Study of local Luminous Compact Blue
Galaxies based on Integral Field Spectroscopy
Africa Castillo-Morales1 , Jorge Pérez-Gallego2 , Jesús Gallego1 and Rafael
Guzmán2
1
2
Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid,
Spain acm@astrax.fis.ucm.es
University of Florida, 211 Bryant Space Science Center,FL 32611-2055,
Gainesville, USA jgallego@astro.ufl.edu
Summary. We present the study of a representative sample of Luminous Compact
Blue Galaxies (LCBGs) within 100 Mpc selected from the DR4 of the SDSS and
the UCM surveys. This project is based on the integral field spectroscopy provided
by the instrument PPAK at 3.5 m CAHA and is part of a major multiwavelength
study which goes from FUV (GALEX) to cm (VLA). Optical PPAK observations
provide us with spatially resolved maps of kinematics, extinction, SFR, and metallicity which allow us to study the star formation history, galactic mass, and the role
of SN feedback in these starbursts. Some of these maps are presented for the galaxy
UCM2325+2318. Our study will define LCBGs as a class and provide a key reference for comparison with similar rest-frame optical spectroscopic studies at high
z obtained in the near future using the new generation of NIRMOS in 10-m class
telescopes.
1 Introduction
The discovery of a starburst galaxy population at z ∼ 2 using the Lymanbreak technique [1, 2] represents a key result in observational cosmology.
The hierarchical galaxy formation paradigm implies these high-z starbursts
will evolve to become today’s population of massive galaxies through major
episodes of star formation and mergers (e.g., [3]). This timeline of galaxy assembly, while predicted by theory, must be determined empirically. The goal
of an empirical understanding of galaxy evolution centers on determining the
evolutionary history of four quantities: dynamical masses, stellar content, star
formation rates, and heavy element abundances.
Several major surveys are being undertaken aimed at characterizing the
nature of high-z starbursts and their relation to today’s galaxy population [4] using 10-m class telescopes. The new generation of NIRMOS, such
as FLAMINGOS-2 in GEMINI and EMIR in GTC, will allow us to study
the rest-frame optical properties of this distant galaxy population, including
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Castillo-Morales et al.
abundances, Hα luminosities, extinctions, and kinematics. Studies of properly
selected local samples of star-forming galaxies are clearly needed for comparison.
In the observational parameter space (M (B), SBe (B), and (B−V )), high-z
starbursts are similar to an ubiquitous population of luminous blue starbursts
that dominate the galaxy number counts at z ∼ 1, but have almost disappeared today. All these objects have luminosities ≥ L? , high effective surface
brightness, and colors bluer than a typical Sbc. They also share other properties such as: metallicities close to solar value, sizes ranging between 2 and
5 Kpc, star formation rates (SFR) around 10-20 M¯/yr, velocity widths of
30-120 km/s, and a wide range of morphologies [5, 2, 6, 7, 8, 9]. Hereafter we
call these galaxies Luminous Compact Blue Galaxies (LCBGs).
Our team (UF-UCM) has started a major multiwavelength study of these
nearby LCBGs which will povide an extensive study of the physical properties of the gaseous component of our LCBGs, including neutral (HI), molecular (CO) and ionized (Hydrogen Balmer series and Oxygen). However, the
key missing piece of the puzzle is to link these properties of the gas (associated mainly with to young starburst component) to those of the underlying,
older stellar component. The physical properties of this component can be
parametrized through observations of optical absorption line indices which
will provide measurements of the velocity dispersion, rotation, age and metallicity of the stellar population.
3D spectroscopy is the most important aspect of our study. 3D maps (see
Figure 2) allow us to test model predictions on the origin and evolution of
these massive starburst, including the role of mergers and supernova galactic
winds (cross-correlating kinematic and morphological information), and the
star formation history (via age/metallicity measurements of the various stellar
components).
2 Observations and data reduction
We have selected a representative sample of LCBGs within 100 Mpc from the
DR4 of the SDSS and the UCM surveys that best resemble the characteristics
of the distant starburst population (MB < −18.5, SBre < 21mag/arcsec2 ,
B − V < 0.6). This sample (40 galaxies), is representative of the LCBG
galaxy population as a class and share the same range in morphological and
spectroscopic properties with the distant LCBGs.
We have observed 20 local LCBGs with PPAK [10], a fiber based system
for integral field spectroscopy operating at 3.5-m CAHA telescope. The field
of view of PPAK, as well as its good spatial and spectral resolution is optimum
for the observation of our sample galaxies with typical half-light diameters of
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around 10 to 30 , and velocity widths ranging from 60 to 120 km/s. PPAK
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consists of 331 scientific fibers each of 2.7 in diameter, convering a hexagonal
Study of local Luminous Compact Blue Galaxies
3
Fig. 1. Nuclear fiber of UCM2325+2318 is shown as an example of the fitting
procedure in order to correct the Balmer emission line intensity for the presence of
the underlying stellar population.
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area of 74 x 65 on the sky. In addition 36 fibers forming groups of 6 fibers
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at the distance of 95 in radius were used to measure the sky simultaneously.
