Star Formation Law at Sub-kpc Scale in the Elliptical ... · ResearchArticle Star Formation Law at Sub-kpc Scale in the Elliptical Galaxy Centaurus A as Seen by ALMA JazeelH.Azeez,1,2

Research ArticleStar Formation Law at Sub-kpc Scale in the Elliptical GalaxyCentaurus A as Seen by ALMA

Jazeel H Azeez12 Zamri Z Abidin1 C-Y Hwang3 and Zainol A Ibrahim1

1Physics Department Faculty of Science University of Malaya 50603 Kuala Lumpur Malaysia2Physics Department Faculty of Science AL-Nahrain University Baghdad 10072 Iraq3Graduate Institute of Astronomy National Central University Chung-Li 32054 Taiwan

Correspondence should be addressed to Jazeel H Azeez jazeelhusseinyahoocom

Received 28 January 2017 Accepted 19 April 2017 Published 13 June 2017

Academic Editor Dean Hines

Copyright copy 2017 Jazeel H Azeez et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

We present an extensive analysis of the relationship between star formation rate surface density (sum SFR) and molecular gas surfacedensity (sumH2) at sub-kpc scale in the elliptical galaxy Centaurus A (also known as NGC 5128) at the distance 38Mpc 12CO (119869 =2-1) data from Atacama Large MillimetreSub-Millimetre Array SV data with very high resolution (2910158401015840 08410158401015840) as well as 24 120583mdata from the Spitzer Space Telescope were usedThis is one of the first studies of the SF law onCentaurus A at this very high spatialresolution The results showed a breakdown in star formation law with a 049 plusmn 005 index relating sum SFR and sumH2 at 185 pc Asignificant correlation exists between surface densities of molecular gas and SFR with very long depletion time (68Gy) In additionwe examined the spatially resolved relationship between velocity dispersion and star formation rate surface density for the outerdisk of this galaxy and we found that the average velocity dispersion is equal to 1178 kmsThe velocity dispersion of the molecularISM for the outer disk is found to follow a power relation with the star formation rate surface density 120590 prop (sum SFR)120573 where 120573 isthe slope from the ordinary least square fitting The value of 120573 is about 1119899 asymp 216 plusmn 040 and 119899 is the power law index of the starformation law

1 Introduction

The present challenge with elliptical galaxies is to resolvetheir formation and evolutionary history Elliptical galaxiesare currently known to contain an interstellar medium (ISM)that varies from that in a spiral galaxy [1] Observations havepreviously detected cold gas dust and even newresidualstar formation in elliptical galaxies [2ndash5] It is well knownthat stars are formed from molecular gas so studying themolecular gas in galaxies provides important indications ofthe primary physical process of massive star formation ingalaxies [6] The molecular gas in early-type galaxies wasstudied by many authors such as [7ndash10]

To understand star formation in galaxies the relationsbetween star formation rate surface density sum SFR and gassurface density sum gas must be determined Schmidt [11] putforth a power law that presents a correlation between sum SFRand sum gas sum SFR prop sum gas119873 Kennicutt Jr [12 13] provedthat the Schmidt law relates gas density to star formation

density within many orders of magnitude and determined apower law relationship with index119873 = 14 This relationshipis usually called the Kennicutt-Schmidt (K-S) law The mostrecent studies on the K-S law examined the index at sub-kpc scales [14ndash17] closing from the intrinsic scale of starformation that is giant molecular clouds size (GMCs) [6]Kennicutt amp Evans [18] reviewed the relation between the starformation at different scale from entire galaxies to individualmolecular clouds at sub-kiloparsec scale they recognized tworegimes the first with low density subthreshold regime andthe second with high density regime They found that theSFR density correlates well with the gas density for the secondregime while it is uncorrelated for the first one The theoriesthat describe and interpret the star formation process in thesetwo regimes were studied by McKee amp Ostriker [19]

One of the most interesting nearby galaxies is NGC 5128(Centaurus A) (see [20] for a review of the properties of CenA) the closest most easily observable giant elliptical galaxy[21] It is the dominant object in the Centaurus A group [22]

HindawiAdvances in AstronomyVolume 2017 Article ID 8416945 8 pageshttpsdoiorg10115520178416945

2 Advances in Astronomy

and lies at a distance of 38Mpc [23ndash25]This distance (where110158401015840 is approximately 185 pc) provides a unique opportunityto study the molecular gas in extraordinary detail NGC5128 shows considerably clear dust lanes that are probablycaused by merger activities in the past (see [26 27]) Themolecular interstellar medium in the dust lane has beenwidely studied [28ndash36] In addition due to it being a strongsource of radio and far infrared and X-ray emissions it hasbeen investigated in detail over a wide part of observablespectrum [37] CO mapping reveals that the disk contains2 times 108119872⊙ of molecular gas [29]

Rare studies covering the K-S law in Centaurus at sub-kpc scale have been reported How do the SFR andH2 surfacedensities relate to each other on sub-kpc spatial resolution inan elliptical galaxy To address this question we determinesumH2 in the central area of the elliptical galaxy Centaurus Aby adopting high spatial resolution (185 pc)

