K. J. Fricke

The Tully-Fisher relation at intermediate redshift

Using the Very Large Telescope in Multi Object Spectroscopy mode, we have observed a sample of 113 field spiral galaxies in the FORS Deep Field (FDF) with redshifts in the range 0.1< z< 1.0. The galaxies were selected based on apparent brightness (R< 23 m ) and encompass all late spectrophotometric types from Sa to Sdm/Im. Spatially resolved rotation curves have been extracted for 77 galaxies and fitted with synthetic velocity fields taking into account all observational e ffects from inclination and slit misalignment to seeing and slit width. We also compared different shapes for the intrinsic rotation curve. To obtain robust values of Vmax, our analysis is focused on galaxies with rotation curves th at extend well into the region of constant rotation velocity at large radii. If the slope of th e local Tully-Fisher relation (TFR) is held fixed, we find evid ence for a mass-dependent luminosity evolution which is as large as up toMB≈ −2 m for the lowest-mass galaxies, but is small or even negligible for the highest-mass systems in our sample. In effect, the TFR slope is shallower at z≈ 0.5 in comparison to the local sample. We argue for a mass-dependent evolution of the mass-to-light ratio. An additional population of blue, low-mass spirals does not seem a very appealing explanation. The flatter tilt we find for the distant TFR is in contradictio n to the predictions of recent semi-analytic simulations.

The blue compact dwarf galaxy I Zw 18: A comparative study of its low-surface-brightness component

Using HSTand ground-based optical and NIR imaging data �� ��� , we investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 possesses an extended low-surface-brightness (LSB) old stellar population underlying its star-forming regions, as is the case in the majority of BCDs. This question is central to the long-standing debate on the evolutionary state of I Zw 18. We show that the exponential intensity decrease observed in the filamentary LSB envelope of the BCD out to >18 �� (>1.3 kpc assuming a distance of 15 Mpc) is not due to an evolved stellar disc underlying its star-forming regions, but rather, due to extended ionized gas emission. Ionized gas accounts for more than 80% of the line-of-sight emission at a galactocentric distance of ∼0.65 kpc (∼ 3e ffective radii), and for >30% to 50% of the R light of the main body of I Zw 18. Broad-band images reveal, after subtraction of nebular line emission, a relatively smooth stellar host extending slightly beyond the star-forming regions. This unresolved stellar component, though very compact, is not exceptional for intrinsically faint dwarfs with respect to its structural properties. However, being blue over a radius range of ∼5 exponential scale lengths and showing little colour contrast to the star-forming regions, it differs strikingly from the red LSB host of standard BCDs. This fact, together with the comparably blue colours of the faint C component, ∼1.6 kpc away from the main body of I Zw 18, suggests that the formation of I Zw 18 as a whole has occurred within the last 0.5 Gyr, making it a young BCD candidate. Furthermore, we show that the ionized envelope of I Zw 18 is not exceptional among star-forming dwarf galaxies, neither by its exponential intensity fall-off nor by its scale length. However, contrary to evolved BCDs, the stellar LSB component of I Zw 18 is much more compact than the ionized gas envelope. In the absence of an appreciable underlying stellar population, extended ionized gas emission dominates in the outer parts of I Zw 18, mimicking an exponential stellar disc on optical surface brightness profiles.

The Evolution of the Tully-Fisher Relation of Spiral Galaxies*

We present the B-band Tully-Fisher relation (TFR) of 60 late-type galaxies with redshifts 0.1–1. The galaxies were selected from the FORS Deep Field with a limiting magnitude of . Spatially resolved rotation curves R p 23 were derived from spectra obtained with FORS2 at the Very Large Telescope. High-mass galaxies with vmax 150 km s 1 show little evolution, whereas the least massive systems in our sample are brighter by ∼1–2 mag compared with their local counterparts. For the entire distant sample, the TFR slope is flatter than for local field galaxies ( vs. ). Thus, we find evidence for the evolution of the slope of the TFR 5.77 0.45 7.92 0.18 with redshift on the 3 j level. This is still true when we subdivide the sample into three redshift bins. We speculate that the flatter tilt of our sample is caused by the evolution of luminosities and an additional population of blue galaxies at . The mass dependence of the TFR evolution also leads to variations for different z 0.2 galaxy types in magnitude-limited samples, suggesting that selection effects can account for the discrepant results of previous TFR studies on the luminosity evolution of late-type galaxies. Subject headings: galaxies: evolution — galaxies: kinematics and dynamics — galaxies: spiral

