is measured based on established methods17, dened as the counter-clockwise angle between north and a line that bisects the velocity eld of gas or stars, measured on the receding side. The solid lines in Fig. 1 show the best t position angle and the two dashed lines show the 1s error. The last two columns show the rotation velocity and velocity dispersion along the major axis.

We matched the MaNGA sample with the literature catalogue18 to obtain the global star formation rate (SFR) and stellar mass (M*) for 1,220 out of 1,351 galaxies. With these two quantities we classify the sample into blue star-forming galaxies, red quiescent galaxies with little star formation and green-valley galaxies between these two extremes (Fig. 2a), as summarized in Table 1. For simplicity, we refer to these three classes as blue, red and green galaxies henceforth. Figure 2b shows the distributions of DPA for these different types of galaxies with nebular emission (required to measure the gas kinematics). Both green (green histogram) and red (red histogram) galaxies have a distribution of the DPA, with the three local peaks at DPA 0, 90 and 150,

while blue galaxies (blue histogram) present a bimodal distribution (the lack of a third peak at 90 being consistent with small number statistics). The grey histogram is for the whole populationthe combination of blue, red and green. In total there are 43 counter-rotators, that is, galaxies with DPA4150.

Considering the completeness correction of the MaNGA sample, the fraction of the counter rotators in blue galaxies is 2% (9 out of 489), while the fractions in red and green galaxies are 10% (16 out of 164) and 6% (18 out of 280), respectively. Our fraction of counter-rotators in the red galaxies is consistent with previous studies4,5,79,19. Thanks to the unbiased MaNGA galaxy sample with respect to morphology, inclination, colour and so on, we can study the incidence as well as the properties of blue counter-rotators for the rst time. The above fractions could be lower limits, since for face-on galaxies, it is not possible to measure rotation.

Properties of blue star-forming counter-rotators. Among nine blue counter-rotators, six of them have strong positive gradients in the 4,000 break (D4000), as shown in Fig. 3, while the remaining show small D4000 across the whole galaxy body,

10 vstar

10 vgas

10 star

10 gas

100 vstar

100 star

20 gas

5 N

5

5

50

80

0

0

0

60

5

5

50

40

E

W

Arcsec

Arcsec

km/s

100

vgas

km/s

5

5

50

80

0

0

0

60

S

5

5

50

40

100 10 5 0 5 10 10 5 0 5 10

20

10 10 5 0 5 10Arcsec

10 10 5 0 5 10Arcsec

Along PA (arcsec)

Figure 1 | An example of a blue star-forming counter-rotating galaxy. The left panel shows the SDSS g, r, iband image, the projected velocity elds of stars (top) and gas (bottom) are shown in the second column, while the third column shows the velocity dispersion maps of stars and gas. The projected velocity and velocity dispersion along major axis (black solid line in the second column) are shown in the last two columns. Dashed black lines represent 1s uncertainties in the major-axis position angle.

2 NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13269 ARTICLE

a

b

1

0

100.0

log SFR/(M yr1 )

1

Number

10.0

2

3

1.0

4

5 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

0.1

0 50 100 150PA (deg)

log M*/M

Figure 2 | SFRs versus stellar masses and DPA distribution. (a) SFRs versus stellar mass. Contours show the SDSS DR7 sample, while the red dots are MaNGA galaxies. The blue dots are the counter-rotators with DPA4150. The two dashed lines separate the galaxies into blue star-formers, green valley and red quiescent galaxies. The black dashed line is adopted from Fig. 11 of ref. 18 as an approximation of the boundary (at the 1s level in scatter) of the star-forming main sequence. The green solid line with log sSFR ( SFR/M*) B 15 remarks red galaxies, in which the SFR can be neglected. The region

between the black and green dashed lines is referred as the green valley. Although galaxies in the green valley have low SFR, they are clearly distinguished from red galaxies. We do not use the colour-magnitude diagram to separate blue from green and red galaxies since the colours are strongly effected by dust extinction. (b) DPA distribution for MaNGA galaxies with nebular emission. The grey histogram is for the whole sample, red for the red quiescent galaxies.