2 Data

Observations of the Centaurus A in 12CO (119869 = 2-1)emission line were undertaken from August 11 2011 at1919 (UTC) to August 12 2011 at 0100 (UTC) by usinga mosaic of 48 pointing with a total integration time ofsim3415min These public data are available at the AtacamaLarge MillimetreSubmillimeter Array (ALMA) Science Por-tal (httpsalmasciencenraoedualma-datascience-verifica-tion) Observations were conducted in band 6 (sim13mm)Four spectral windows were used each with a 188GHzbandwidth 256 channels and unique polarization Thedata have a rms of 266mJy with a spectral resolutionof 20 km sminus1 The observation parameters are listed inTable 1 The data were reduced and images were processedusing the Common Astronomy Software Applicationpackage (CASA) The continuum was first subtracted inthe visibility domain The images were processed using theCLEAN algorithm with a robust = 05 weighting (Briggsweighting) of the visibilities The CO data were binnedspectrally into channels with 20 km sminus1 wide The procedureis described in the scripts available on the ALMA SciencePortal (httpsalmasciencenraoedualmadatasciverCenA-Band6)

To estimate the amount of star formation obscured bydust we used 24120583m data obtained by SPITZER (MIPS) Thedata was obtained from the public archive (httpshaipaccaltecheduapplicationsSpitzerSHA) The resolution ele-ment (beam size) of MIPS band is sim610158401015840 with pixel size of25510158401015840 The 24120583m data has a sim51015840 square field of view

3 Results and Discussion

31 COMaps and Line EmissionDistribution Themaps of theintegrated intensity and velocity dispersion of the emissionline 12CO (119869 = 2-1) are shown in Figures 1 and 2 We can seefrom Figure 1 that the distribution can be divided into threeregions a central emission region and two regions (northeastNE and southwest SW one on either side of the centre)forming an S-like shape The NE region takes an arc shape

Table 1 ALMA observational parameters

Parameter ValueTarget Centaurus A (CenA NGC 5128)Observing date 2011 August 11 12Total integration time sim3415minField centerRA (J2000) 12h 25m 2761sDec (J2000) minus43∘ 011015840 08810158401015840Number of antenna 15System velocity 542 plusmn 7 km sminus1

Rest frequency 230GHz

Restoring beam (Major minor PA)(2910158401015840 08410158401015840 8265∘)

Total bandwidth 188GHzTotal channels 256Velocity resolution 20 km sminus1

Integrated flux density 632809 Jy km sminus1

Redshift (119911) 000183

J2000

decli

natio

n

J2000 right ascension

5

10

15

20

25

30

(Jyb

eam

middotkm

s)

02

01

minus43∘

13Ｂ25Ｇ39Ｍ 33Ｍ 30Ｍ 27Ｍ 24Ｍ 21Ｍ 18Ｍ

Figure 1 ALMA 12CO (119869 = 2-1) integrated intensity images ofCentaurus A (NGC 5128)

that extends to the central region while the SW region takesa semiarc shape that extends from the central region andbecomes fainter as it goes radially outwards It is clear that theNE region is brighter than the SW region because the latteris CO poorer These two regions have also been identified byBland et al [38] who observed ionized gas with CentaurusA His results revealed that the HII regions were embeddedin a faint component of diffusely ionized gas and confined toan envelope which has the form of a hysteresis loop From thesame figure it is noticed that the CO distribution is centrallypeaked and this is also observed in most Sc spiral galaxies[39 40]

32 Molecular GasMass and Surface Density Themass of themolecular gas was calculated from the 12CO(2-1) emission

Advances in Astronomy 3J2000

decli

natio

n


02

01

minus43∘

13Ｂ25Ｇ39Ｍ 33Ｍ 30Ｍ 27Ｍ 24Ｍ 21Ｍ 18Ｍ

20

40

60

80

100

120

140

(km

s)

Figure 2 ALMA 12CO (119869 = 2-1) velocity dispersion images ofCentaurus A (NGC 5128)

line as tracer for molecular hydrogen gas using the followingformula

(119872H2

119872⊙ ) = 3 sdot 92 times 10minus17119883CO ( 119878COJy km sminus1

)( 119863Mpc)2

(1)

We adopt a CO(2-1)CO(1-0) line ratio (11987721) of 055 [41]Themolecular hydrogenmass has been calculated in a regionof (sim15 kpc) diameter to be sim2 times 108119872⊙ by adopting thedistance 119863 = 38Mpc By comparing the data with the singledish [28] value of 1 times 109119872⊙ (with 119863 = 7Mpc) a differenceof 33 is found by using the same distance This differenceis attributed to missing flux because this data does notcontain short spacing information (ACA and single dish) inaddition the relatively short observation time and the highuncertainties in data calibration for the mentioned singledish may also participate in increasing the missing flux Inthe present study the central region of Centaurus A wasdivided into many boxes each one with angular resolutionsof 1010158401015840times 1010158401015840 The angular to linear scale is 110158401015840 = 185 pc thusthese boxes correspond to the 185 pc times 185 scale In each boxthe molecular gas mass surface density was calculated usingconversion factor 119883CO = 2 times 1020 cmminus2 (K km sminus1)minus1 [42 43]Themolecular gasmass and surface density of each region arelisted in Table 2 which shows that some regions have hugeamount of molecular gas mass (1ndash14 times 106119872⊙) this indicatesthat several GMC associations are located there [44]

