jari j. e. kajava et al- star formation and supernovae in starburst galaxies

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Mon. Not. R. Astron. Soc. 000, 1–10 (2009) Printed 18 August 2009 (MN LATEX style file v2.2)

Star formation and supernovae in starburst galaxies

Jari J. E. Kajava1 , Genoveva Micheva2†, Olesja Smirnova3‡, Auni Somero4§1Department of Physical Sciences, Astronomy division, P.O.Box 3000, 90014 University of Oulu, Finland 2Oskar Klein Centre for Cosmoparticle Physics, Department of Astronomy, Stockholm University, 10691 Stockholm, Sweden 3Institute of Astronomy, University of Latvia, Raina bulv. 19, LV-1586 Riga, Latvia 4Tuorla Observatory, University of Turku, V¨ ais¨ al¨ antie 20, FIN-21500 Turku, Finland

Accepted - – –. Received - – –; in original form - – –

ABSTRACT

We have studied a sample of five nearby IR-luminous starburst galaxies by meansof optical, near-IR and Hα imaging, optical long-slit spectroscopy and millimeterCO (1-0) imaging, performed with NOT and Onsala 20m telescopes. The star forma-tion rate (SFR) and supernovae rate (rSN ) estimations for these galaxies are derivedfrom the Hα imaging. The surface brightness profiles of the sample measured fromK S imaging are presented, offering the spatial distribution of star formation regions.Using radio-observations together with archival X-ray data, the total mass of gas inM82 star-forming region is estimated to be M gas ≈ 2.1 × 108 M, yielding the starformation efficiency SFE ∼ 0.6. The extinction was examined in four regions nearthe M82 nucleus from the spectroscopic data, using the relative ratios of the H α andH β lines, leading to the values 3 − 6 mag. Spectroscopic age-dating by means of Hβ line equivalent width measurement gives values of 6 − 7 Myr for studied regions. We

did a comparison of the near-infrared images to older data in order to find new su-pernovae in our galaxies, but nothing was found. In addition, two recently discoveredsupernovae SN2009gf and SN2009fv were spectroscopically defined to be of type Ia.

Key words: Random keywords...

1 INTRODUCTION

The first quantitative studies of star formation in nearbygalaxies was done by Tinsley (1968), who derived the starformation rates (SFR) from evolutionary synthesis models of galaxy colors. Further modelling by Larson & Tinsley (1978)

showed that evolution of low-mass galaxies and interactinggalaxies are strongly influenced by a ‘burst’ mode of starformation; hence the name starburst galaxy. The develop-ment of other SFR diagnostic methods such as integratedemission line fluxes (Cohen 1976), near-ultraviolet contin-uum fluxes (Donas & Deharveng 1984) and infrared (IR)continuum fluxes (Harper & Low 1973) have allowed themeasurements of SRFs in large samples of nearby galaxies.

SFRs in galaxies show a large range from basicallyzero in gas-poor ellipticals, S0 and dwarf galaxies to ∼

20 M year−1 in gas rich spirals (see Kennicutt 1998, forreview). Also, star formation takes place in two distinctphysical environments: in the discs of spiral and irregular

E-mail: [email protected]† E-mail: [email protected]‡ E-mail: [email protected]§ E-mail: [email protected]

galaxies and in compact, dense gas discs in the centers of galaxies (Kennicutt 1998). In galaxies with lower SFRs, nu-clear star formation typically occurs at physically similarconditions as in the disc HII regions, with a mean extinc-tion AHα ∼ 0.8 − 1.1 mag (Kennicutt 1983; Niklas et al.1997). However, the IR observations have revealed a pop-

ulation of more luminous nuclear regions, with SFRs upto ∼ 1000M year−1 (Rieke & Low 1972; Devereux 1987).These luminous nuclear starbursts are uniquely associatedwith dense molecular gas discs (Young & Scoville 1991, andreferences therein) and they are not seen in optical becausethe visible extinction, due dust and gas absorption, becomesorders of magnitude higher. These nuclear bursts, when theyoccur, dominate the star formation of the entire galaxy.One of the most dramatic consequence of such star form-ing conditions is the rate of supernova explosions (SNe) itinduces. SNe rates as large as 1.5 year−1 has been deducedfor Arp 220 (Mattila & Meikle 2001). Therefore, these star-burst galaxies have been targets of several SNe observing

campaigns (Mattila 2002; Mattila et al. 2002, 2003, 2004,2007; Kankare et al. 2008).