Table 1 | Classication of the MaNGA sample.

Type Number (number with EML) Misalignment Counter-rotators

(DPA430) (DPA4150)

Blue 489 (489) 10 9

Green 377 (280) 26 18 Red 354 (164) 30 16 Total 1220 (933) 66 43

This table gives the number of galaxies in each catagory. blue: blue star forming galaxies; green: green valley; red: red quiescent galaxies. Misalignment and counter-roators are classied by DPA given in the table. EML means galaxies with emission lines; the number of galaxies with line emission is in parenthesis.

indicating young stellar populations existing in the central regions. The map of the Ha ux further shows ongoing star formation in the central region. We checked the emission line ratio diagnostic20 to assure that the Ha radiation is dominated by star formation instead of active galactic nuclei (AGN; Fig. 5). In contrast to the blue counter-rotators, all the green and red counter-rotators have negative D4000 gradients with older stellar populations in the central regions. Although the Ha ux also peaks at the center for the green and red counter rotators, it is primarily contributed by the AGN based on the emission-line diagnostic20.

To further quantify the importance of the ongoing star formation in growing the central region, we introduce the star formation activity parameter21 as aSF 1/(sSFR (tH(z) 1 Gyr)),

where tH(z) is the Hubble time at the redshift of the galaxy, and 1 Gyr is subtracted to account for the fact that star formation mainly occurred after reionization. If a galaxys current SFR is equal to its past average (M*/((tH(z) 1 Gyr)) then aSF 1; values

less than one indicate that the current SFR is higher than the past average. As shown in Fig. 4, all nine galaxies present a steep rising aSF with increasing distances from the galaxy center. The grey shaded regions show the 1s range of aSF for the central 1 kpc of local star forming galaxies with DPAo30. Grey lines mark the median value of B0.75. Focusing on the central 1 kpc, we nd six of the blue counter rotators have aSF about one order of magnitude

smaller than the average value (the grey line), indicating fast growth of the central components of these galaxies.

Both the D4000 and star formation activity parameter aSF

suggest signicant ongoing growth of the central region (o1 kpc) of these blue counter-rotators by star formation. For nine blue counter-rotators, we t the r-band surface brightness proles (Figs 6 and 7) and found that ve of them already have photometric bulge-like components (above an exponential disk-like component). In addition, the SDSS images show no signs of strong galaxy interactions or major merging, indicating accretion of gas from intergalactic medium or dwarfs (minor mergers) as the origin of the counter-rotating gas. This is also consistent with their environments, as both the neighbour number (N) and the tidal strength parameter22 (Qlss) indicate that the blue counter rotators tend to be located in more isolated environments. By matching our galaxies with the MPA-JHU catalogue (http://wwwmpa.mpa-garching.mpg.de/SDSS/DR7/oh.html

Web End =http://wwwmpa.mpa-garching.mpg.de/SDSS/DR7/oh.html), we obtained the metallicity for eight blue counter-rotators. Four of them follow the stellar mass versus metallicity relation of the general population23, while another four lie 0.20.3 dex above the stellar-mass versus metallicity relation.

DiscussionWe suggest the following scenario to explain the above observational facts: (i) The progenitor accretes counter-rotating gas from a gas-rich dwarf or cosmic web. (ii) Redistribution of angular momentum occurs from gasgas collisions between the pre-existing and the accreted gas largely accelerates gas inow, leading to a fast centrally-concentrated star formation.(iii) Higher metallicity is a puzzle, one possibility is due to the enrichment from star formation. In a closed-box model24, the metallicity will mainly depend on the gas mass fraction fgas ( Mgas/(Mgas M

stars)), so the abundances get elevated instantaneously as a large fraction of the available gas turns into stars. The low D4000 at the center is a hint that such stars exist. However, we keep in mind that the external gas likely had low metallicity and the closed-box model is a strong assumption, future simulations are necessary in helping us to understand the gas enrich process.