33 Star Formation Rate Surface Density and Star FormationLaw The infrared data are used as star formation tracersTheinfrared data are obtained from the Spitzer Space TelescopeTo estimate the amount of star formation obscured by dustwe use 24120583m data obtained from the public archive whichis taken with the MIPS instrument According to Helou [45]and Eckart et al [29] the Centaurus A has cirrus like colourshowever its effect is weak on our data due to two reasonsthe first is the used wavelength is 24120583m Bendo et al [46]made an overview of ancillary 24 70 and 160 120583m data fromthe Multiband Imaging Photometer for Spitzer (MIPS) for

00 05 10 15 20 25 30 35minus40

minus35

minus30

minus25

minus20

minus15

minus10

logsum gas (M⊙Ｊ＝2)

logsum

SER

(My

rminus1 middot

kpc2 )

⊙

Figure 3 Relation between the surface density of molecularhydrogensumH2 and surface density of star formation ratesum SFRThesolid line represents the linear square fit line

samples of nearby galaxies (including Centaurus A) In theirstudy they concluded that the cirrus like colour in CentaurusA and in the other galaxies is only effective at the wavelengths70 and 160 120583m and its effect is not influential in 24 120583mThe second reason as Eckart et al [29] concluded is thatthe cirrus clouds emission is due to cold dust with largerspatial extent than the molecular disk In the current data weonly focused on the molecular disk Accordingly the cirrusclouds emission outside the disk has no effect on the observedemission and as a result on star formation

Wu et al [47] showed that the SFR can be estimated from24 120583m luminosity using the following formula

sum SFR119872⊙ yrminus1 =

V 119871V [24 120583m]666 times 108119871⊙ (2)

where 119871V is the 24 120583m dust luminosity The archival 24120583mdata from MIPS show that there is very weak star formation(few 10minus5ndash10minus4119872⊙ yrminus1) The total SFR in this galaxy 389times 10minus3119872⊙ yrminus1 was found to be lower than the previousestimates The relationship between the SFR and moleculargas surface densities which is known as star formation lawor (Kennicutt-Schmidt law) is drawn for NGC 5128 with thespatial resolution of 1010158401015840times 1010158401015840 as shown in Figure 3 Thederived values of star formation rate surface density at thisspatial resolution are shown in Table 2 12CO (119869 = 2-1) wasused as amolecular gas density indicator All data points fromthe boxes at this spatial resolution were combined to fit aline in log-log space using ordinary least square fitting (solidline in Figure 3) We found a high significant correlation withcorrelation coefficient 075 with a probability 119901 = 100 Thepower law index of the sum SFR-sumH2 relation for the ellipticalgalaxy NGC 5128 data is about 049 plusmn 005 This value issignificantly lower than a typical power law index of the K-S law In the Milky Way young clusters are known to recedefrom their parents at a rate of 10 km sminus1 [48] As they evolvethey recede from their parent GMCs by a scale of 100 pc over


Table 2 CO intensity molecular gas density and mass star formation rate and star formation rate surface density With spatial resolution of185 pc times 185 pcBox 119878CO(2-1)Δ119881 119872(H2) sumH2 SFR sum SFR

(Jy km sminus1) 119872⊙times 106 (119872⊙ pcminus2) (119872⊙ yrminus1) times 10minus5 (119872⊙ yrminus1 kpcminus2) times 10minus31 1482 076 2229 257 0752 1647 085 2477 276 0813 1081 056 1625 229 0674 1443 074 2170 207 0605 922 047 1386 203 0596 2691 138 4046 359 1057 7444 383 11193 418 1228 2501 129 3761 364 1069 1979 102 2975 254 07410 1687 087 2537 224 06511 2213 114 3327 248 07212 6881 354 10346 463 13513 7783 401 11703 542 15814 3803 196 5718 422 12315 2597 134 3905 273 08016 2773 143 4169 261 07617 6621 341 9955 385 11218 9744 501 14650 563 16419 9171 472 13789 491 14320 3103 160 4666 293 08521 4032 208 6063 340 09922 14517 747 21827 588 17223 20296 1044 30517 681 19924 6485 334 9750 325 09525 2244 115 3374 238 06926 3845 198 5781 273 08027 17401 895 26164 501 14628 11014 567 16560 492 14429 7656 394 11511 393 11530 7107 366 10686 256 07531 3616 186 5437 258 07532 17050 877 25636 491 14433 6495 334 9765 472 13834 5927 305 8912 675 19735 3824 197 5750 314 09236 11858 610 17829 352 10337 11387 586 17121 516 15138 8684 447 13057 961 28139 7334 377 11027 803 23540 3285 169 4939 279 08241 1531 079 2302 260 07642 6013 309 9040 345 10143 29090 1497 43738 1097 32044 17802 916 26767 6735 196845 7969 410 11982 651 19046 4040 208 6074 285 08347 4085 210 6142 347 10148 9088 468 13664 1099 32149 22849 1176 34354 774 22650 4507 232 6777 479 14051 2276 117 3423 254 074