The SFR measurements using different diagnostics yielddifferent results (Bell & Kennicutt 2001), mainly because of

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2 Jari J. E. Kajava, Genoveva Micheva, Olesja Smirnova, Auni Somero

the effect of dust obscuration. In principle, the SFR can bedirectly measured from the H α observations using calibra-tions of Kennicutt et al. (1994)

SF R( M year−1

) = 7.9× 10−42

L(H α)ergs−1

. (1)The reason why this method is so attractive, is that the starformation in nearby galaxies can be mapped at high resolu-tion, but we must keep in mind that the dust obscurationmay lead to a significant underestimation of the true SFR.However, it is possible to correct for this by measuring theH α/H β line ratio from optical spectroscopy, and comparingit the value expected (Osterbrock 1989b)

H α/H β = 2.86 (2)

in the case where no absorption exist. The only drawbackof this method is that the most luminous nuclear starburstsmight be completely obscured by the dust in the visual band.

Here we report on our NOT broad- (R and K S) andnarrow band (Hα) imaging observations and Onsala 20mCO(1-0) line observations of nearby starburst galaxies. Wederive SFRs for the sample from the Hα images and as acase study, we also estimate the effects of dust absorptionon our SFR estimates using low resolution optical long-slitspectroscopy for the nearest starburst galaxy M82. In addi-tion, we estimate the star formation efficiency (SFE) usingthe total gas mass estimates from the CO(1-0) line observa-tions of M82. We also use the near infrared (NIR) K S imag-ing observations to make a blind search for “dust buried”supernovae, by comparing the K S images on previous ob-servations of Mattila et al. (2004).

2 OBSERVATIONS AND DATA REDUCTION

The observations were done remotely from Tuorla Observa-tory in Finland as part of the NordForsk Nordic-Baltic Op-tical/NIR and Radio Astronomy Summer School. The usedtelescopes were the 2.56m Nordic Optical Telescope (NOT)situated at the Observatorio de Roque de los Muchachos onLa Palma and the 20m Onsala radio telescope in Sweden.

2.1 NOT Observations

We used two instruments at the NOT: ALFOSC (Andalu-

cia Faint Object Spectrograph and Camera) was used foroptical imaging and spectroscopy, and NOTCam, the near-infrared camera for imaging and spectroscopy, was used toobtain infrared imaging. The original plan was to observe2.5 hours with each instrument on separate nights. However,there was some leftover observing time that was granted toour group. Thus we ended up observing twice 2.5 hours withboth NOT instruments: NOTCam was used on 2009 June10 and 11, and ALFOSC on 2009 June 12 and 15. All nightswe were observing in the beginning of the night, correspond-ing approximately to 21:20-23:50 UT. On ALFOSC nightswe could start observing slightly earlier than on NOTCamnights. This was because on ALFOSC nights we had to fo-cus the telescope ourselves, when at the NOTCam nights it

was done by the local staff present at the telescope.We had a sample of 19 star burst galaxies of which we

chose the targets to be observed based on their visibility inLa Palma. The observed galaxies are listed in Table 1. Only

five of the galaxies were observed both in near-infrared andoptical.

In optical we did Hα imaging and for calibration pur-poses we also took R images. We used two different Hα fil-

ters: the NOT filter #21 (λcen=6564 A, FWHM=33 A) wasused for M82 and the filter #49 (λcen=6610 A, FWHM=50A) for the rest of the galaxies. The R filter used was BesselR (λcen=6500 A, FWHM=1300 A). We also observed onespectrophotometric standard star, Feige 34, in those filters.The basic calibration frames, biases and skyflats, were pro-vided by the telescope staff.

In addition to imaging, we did optical spectroscopy. Wetook one low-resolution spectrum of M82 with the grism#4 (λλ=3200-9100 A, R=710) and a 1.0” slit. The slit wasoriented along the parallactic angle, i.e. perpendicular to thehorizon, in order to prevent light losses due to atmosphericdispersion. The exposure time of the spectrum was 300s.

On the second ALFOSC night the weather was not verygood: there were some clouds and the seeing was bad. Thuswe decided to skip the Hα imaging and to concentrate onspectroscopy instead. We took spectra of 2 recently discov-ered supernovae, SN2009gf and SN2009fv (see Section 3.1and Table 4), in order to type them. The spectra were takenwith the same setup as the M82 spectrum (grism #4 and1.0” slit) and the exposure time was 600s in both cases. Alsoarc and flat lamp spectra were obtained pointing at the tar-get just after the science exposure. In addition a spectrumof the spectrophotometric standard star SP1234+254 wastake for flux calibration purpose.

