Combined color indexes and photometric structure of galaxies NGC 834 and NGC 1134

3 Pages·0.18 MB·English

by  D. Bizyaev

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COMBINED COLOR INDEXES AND PHOTOMETRIC STRUCTURE OF GALAXIES NGC 834
AND NGC 1134
D.Bizyaev1, A.A.Zasov1, and S.Kajsin2
1
SternbergAstronomical Institute– Universitetskiyprospect 13 Moscow, Russia
2
Special Astrophysical Observatory of Russian Academy of Sciences – Nizhniy Arkhyz,Karachaevo-Cherkessia, Russia
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Abstract 2. Observations
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0 We present the results of BVRI photometry of two
Two spiral galaxies - NGC 834 and NGC 1134 were ob-
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galaxies with active star formation: NGC 834 and NGC
served in BVRI colors (Cousins system) 21-22 Jan 1988
n 1134.CombinedcolorindexQBVI wasusedtoinvestigate at 1-m reflector of Special Astrophysical Observatory of
a
J the photometrical structure of the galaxies. Index QBVI Russian Academy of Science. CCD camera 512x512 with
7 is not affected by internal extinction and is sensitive to scale0.”37/pixwasused.Dataprocessingwascarriedout
the presence of blue stars.
1 with the help of MIDAS package.
Ring-likeregionwithactivestarformationat15”from Asymmetric distribution of colors on the color maps
1 the center reveals itself in the QBVI map of NGC 834.
of NGC 834 and NGC 1134 gives evidence that they are
v Three-arm spiral structure is well-seen on the QBVI map dusty objects.
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of NGC 1134.
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2 We propose to use the combined indexes QBVI and
1 similarly defined indices as a tracers of Star Formation 3. Data analysis
0 activity and structure of dusty galaxies.
0 Combined colors indices do not describe neither ”real”
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Key words: Galaxies: star formation, photometry colors nor the value of the extinction. They may be con-
/
h sidered as some conditional color indices free (or nearly
p free) of color excesses. The values of QABC depend on
-
o stellar population, stellar abundances and, if R band is
r 1. Introduction involved, on the presence of Hα emission (see Zasov &
t
s Moiseev 1998).
:a Observationsofgalaxiesinopticalwavebandsarestrongly Asanillustration,Fig.1ashowshowthevaluesofQBVR,
v influenced by selective extinction which is hard to take QVRI, QBVI and QBRI change when young stars formed
Xi into account due to inhomogeneous distribution of dust. in the single burst with the aget=0.1 Gyr are overlapped
By this reasonthe maps of color and brightness may give onto the the old population (t=13 Gyr) for different rela-
r
a distorted picture of photometrical structure of galaxies tive mass of young stars. The evolution program by
and distribution of star-formationtracers. G.Worthey(seeWorthey1994)wasusedadoptingSalpeter
In paper Zasov & Moiseev (1998) it was proposed to IMF and [Fe/H]=0. Fig.1b illustrates the dependence of
use the combined color index QVRI whose value doesn’t Q’s on the burst ages for a fixed (1%) fraction of young
dependonselectiveattennuationoflighttotracethestruc- stars. Fig 1c shows a variation of Q’s with the adopted
ture of galaxies. In general case for photometric bands metallicity of the old population for the same parameters
A,B,C one can define combined index QABC as of burst as in Fig 1b.
As it follows from Figures 1a - 1c, QBVI is the most
QABC =(A−B)− EAB (B−C) sensitive to the fraction of young stars. The change of
EBC
QVRI is less significant, but on the other hand this index
tomakeitasfarinsensitivetoextinctionaspossible.Here should allow to localize giant emission regions where Hα
EAB is the ratio of color excesses. We have assumed its line locally increases R luminosity of the disk.
EBC
values for VBRI system to be equal to standart values In Fig.2 and 3 the maps of QBVI and QVRI distribu-
for our Galaxy according to Cardelli et al. (1989): EEBVVI tion for our galaxies are presented. Maps are bounded by
= 0.840, EEVVRI = 0.413, EEBVVR = 2.033, EEBBRI = 0.681. As the ellipses whose major axes equal to D25. They appear
it was shown in Zasov& Moiseev (1998),these ratioes do to be muchmoresymmetricallydistributed thancolorin-
not depend practically on whether we observe the source dices. Curiouslya ring-likezone at about15”(4.7 kpc for
through the dust screen or dust and stars are well mixed. H0 =75 kms−1Mpc−1) from the center appears in NGC
Similar approach was later used in Vansevicius (1999) to 834whichisnotnoticeableatthecolormaps.Itenablesto
investigate the structure of M 51. consider this system as the galaxy, where star formation
Proc. of 33rd ESLAB Symp.“Star formation from the small to the large scale” (F. Favata, A.A.Kaas & A. Wilson eds., ESA