Advances in Astronomy 5

Table 2 Continued

Box 119878CO(2-1)Δ119881 119872(H2) sumH2 SFR sum SFR(Jy km sminus1) 119872⊙times 106 (119872⊙ pcminus2) (119872⊙ yrminus1) times 10minus5 (119872⊙ yrminus1 kpcminus2) times 10minus3

52 7414 382 11147 567 16653 14776 760 22217 643 18854 2792 144 4198 346 10155 3612 186 5431 408 11956 18248 939 27437 563 16457 4971 256 7474 354 10458 9237 475 13889 467 13759 10344 532 15553 584 17160 4743 244 7132 417 12261 1906 098 2866 284 08362 5073 261 7628 429 12563 11524 593 17327 595 17464 6564 338 9869 583 17065 3663 188 5508 357 10466 1423 073 2139 254 07467 5309 273 7982 412 12068 8509 438 12793 541 15869 9566 492 14384 673 19770 2883 148 4335 374 10971 3092 159 4649 274 08072 3706 191 5573 373 10973 5865 302 8818 430 12674 2450 126 3683 317 09375 1690 087 2540 204 06076 2026 104 3045 232 068Note Column 1 region name Column 2 intensity of 12CO (119869 = 2-1) emission Column 3 molecular gasmass Column 4 surface density of molecular hydrogenmass Column 5 star formation rate Column 6 Star formation rate surface density

a time period of 10Myr The drift scale of 100 pc is close toour scale of 185 pc This result indicates that the traditionalK-S law in this elliptical galaxy holds at greater than 200 pcthus when the spatial resolution is below this value the K-Slaw for this galaxy may be not valid As proposed by Calzettiet al [49] at such scales below 500 pc the sampling effectsare more clear than they are for larger scales More thoroughstudies are needed to figure out the effect of sampling effectsversus scales especially for the high spatial resolution (smallregion size) In addition the type of fitting method has shownto have an effect on the results of star formation law asproved by Rahmani et al [50] so it will be more worthyto try different fitting methods at high spatial resolutionsWe also compared the CO masses with the SFR estimates inorder to measure a star formation efficiency (depletion time)Using a conversion factor 119883CO = 2 times 1020 cmminus2 (K km sminus1)minus1led to long depletion time (68Gyr) which indicates that thestar formation is inefficient in this galaxy It is clear thatthe depletion time of the molecular gas in this galaxy islonger than theHubble timeThismeans a very low efficiencyof forming stars The efficiency may also depend on thepressure and the surface density of star (eg [51]) which is onaverage low in this galaxy The solid red triangle in Figure 3represents the nuclear region of Centaurus A It is clear that

the molecular gas at this point is obviously offset from theKS law suggesting that this points is mainly affected by AGNactivities instead of star formation [17] This means that thispoint is powered by kinetic energy injection from the AGNjetwind and leading to molecular gas reservoir not formingstar efficiently [52]

Figure 4 shows our elliptical galaxy Centaurus A and thenormal spiral and starburst galaxies of Kennicutt [13] on thesame plot for comparison We then went on to investigatethe position of Centaurus A on the Kennicutt-Schmidt (KS)relation We found that this galaxy has comparable starformation surface densities to normal spiral galaxy centresbut it lies systematically offset from the KS relation havinglower star formation surface density This is in agreementwith Davis et al [53] who compared the Kennicutt-Schmidt(KS) relation [13] of nearby spiral galaxies with that of theCO-detected ETGs They found that the ETGs had loweraverage SFR surface densities at a givenmolecular gas surfacedensity compared to spirals This is in agreement with recentsimulations by Martig et al [54] who predicted a decrease inthe SFEs of ETGs

As we know the velocity dispersion of the gas is alsoan important parameter for the star formation law [55]We present here a measurement of the velocity dispersion


00 05 10 15 20 25 30 35 40 45minus4

minus3

minus2

minus1

0

1

2

3


logsum

SER

(My

rminus1 middot

kpc2 )

⊙

Figure 4 Relation between the surface density of molecular gassumH2 and sum SFR for normal galaxies (blue squares) starburstgalaxies (green triangle) and the elliptical galaxy Centaurus A(red circles) The gas surface density is derived from CO(2-1)emission The normal and starburst galaxy samples are obtainedfrom Kennicutt [13]

minus33 minus32 minus31 minus3 minus29 minus28 minus27 minus26

sum SER (M⊙yrmiddotkpc2)

minus300

minus250

minus200

minus150

minus100

minus050

000

050

100

120590(k

ms

)