The near-infrared imaging was done by using the Ksfilter (NOT #207) that has the central wavelength of 2.14

µm and it covers the wavelength range of 1.999-2.282 µm.As the sky background is very high in near-infrared (due toatmospheric emission and thermal emission), we executedthe observations by using a dithering technique. For cali-bration, sky flats were again provided by the telescope staff.As photometric standard stars we observed AS33 and AS40on 2009 June 10 and AS29 on 2009 June 11.

The observations were planned with the help of thetools, e.g. visibility plot creator and exposure time calcu-lator, provided on the NOT web page.1

2.2 Optical reduction

In the optical, the data is reduced in the following way:the raw images were cleaned from bad pixels with a badpixel mask created from a flatfield image with the IRAFCCDMASK procedure. Then the bias was subtracted witha masterbias created from 11 bias frames taken the samenight. The data was then trimmed, flatfielded with a nor-malized masterflat created from 3 skyflats in each filter, andsky subtracted with a flat surface interpolated with a firstorder polynomial. In the cases where the galaxy is too bigand takes up the entire frame, no sky subtraction was per-formed since the sky regions could not be identified. We havenot cleaned the data from bad pixels or cosmic rays sincewe had only one frame per filter per galaxy. The exposuretimes were short, however, so our data does not suffer toomuch cosmic ray contamination. We neglected to subtract

1 http://www.not.iac.es/observing/tools.html

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Star formation and supernovae in starburst galaxies 3

Table 1. The NOT observing log

Galaxy RA Dec cz [km/s] Filter Exposure time [s] Date

M82 09:55:52.7 +69:40:46 203 K S 280a 2009 06 10Hα 300 2009 06 12R 50 2009 06 12

NGC3310 10:38:45.8 +53:30:12 993 K S 375b 2009 06 11NGC3471 10:59:09.0 +61:31:50 2109 K S 300c 2009 06 11

Arp299 11:28:30.4 + 58:34:10 3 088 K S 150d 2009 06 10Hα 300 2009 06 12R 100 2009 06 12

NGC4194 12:14:09.5 +54:31:37 2501 K S 300c 2009 06 10Hα 300 2009 06 12R 100 2009 06 12

NGC4536 12:34:27.0 +02:11:17 1808 K S 300e 2009 06 11NGC5218 13:32:10.4 +62:46:04 2933 K S 300c 2009 06 10NGC5430 14:00:45.7 +59:19:42 2961 K S 375b 2009 06 11NGC5929/30 15:26:07.0 +41:40:24 2539 K S 300c 2009 06 10

Hα 300 2009 06 12R 100 2009 06 12

NGC6181 16:32:20.9 +19:49:36 2375 K S 300c 2009 06 11Hα 300 2009 06 12R 100 2009 06 12

NGC6764 19:08:16.4 +50:56:00 2416 K S 375b 2009 06 11

a 8×7s exposures in 5 different positions using beamswitchingb 25s ramp-sampling exposure in 5 different positions, cycle repeated 3 timesc 3×20s exposures using 5-pont (dice) ditheringd 10×3s exposures using 5-point (dice) ditheringe 3×20s exposures in 5 different positions using beamswitching

the dark current since it is negligible for ALFOSC and sub-tracting it would only introduce noise.

To obtain the final Hα images we subtracted the con-tinuum in the following way:

Hα = Hα∗−Rcont · ρ (3)

where Hα∗ is the reduced Hα frame containing the contin-uum, Rcont is the frame containing only continuum and ρ isthe scaling constant. ρ was obtained from the mean ratio of

the total flux in point sources in the Hα and R bands whichhad identical point spread functions. Using the R filter ascontinuum is not ideal because it is too broad and containsnot only the Hα line itself but also additional lines, thusmaking the estimation of the continuum level inaccurate.

In order to calibrate the continuum-subtracted Hαframes, we also reduced the images of the spectrophoto-metric standards in each Hα filter. Then we convolved theflux-calibrated spectrum of the standard star with the filtertransmission and thus obtained the total flux in each filterin units of erg s−1cm−2. The filter transmission and thespectrum did not have the same stepsize, so we interpolatedthe missing values. Next, we measured the total flux in unitsof counts s−1 by doing aperture photometry on the reduced

images of the standard stars. This gave us a conversion fac-tor between erg s−1cm−2 and counts s−1 which we usedto calibrate our Hα frames. The flux correction factors foreach filter are summarized in Table 2.

