Properties in the u band of local star-forming
galaxies
Cristina Dı́az1 , Jaime Zamorano1, Jesús Gallego1 , P.G. Pérez-González1,
and A. Gil de Paz1
Departamento de Astrofı́sica, Universidad Complutense de Madrid, Avda.
Complutense s/n 28040 Madrid, Spain
cdl@astrax.fis.ucm.es
Summary. We present u0 Sloan photometry for a representative sample of local
star-forming UCM galaxies. We perform a comparison with previous photometry,
showing that galaxy colors are those typical for late type galaxies; the study of
structural parameters (effective radii and light concentration indexes) points to a
mainly central location for the star-forming bursts.
1 Introduction
The Universidad Complutense de Madrid Survey (list I, Zamorano et al. 1994;
list II, Zamorano et al. 1996; list III, Alonso et al. 1999) constitutes a representative and fairly complete sample of current star-forming galaxies in the
local Universe. The survey was carried out with the 80/120 cm f/3 Schmidt
telescope at the German-Spanish Observatory of Calar Alto (Almerı́a, Spain).
The objects were selected by the presence of Hα λ6563 + [NII] λ6584 emission in their spectra. A total number of 191 objects were cataloged as UCM
galaxies in List I and List II.
The galaxies included in UCM lists I and II (hereafter the UCM Survey) have been deeply analyzed in optical and infrared bandpass: Johnson B
(Pérez-González et al., 2000, 2001), Gunn r (Vitores et al., 1996a,b), J and
K nIR (Alonso-Herrero et al., 1996; Gil de Paz et al., 1998), narrow band Hα
(Pérez-González et al., 2003c). The spectroscopic analysis was performed by
Gallego et al. (1996, 1997) and it has also been used to determine the Hα luminosity function and star formation rate density in the Local Universe (Gallego et al., 1995; Pérez-González et al., 2003c). Spectroscopically, all types of
star-forming galaxies are represented, mainly low-excitation, high-metallicity
starburst-like galaxies (57%), but also high-excitation, low-metallicity HIIlike galaxies (32%), and a small fraction (8%) of AGN. Morphologically, the
sample is dominated by late-type spirals, with less than 10% showing typical
parameters of earlier types (Vitores et al., 1996a; Pérez-González et al., 2001).
2
Dı́az, C., et al.
70
60
50
25
40
20
30
15
20
10
10
5
0
-23
-22
-21
-20
-19
-18
-17
-16
-15
0
-14
MB
60
30
25
-0.2
0.2
0.6
1
1.4
1.8
2.2
UCM Survey
This subsample
u + ha obs
20
Ngal
Ngal
40
30
15
20
10
10
5
0
-0.6
B-r
UCM Survey
This subsample
u + ha obs
50
UCM Survey
This subsample
u + ha obs
30
Ngal
Ngal
35
UCM Survey
This subsample
u + ha obs
50 100 150 200 250 300 350 400 450 500
EW (Hα)
0
-26 -25 -24 -23 -22 -21 -20 -19 -18 -17
MK
Fig. 1. Histograms for the UCM Survey galaxies and the galaxies involved in this
study. Most of the galaxies observed at the u band have Hα images.
The analysis of the properties of the stellar populations in the UCM galaxies has been tackled in Pérez-González et al. (2003a,b). An average UCM
galaxy has subsolar metallicity, total stellar ∼ 1010 M , and harbored an instantaneous burst about 5 Myr ago, involving ∼ 5% of the total stellar mass.
Here we present u0 Sloan band photometry for a representative subsample
of galaxies of the UCM Survey. This band has been chosen because it yields
in the optical range for high-z populations and, while LU V (λ . 2500 Å)
is commonly used as an SFR tracer (e.g. Sullivan et al., 2000; Schiminovich
et al., 2005), the luminosity at the u-band (λ ≈ 3600 Å) is similarly dominated
in starburst galaxies by young stellar populations, and so it may be used as
and SFR indicator (Hopkins et al., 2003) for high-z objects.
2 The data
The UCM Survey includes 191 galaxies in regions of 12h−16h and 22h−2h on
RA, respectively, covering a 10◦ width strip centered at DEC 20◦ . This work
presents the u0 observations for a subsample of 71 galaxies (40% of the total).
According to the Kolmogorov-Smirnov test, the subset is representative of
the whole sample with over 85% of confidence in properties such as equivalent
width of the Hα line, K and B band magnitude and B − r color (figure 1).
Properties in the u band of local star-forming galaxies
3
The galaxies were observed during three runs and two service nights at the
CAHA 2.2m Telescope1 , and another run at the NOT Telescope2 . At the 2.2m
telescope in Calar Alto, the instrument BUSCA was used to get simultaneous
images of the UCM galaxies in u, g, r Gunn-Thuan and I Johnson with a scale
of 0.34 00 /pix, ensuring a good sampling of the objects, with seeing values from
000. 9 to 200. 5. The effective wavelength and the FWHM in the u channel are
3633 and 400 Å. At the NOT telescope, the ALFOSC instrument provide
images at the u’ Sloan band with 0.19 00 /pix.
