1. Introduction: Strong lensing and the 1E0657 arc core
Lyman-Break Galaxies are particularly small and faint, even for being at high redshift, with typical half-light radii of . 0.300 (Giavalisco, Steidel, & Macchetto 1996). Studying the spatially-
resolved properties (e.g. dynamics, metallicities) of their rest-frame optical emission line gas in detail means driving the current generation of instrumentation to its limits, as will become clear from the next Chapter. Results based on this type of measurement are biased towards the strongest line emitting sources (e.g., starbursts) which may not be representative of the overall population or only sample selected regions within each source (e.g., concentration of the line emission to the central regions, variation in the extinction, etc.; Lehnert & Heckman 1996b). This bias may in turn lead to intrinsic systematic uncertainties (e.g., not sampling the full rotation curve or being sensitive to hydrodynamic effects like galactic-scale outflows or complex dynamics due to mergers). Such worries are not without a substantial basis. It is known that LBGs are actively forming stars, and their morphologies are often complex and do not resemble any of the Hubble types in the nearby Universe. This suggest that mergers, extinction and the distribution of the emission line gas may be important factors in interpreting the observed dynamics ofz∼3 LBGs.
Some of these worries could be mitigated against if it was possible to “zoom into” a small region around the center of a z ∼3 LBG. A highly magnified circum-nuclear region with a well- defined rotation curve would be concrete evidence that at least some sources in this population are dominated by rotation. Although a galaxy in the throws of a major merger can have complex dynamics even in its core, on small scales, it is hard to conceive how it could be mimicking a “rotation curve”. A merger would have broad lines and proportionally small velocity gradient which should be obvious. On the other hand, on large scale, blurred by seeing, it is easily possible for a merger to masquerade as a rotating disk due to the large relative velocity offsets and the seeing merging the velocities into a smooth curve.
The only way currently available to zoom into high-redshift galaxies at size scales below∼1 kpc 125
is through the observation of strongly lensed sources within the critical radius of massive galaxy clusters at intermediate redshift. In spite of the increasing number of detected strong gravitational arcs, spectroscopic follow-up observations with good resolution and signal-to-noise ratios have been carried out only for very few sources. The number is virtually zero if one considers strongly lensed galaxies with high quality, spatially-resolved near-infrared spectra. The sample with detailed observations is quite inhomogeneous, ranging from low-extinction Lyman Break Galaxies (such as MS1512-cB58 at z = 2.7; Yee et al. 1996) to dusty submillimeter sources (e.g. MS0451.6-0305 at
z = 2.9; Borys et al. 2004). Another spectacular recent example is the Lynx arc, a lensed HII galaxy at z= 3.4 (Fosbury et al. 2003).
Strongly lensed sources are a necessary complement to the unmagnified ones. They make it possible to observe parts of galaxies at very high physical resolution. Given the debate as to whether or not spatially resolved dynamics is due to rotation or something more complex, this is not a mere academic advantage. Unfortunately, the regions magnified are effectively random cutouts of the galaxy – results might not at all be representative for the integrated properties of the object. In addition, the detailed modelling of the lensing potential is crucial. This can be a problem since sometimes there are insufficient constraints to construct accurate lensing models. Consequently, morphologies, size scales, and how sources are “parceled” may be highly uncertain and are certainly model-dependent.
Spatially-resolved kinematics of gravitational arcs are only rarely addressed in the literature. Franx et al. (1997) obtained spatially resolved Lyαand SIIλ1260 emission line spectra of az= 4.0 lensed starburst galaxy in the CL 1358+62 field, finding an overall blueshift of SII with respect to Lyα, and velocity differences of about 300 km s−1 over a diameter of a few kpc. Unfortunately, Lyα is susceptible to optical depth effects which may dominate the line centers and widths. Thus Lyα cannot generally be used to track the dynamical masses of galaxies. Franx et al. (1997) conclude that the line emission most likely traces an outflow, with Lyαbeing apparently redshifted due to absorption by neutral gas, and is not indicative of the large-scale gravitational motion. Lemoine-Busserolle et al. (2003) studied a pair of z≈1.9 galaxies, lensed by the cluster AC114, and separated by ∼ 34 kpc in the source plane. In one of the two cases, they spatially resolved the Hα and [OIII]λ5007 lines for one of the galaxies (AC114-S2) and derived a mass of approximately 1×1010 M
¯within the inner 1 kpc (actually beyond their velocity measurements). However, these
are not galaxies in the “classical” redshift range of Lyman break galaxies, and they have different physical and photometric properties. Finally, from a rest-frame optical integrated spectrum of the strongly lensed LBG MS 1512-cB58, Teplitz et al. (2000) estimate a dynamical mass of 1.2×1010 M¯ from the velocity dispersion estimated using the Balmer emission lines.