Though the amount of pre-existing and accreted gas in the nine galaxies is uncertain, collision between pre-existing and

NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 3

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13269

D4000

5 0 5

H flux

5 0 5

25.0 D4000

5 0 5

H flux

5 0 5

2.0

2.0

19.4

1-337898

1-113698

5

5

0

1.5

12.5

0

1.5

9.7

5

5

1.0

0.0

1.0

0.0

2.0

45.2

2.0

82.2

1-138140

1-137890

10

5

5

1.5

22.6

0

1.5

41.2

0

5

5

10 10 5 0 5 10

1.0

0.0

1.0

0.1

10 5 0 5 10

5 0 5

5 0 5

2.0

26.2

2.0

9.9

1-134004

1-251198

10

5

5

0

1.5

13.1

1.5

4.9

0

5

5

1.0

0.0

1.0

0.0

5 0 5

5 0 5

10 10 5 0 5 10

10 5 0 5 10

2.0

69.9

2.0

42.8

1-255878

1-419380

10

5

5

0

1.5

35.0

1.5

21.4

0

5

5

1.0

0.2

1.0

0.1

5 0 5

5 0 5

10 10 5 0 5 10Arcsec

10 5 0 5 10Arcsec

2.0

8.2

1-487046

10

1.5

(1017 erg s1 cm2 )

5

4.1

0

5

1.0

0.0

10 10 5 0 5 10Arcsec

10 5 0 5 10Arcsec

Figure 3 | The D4000 and Ha ux maps for nine star-forming counter rotators. The MaNGA-ID for each galaxy is shown in the D4000 map. The Ha ux is in the unit of 10 17 erg s 1 cm 2.

accreted gas is unavoidable, leading to redistribution of angular momentum and dissipation of kinetic energy. The impact on both the morphology and dynamics of the inner parts of the galaxy may thus be associated with the observed slight increase of the gas velocity dispersion. We nd the typical gas velocity dispersion (4060 km s 1) in the disk region of these nine galaxies is about 20 km s 1 larger than a control sample of star-forming galaxies

with aligned gas and stellar kinematics (DPAo30), closely matched in SFR, M* and redshift. The typical errors of gas velocity dispersion is about 10 km s 1.

In summary, redistribution of angular momentum through the collisions between accreted and pre-existing gas is thus an efcient way for gas to migrate to the centre, indicating that accretion of counter-rotating gas into disk galaxies is an effective

4 NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13269 ARTICLE

10.0

1.0

0.1

0 1 2 3

0.0 0.5 1.0 1.5 2.0 0 2 4 6 8

SF= 1/[sSFR (t H(z)1 Gyr)]

10.0

1.0

0.1

0 1 2 3 4

0.0 0.5 1.0 1.5 2.0 2.5 0 2 4 6

10.0

1.0

0.1

0 1 2 3 4R (kpc)

0 1 2 3

0 1 2 3

Figure 4 | The star formation activity parameter aSF versus radius for the nine blue star forming counter rotators. The circles are our data points while the red lines show the median. The grey shaded regions show the 1s range of aSF for the central 1kpc of local star forming galaxies with DPAo30. Grey lines mark the median value of B0.75.

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

way to grow the central region. This mechanism may be more effective in growing the central component of galaxies at zB12 where external gas acquisition is more frequent25,26.