Table 2. Flux correction factors from the spectrophotometricstandard star Feige34 for the two Hα filters

Filter Flux Flux Correction[ergs/s/cm2] [counts/s] factor

#49 2.6e-10 11414.3 2.29e-14#21 1.4e-10 6142. 2.28e-14

2.3 Optical spectra reduction

The optical long-slit spectra reduction was performed using

the tasks in the IRAF package. It included overscan andmasterbias subtraction and flat-field correction using nor-malized masterflat produced from the halogen lamp flatsto remove the fringing. Then the spectra were extracted,wavelength calibrated by using a reference HeNe arc lampspectrum and finally flux calibrated with the help of thespectroscopic flux-standard star data.

For M82 spectrum background sky subtraction, extrac-tion of apertures, wavelength calibration, and flux calibra-tion were made using IRAF DOSLIT. Night-sky spectra at∼ 1.5 arcmin distance on both sides of any notable galaxyemission were interpolated in the region of the galactic spec-trum and subtracted. In the spatial direction perpendicularto the dispersion axis, five pixel rows were averaged accord-

ing to the seeing (∼ 1.0 arcsec) to enhance the S/N with-out loosing any spatial information. Thus, each resultingextracted spectrum of the spatially resolved emission corre-sponds to 1× 1 along and perpendicular to the slit direc-

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1’

a) N

E

N

E

NE

NE

b)

d)c)

A1

A2

A3

A4

5’’

A1

A2

A3

A4

5’’

A1

A2

A3

A4

5’’

Figure 1. a) R-band image of M82 with the slit position of our observations at P.A. = 65.56 deg indicated by the line. b), c), d) TheR-band, Hα and K S-band images, respectively, of the central 0.6×0.5 arcmin region of M82. The ALFOSC 1” long-slit has been overlaid,with centers of four extraction regions marked as A1, A2, A3 and A4.

tion. In total, out of 11 apertures extracted from the frameonly four were selected for further analysis: those situatednear the Hα-luminous M82 nuclear region and having Hβ line intensity above the noise. The positions of these aper-tures in slit projected on our M82 images taken at differentphotometrical bands are shown in Figure 1(b-d).

Following the wavelength calibration and flux calibra-tion (using the standard star Feige 34), the foreground Milky

Way reddening was corrected using values from Schlegelet al. (1998) as listed in NED (AV = 0.526 mag) and theextinction law from Cardelli et al. (1989). The spectrum of aperture A3 reduced in this way is presented in Figure 2.

2.4 NIR reduction

In the NIR, we used the NOTCAM package for IRAF but wewill briefly outline the major reduction steps. The night skyin the NIR is usually much brighter than the target. Remov-ing the sky correctly is therefore of highest priority. A sloppysky subtraction can lead to systematic errors that dominatethe luminosity output of the target, introduce features in the

radial surface brightness profile, give spurious target colors,etc. The targets were observed in the Ks band both in chop-ping mode (beam switching) and dithering mode, dependingon their size. The first step of the reduction was then to re-

4 5 0 0 5 0 0 0 5 5 0 0 6 0 0 0 6 5 0 0 7 0 0 0

I

F

l

u

x

(

1

0

e

r

g

s

c

m

Å

1

)

v e l e n g t h ( Å )

H

H

I

S i I I

Figure 2. Flux calibrated ALFOSC long-slit spectrum of M82aperture A3.

move the sky from the raw frames either by subtracting asky frame or by subtracting a neighboring dithered galaxyframe. In either case the resulting sky subtracted frame con-

tains sources that appear negative. The data was then flat-fielded with a masterflat created from 3 skyflats. Dark cur-rent and bias are not explicitly subtracted during reductionsin the NIR because they automatically cancel out during the

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Table 3. Zero point correction factors obtained from individualgalaxy frames at different times during the night. The large temporalvariation of the zero point is due to the non-photometric conditionsof the night.