The strongly lensed z = 3.241 galaxy behind the z = 0.3 X-ray cluster 1E0657-56 (Tucker et al. 1998) is different from most other well-studied high-redshift gravitational arcs. Its high magnification (> 20) presents a good opportunity to investigate the properties of LBGs at high physical and spectral resolution. Rest-frame UV data of the∼1400long 1E0657-56 gravitational arc
were initially obtained by Appenzeller et al. (1998) during the commissioning ofFors1. Photometry in B, g, R and I was taken, as well as longslit spectra (Mehlert et al. 2001). Its lensing configuration suggests the simultaneous magnification of a high surface brightness region at the south-eastern tip of the source that may be associated with the “core” of the galaxy as well as a more highly magnified, lower surface brightness region apart from the core (“arc” Mehlert et al. 2001). The arc
1Using the flat concordance cosmology with Ω
Λ=0.7 and H0 = 70kms−1M pc−1 leads to DL = 27.9 Gpc and
DA= 1.5 Gpc atz= 3.24 (the redshift of the arc and core). The size scale is 7.5 kpc/00. The age of the universe at
shows a complex substructure: Mehlert et al. (2001) identify three faint knots of similar surface brightness within the arc, each separated by a few arcseconds. These 3 knots lie to the northwest of the core. The knots and core are spatially resolved along the axis of greatest magnification, and unresolved perpendicular to it in the ground-based imaging of Appenzeller et al. (1998). Mehlert et al. (2001) propose that the central highest surface brightness region of the lensed galaxy, lying near, but outside the cusp-caustic, is seen as the bright arc core, whereas a fainter outer region of the same galaxy, which touches the cusp-caustic, is split into three merging images, and constitutes the full extent of the arc. Their modeling suggests a magnification factor of ∼20, or 3.25 mag for the core and higher by an unspecified amount for the arc.
The rest-frame UV spectra in Mehlert et al. (2001) show absorption lines of moderate strength, with strong Lyαand Lyβabsorption. No emission line is seen, arguing against a dominant spectral contribution from hot O stars.
Given the interesting configuration of this source and the wealth of supplementary data, it was observed in one of the Spiffi“Guest Instruments” runs. TheSpiffi-GI commissioning-team obtained deep K band data of 1E0657-56 arc core, covering the core as well as the neighboring and parts of the subsequent bright arc knot. Observations between April, 6th and 11th 2003, were carried out under variable conditions, so that only 190 out of 300 minutes total integration time were used in the final cube. The size of the seeing disk was determined from the light profile of the standard star to be 0.600 and 0.400 in right ascension and declination, respectively.
2. Evidence that “Arc + Core” is a LBG
Undoubtedly, one of the best-studied class of galaxies at high redshift are the Lyman break galaxies (LBGs) at z∼3. As discussed by Pettini et al. (2001) and Shapley, Steidel, Pettini, & Adelberger (2003), for example, these are low-metallicity, actively star-forming galaxies of suffi- cient number density that they may represent the phase of bulge/spheroid growth of M? galaxies. However, their small spatial extent (∼0.300in the rest-frame UV continuum, e.g., Giavalisco, Steidel,
& Macchetto 1996) makes it difficult or even impossible to obtain high-quality spatially resolved spectra of these sources. Relatively weak velocity gradients have been found for two LBGs (Pet- tini et al. 2001), but these tend to have large projected angular sizes and irregular morphologies making the interpretation of the velocities difficult. Although rest-frame UV morphologies of local galaxies are often very irregular, and dominated by star-forming regions and complexes, those in- homogeneities are typically on scales of 10 to a few 100 parsecs (in the adopted cosmology, angular sizes of <0.1 arcsec). At the redshifts of the LBGs, such scales are smoothed by the seeing in typical conditions and are no longer resolved. The light profiles of these galaxies, however, often appear elongated, and dominated by two unresolved rest-frame UV bright spots at distances of a few kpc. Such a geometry and scales are more consistent with galaxy pairs or mergers. How- ever, the small spatial extent and faintness of these sources make it difficult to distinguish between these two hypotheses, especially when only ground-based observations (without adaptive optics) are available.