Methods

Observations and data reduction. The data used in this work comes from the ongoing MaNGA survey16,2729 using the SDSS 2.5-in telescope30 and Baryon oscillation spectroscopic survey spectrographs31. As one of three programs comprising the SDSS-IV, MaNGA is obtaining spatially resolved spectroscopy for about 10,000 nearby galaxies with log M =M 9, and a median redshift of

zE0.04. The r-band signal-to-noise ratio (S/N) in the outskirts of MaNGA galaxies is 48 1, and the wavelength coverage is 3,600 10,300 . MaNGAs effective

spatial and spectral resolution is 200.4 (Full Width at Half Maximum, FWHM) and sB60 km s 1, respectively. The MaNGA sample and data products used here were drawn from the internal MaNGA Product Launch-4 (MPL-4), which includes

E1,400 galaxies observed through July 2015 (the rst year of the survey).

The MaNGA data analysis pipeline, which uses pPXF32 and the MIUSCAT stellar library33, ts the stellar continuum in each spaxel and produces estimates of the stellar kinematics. Ionized gas kinematics, vgas and sgas, as well as the ux were

estimated by tting a single Gaussian to the emission lines after stellar continuum subtraction. The observables used in this work, that is, vgas and sgas, D4000,

emission line ux, are from data analysis pipeline.

Redshift distributions of the samples. In Fig. 5, we show the redshift distributions of the whole MaNGA sample (black histogram), the blue (blue histogram),

1.0

0.8

0.6

N

0.4

0.2

0.0

Redshift

Figure 5 | The redshift distributions of the samples. The grey histogram is for the whole MaNGA sample; the blue, red and green histograms show the redshift distributions for the blue, red and green sub-samples, respectively; the nine blue counter rotators are shownin cyan histogram.

NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 5

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13269

1-337898

Vstar

5 0 5Arcsec

Vgas

5 0 5Arcsec

Resolved BPT

5 0 5Arcsec

5 N

18

150.0

150.0

5

Mag/arcsec2

Mag/arcsec2 Mag/arcsec2

20

E

22

Arcsec

W

0

0.0

0.0

24

5

S

150.0

150.0

26 0 5 10 15 20Arcsec

1-113698

5

5

5

5

N

18

120.0

120.0

5

20

E

W

Mag/arcsec2

Mag/arcsec2

22

Arcsec

0

0.0

0.0

24

5

S

120.0

120.0

26 0 5 10 15 20Arcsec

5 0 5Arcsec

5 0 5Arcsec

5 0 5Arcsec

1-138140

N

18

120.0

120.0

10

20

5

E

W

22

Arcsec

0.0

0.0

0

24

5

S

120.0

120.0

26 0 5 10 15 20Arcsec

10 10 5 0 5 10Arcsec

10 5 0 5 10Arcsec

10 5 0 5 10Arcsec

1-137890

N

18

120.0

100.0

5

20

E

22

Arcsec

W

0

0.0

0.0

24

5

S

26

120.0

100.0

0 5 10 15 20 Arcsec

5 0 5Arcsec

5 0 5Arcsec

5 0 5Arcsec

1-134004

N

18

130.0

80.0

5

20

E

22

Arcsec

W

0

0.0

0.0

24

5

S

130.0

80.0

26 0 5 10 15 20Arcsec

5 0 5Arcsec

5 0 5Arcsec

5 0 5Arcsec

Figure 6 | Properties of the blue counter rotators. Left: the SDSS false-colour image; second column: the surface brightness prole, black is the data, green is the best t model. Except for the rst object, all the others are tted by two components (red blue); the third and fourth columns show the

velocity elds of stars and gas, respectively. The velocities are in the unit of km s 1. The spatial resolved BPT diagram20 is shown in the last column, blue represents star forming region, red represents Seyfert, green is the composite of AGN and star formation and yellow represents Low-Ionization Emission-line Region (LIER).