The FORS Deep Field: Field selection, photometric observations and photometric catalog ,

The FORS Deep Field project is a multi-colour, multi-object spectroscopic investigation of a ∼7 � × 7 � region near the south galactic pole based mostly on observations carried out with the FORS instruments attached to the VLT telescopes. It includes the QSO Q 0103-260 (z = 3.36). The goal of this study is to improve our understanding of the formation and evolution of galaxies in the young Universe. In this paper the field selection, the photometric observations, and the data reduction are described. The source detection and photometry of objects in the FORS Deep Field is discussed in detail. A combined B and I selected UBgRIJKsphotometric catalog of 8753 objects in the FDF is presented and its properties are briefly discussed. The formal 50% completeness limits for point sources, derived from the co-added images, are 25.64, 27.69, 26.86, 26.68, 26.37, 23.60 and 21.57 in U, B, g, R, I, J and Ks(Vega-system), respectively. A comparison of the number counts in the FORS Deep Field to those derived in other deep field surveys shows very good agreement.

Extremely metal-poor star-forming galaxies - New detections and general morphological and photometric properties

Context. Extremely metal-deficient [12 + log(O/H) ≤ 7.6] emission-line galaxies in the nearby universe are invaluable laboratories of extragalactic astronomy and observational cosmology since they allow us to study collective star formation and the evolution of galaxies under chemical conditions approaching those in distant protogalactic systems. However, despite intensive searches over the last three decades, nearby star-forming (SF) galaxies with strongly subsolar metallicity remain extremely scarce. Aims. We searched the Sloan Digital Sky Survey (SDSS) and the Six-Degree Field Galaxy Redshift Survey (6dFGRS) for promising low-metallicity candidates using a variety of spectroscopic criteria. Methods. We present long-slit spectroscopy with the 3.6 m ESO telescope of eight Hll regions in seven emission-line dwarf galaxies, selected from the Data Release 4 of SDSS (six galaxies) and from 6dFGRS (one galaxy). In addition, we use SDSS imaging data to investigate the photometric structure of the sample galaxies. Results. From the 3.6 m telescope spectra, we determine the oxygen abundance of these systems to be 7.3 ≤ 12 + log(O/H)≤7.6, placing them among the most metal-poor star-forming galaxies ever discovered. Our photometric analysis reveals a moderately blue, stellar host galaxy in all sample galaxies. Conclusions. The detection of a stellar host in all galaxies studied here and all previously studied extremely metal-deficient SF galaxies implies that they are unlikely to be forming their first generation of stars. With regard to the structural properties of their host galaxy, we demonstrate that these systems are indistinguishable from blue compact dwarf (BCD) galaxies. However, in contrast to the majority (>90%) of BCDs that are characterised by red elliptical host galaxies, extremely metal-poor SF dwarfs (hereafter XBCDs) reveal moderately blue and irregular hosts. This is consistent with a young evolutionary status and in the framework of standard star formation histories implies that several XBCDs formed most of their stellar mass in the past ∼2 Gyr. A large fraction of XBCDs reveal a cometary morphology due to the presence of intense SF activity at one edge of an elongated host galaxy with a gradually decreasing surface brightness towards its antipodal end.

New insights to the photometric structure of Blue Compact Dwarf galaxies from deep Near-Infrared studies - I. Observations, surface photometry and decomposition of surface brightness profiles