SP-445, 2000)
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Figure2. QBVI and QVRI maps for NGC 834. They ap-
pear to bemuch more symmetrically distributed than color
Figure1. (a) Variation of model values of QBVR, QVRI, indices.Thering-likezoneatabout4.7kpcfromthecenter
QBVI and QBRI with relative mass of young population. appears on QBVI map.
Young stars formed in the single burst with the age t=0.1
Gyr are overlapped onto the the old population (t=13
Gyr).Theevolution enginebyG.Wortheywasusedadopt- Dashed curve shows the values of the combined in-
ing Salpeter IMF and [Fe/H]=0. (b)Dependence of Q’s diceswhentheburstofSFisaddedtothepreviousmodel
ontheburstagesforthefixed(1%)fractionofyoungstars. (young population contains 30% of the mass of the back-
(c)Variation of Q’s with the adopted metallicity of the old groundpopulation,hassolarmetallicityandtheageof0.1
population for the same parameters of burst as in Fig 1b Gyr). The arrow in Fig.4. shows how Hα emission shifts
(burst age 0.1 Gyr). the point in the diagramm.
Themodelcurvesshowthattheincreasingofthefrac-
is enhanced in the ring. Note that NGC 834 is absent in tion of youngstellar population change mostlyQBVI. On
catalog of a ring galaxies by Buta & Crocker 1993 ). theotherhandtheHashiftspointshorizontally.Itenables
to use the diagram as the diagnostic one for comparizon
QVRI mapofNGC1134revealsaclear3-armedspiral-
of star formation in different parts of a galaxy.
likestructurewherethevaluesofQVRI havelocalmaxima
whichmaybeexplainedbyenhancedemissioninHα(local As an example we put average values of Q’s for more
equivalent width WHα ≈ 100 ˚A). However blue stars do than hundred points of central region (diamond), regions
not form a clear spiral structure in this region as one can of SF-ring (triangle) and outer regions (square) of NGC
see from QBVI map of the galaxy. 834 on the diagram in Fig.4.
Wecancomparetherelativeintensityofstarformation Mean errors of position of the points is about 0m.003.
in different regions of a galaxy using QBVI – QVRI dia- Regions in the center and in the SF-ring have the same
gram.Thetoy-modelofagalaxywithexponentialdecreas- QBVI which enables to conclude that they have similar
ing SFR ∼ exp(−t/τ) was developed using Worthey’s fractionofyoungstars.ThedifferenceinQVRI maybeex-
program (from Worthey 1994). The model values of Q’s plainedbythepresenceofHaemission(localWHα ∼ 50˚A).
whichdo nottakeinto accountrecentlyformedstars(t < The outer regions of the galaxy contain a less amount of
109 yr) are shown by continious curve in Fig.4. blue stars.
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Figure3. QBVI and QVRI maps for NGC 1134. QVRI
map of NGC 1134 reveals 3-armed spiral-like structure
where the values of QVRI have local maxima which may Figure4. Diagram QBVI – QVRI. Average value of Q’s
be explained by enhanced emission in Hα. forthepointsofcentralregionofNGC834isshownbythe
diamond, value for the regions on the SF-ring are shown
bythetriangleandvaluesfortheouterregionsaredenoted
Theseexamplesillustratetheabilitytoinvestigatestruc- bythesquare. Curves: 1.Thetoy-modelofgalaxywith
ture and distribution of star formation tracers in dusty decreasing SFR ∼ exp(−t/τ) was computed for different
galaxies using optical broad-band photometry. values of τ. There are no recent bursts. 2. Dashed curve
shows the values of the combined indices when the burst
4. Conclusion of SF is added to the previous model.
Combined photometrical indices QBVI and QVRI which
are weakly affected by the selective extinction may be This research was suported by russian grants RFBR
used successfully to restore the photometrical structure 98-02-17102andFederalprogram”Astronomy”.D.B.very
of galaxies with non-homogeneousdust distribution, even appreciates financial support of European Space Agency
if their dust-free colors remainunknown. Index QBVI de- to participate to the 33rd ESLAB Symposium.
pendsonthepresenceofbluestarswhichenablestouseit
to localize regions of recent star formation. On the other References
hand,QVRI weaklydepends onthe lightofblue starsbut
issensitivetoyoungstarpopulationthroughtheemission Buta R., Crocker D. 1993, AJ105, 1344
Cardelli J., Clayton G., Mathis J. 1989, ApJ345, 245
in Hα line. Whereas color distribution is asymmetric in
Vansevicius V. 1999, astro-ph/9906352
NGC 834andNGC 1134due tointernalabsorption,their
Worthey G. 1994, ApJSuppl 95, 107 Model was taken from
QBVI andQVRI mapsarerelativelysymmetricandallow
http://astro.sau.edu/~worthey/dial/dial_a_pad.html
to reveal structure details hidden by the dust. In NGC
Zasov A., Moiseev A. 1998, Astronomy Letters 24, 584
834 a ring-like region evidently related to active star for-
mation is clearly visible at Q-maps. In NGC 1134 three
armed spiral structure and active star formation reveal
themselves in QVRI map of the galaxy.