Galaxy ZP ZP correction[mag] factor

Arp299 23.8 (weighted mean) 2.86e-10M82 24.4 1.74e-10NGC5929/30 25.3 7.29e-11NGC4194 25.2 8.25e-11NGC6181 25.0 (weighted mean) 9.86e-11

pair subtraction. The frames were aligned and stacked, usinga median combine. This also removes all negative sources if there are enough frames and the sky subtraction on each

frame was successful.The calibration was performed against 2MASS sources

located in each reduced galaxy frame, which had suitable2MASS photometry flags. Such sources were not freely avail-able, so we used sources with large photometric errors, e.g.σK = 0.2 mag, and in the case of M82 and Arp299, sourcessuffering from galaxy contamination with flags gal contam

= 2.0 (instead of 0), rd flag = 2 and cc flag = 0. Thezero-points for each frame are summarized in Table 3.

2.5 Radio observations and reduction

We observed the core regions of M82 with the Onsala 20m

telescope on 2009 June 14 to estimate the mass of moleculargas within the galaxy. Because of the limited capability of the telescope to observe faint extragalactic sources, we choseto observe the CO(1-0) line at 115 GHz. Other moleculargas tracers such as the CO(2-1) line would have been betterthan the CO(1-0) line because they remain optically thinfor higher densities and would therefore give more accurateresults. The beam size of the observations at 115 GHz was∼ 30”. We observed the central 1’ region of M82 using a5 × 5 grid with 15” spacing between the different pointingsto assure that the intensity map is Nyquist sampled.

The reduction of the radio data was done using the XSsoftware as follows. We first read the 5×5 map into the XS.

We went to multiple view mode and changed the scale tovelocity. We then averaged the individual scans accordingto the “system temperature and integration time”. Becauseour data was very noisy, we decided to increase our signalby “lowering” the resolution. Our CO(1-0) line was verybroad, so we decided set a 20 km/s resolution by using aGaussian weight function in rebinning the data. This gaveus a clear signal for most of the individual pointings. Ourbaseline was rather well behaved in most of the observations.We therefore chose to use two baseline boxes around theemission line and used a first order polynomial to subtractthe baseline. We then added a moment box, that coveredthe observed emission line.

After these reduction steps, we plotted the Pos&Pos

contour map. We then played around with the contour levelsto get an esthetically pleasing map (see Fig. 6). We choseto use a false color map with linear scaling, with a secondorder bi-cubic interpolation over the different pointings.

Table 4. The observed supernovae.

Supernova RA Dec Host galaxy cza[km/s]

SN2009fv 16:29:44.22 +40:48:41.8 NGC 6173 8784SN2009gf 14:15:37.12 +14:16:48.7 NGC 5525 5553

a recessional velocity of the host galaxy

3 RESULTS AND DISCUSSION

3.1 Typing of supernovae

The spectra of the two observed supernovae, SN2009fv andSN2009gf, are presented in Figure 3. We identify both of the supernovae to be of type Ia. This is because the spectralack hydrogen lines and especially because they exhibit a

strong Si II absorption line (6355 A). Also other spectralfeatures characteristic for SNe Ia are present, e.g. a Ca IIH&K doublet at 3934Aand 3968A(Filippenko 1997).

We fitted a Gaussian profile to the most prominent fea-tures of the spectra and measured the central wavelengthof these lines. The fitting was done in IRAF with the tasksplot and the key command k. Combining this informationwith the recessional velocity of the host galaxy (see Table4), we could define the velocity of the absorbing medium,i.e. the expansion velocity of the supernova ejecta.

For SN2009fv we measured a blueshifted position of SiII at 6318.62A, yielding a velocity of 10499 km/s of theexpanding gas. For SN2009gf we found Si II at 6246.6A,corresponding to a velocity of 10666 km/s.

We reported these results in an immediately publishedCBET telegram (Somero et al. 2009).

Supernovae Ia are produced from accreting binary sys-tems when the mass of the accreting white dwarf exceedsthe critical Chandrashekar mass (1.4M ) causing a ther-monuclear runaway. The other types of supernovae are socalled core collapse supernovae where a single star collapsesunder its own gravitation when the fission reactions can notcontinue in the centre and thus the radiation pressure isnot sufficient to keep the star in hydrodynamical equilib-rium. Supernovae Ia occur in all type of galaxies whereasthe other types are found mostly in spiral galaxies and star-burst galaxies.

3.2 H α imaging

In Table 5 we present the SFRs and other basic parameters,calculated for our sample galaxies using Hα-imaging. Thetotal (calibrated) Hα flux of the entire galaxy was obtainedusing IRAF IMEXAM task. For comparison the values foundin the literature, SFRL(H α), for these galaxies are listed. Wealso made an estimation of the supernova rates based on thecalculated SFRs of the galaxy sample. For this we used theEquation (3) in Mattila & Meikle (2001). These values arepresented in Table 5 as well.