6 NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13269 ARTICLE

1-251198

Vstar

10 10 5 0 5 10Arcsec

Vgas

10 5 0 5 10Arcsec

Resolved BPT

10 5 0 5 10Arcsec

5 N

18

200.0

200.0

10

20

E

5 N

W

Mag/arcsec2

Mag/arcsec2

Mag/arcsec2

Mag/arcsec2

5

22

Arcsec

0.0

0.0

0

24

5

S

200.0

200.0

26 0 5 10 15 20Arcsec

1-255878

18

80.0

80.0

5

20

E

5 N

W

22

Arcsec

0

0.0

0.0

24

5

S

26

80.0

80.0

0 5 10 15 20 Arcsec

5 0 5Arcsec

5 0 5Arcsec

5 0 5Arcsec

1-419380

18

100.0

100.0

10

20

5

E

5 N

Arcsec

W

22

0.0

0.0

0

24

5

S

100.0

100.0

26 0 5 10 15 20Arcsec

10 10 5 0 5 10Arcsec

10 5 0 5 10Arcsec

10 5 0 5 10Arcsec

1-487046

18

100.0

100.0

10

20

5

E

W

22

Arcsec

0.0

0.0

0

24

5

S

100.0

100.0

26 0 5 10 15 20Arcsec

10 10 5 0 5 10Arcsec

10 5 0 5 10Arcsec

10 5 0 5 10Arcsec

Figure 7 | Properties of the blue counter rotators. Same as Fig. 6, but with more objects.

red (red histogram) and green (green histogram) subsamples, as well as the nine blue counter rotators (cyan histogram).

Sample completeness correction. An issue with every data set is the selection of weights to correct for missing galaxies. The MaNGA target sample is selected to lie within a redshift range, zminozozmax, that depends on absolute iband magni

tude in the case of the Primary and Secondary samples, and absolute iband magnitude and NUVr colour in the case of the colour-enhanced (CE) sample. zmin and zmax are chosen to yield both the same number density of galaxies and angular size distributions, matched to the IFU sizes, at all absolute iband magnitudes (or magnitudes and colours for the CE sample). This results in lower, and narrower, redshift ranges for less luminous galaxies and higher and wider redshift ranges for more luminous galaxies.

At a given Mi (or Mi and NUVr colour for the CE sample) the sample is effectively volume limited in that all galaxies within zmin(Mi)ozozmax(Mi) are

targeted irrespective of their other properties. However, that volume varies with Mi. Therefore in any analysis of the properties of MaNGA galaxies as a function of anything other than Mi we must correct for this varying selection volume, Vs(Mi) the volume with zmin(Mi)oz ozmax(Mi). The simplest approach is just to correct

the galaxies back to a volume-limited sample by applying a weight (W) to each galaxy in any calculation such that W Vr/Vs, where Vr is an arbitrary reference

volume. Since the zmin and zmax for each MaNGA galaxy are provided in the

MaNGA sample catalogue (Wake et al., in preparation), we can easily estimate the fraction of galaxies with decoupled gas and star kinematics in a complete sample by applying this volume correction.

Global SFR and M*. Combining SDSS and wide-eld infrared survey explorer photometry for the full SDSS spectroscopic galaxy sample, the spectral energy distributions that cover l 0.422 mm has been created for a sample of 858,365

present-epoch galaxies18. Using MAGPHYS34, they then model both the attenuated stellar spectral energy distributions and the dust emission at 12 and 22 mm, producing new calibrations for monochromatic mid-IR SFR proxies, as well as M*.

Spatially resolved SFR and M*. Principal component analysis (PCA) is a standard multivariate analysis technique, designed to identify correlations in large data sets. Using PCA, a new method35 has been generated to estimate stellar masses, mean stellar ages, star formation histories, dust extinctions and stellar velocity dispersions for galaxies from Baryon oscillation spectroscopic survey . To obtain these results, we use the stellar population synthesis models of BC03 (ref. 36) to generate a library of model spectra with a broad range of star formation histories, metallicities, dust extinctions and stellar velocity dispersions. The PCA is run on this library to identify its principal components (PC) over a certain rest-frame

NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 7

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13269

wavelength range 3,700 5,500 . We then project both the model spectra and the

observed spectra onto the rst seven PCs to get the coefcients of the PCs, which represents the strength of each PC presented in the model or observed spectra. We derive statistical estimates of various physical parameters by comparing the projection coefcients of the observed galaxy to those of the models as follows. The w2 goodness of t of each model determines the weight Bexp( w2/2) to be

assigned to the physical parameters of that model, when building the probability distributions of the parameters of the given galaxy. The probability density function of a given physical parameter is thus obtained from the distribution of the weights of all models in the library. We characterize the probability density function using the median and the 1684% range (equivalent to 1s range for

Gaussian distributions). In this work, we directly apply this PCA method to the MaNGA data to get the stellar mass for each spaxel.