3 Galaxy photometry
3.1 Data reduction and flux calibration
Standard reduction procedures for CCD photometry were applied. Raw images were bias subtracted by using a master bias for each run (an average of
the nightly 15-20 bias images). Flat field correction was applied after combining at least seven sky-flats images. Cosmic rays were removed using the
laplacian cosmic ray removal algorithm of van Dokkum (2001) and near foreground stars masked using the cr utils IRAF3 package.
For the purposes of photometric calibration, standard stars were observed
during each night under different airmasses. The standards were chosen between those standard stars that define the SDSS u0 , g 0 , r0 , i0 , z 0 photometric
system (Smith et al., 2002). Standard stars fluxes were measured using the
IRAF apphot package, and the differences between the u0 filter and the one
used at CAHA runs were taken into account in the color term of the Bouguer
equation to transform instrumental to u0 magnitudes:
mu0 + 2.5 · log(Fu ) = Cu − Ku · X + K(u0 −g0 ) · (u0 − g 0 )
0
(1)
−1
where mu0 is SDSS u magnitude, Fu is the flux in counts s , C is the
instrumental constant, Ku0 the extinction, X the airmass, and K(u0 −g0 ) the
color constant referred to the SDSS u0 − g 0 color.
3.2 Integrated photometry
The measurement of galaxy fluxes has made use of the IRAF task polyphot:
we built polygons around the galaxies including the whole object but also
1
2
3
Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly
by the Max-Planck Institut für Astronomie and the Instituto de Astrofı́sica de
Andalucı́a (CSIC)
Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark,
Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de
los Muchachos of the Instituto de Astrofı́sica de Canarias
IRAF is distributed by the National Optical Astronomy Observatories, which
are operated by the Association of Universities for Research in Astronomy, Inc.,
under cooperative agreement with the National Science Foundation
 Sab
 S0
 Scd
 Sbc
0.2
 Irr
0.2
E
 Irr
0.25
 Sab
 S0
Dı́az, C., et al.
 Scd
 Sbc
4
N/Ntot
N/Ntot
0.15
0.15
0.1
0.05
0.05
0
0.1
0
0
0.5
1
1.5
2
0
u-b
0.5
1
u-r
1.5
2
Fig. 2. Normalized histograms for the u − B and u − r colors. Median, first and
third quartiles are shown at the top of each panel. Mean values for morphological
types according to Fukugita et al. (1995).
minimizing the area of sky also included. Object fluxes were determined as an
average of several measurements with at least two different polygons, taking
also into account the Poisson noise, the uncertainty in the sky determination,
the readout noise of the CCD and the errors associated to the Bouguer fit to
compute the final error for each galaxy magnitude determination. At the end
of the process, we compute apparent total galaxy magnitudes within an error
interval of 0.15 mag, with a mean error of 0.05 mag.
The apparent magnitudes were converted into absolute magnitudes. Galactic extinction correction was applied (Schlegel et al., 1998); K-correction are
small at the redshifts of the galaxies and a linear dependency with z is enough
for our purposes. Internal extinction correction is not applied. We also compute u − B and u − r colors.
3.3 Effective radii and concentration indexes
The effective radius is the one containing half of the total light of a galaxy.
We have calculated an equivalent half light radius in arcsec by measuring the
flux of the galaxy inside circular apertures and selecting the one containing
half of the light. The value in kpc is computed considering the concordance
cosmology values (H0 = 70 km s−1 Mpc−1 , ΩM = 0.3, ΩΛ = 0.7).
A similar process allows to measure radius containing several percentages
of total flux and to compute the light concentration indexes c31 and c42 :
r75
r80
c31 =
;
c42 = 5 log
(2)
r25
r20
4 Data analysis and results
In figure 2 we plot the histograms for u − B, u − r colors. By comparing with
mean colors in Fukugita et al. (1995), we can see that the colors for the UCM
Properties in the u band of local star-forming galaxies
1.5
0.25
S0
Sa
Sb
Sc+
INT
BCD
0.2
E
S0
Sab
0.15
Sbc
Sc+
Scd
Sa
B-r
N/Ntot
1
0.1
0.5
INT
Sb
BCD
0.05
0
5
Irr
0
-24-23-22-21-20-19-18-17-16-15-14-13
Mu
0
0.5
1
1.5
2
u-B
Fig. 3. u absolute magnitude histogram and B −r vs. u−B color-color plot. Dashed
line is the separation early - late types found by Strateva et al. (2001). Ellipses show
mean and std deviation values for Hubble types. Mean values of Fukugita et al.