3.3 M82 long-slit spectroscopy

To determine the extinction from material associated withM82 in our selected regions we used the observed Balmer

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Figure 3. The spectra of the supernovae SN2009fv and SN2009gf.

Table 5. The physical properties and SFRs of sample galaxies derived from the Hα imaging. SFRL(H α) is the starformation rate found in literature, rSN is the estimated supernova rate calculated using the SFRs.

Galaxy Distance Flux Luminosity SFR(H α) SFRL(H α) rSN (Mpc) (erg/s/cm2) (erg/s) (M/yr) (M/yr) (yr−1)

Arp299 41 1.4E-10 2.8E+43 222 60-300a 1.55M82 4.5 1.87E-9 4.5E+42 36 * 0.25

NGC4194 36.5 6.3E-11 1.0E+43 79 49b 0.55NGC6181 33.55 1.5E-10 2.0E+43 159 * 1.11NGC5929/30 38.5 6.8E-12 1.2E+42 9.5 6c 0.07

a Alonso-Herrero et al. (1998)b Hancock et al. (2006)c Bower & Wilson (1995)

line flux ratios R = F (H α)/F (H β) together with Case B re-combination theory (Hummer & Storey 1987). The observedBalmer decrements are given in Table 6. These decrementshave been converted into the reddening, E (B−V ), using thestandard interstellar extinction curve of Osterbrock (1989a).This interstellar extinction curve leads to the expression

E (B − V ) = a log(R/Rintr) (4)

where a = 2.21 is a constant, R is the observed Balmerdecrement, Rintr = 2.86 is the intrinsic Balmer decre-ment. The significant uncertainties in our measurement of E (B−V ) can be expected by several reasons. The Hα line isblended with the NII lines and so an unambiguous determi-nation of the Hα flux is difficult without higher resolutiondata. In addition, the Hβ flux is uncertain due to the factthat it is weak compared with the local continuum and so itis sensitive to the modelling of that continuum.

Using the obtained reddening we calculated the corre-sponding visual extinction AV values assuming a standardGalactic extinction law with RV = AV /E (B − V ) = 3.1(Rieke & Lebofsky 1985).

After the de-reddening of the spectra, the equivalent

widths of hydrogen recombination lines, particularly Hβ ,can be used as an accurate indicator of the age of the pop-ulation in the case of an instantaneous burst of star forma-tion, as first pointed out by Copetti et al. (1986). Table 6

Table 6. The F (Hα)/F (Hβ ) ratios, inferred reddening and ex-tinction values, equivalent widths of H β lines and correspondingages of the four M82 apertures.

Aper- R E (B − V ) AV W λ(H β) Ageture (mag) (mag) (A) (Myr)

A1 21 1.9 5.9 4.4 6.8A2 8 1.0 3.1 12.2 5.9

A3 9 1.1 3.5 14.6 5.8A4 14 1.5 4.7 22.7 5.6

presents the measured Hβ equivalent widths W λ(Hβ ) foreach of our four apertures. We used Schaerer & Vacca (1998)evolutionary synthesis models to determine the ages of re-gions, assuming a solar metallicity following the McLeodet al. (1993a).

We can compare our results on extinction and age-dating to the recently obtained by Konstantopoulos et al.(2009) during the spectroscopical study of M82 stellar clus-ter population, since the apertures inside the slits #67 inthose research were located close to our studied regions.

The authors obtained extinction values AV from photomet-rical data that, range between 1.7 − 1.9 mag (corrected forforeground galactic extinction). These values are somewhatsmaller that we found, but taking into account uncertainties

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Table 7. Parameters for the surface brightness profiles. The widthof the elliptical rings is 2 pixels for all galaxies. The position anglesP.A. are measured North through East. e is the ellipticity.

Galaxy RA DEC e P.A.

Arp299 11:28:30.7 +58:33:04.4 0.0 0◦

M82 09:55:55.2 +69:40:48.0 0.75 68◦

NGC4194 12:14:09.6 +54:30:00.0 0.33 −15◦

NGC6181 16:32:21.6 +19:49:48.0 0.5 0◦

NGC5929/30 15:26:07.2 +41:40:48.0 0.0 0◦

and the spatial discrepancy of studied regions we can con-sider our results suitable. However, our estimations of agesfor studied regions are consistent with values of 4− 10 Myrfound by Konstantopoulos et al. (2009) from the spectro-scopic data.