The SFR for each spaxel is derived from the dereddened Ha luminosity (LHa) as SFR (M yr 1) 7.9 10 42 LHa(erg s 1). We use Balmer decreasement for

dust extinction correction.

Environment. We characterize the environment with two parameters, the neigh-bour number (N) and the tidal strength parameter Qlss. The neighbour number is dened as the count of galaxies brighter than 19.5 mag in r-band absolute

magnitude within a xed volume of 1 Mpc in projected radius and 500 km s 1 in redshift to the primary galaxy. Given the neighbour number is independent of the stellar mass and cannot account for the interaction a galaxy suffering from its satellites, we also use the tidal strength parameter Qlss to depict the effect of total interaction strength produced by all the neighbours within the xed volume22,37; the higher the parameter, the stronger the interaction. The parameter Qlss is

dened as

Qlss log

XiMiMpDp di

3

" #

1

where Mi and Mp are the stellar masses of the ith neighbour and the primary galaxy. di is the projected distance from the primary galaxy to the ith satellite and Dp is the estimated diameter of the central galaxy22. Both the number of neighbours and Qlss are drawn from the catalogue generated by Argudo-Fernndez et al.

Surface brightness prole. We t the surface brightness proles of the nine blue counter rotators with three different models: (1) single Sersic; (2) double Sersic;(3) Sersic bulge exponential disk. The best tting results are shown in Figs 6 and

7.

Data availability. The data supporting the ndings of this study are available through SDSS Data Release Thirteen which can be downloaded from http://www.sdss.org/dr13/manga/

Web End =http://www.sdss.org/dr13/manga/ .

References

1. Rubin, V. C. Kinematics of NGC 4826: a sleeping beauty galaxy, not an evil eye. Astron. J. 107, 173183 (1994).

2. Schweizer, F. & Seitzer, P. Ages and metallicities of Young globular clusters in the merger remnant NGC 7252. Astron. J. 116, 22062219 (1998).

3. Galletta, G. Detection of retrograde gas streaming in the SB0 galaxy NGC 4546. Astrophys. J. 318, 531535 (1987).

4. Bertola, F. et al. The external origin of the gas in S0 galaxies. Astrophys. J. Lett. 401, L79L81 (1992).

5. Kuijken, K., Fisher, D. & Merrield, M. R. A search for counter-rotating stars in S0 galaxies. Mon. Not. R. Astron. Soc. 283, 543550 (1996).

6. Kannappan, S. J. & Fabricant, D. G. A broad search for counterrotating gas and stars: evidence for mergers and accretion. Astron. J. 121, 140147 (2001).7. Davis, T. A. et al. The ATLAS3D projectX. On the origin of the molecular and ionized gas in early-type galaxies. Mon. Not. R. Astron. Soc. 417, 882899 (2011).

8. Sarzi, M. et al. The SAURON projectV. Integral-eld emission-line kinematics of 48 elliptical and lenticular galaxies. Mon. Not. R. Astron. Soc. 366, 11511200 (2006).

9. Barrera-Ballesteros, J. K. et al. Tracing kinematic (mis)alignments in CALIFA merging galaxies. Stellar and ionized gas kinematic orientations at every merger stage. Astron. Astrophys. 582, A21 (2015).

10. Ciri, R., Bettoni, D. & Galletta, G. A massive counter-rotating gas disk in a spiral galaxy. Nature 375, 661663 (1995).