The study of gravitationally lensed Lyman break galaxies is one way to overcome the limited spatial resolution of ground-based data without adaptive optics. However, only one strongly lensed LBG has so far been identified, MS1512-cB58 at z∼2.73, a rather low redshift for this population. Since the redshift z = 3.24 of the 1E0657 arc and core galaxy is near the average redshift of the LBG sample (z= 3.13±0.06), and its physical properties are very similar, it is of major interest to investigate whether this target quantitatively satisfies the LBG color selection at z∼3. Therefore
initially, data sets previously obtained by Mehlert et al. (2001), Appenzeller et al. (1998) will be reviewed and reanalyzed, and a currently unpublished ACS image2.
2.1. Rest-frame UV colors
The Fors1 rest-frame UV spectrum covers the full spectral range used for the Lyman-break color selection. By convolving the spectrum with the corresponding filter curves used to select LBGs (digitized from Fig. 1 in Steidel et al. (2003)), and deriving the colors directly from the flux ratios, it is possible to measurea posterioriwhether the gravitational arc and core in 1E0657 fulfill the Lyman break criteria3. In an analogous way, the magnitude measured in theForsR-band can be translated into the R band magnitude used by Steidel et al. (2003). To illustrate the method graphically, the SED of the core together with the digitized filter curves is shown in Fig. 1. For the 1E0657 giant arc one obtains the colors and magnitudes:
R= 21.53 G− R= 0.94 U− G= 2.73(>G− R+ 1)
Fig. 1.—Transmission curves for the filters used in the Lyman-Break color selection (US,G,RLBG), and ob-
served optical spectrum of the core (gray shaded area). RF denotes the filter curve of the Fors R-band filter.
Comparison with the Lyman-break crite- ria given in Steidel et al. (1999) shows that all three selection criteria are fulfilled, i.e. the gravitational arc in 1E0657 is a Lyman-break galaxy. The R band mag- nitude is not corrected for the M ∼ 3.3 mag magnification, because it is mainly an observational limit to ensure a homo- geneous selection in spite of varying sen- sitivities and fluxes in the different wave- bands, and to minimize color biases at the faint end. However, even if the magnitude is corrected for lensing, the R-band mag- nitude of the arc, R = 24.83, is brighter than the Lyman-break selection cutoff of
R = 25.5. The SEDs of the arc and core are very similar, so that it is not very sur- prising that also the core is within the Lyman-break range:
R= 21.15 G− R= 0.92 U− G= 2.71(>G− R+ 1)
2The image was kindly, quickly, and generously provided by Christine Forman-Jones before its release into the
HST archive
Integrated R-band fluxes within the 26 mag isophote were used for this comparison (see Table 1 of Mehlert et al. 2001). However, similar results are obtained for all magnitudes given in Mehlert et al. (2001).
2.2. Rest-frame UV spectroscopy
Rest-frame UV spectroscopy of the arc and core with Fors1 on the VLT were described by Mehlert et al. (2001) and allows a detailed comparison of the UV emission with that in the K-band from Spiffi.
The rest-frame UV spectra were integrated over the core and an identical aperture was used to extract theSpiffi data. In addition, a longslit spectrum of Mehlert et al. (2001) extends over the full extent of the arc (which is about twice the surface covered with Spiffi). At a redshift of
z ∼ 3.24, wavelengths between ∼1000 ˚A and ∼1800 ˚A fall within the Fors bandpass and were obtained with good signal-to-noise ratios. The spectra are dominated by continuum emission, with a comparably broad, asymmetric Lyα absorption line. No significant Lyα emission was detected either in the arc nor in the core. The asymmetric, broad blue wing of the Lyαline is typical for high- redshift galaxies, and is due to absorption by neutral hydrogen in the ISM. A number of narrow metal absorption lines are superimposed, which originate from stellar winds in the atmospheres of hot, massive stars and from the ambient interstellar medium. Unfortunately, these spectra have signal-to-noise ratios too low to robustly investigate the significant UV stellar photospheric diagnostic lines, especially in regions of night sky lines. Mehlert et al. (2001) investigated the strong stellar wind lines ofSiIV 1400 andCIV1550 to determine the metallicities of the gas.