Figure 5The velocity dispersion for each pixel 120590 of Centaurus A asa function of the SFR surface averaged (in units of119872⊙yrsdotkpc2)

for molecular components of the ISM for the outer disk ofCentaurus A In Figure 5 we investigate the correlation of thevelocity dispersion 120590 ofmolecular gas with the star formationrate surface density sum SFR of the galaxy The former one isestimated from the second moments map (Figure 2) of thegalaxy using the data from ALMA The velocity dispersionin outer disk ranges from 002 to 3150 kms with an averagevalue of 1178 kms This average value of velocity dispersionis consistent with previous measurements for the Milky Way[56] M33 [57] and other nearby galaxies [58ndash60] We finda significant correlation coefficient 119903 = 059 with probabilityp = 9999 Generally the results indicate low values for thevelocity dispersion According to Krumholz amp Dekel [61] thelow velocity dispersion values are usually associated with lowSFR values The determined average value which is close to10 indicates low star formation for this elliptical galaxy This

indicates that there is no gravitational collapse accordinglythere is no star formation We can see from Figure 5 that thevelocity dispersion of the molecular ISM for the outer diskfollows a power relation 120590 prop (sum SFR)120573 where 120573 is the slopefrom the ordinary least square fittingThe value of 120573 is foundto be asymp1n = 216plusmn04 and 119899 is the power low index of the starformation law This power law correlation may be a naturalconsequence of the gas and star formation surface densityscaling laws [62]

4 Summary and Conclusions

We used a high spatial resolution mosaic of ALMA obser-vations of the 12CO (119869 = 2-1) line emission as a moleculargas indicator in the central region of the elliptical galaxyCentaurus A (NGC 5128) at a distance of 38Mpc (110158401015840 =185 pc) We measured sum SFR with 24120583m data from the(MIPS) Spitzer Space Telescope We investigated the SF law(SFL) for this elliptical galaxy at very high spatial resolution(185 pc) and the result showed a breakdownSFL at this spatialresolutionThebest fit parameter for the star formation powerlaw index is 049 plusmn 005 The second moment map fromALMA data for this galaxy was used to find the velocitydispersion of the molecular component for the interstellarmedium The results for the average velocity dispersion areclose to 10 kmsminus1 which is associated with low star formationdue to no gravitational collapse which leads to no starformation This result is supported by the long depletiontime (68Gyr) (low star formation efficiency) In addition wefind a significant correlation between velocity dispersion foreach pixel and star formation surface density which followsa power law relation 120590 prop (sum SFR)120573 where the value of 120573is approximately asymp1n and n is set by the Kennicutt-Schmidtlaw sum SFR prop (sum gas)119899 This power law correlation may bea natural result of the gas and star formation surface densityscaling laws

We can see that the star formation efficiency (SFE) isvery low in this galaxy despite the fact that the amount ofmolecular gas reservoir available is huge when compared tostandard star formation efficiencies This suggests that someprocesses may prevent the star formation to proceed in thecold gas These processes can be due to high kinetic energyinjection from the AGN that suppress the star formationprocesses [44]

Conflicts of Interest

The authors declare that they have no conflicts of interest

Acknowledgments

This paper makes use of the following ALMA dataADSJAOALMA2011000008SVALMA is a partnership ofESO (representing its member states) NSF (USA) and NINS(Japan) together with NRC (Canada) and NSC and ASIAA(Taiwan) and KASI (Republic of Korea) in cooperation withthe Republic of Chile The Joint ALMA Observatory is oper-ated by ESO AUINRAO and NAOJ The coauthor Zamri


Z Abidin would also like to acknowledge the University ofMalayarsquos HIR Grant UMS6253HIR28 for their funding

References

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[3] R Morganti P T De Zeeuw T A Oosterloo et al ldquoNeutralhydrogen in nearby elliptical and lenticular galaxies the con-tinuing formation of early-type galaxiesrdquoMonthly Notices of theRoyal Astronomical Society vol 371 no 1 pp 157ndash169 2006

[4] S Kaviraj K Schawinski J E G Devriendt et al ldquoUV-optical colors as probes of early-type galaxy evolutionrdquo TheAstrophysical Journal Supplement Series vol 173 no 2 p 6192007

[5] M Sarzi R Bacon M Cappellari et al ldquoRecent star formationin nearby early-type galaxiesrdquo in Pathways Through an EclecticUniverse J H Knapen T J Mahoney and A Vazdekis Edsvol 390 of Astronomical Society of the Pacific Conference Seriesp 218 2008

[6] S Onodera N Kuno T Tosaki et al ldquoBreakdown ofkennicuttndashSchmidt law at giant molecular cloud scales inM33rdquoThe Astrophysical Journal Letters vol 722 p L127 2010

[7] L H Wei S N Vogel S J Kannappan A J Baker D VStark and S Laine ldquoThe relationship between molecular gasand star formation in low-mass ES0 galaxiesrdquo AstrophysicalJournal Letters vol 725 no 1 pp L62ndashL67 2010