3.4 NIR observations

The near-infrared K S images of all the observed galax-ies were compared to older K S images provided us bySeppo Mattila. The images had been taken during the years2001-2006 with different near-infrared instruments at LaPalma (NOTCam at the NOT, or LIRIS and INGRID atthe William Herschel Telescope operated by Isaac NewtonGroup of Telescopes2).

First the images of the same galaxy were shifted andscaled to match with each others in orientation and scaling.This was done with an CL script written by Seppo Mat-

tila. Further image subtraction was done with ISIS imagesubtraction package (Alard 2000). As a result of the imagesubtraction, we did not find any new sources in the images.

In the K S band we also plotted the surface brightnessprofiles of the galaxies (Fig. 4). Tables 7 and 8 summa-rize the integration parameters for each galaxy and the to-tal luminosity in Vega magnitudes, respectively. The surfacebrightness profiles indicate that NGC 4194 and NGC 6181are not undergoing a (major) merger since their profiles area smoothly decreasing curve with no additional peaks. Therest of the profiles, however, show the presence of compan-ion galaxies close to the center of integration, indication amajor merger. For Arp299 the two extra peaks in the sur-face brightness profile come from the two cores marked in

Figure 5. The M82 profile indicates resolved structure in thecore, giving rise to the bumpy features. The galaxy obvio-suly covers the entire image, with high signal-to-noise outto and beyond 40 kpc, which is why the errorbars for thatprofile are negligible. For NGC 5929/30, the center of inte-gration is at NGC 5930 and NGC 5929 is clearly visible asthe sharp peak around 29”.

3.5 Radio observations

To extract the intensity over the region where most of theemission was observed to originate, we used a 30” extractionregion radius with -5” and -5” offsets in right ascension and

declination (see Fig. 6). We measured an intensity

2 http://www.ing.iac.es

Table 8. Total luminosity in Vega magnitudes for all galaxiesobtained by summing up the flux inside each elliptic ring duringthe integration.

Galaxy Total luminosity(mag)

Arp299 8.73M82 4.69NGC4194 9.26NGC6181 11.02NGC5929/30 8.34

Figure 5. Ks image of Arp299. The red circle marks the center of integration, which is the brightest pixel in the image. The blackcircles mark the cores whose contribution to the Arp299 surfacebrightness profile can be seen in the two extra peaks around 10

”3 and 25 ” in Figure 4.

< I >= 117Kkms−1, (5)

which corresponds to I = 3.3 × 105 K km s−1 arcsec2. Wemust caution, that the the CO(1-0) was optically thick at thecenter of our extraction radius. Therefore, we have under-estimated the intensity slightly and the following equationsdo not strictly apply and the results must be taken as orderof magnitude estimates.

We estimated the hydrogen column density of the emit-ting molecular gas from the CO(1-0) emission using the con-version factor

N (H 2) = X × 1020

T COdV cm−2, (6)

where X ∼ [ 0.5 − 3 ] is the so called “X factor” (see Bellet al. 2006, for uncertainties of this factor) and

T COdV =<

I >= 117 Kkms−1 in our case. Plugging in the measuredvalue we get

N (H 2) ∼ 1.2× 1022 cm−2, (7)

assuming that X ∼ 1.The column density of neutral hydrogen can be esti-

mated from the HI line

N (H ) = 1.82 × 1018 T BdV cm−2, (8)

where T B is given in K km s−1. However, as we didn’t observethis emission line, we must estimate this using an indirectmethod. One possibility to measure the neutral hydrogen

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Figure 4. Surface brightness profiles for all galaxies. The parameters of each integration can be seen in Table 7.

column density is through soft X-ray observations of the lu-minous X-ray sources within the star forming region. Wetherefore modeled the available Chandra data of the mostluminous X-ray sources to which have a power-law spectrawithin the observed X-ray band. As these spectra are in-fluenced by the line of sight hydrogen only up to photonenergies of ∼ 2 keV (Chandra data goes up to ∼ 8 keV), wewere able to derive a best fitting hydrogen column densityof

N (H ) ∼ 0.6× 1022 cm−2, (9)

which is an order of magnitude higher than the Galacticabsorption column (Dickey & Lockman 1990). Therefore, weassociate this value to be intrinsic to the gas within M82.