11. Bertola, F. et al. Counterrotating stellar disks in early-type spirals: NGC 3593. Astrophys. J. Lett. 458, L67 (1996).

12. Thakar, A. R., Ryden, B. S., Jore, K. P. & Broeils, A. H. NGC 4138: a case study in counterrotating disk formation. Astrophys. J. 479, 702722 (1997).

13. de Zeeuw, P. T. et al. The SAURON projectII. Sample and early results. Mon. Not. R. Astron. Soc. 329, 513530 (2002).

14. Pizzella, A., Corsini, E. M., Vega Beltrn, J. C. & Bertola, F. Ionized gas and stellar kinematics of seventeen nearby spiral galaxies. Astron. Astrophys. 424, 447454 (2004).

15. Galletta, G. Counterrotation and barred galaxies. ASPC 91, 429 (1996).

16. Bundy, K. et al. Overview of the SDSS-IV MaNGA Survey: mapping nearby Galaxies at Apache Point Observatory. Astrophys. J. 798, 7 (2015).17. Krajnovi, D., Cappellari, M., de Zeeuw, P. T. & Copin, Y. Kinemetry:a generalization of photometry to the higher moments of the line-of-sight velocity distribution. Mon. Not. R. Astron. Soc. 366, 787802 (2006).

18. Chang, Y.-Y., van der Wel, A., da Cunha, E. & Rix, H.-W. Stellar masses and star formation rates for 1M galaxies from SDSS WISE. Astrophys. J. Suppl.

219, 8 (2015).19. Krajnovi, D. et al. The ATLAS 3D projectII. Morphologies, kinemetric features and alignment between photometric and kinematic axes of early-type galaxies. Mon. Not. R. Astron. Soc. 414, 29232949 (2011).

20. Baldwin, J. A., Phillips, M. M. & Terlevich, R. Classication parameters for the emission-line spectra of extragalactic objects. Publ. Astron. Soc. Pac. 93, 519 (1981).

21. Dav, R. The galaxy stellar mass-star formation rate relation: evidence for an evolving stellar initial mass function? Mon. Not. R. Astron. Soc 385, 147160 (2008).

22. Argudo-Fernndez, M. et al. Catalogues of isolated galaxies, isolated pairs, and isolated triplets in the local Universe. Astron. Astrophys. 578, A110 (2015).

23. Tremonti, C. et al. The origin of the mass-metallicity relation: insights from 53,000 star-forming galaxies in the sloan digital sky survey. Astrophys. J. 613, 898913 (2004).

24. Dalcaton, J. The metallicity of galaxy disks: infall versus outow. Astrophys. J. 658, 941959 (2007).

25. Genzel, R. et al. The SINS/zC-SINF survey of zB2 galaxy kinematics: evidence for gravitational quenching. Astrophys. J. 785, 75 (2014).

26. Danovich, M., Dekel, A., Hahn, O., Ceverino, D. & Primack, J. Four phases of angular-momentum buildup in high-z galaxies: from cosmic-web streams through an extended ring to disc and bulge. Mon. Not. R. Astron. Soc. 449, 20872111 (2015).

27. Drory, N. et al. The MaNGA integral frield unit ber feed system for the Sloan2.5 m telescope. Astron. J. 149, 77 (2015).28. Law, D. et al. Observing strategy for the SDSS-IV/MaNGA IFU galaxy survey. Astron. J. 150, 19 (2015).

29. Yan, R. B. et al. SDSS-IV/MaNGA: spectrophotometric calibration technique. Astron. J. 151, 8 (2016).

30. Gunn, J. E. et al. The 2.5 m telescope of the Sloan Digital Sky Survey. Astron. J. 131, 2332 (2006).

31. Smee, S. A. et al. The multi-object, ber-fed spectrographs for the sloan digital sky survey and the baryon oscillation spectroscopic survey. Astron. J. 146, 32 (2013).