[8] L M Young M Bureau T A Davis et al ldquoThe ATLAS3119863project ndash IV The molecular gas content of early-type galaxiesrdquoMonthly Notices of the Royal Astronomical Society vol 414 no2 pp 940ndash967 2011

[9] A Crocker M Krips M Bureau et al ldquoThe ATLAS3119863 project ndashXI Dense molecular gas properties of CO-luminous early-typegalaxiesrdquoMonthly Notices of the Royal Astronomical Society vol421 no 2 pp 1298ndash1314 2012

[10] T A Davis K Rowlands J R Allison et al ldquoMolecularand atomic gas in dust lane early-type galaxies ndash I Low starformation efficiencies in minor merger remnantsrdquo MonthlyNotices of the Royal Astronomical Society vol 449 no 4 pp3503ndash3516 2015

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[30] A Eckart M Cameron R Genzel et al ldquoMolecular absorptionlines toward the nucleus of Centaurus ArdquoAstrophysical Journalvol 365 no 2 pp 522ndash531 1990

[31] F P Israel E F van Dishoeck F Baas J Koornneef J HBlack and T de Graauw ldquoH2 emission and CO absorption inCentaurus A - evidence for a circumnuclear molecular diskrdquoAstronomy amp Astrophysics vol 227 p 342 1990

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[35] V Charmandaris F Combes and J M van der Hulst ldquoFirstdetection of molecular gas in the shells of CenArdquo Astronomyamp Astrophysics vol 356 p L1 2000


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[38] J Bland k Taylor and P D Atherton ldquoThe structure andkinematics of the ionized gas within NGC 5128 (Cen A) ndash ITAURUS observationsrdquoMonthly Notices of the Royal Astronom-ical Society vol 228 no 3 pp 595ndash621 1987

[39] J S Young Star Forming Regions Springer Netherlands Dor-drecht 1987

[40] A K Leroy F Walter F Bigiel et al ldquoHeracles the hera co lineextragalactic surveyrdquoTheAstronomical Journal vol 137 p 46702009

[41] R Morganti T Oosterloo J B Raymond Oonk F Santoro andC Tadhunter ldquoThe missing link tracing molecular gas in theouter filament of Centaurus Ardquo Astronomy amp Astrophysics vol592 p L9 2016

[42] D Espada S Matsushita A Pecket et al ldquoDisentangling thecircumnuclear environs of centaurus A I High-resolutionmolecular gas imagingrdquo The Astrophysical Journal vol 695 p116 2009

[43] J H Azeez C-Y Hwang Z Z Abidin and Z A IbrahimldquoKennicutt-Schmidt law in the central region of NGC 4321 asseen by ALMArdquo Scientific Reports vol 6 p 26896 2016

[44] Q Salome P Salome F Combes and S Hamer ldquoAtomic-to-molecular gas phase transition triggered by the radio jet inCentaurus Ardquo Astronomy amp Astrophysics vol 595 p A65 2016

[45] G Helou ldquoThe IRAS colors of normal galaxiesrdquo AstrophysicalJournal vol 311 pp L33ndashL36 1986

[46] G J Bendo F Galliano and S C Madden ldquoMIPS 24-160 120583mphotometry for the Herschel-SPIRE local galaxies guaranteedtime programsrdquo Monthly Notices of the Royal AstronomicalSociety vol 423 no 1 pp 197ndash212 2012

[47] H Wu C Cao C-N A Hao et al ldquoPAH and MID-infraredluminosities as measures of star formation rate in spitzer firstlook survey galaxiesrdquo Astrophysical Journal vol 632 no 2 ppL79ndashL82 2005

[48] D Leisawitz F N Bash and P Thaddeus ldquoA CO surveyof regions around 34 open clustersrdquo Astrophysical JournalSupplement Series vol 70 pp 731ndash812 1989

[49] D Calzetti G Liu and J Koda ldquoStar formation lawsThe effectsof gas cloud samplingrdquo Astrophysical Journal vol 752 no 2article 98 2012

[50] S Rahmani S Lianou and P Barmby ldquoStar formation laws intheAndromeda galaxy gas starsmetals and the surface densityof star formationrdquo Monthly Notices of the Royal AstronomicalSociety vol 456 no 4 pp 4128ndash4144 2016

[51] L Blitz and E Rosolowsky ldquoThe role of pressure in GMCformation II The H2-pressure relationrdquo Astrophysical Journalvol 650 no 2 I pp 933ndash944 2006

[52] Q Salome P Salome F Combes S Hamer and I HeywoodldquoStar formation efficiency along the radio jet in Centaurus ArdquoAstronomy amp Astrophysics vol 586 p A45 2016

[53] T A Davis L M Young A F Crocker et al ldquoThe ATLAS3DProject ndash XXVIII Dynamically driven star formation sup-pression in early-type galaxiesrdquo Monthly Notices of the RoyalAstronomical Society vol 444 no 4 pp 3427ndash3445 2014