Fig. 2.— The rest-frame UV SED of the arc (gray line) and the best-fit Starburst99 model (black line) assuming an instantaneous burst 8×107 yr ago and no
extinction. The green and yellow bands indicate the 1 and 2-σ uncertainties of the data.
The continuum slope is a reasonable age and extinction indicator in high redshift galaxies. Given that most of the strong ab- sorption lines in the UV are due to stellar winds and absorption by gas in the ambi- ent ISM, they are not very useful for de- termining the ages of galaxies. These lines are more a probe of the metallicity, ex- tinction, luminosity, and gas dynamics of starburst galaxies (Heckman et al. 1998). In addition, the continuum slope is much better defined in these data, which can be fitted with population synthesis mod- els, such as starburst 99 (Leitherer et al. 1999) to make an approximate age de- termination (although it is still degener- ate with the extinction). starburst 99 was chosen for this particular source, be- cause it concentrates on the rest-frame UV emission of starburst galaxies and is widely
used within the astrophysics community. Population synthesis is described in more detail in Sec- tion 2 of Chapter 4.
In a burst of star-formation, the presence of a strong UV continuum in itself restricts the age of the dominant population to less than a few hundred Megayears, due to the short lifetimes of the most massive stars that have hot enough atmospheres to emit a substantial number of UV photons. From the analysis of the rest-frame optical lines, it is known that 1E0657 arc+core have subsolar metallicities, hence the model used metallicities measured in the Small Magellanic Cloud. A Salpeter IMF was used, with 1 M¯and 100 M¯lower and upper mass cut-offs. An equally-spaced grid of ages between 0.5 and 15×107 yr was modelled in steps of 5 Myr. The best fit was found in both the arc and the core for a population that formed in an instantaneous burst 8×107 yr ago, and has no extinction. The fit to the core together with the data, convolved to match the rather coarse resolution of the model (∼ 10 ˚A) is shown in Fig. 2. The green and yellow bands indicate the 1 and 2-σ uncertainties of the data.
Although the presence of the UV continuum alone makes the fit rather unique in a burst model, it is interesting to ask how well the model is constrained. A detailed, statistical assessment of this question is difficult due to the large number of partly degenerate parameters and correlations within the data. However, a rough estimate of at least parts of the parameter space can be given by varying the input parameters (especially the age) and examining from which values on the disagreement between data and model will systematically exceed 3σ. It was found that within the calculated grid of models, systematic discrepancies either in the blue or red wavelength range (or both) were found for ages below 6×107 yr in the arc (5×107 yr in the core due to the smoother spectrum) and for ages above 1.1×108 yr in either part of the source. For the concordance cosmology, this means the burst occurred at z.3.4. The stellar mass estimate follows directly from the modeled luminosity and the distance modulus. The according ranges of stellar mass within the arc are 3.5−10×109 M¯ and 2−7×109 M¯ in the core. These values are not corrected for lensing.
2.3. Continuum morphology and the half-light radius of the core
High-resolution imaging of the arc and core have been recently obtained with the ACS camera on-board the HST. The image taken through the F814W filter (roughly I-band) is shown in Fig. 12. To estimate the spatial resolution, an artificial PSF was created using the TinyTim package (Krist & Hook 1997). Sizes were measured by fitting a two-dimensional Gaussian. FWHMs are 0.08900 in
both right ascension and declination. The profiles along right ascension for the PSF and the arc are compared in Fig. 3.
Both arc and core are spatially resolved in the direction perpendicular to the magnification axis. Intrinsic profile widths are determined by deconvolving the observed diameter with the diameter of the PSF generated by TinyTim:
FWHMintr=
q
FWHM2obs−FWHM2psf
The unlensed diameter of the arc varies between ∼0.3−0.600(2.4−4.8 kpc), the core reaches an
unlensed diameter of 0.11300 (0.9 kpc) across its brightest part.
The ACS image reveals the complexity of the source. The arc is composed of irregular high surface-brightness patches, maybe giant star-forming HII regions, embedded into a continuous structure with lower surface brightness, which itself also does not have a very smooth, regular shape. The core has an overall higher surface brightness, with a bright, unresolved spot in the