We have analyzed deep Near Infrared (NIR) broad band images for a sample of Blue Compact Dwarf Galaxies (BCDs), observed with the ESO NTTand Calar Alto �� 3.6 m telescopes. The data presented here allows for the detection and quantitative study of the extended stellar low-surface brightness (LSB) host galaxy in all sample BCDs. NIR surface brightness profiles (SBPs) of the LSB host galaxies agree at large galactocentric radii with those from optical studies, showing also an exponential intensity decrease and compatible scale lengths. At small to intermediate radii (within 1-3 exponential scale lengths), however, the NIR data reveals for more than one half of our sample BCDs evidence for a significant flattening of the exponential profile of the LSB component. Such profiles (type V SBPs, Binggeli & Cameron 1991) have rarely been detected in the LSB component of BCDs at optical wavelengths, where the relative flux contribution of the starburst, being stronger than in the NIR, can readily hide a possible central intensity depression in the underlying LSB host. The structural properties, frequency and physical origin of type V LSB profiles in BCDs and dwarf galaxies in general have not yet been subject to systematic studies. Nevertheless, the occurrence of such profiles in an appreciable fraction of BCDs would impose important new observational constraints to the radial mass distribution of the stellar LSB component, as well as to the photometric fading of these systems after the termination of star-forming activities. We test the suitability of two empirical fitting functions, a modified exponential distribution (Papaderos et al. 1996a) and the Sersic law, for the systematization of the structural properties of BCD host galaxies which show a type V intensity distribution. Either function has been found to satisfactorily fit a type V distribution. However, it is argued that the practical applicability of Sersic fits to the LSB emission of BCDs is limited by the extreme sensitivity of the achieved solutions to, e.g., small uncertainties in the sky subtraction and SBP derivation. We find that most of the sample BCDs show in their stellar LSB host galaxy optical-NIR colors indicative of an evolved stellar population with subsolar metallicity. Unsharp-masked NIR maps reveal numerous morphological details and indicate in some cases, in combination with optical data, appreciable non-uniform dust absorption on a spatial scale as large as ∼1 kpc.

SBS 0335-052E+W: deep VLT/FORS+UVES spectroscopy of the pair of the lowest-metallicity blue compact dwarf galaxies

Context. We present deep archival VLT/FORS1+UVES spectroscopic observations of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS 0335-052W. Aims. Our aim is to derive element abundances in different H II regions of this unique system of galaxies and to study spatial abundance variations. Methods. The electron temperature T-e (O III) in all H II regions, except for one, is derived from the [O III] lambda 4363/(lambda 4959+lambda 5007) flux ratio. We determine ionic abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon and iron. The empirical relations for ionization correction factors are used to derive total abundances of these elements. Results. The oxygen abundance in the brighter eastern galaxy varies in the range 7.11 to 7.32 in different H II regions supporting previous findings and suggesting the presence of oxygen abundance variations on spatial scales of similar to 1-2 kpc. Good seeing during FORS observations allowed us to extract spectra of four H II regions in SBS 0335-052W. The oxygen abundance in the brightest region No. 1 of SBS 0335-052W is 7.22 +/- 0.07, consistent with previous determinations. Three other H II regions are much more metal-poor with an unprecedently low oxygen abundance of 12 + log O/H = 7.01 +/- 0.07 (region No. 2), 6.98 +/- 0.06 (region No. 3), and 6.86 +/- 0.14 (region No. 4). These are the lowest oxygen abundances ever derived in emission-line galaxies, supporting earlier conclusions that SBS 0335-052W is the lowest-metallicity emission-line galaxy known. Helium abundances derived for the brightest H II regions of both galaxies are mutually consistent. We derive weighted mean He mass fractions of 0.2485 +/- 0.0012 and 0.2514 +/- 0.0012 for two different sets of He I emissivities. The ratios of neon and sulfur to oxygen abundance are similar to the respective ratios obtained for other emission-line galaxies. On the other hand, the chlorine-to-oxygen abundance ratio in SBS 0335-052E is lower, while the argon-to-oxygen abundance ratio is higher than those in other low-metallicity galaxies. The Fe/O abundance ratios in different regions of SBS 0335-052E are among the highest for emission-line galaxies implying that iron is almost entirely not depleted onto dust grains despite dust being detected in this galaxy in earlier ISO and Spitzer observations. The N/O abundance ratio in both galaxies is slightly higher than that derived for other BCDs with 12 + logO/H < 7.6. This implies that the N/O in extremely metal-deficient galaxies could increase with decreasing metallicity.