[54] M Martig et al ldquoThe ATLAS3D project - XXII Low-efficiencystar formation in early-type galaxies hydrodynamic modelsand observationsrdquo Monthly Notices of the Royal AstronomicalSociety vol 432 2013

[55] R C Kennicutt Jr ldquoThe star formation law in galactic disksrdquoAstrophysical Journal Part 1 vol 344 pp 685ndash703 1989

[56] A A Stark and J Brand ldquoKinematics of molecular clouds II- New data on nearby giant molecular cloudsrdquo AstrophysicalJournal Part 1 vol 339 pp 763ndash771 1989

[57] C D Wilson and N Z Scoville ldquoThe properties of individualgiant molecular clouds in M33rdquo Astrophysical Journal Part 1vol 363 pp 435ndash450 1990

[58] F Combes and J-F Becquaert ldquoPerspectives for detecting coldH2 in outer galactic disksrdquo Astronomy amp Astrophysics vol 327p 453 1997

[59] W Walsh R Beck G Thuma A Weiss R Wielebinski andM Dumke ldquoMolecular gas in NGC 6946rdquo Astronomy andAstrophysics vol 388 no 1 pp 7ndash28 2002

[60] C D Wilson B E Warren J Irwin et al ldquoThe JCMTNearby Galaxies Legacy Survey ndash IV Velocity dispersions inthe molecular interstellar medium in spiral galaxiesrdquo MonthlyNotices of the Royal Astronomical Society vol 410 no 3 pp1409ndash1422 2011

[61] M R Krumholz and A Dekel ldquoSurvival of star-forming giantclumps in high-redshift galaxiesrdquo Monthly Notices of the RoyalAstronomical Society vol 406 no 1 pp 112ndash120 2010

[62] A M Swinbank I Smail D Sobral T Theuns P N Bestand J E Geach ldquoThe properties of the star-forming interstellarmedium at z = 08ndash22 fromHiZELS star formation and clumpscaling laws in gas-rich turbulent disksrdquo Astrophysical Journalvol 760 no 2 article 130 2012

Submit your manuscripts athttpswwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


FluidsJournal of

Atomic and Molecular Physics

Journal of



Advances in Condensed Matter Physics

OpticsInternational Journal of



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Solid State PhysicsJournal of

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Hindawi Publishing Corporationhttpwwwhindawicom

AerodynamicsJournal of

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PhotonicsJournal of


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Biophysics


ThermodynamicsJournal of


and lies at a distance of 38Mpc [23ndash25]This distance (where110158401015840 is approximately 185 pc) provides a unique opportunityto study the molecular gas in extraordinary detail NGC5128 shows considerably clear dust lanes that are probablycaused by merger activities in the past (see [26 27]) Themolecular interstellar medium in the dust lane has beenwidely studied [28ndash36] In addition due to it being a strongsource of radio and far infrared and X-ray emissions it hasbeen investigated in detail over a wide part of observablespectrum [37] CO mapping reveals that the disk contains2 times 108119872⊙ of molecular gas [29]

Rare studies covering the K-S law in Centaurus at sub-kpc scale have been reported How do the SFR andH2 surfacedensities relate to each other on sub-kpc spatial resolution inan elliptical galaxy To address this question we determinesumH2 in the central area of the elliptical galaxy Centaurus Aby adopting high spatial resolution (185 pc)

2 Data

Observations of the Centaurus A in 12CO (119869 = 2-1)emission line were undertaken from August 11 2011 at1919 (UTC) to August 12 2011 at 0100 (UTC) by usinga mosaic of 48 pointing with a total integration time ofsim3415min These public data are available at the AtacamaLarge MillimetreSubmillimeter Array (ALMA) Science Por-tal (httpsalmasciencenraoedualma-datascience-verifica-tion) Observations were conducted in band 6 (sim13mm)Four spectral windows were used each with a 188GHzbandwidth 256 channels and unique polarization Thedata have a rms of 266mJy with a spectral resolutionof 20 km sminus1 The observation parameters are listed inTable 1 The data were reduced and images were processedusing the Common Astronomy Software Applicationpackage (CASA) The continuum was first subtracted inthe visibility domain The images were processed using theCLEAN algorithm with a robust = 05 weighting (Briggsweighting) of the visibilities The CO data were binnedspectrally into channels with 20 km sminus1 wide The procedureis described in the scripts available on the ALMA SciencePortal (httpsalmasciencenraoedualmadatasciverCenA-Band6)

To estimate the amount of star formation obscured bydust we used 24120583m data obtained by SPITZER (MIPS) Thedata was obtained from the public archive (httpshaipaccaltecheduapplicationsSpitzerSHA) The resolution ele-ment (beam size) of MIPS band is sim610158401015840 with pixel size of25510158401015840 The 24120583m data has a sim51015840 square field of view