32. Cappellari, M. & Emsellem, E. Parametric recovery of line-of-sight velocity distributions from absorption-line spectra of galaxies via penalized likelihood. Publ. Astron. Soc. Pac. 116, 138147 (2004).

33. Vazdekis, A. et al. MIUSCAT: extended MILES spectral coverageI. stellar population synthesis models. Mon. Not. R. Astron. Soc. 424, 157 (2012).

34. da Cunha, E., Charlot, S. & Elbaz, D. A simple model to interpret the ultraviolet, optical and infrared emission from galaxies. Mon. Not. R. Astron. Soc. 388, 15951617 (2008).

35. Chen, Y.-M. et al. Evolution of the most massive galaxies to z 0.6I. A new

method for physical parameter estimation. Mon. Not. R. Astron. Soc. 421, 314332 (2012).36. Bruzual, G. & Charlot, S. Stellar population synthesis at the resolution of 2003. Mon. Not. R. Astron. Soc. 344, 10001028 (2003).

37. Verley, S. et al. The AMIGA sample of isolated galaxies. V. Quantication of the isolation. Astron. Astrophys. 472, 121130 (2007).

Acknowledgements

Y.M.C. acknowledges support from NSFC grant 11573013, 11133001, the Natural Science Foundation of Jiangsu Province grant BK20131263, the Opening Project of Key Laboratory of Computational Astrophysics, National Astronomical Observatories, Chinese Academy of Sciences. Y.S. acknowledges support from NSFC grant 11373021, the CAS Pilot-b grant no. XDB09000000 and Jiangsu Scientic Committee grant BK20150014. C.A.T. acknowledges support from National Science Foundation of the United States grant no. 1412287. Funding for the Sloan Digital Sky Survey IV has been provided by the AlfredP. Sloan Foundation, the U.S. Department of Energy Ofce of Science and the Participating Institutions. SDSS-IV acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is http://www.sdss.org

Web End =www.sdss.org . SDSS-IV is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofsica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU)/University of Tokyo, Lawrence Berkeley National Laboratory, Leibniz Institut fr Astrophysik Potsdam (AIP), Max-Planck-Institut fr Astronomie (MPIA Heidelberg), Max-Planck-Institut fr Astrophysik (MPA Garching), Max-Planck-Institut fr Extraterrestrische Physik (MPE), National Astronomical Observatory of China, New Mexico State University, New York University,

8 NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13269 ARTICLE

University of Notre Dame, Observatrio Nacional/MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autnoma de Mxico, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University and Yale University.

Author contributions

Y.M.C. discovered these sources, studied their properties and led the writing of the manuscript. Y.S., C.A.T., M.B., M.M. and E.E. provided the picture to explain all the observation results. Y.S. and M.M. also helped the writing of the manuscript. Y.F.J. helped in making plots. S.H. tted the surface brightness prole. D.A.W. helped with the sample completeness correction. D.S. searched for deeper images of these galaxies. L.L and M.A.F. provided the environment parameters. K.B., R.B.Y., M.B., N.D., D.A.W., D.T. and D.B. contributed to the design and execution of the survey. All authors commented on the manuscript and contributed to the interpretation of the observations.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications

Web End =http://www.nature.com/ http://www.nature.com/naturecommunications

Web End =naturecommunications

Competing nancial interests: The authors declare no competing nancial interests.

Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/

Web End =http://npg.nature.com/ http://npg.nature.com/reprintsandpermissions/

Web End =reprintsandpermissions/

How to cite this article: Chen, Y.-M. et al. The growth of the central region by acquisition of counterrotating gas in star-forming galaxies. Nat. Commun. 7, 13269doi: 10.1038/ncomms13269 (2016).

Publishers note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afliations.

This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the articles Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

Web End =http://creativecommons.org/licenses/by/4.0/

r The Author(s) 2016

NATURE COMMUNICATIONS | 7:13269 | DOI: 10.1038/ncomms13269 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 9