Using Equations 7 and 9, we can derive the visual ex-tinction within the star forming region from a relation

AV ≈ 5× 10−22(N (H ) + 2N (H 2)) ∼ 15mag, (10)

which is in good agreement with the estimation of AV =[ 12 − 27 ] by McLeod et al. (1993b). We can also estimatethe extinction in the K S band using the scaling by Draine(1989)

AK ≈ 0.1×AV ∼ 1.5mag, (11)

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Figure 6. CO(1-0) intensity map of the core of M82. The extrac-tion region for Eq. 5 is shown as a red dashed line.

Therefore, we see that we can “penetrate” the star formationregion in the K S band, while in the R band the interstellargas absorbs most of the radiation, which is also evident fromour photometry (add ref to figures). This fact has to be takeninto account when we estimate the star formation rate from

our H α images. Otherwise the “true” star formation ratewill be underestimated.

Finally, we can estimate the amount of gas within M82from the measured column densities. As the distance to M82is 3.9 Mpc, the surface area from where we extracted theCO(1-0) intensity is

A = πR2≈ 9.7× 1042 cm2, (12)

because the 30” extraction radius at the distance of M82corresponds to ∼ 570 pc. Therefore, assuming that most of the mass is in hydrogen we have

M gas ≈ mHA(N (H ) + 2N (H 2)) ≈ 2.1× 108 M, (13)

where mH is the mass of hydrogen, which is very close tothe proton mass mp. This mass estimate is very close to thevalues given in Hughes et al. (1990) who gave an estimateof M gas ≈ [ 2.2 − 9 ] × 108 M. The reason why our massestimate is slightly below the values is most likely due to thefact that our 30” extraction radius doesn’t cover the entireemission of the M82. Also, as our CO(1-0) line was opticallythick, we have underestimated the amount of N (H 2), whichdecreases our mass estimate.

Now, using previously derived SFR value for M82, thestar formation efficiency (SFE) can be estimated as

SF E = (SF R) tsf /M gas, (14)

where tsf is typical timescale of star formation. We adopthere the value of tsf = 3.3 × 106 yr found by Inoue et al.(2000). Consequently we estimated the SFE to be ∼ 0.6.

4 CONCLUSIONS

We have carried out the investigation of five starburst galax-ies using the optical and near-infrared imaging, optical long-

slit spectroscopy and millimeter CO (1-0) imaging. The fol-lowing main results were obtained from our studies:1) The SFRs and supernovae rates of sample galaxies

are derived and surface brightness profiles and total lumi-nosities are found from K S images.

2) The estimation of the hydrogen column densityN (H 2) of the emitting molecular gas in the core of M82galaxy is performed using the observed intensity of CO (1-0) line, leading to the value of N (H 2) ∼ 1.2×1022 cm−2. Alsothe neutral hydrogen column density value N (H ) ∼ 0.6 ×1022 cm−2 is derived using X-ray observational data. Thatallows us to estimate the visual extinction and hydrogenmass values, yielding AV ∼ 15 mag and M gas ≈ 2.1×108 M

respectively. The SFE value of ∼ 0.6 was found using previ-

ously derived parameters.3) The visual extinction in four regions near M82 nu-

cleus is estimated using the observed H α/H β ratios, leadingto the value of AV ∼ 3 − 6 mag, which is slightly higherthan that measured by other authors in nearby regions. Theestimations of ages of studied regions using Hβ equivalentwidths and Schaerer & Vacca (1998) evolutionary synthesismodels yields values of ∼ 6 − 7 Myr, consistent with otherauthors’ estimations.

4) The search for highly obscured core-collapse super-novae was performed for the near-IR images by comparingolder near-IR images of the same galaxies to our data; as aresult no new sources were registered.

5) The spectra of the two observed supernovae,SN2009fv and SN2009gf, are classified as type Ia accordingto the lack of hydrogen lines and the presence of strong SiIIabsorption line (6355 A). The measured blueshifted posi-tions of SiII line suggest the expansion velocity of the ejectaof SN2009fv and SN2009gf to be 10499 and 10666 km/s,respectively.

ACKNOWLEDGMENTS

We acknowledge the organisers of the NordForsk summerschool as well as the NOT staff for all help with the ob-servations. The data presented here have been taken using

ALFOSC, which is owned by the Instituto de Astrofisica deAndalucia (IAA) and operated at the Nordic Optical Tele-scope under agreement between IAA and the NBIfAFG of the Astronomical Observatory of Copenhagen. Based on ob-servations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Ice-land, Norway, and Sweden, in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrofisica deCanarias.

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