3 Results and Discussion

31 COMaps and Line EmissionDistribution Themaps of theintegrated intensity and velocity dispersion of the emissionline 12CO (119869 = 2-1) are shown in Figures 1 and 2 We can seefrom Figure 1 that the distribution can be divided into threeregions a central emission region and two regions (northeastNE and southwest SW one on either side of the centre)forming an S-like shape The NE region takes an arc shape

Table 1 ALMA observational parameters

Parameter ValueTarget Centaurus A (CenA NGC 5128)Observing date 2011 August 11 12Total integration time sim3415minField centerRA (J2000) 12h 25m 2761sDec (J2000) minus43∘ 011015840 08810158401015840Number of antenna 15System velocity 542 plusmn 7 km sminus1

Rest frequency 230GHz

Restoring beam (Major minor PA)(2910158401015840 08410158401015840 8265∘)

Total bandwidth 188GHzTotal channels 256Velocity resolution 20 km sminus1

Integrated flux density 632809 Jy km sminus1

Redshift (119911) 000183

J2000

decli

natio

n


5

10

15

20

25

30

(Jyb

eam

middotkm

s)

02

01

minus43∘

13Ｂ25Ｇ39Ｍ 33Ｍ 30Ｍ 27Ｍ 24Ｍ 21Ｍ 18Ｍ

Figure 1 ALMA 12CO (119869 = 2-1) integrated intensity images ofCentaurus A (NGC 5128)

that extends to the central region while the SW region takesa semiarc shape that extends from the central region andbecomes fainter as it goes radially outwards It is clear that theNE region is brighter than the SW region because the latteris CO poorer These two regions have also been identified byBland et al [38] who observed ionized gas with CentaurusA His results revealed that the HII regions were embeddedin a faint component of diffusely ionized gas and confined toan envelope which has the form of a hysteresis loop From thesame figure it is noticed that the CO distribution is centrallypeaked and this is also observed in most Sc spiral galaxies[39 40]

32 Molecular GasMass and Surface Density Themass of themolecular gas was calculated from the 12CO(2-1) emission


decli

natio

n


02

01

minus43∘

13Ｂ25Ｇ39Ｍ 33Ｍ 30Ｍ 27Ｍ 24Ｍ 21Ｍ 18Ｍ

20

40

60

80

100

120

140

(km

s)



(119872H2


)( 119863Mpc)2

(1)



00 05 10 15 20 25 30 35minus40

minus35

minus30

minus25

minus20

minus15

minus10


logsum

SER

(My

rminus1 middot

kpc2 )

⊙





V 119871V [24 120583m]666 times 108119871⊙ (2)






Table 2 Continued








00 05 10 15 20 25 30 35 40 45minus4

minus3

minus2

minus1

0

1

2

3


logsum

SER

(My

rminus1 middot

kpc2 )

⊙




minus300

minus250

minus200

minus150

minus100

minus050

000

050

100

120590(k

ms

)









Acknowledgments




References





































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




decli

natio

n


02

01

minus43∘

13Ｂ25Ｇ39Ｍ 33Ｍ 30Ｍ 27Ｍ 24Ｍ 21Ｍ 18Ｍ

20

40

60

80

100

120

140

(km

s)



(119872H2


)( 119863Mpc)2

(1)



00 05 10 15 20 25 30 35minus40

minus35

minus30

minus25

minus20

minus15

minus10


logsum

SER

(My

rminus1 middot

kpc2 )

⊙





V 119871V [24 120583m]666 times 108119871⊙ (2)






Table 2 Continued








00 05 10 15 20 25 30 35 40 45minus4

minus3

minus2

minus1

0

1

2

3


logsum

SER

(My

rminus1 middot

kpc2 )

⊙




minus300

minus250

minus200

minus150

minus100

minus050

000

050

100

120590(k

ms

)









Acknowledgments




References





































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics







Table 2 Continued








00 05 10 15 20 25 30 35 40 45minus4

minus3

minus2

minus1

0

1

2

3


logsum

SER

(My

rminus1 middot

kpc2 )

⊙




minus300

minus250

minus200

minus150

minus100

minus050

000

050

100

120590(k

ms

)









Acknowledgments




References





































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




Table 2 Continued








00 05 10 15 20 25 30 35 40 45minus4

minus3

minus2

minus1

0

1

2

3


logsum

SER

(My

rminus1 middot

kpc2 )

⊙




minus300

minus250

minus200

minus150

minus100

minus050

000

050

100

120590(k

ms

)









Acknowledgments




References





































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




00 05 10 15 20 25 30 35 40 45minus4

minus3

minus2

minus1

0

1

2

3


logsum

SER

(My

rminus1 middot

kpc2 )

⊙




minus300

minus250

minus200

minus150

minus100

minus050

000

050

100

120590(k

ms

)









Acknowledgments




References





































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics





References





































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics








FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics



Documents

Star Formation Law at Sub-kpc Scale in the Elliptical ... · ResearchArticle Star Formation Law at Sub-kpc Scale in the Elliptical Galaxy Centaurus A as Seen by ALMA JazeelH.Azeez,1,2