An Imaging and Spectroscopic Study of the Very Metal-deficient Blue Compact Dwarf Galaxy Tol 1214−277*

We present a spectrophotometric study based on VLT/FORS I observations of one of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol 1214-277 (Z ~ Z⊙/25). The data show that roughly half of the total luminosity of the BCD originates from a bright and compact starburst region located at the northeastern tip of a faint dwarf galaxy with cometary appearance. The starburst has ignited less than 4 Myr ago and its emission is powered by several thousand O7V stars and ~170 late-type nitrogen Wolf-Rayet stars located within a compact region with 500 pc in diameter. For the first time in a BCD, a relatively strong [Fe V] λ4227 emission line is seen which together with intense He II λ4686 emission indicates the presence of a very hard radiation field in Tol 1214-277. We argue that this extraordinarily hard radiation originates from both Wolf-Rayet stars and radiative shocks in the starburst region. The structural properties of the low surface brightness (LSB) component underlying the starburst have been investigated by means of surface photometry down to 28 B mag arcsec-2. We find that, for a surface brightness level fainter than ~24.5 B mag arcsec-2, an exponential fitting law provides an adequate approximation to its radial intensity distribution. The broadband colors in the outskirts of the LSB component of Tol 1214-277 are nearly constant and are consistent with an age below one Gyr. This conclusion is supported by the comparison of the observed spectral energy distribution (SED) of the LSB host with theoretical SEDs.

On faint companions in the close environment of star-forming dwarf galaxies - Possible external star formation triggers?

We have searched for companion galaxies in the close environment of 98 star{forming dwarf galaxies (SFDGs) from eld and low density environments, using the NASA Extragalactic Database. Most of the compan- ions are dwarf galaxies which due to observational selection eects were previously disregarded in environmental studies of SFDGs. A subsample at low redshift, cz < 2000 km s 1 , was chosen to partially eliminate the ob- servational bias against distant dwarf companions. We nd companion candidates for approximately 30% of the objects within a projected linear separation sp < 100 kpc and a redshift dierence cz < 500 km s 1 . The limited completeness of the available data sets, together with the non-negligible frequency of H i clouds in the vicinity of SFDGs indicated by recent radio surveys, suggest that a considerably larger fraction of these galaxies may be accompanied by low{mass systems. This casts doubt on the hypothesis that the majority of them can be consid- ered truly isolated. The velocity dierences between companion candidates and sample SFDGs amount typically to <250 km s 1 , and show a rising distribution towards lower cz. This is similarly found for dwarf satellites of spiral galaxies, suggesting a physical association between the companion candidates and the sample SFDGs. SFDGs with a close companion do not show signicant dierences in their H equivalent widths andB V colours as compared to isolated ones. However, the available data do not allow us to rule out that interactions with close dwarf companions can influence the star formation activity in SFDGs.

Balmer jump temperature determination in a large sample of low-metallicity Hii regions ⋆,⋆⋆

Aims. Continuing the systematic determination of the electron temperature of H II regions using the Balmer and/or Paschen discontinuities by Guseva et al. (2006, ApJ, 644, 890) we focus here on 3.6 m ESO telescope observations of a large new sample of 69 H II regions in 45 blue compact dwarf (BCD) galaxies. This data set spans a wide range in metallicity (Z ⊙ /60 S Z ≥ Z ⊙ /3) and, combined with the sample of 47 H II regions from Guseva et al. (2006), yields the largest spectroscopic data set ever used to derive the electron temperature in the H + zone. Methods. In the same way as in Guseva et al. (2006) we have used a Monte Carlo technique to vary free parameters and to calculate a series of model spectral energy distributions (SEDs) for each H II region. The electron temperature in the H + zones was derived from the best fitting synthetic and observed SEDs in the wavelength range ∼ 3200-5100 A, which includes the Balmer jump. Results. On the base of the present large spectroscopic sample we find that in hot (T e (H + ) ≥ 11 000 K) H II regions the temperature of the O 2+ zone, determined from doubly ionised oxygen forbidden lines, does not differ statistically from the temperature of the H zone. Thus, we confirm and strengthen the finding by Guseva et al. (2006). We emphasize that due to a number of modelling assumptions and the observational uncertainties for individual objects, only a large, homogeneous sample, as the one used here, can enable a conclusive study of the relation between T e (H + ) and T e (O III).