We have chosen two instrumental setups per galaxy. V1200 grating at first
order and centered at 5040Å covers the Hβ and [O III] spectral range, i.e.
from 4669 to 5395Å. This intermediate resolution setup provides a spectral
resolution of FWHM=2.5Å(σ ∼ 65 km/s) which allow us: (i) to derive the
velocity field of the gaseous component using emission lines centroids ([O III]
and Hβ), including rotation velocities typically ranging from 50 to 120 km/s,
and any extra kinematic components associated to supernova galactic winds
ranging from 30 to 100 km/s; and (ii) to measure stellar velocity dispersions
using absorption lines (Mg and Fe), with typical values of σ = 60 to 120
km/s. V300 grating at first order and centered at 5316Å covers the spectral
range between 3591 and 6996 Å. This low resolution setup (FWHM=9Å)
allow us to measure all the emission line ratios and absorption line indices in
one single spectrum, avoiding uncertainties associated to flux calibration and
spectral response between different spectral ranges obtained with different
configurations.
The data reduction consists of three main steps: (1) the standard reduction of two-dimensional fiber spectra, (2) the line fitting procedure and (3)
the generation of maps of spectral features (e.g intensity and velocities fields)
from the calibrated spectra. The basic reduction was done using the IRAF environment and R3D software following [11]. The standard procedure consists
of: bias subtraction, cosmic rays removal, tracing and extraction of each individual spectrum, distortion correction and determination of the wavelength
solution, correction of the differences between fiber-to-fiber transmission, sky
emission subtraction, flux calibration and differential atmospherics refraction
correction. For each spectrum, the total flux, the radial velocities and the velocity dispersions were measured by fitting single Gaussian functions to the
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Castillo-Morales et al.
observed emission-line profiles using own software. A two-component (emission and absorption) gaussian fit was performed in order to correct the Balmer
lines for underlying absorption (see Figure 1). We use a two-dimensional interpolation included in the software package E3D [12] to generate maps. When
observing with v300 grating, we use the observational technique of dithering
in order to cover the data gaps between fibers. Therefore, there are regions in
the field of view which are sampled once, twice and three times. In this case
no interpolation is needed and we generate maps from the data cube using a
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grid of pixels 1 size. In each pixel the average spectrum (of three, two or one
spectrum) weighted with the area of the pixel occupied by the corresponding
dithering is assigned.
3 Results
The specific objectives of our study are: (i) to study the kinematics of the
ionized gas using [O III] λ5007; (ii) to measure the stellar velocity field using
the Mg2 index; (iii) to parameterize the star formation rate using Hβ luminosities; (iv) to estimate the age and metallicity of the starburst using O, N,
and Balmer emission lines, (v) to estimate the age and metallicity of the underlying older population from the Mg lines in absorption; (vi) to determine
the distribution of the interstellar dust using the Balmer decrement; (vii) to
compare the properties of the ionized gas to those of the molecular gas and
the neutral atomic gas using IRAM and VLA data already at hand; and (viii)
to simulate observations of this galaxy sample at z ∼ 2.
At the moment our galaxy sample is under reduction and some results on
UCM2325+2318 (NGC 7673) which are under analysis are presented here.
High-quality ground-based images indicated this galaxy consists of giant
star-forming clumps embedded in a diffuse halo, and thus NGC 7673 was
classified as a clumpy irregular galaxy [13, 14]. The spiral pattern in this
galaxy becomes clearer in images taken with WFPC2 on the HST [15]. NGC
7673 shows an hyperactive star formation within the clumps [16] with L(Hα)
= 2-8 x 108 L¯ for the three largest clumps, comparable to the total L(Hα)
seen in many starburst galaxy nuclei.
A continuum-subtracted Hα image from PPAK data is shown in Figure 2
(a) and can be compared with the Hα image from NOT telescope at La Palma
(Figure 2(b)). There are three main H II regions: the central ”nucleus,” the
large clump to the northeast, and a smaller clump situated between the two.
The extinction map is computed from the ratio of the observed PPAK
Hα and Hβ emission line fluxes (see Figure 2 (c)). We find a low extinction
gradient in the galaxy with the maximum extincition value AHα ≈ 1 mag
(Castillo-Morales et al. in preparation). The location of the main bursts are
not coincident with the maximum extinction computed. The nuclear burst
seems to be located in a region of lower extinction surrounded by a higher
extinction shell.
Study of local Luminous Compact Blue Galaxies
5
The Hα velocity field computed with PPAK data is shown in Figure 2 (d).
This map shows the presence of a rotating disk with signs of perturbations
(Pérez-Gallego et al. in preparation). In spite of these perturbations caused by
the star formation processes and the spiral pattern, the galaxy has survived the
event that triggered the starburst and appears to be rotating smoothly. The
shallow velocity gradient (≈ 150 km/s from the receding to the approaching
part) is very similar to the values found from HI data [17].
Fig. 2. UCM2325+2318 (NGC 7673). Continuum-subtracted Hα image from PPAK
data (a) compared with the Hα image from the NOT telescope (b). Hα velocity field from PPAK data (c). Hα/Hβ ratio used to computed the extinction in
UCM2325+2318 (d).
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