(1995) (red). The arrow marks mean dust correction following Calzetti et al. (2000).
galaxies are those typical for intermediate and late spiral. Figure 3 shows the
u absolute magnitudes and the u − B vs. u − r color-color plot; UCM galaxies
are located were expected for late type galaxies, according to studies involving
thousand of galaxies of the SDSS survey (Strateva et al., 2001).
The values of effective radii and concentration indexes at the u0 band are
directly comparable to previous measurements of these parameters at larger
wavelenghts (B, Pérez-González et al. 2001; r, Vitores et al. 1996b). Figure 4
shows the histograms for effective radii and concentration index c31 in the
three bands, and table 1 includes the mean values, for the spectroscopic disklike and HII-like galaxy types and the whole subsample. The young stars,
responsible for the most of the luminosity at the u band, are mainly located
at the central regions of the galaxies, as pointed by the low values for ref f ;
concentration indexes c31 and c42 have however lower values than in bluer
bands. This two facts point to a different correlation between these structural
parameters and morphological types (usually performed with the values at B
band) and has to be taken into account in studies involving high-z galaxies.
Table 1. Statistic on surface photometry of UCM galaxies.
r1/2 (kpc)
c31
c42
u
B
r
u
B
r
u
B
r
Disk-like
1.48 ± 0.10
5.86 ± 0.38
5.09 ± 0.33
2.97 ± 0.13
3.36 ± 0.09
3.18 ± 0.08
2.85 ± 0.10
3.21 ± 0.06
3.07 ± 0.05
HII-like
1.19 ± 0.15
4.03 ± 0.27
3.55 ± 0.17
2.49 ± 0.18
3.48 ± 0.12
3.07 ± 0.11
2.41 ± 0.19
3.31 ± 0.08
2.98 ± 0.08
Total
1.39 ± 0.09
5.16 ± 0.27
4.49 ± 0.23
3.22 ± 0.39
3.40 ± 0.07
3.12 ± 0.06
2.80 ± 0.11
3.25 ± 0.05
3.02 ± 0.04
6
Dı́az, C., et al.
0.20
u
0.30
u
0.15
0.20
0.10
0.10
0.05
0.00
0.00
0.15
B
0.15
N/Ntot
N/Ntot
0.20
0.10
0.05
0.05
0.00
0.30
0.00
0.25
r
0.25
r
0.20
0.20
0.15
0.15
0.10
0.10
0.05
0.05
0.00
B
0.10
0
1
2
3
4
5
Reff (kpc)
6
7
8
0.00
1
2
3
4
5
c31
6
7
8
Fig. 4. Normalized histograms for the effective radii and concentration indexes.
References
Alonso, O., Garcı́a-Dabó, C. E., et al. 1999, ApJS, 122, 415
Alonso-Herrero, A., Aragón-Salamanca, A., et al. 1996, MNRAS, 278, 417
Calzetti, D., Armus, L., et al. 2000, ApJ, 533, 682
Fukugita, M., Shimasaku, K., & Ichikawa, T. 1995, PASP, 107, 945
Gallego, J., Zamorano, J., et al. 1995, ApJL, 455, L1
Gallego, J., Zamorano, J., et al. 1996, A&AS, 120, 323
Gallego, J., Zamorano, J., et al. 1997, ApJ, 475, 502
Gil de Paz, A., Gallego, J., et al. 1998, Ap&SS, 263, 147
Hopkins, A. M., Miller, C. J., et al. 2003, ApJ, 599, 971
Pérez-González, P. G., Gallego, J., et al. 2001, A&A, 365, 370
Pérez-González, P. G., Gil de Paz, A., et al. 2003a, MNRAS, 338, 508
Pérez-González, P. G., Gil de Paz, A., et al. 2003b, MNRAS, 338, 525
Pérez-González, P. G., Zamorano, J., et al. 2003c, ApJ, 591, 827
Pérez-González, P. G., Zamorano, J., et al. 2000, A&AS, 141, 409
Schiminovich, D., Ilbert, O., et al. 2005, ApJL, 619, L47
Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525
Smith, J. A., Tucker, D. L., et al. 2002, AJ, 123, 2121
Strateva, I., Ivezić, Ž., et al. 2001, AJ, 122, 1861
Sullivan, M., Treyer, M. A., et al. 2000, MNRAS, 312, 442
van Dokkum, P. G. 2001, PASP, 113, 1420
Vitores, A. G., Zamorano, J., et al. 1996a, A&AS, 118, 7
Vitores, A. G., Zamorano, J., et al. 1996b, A&AS, 120, 385
Zamorano, J., Gallego, J., et al. 1996, ApJS, 105, 343
Zamorano, J., Rego, M., et al. 1994, ApJS, 95, 387