arXiv subjects

Ulrich Hopp

Publications and source records attributed to Ulrich Hopp.

68 records · Page 4Linked to original sources

arXiv2000

A Compact Population of Red Giants in the Blue Compact Dwarf Galaxy UGCA 290

We present HST/WFPC2 single-star photometry for the blue dwarf galaxy UGCA 290, whose morphology is intermediate between classic iE Blue Compact Dwarfs and blue dwarfs which exhibit no red background sheet of older stars. The color-magnitude diagram of this galaxy in V and I, extending over six magnitudes, is remarkably similar to that of the star-forming region in the iE Blue Compact Dwarf VII Zw 403. There is no evidence for gaps in its star-formation history over the last billion years, and the color of its red giant branch indicates a very metal-poor stellar population. From the magnitude of the tip of the red giant branch, we derive a distance of 6.7 Mpc, more than twice the distance estimated from the brightest blue supergiants.

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arXiv2000

A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: NICMOS Detection of Red Giant Stars in the Wolf-Rayet Galaxy Mrk 178

We observed the Blue Compact Dwarf/Wolf-Rayet galaxy Mrk 178 with the NICMOS camera aboard HST. The galaxy is well resolved into individual stars in the near-IR; photometry in J and H yields color-magnitude diagrams containing 791 individual point sources. We discuss the stellar content, drawing particular attention to the intermediate age and/or old stars. Mrk 178 is only the second Blue Compact Dwarf galaxy in which the red giant branch has been resolved, indicating stars with ages of at least 1-2Gyr. This allows us to derive a distance of $\geq$4.2($\pm$0.5)Mpc. The near-IR color-magnitude diagram also exhibits an abundance of luminous, asymptotic giant branch stars. We find that this requires vigorous star formation several hundred Myr ago. Some candidate carbon stars are identified via their extreme near-IR color. We argue that Mrk178 is fundamentally an old galaxy, based on the NICMOS detection of red giants underlying the blue, starburst core, and its extended, faint halo of redder color.

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arXiv1999

A spectrophotometric catalogue of HII galaxies

We present a spectrophotometric catalogue of 90 emission-line galaxies (ELGs) discovered during an objective - prism survey that aimed to search for dwarf galaxies within the voids (Popescu et al. 1996, 1998). The paper presents line ratios, equivalent widths and absolute fluxes for the emission-lines seen in the spectra of the galaxies. A list of newly discovered Wolf-Rayet galaxies is presented. Many objects included in the catalogue have low metallicity and the extreme cases are proposed as candidates for very low metallicity galaxies.

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arXiv1999

A Near-Infrared Stellar Census of the Blue Compact Dwarf Galaxy VII~Zw~403

We present near-infrared single-star photometry for the low-metallicity Blue Compact Dwarf galaxy VII~Zw~403. We achieve limiting magnitudes of F110W~$\approx$~25.5 and F160W~$\approx$~24.5 using one of the NICMOS cameras with the HST equivalents of the ground-based J and H filters. The data have a high photometric precision (0.1 mag) and are $>95$% complete down to magnitudes of about 23, far deeper than previous ground-based studies in the near-IR. The color-magnitude diagram contains about 1000 point sources. We provide a preliminary transformation of the near-IR photometry into the ground system...

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arXiv1999

On the star-formation properties of emission-line galaxies in and around voids

We present a study of the star formation properties of a sample of emission line galaxies (ELGs) with respect to their environment. This study is part of a bigger project that aimed to find galaxies in voids and to investigate the large scale structure of the ELGs. A survey for ELGs was therefore conducted with the result that 16 galaxies have been found in very low density environments, of which 8 ELGs were found in two very well defined nearby voids. The sample presented here contains some galaxies identified in voids, as well as in the field environment that delimited the voids. These ELGs are all Blue Compact Galaxies (BCGs), and all void galaxies are also dwarfs. Both void and field sample contain the same mixture of morphological subtypes of BCDs, from the extreme Searle-Sargent galaxies to the Dwarf-Amorphous Nuclear-Starburst galaxies. The main result of this study is that field and void galaxies seem to have similar star formation rates (SFR), similar ratios between the current SFR and their average past SFR and similar mean SFR surface densities. There is no trend in metallicity, in the sense that void galaxies would have lower metallicities than their field counterparts. The field-cluster dichotomy is also discussed using available results from the literature, since our sample does not cover the cluster environment.

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arXiv1999

A Stellar Population Gradient in VII Zw 403 - Implications for the Formation of Blue Compact Dwarf Galaxies

We present evidence for the existence of an old stellar halo in the Blue Compact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarf galaxy for which a clear spatial segregation of the resolved stellar content into a "core-halo" structure is detected. Multicolor HST/WFPC2 observations indicate that active star formation occurs in the central region, but is strikingly absent at large radii. Instead, a globular-cluster-like red giant branch suggests the presence of an old (> 10 Gyr) and metal poor (<[Fe/H]>=-1.92) stellar population in the halo. While the vast majority of Blue Compact Dwarf galaxies has been recognized to possess halos of red color in ground-based surface photometry, our observations of VII Zw 403 establish for the first time a direct correspondence between a red halo color and the presence of old, red giant stars. If the halos of Blue Compact Dwarf galaxies are all home to such ancient stellar populations, then the fossil record conflicts with delayed-formation scenarios for dwarfs.

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arXiv1998

Fundamental properties of the new dwarf galaxy And VI - alias Pegasus Dwarf - another companion of M31

We present medium deep CCD imaging in B, V, and I of the Pegasus Dwarf galaxy (And VI) which was recently found by Karachentsev & Karachentseva (1998), and independently also by Armandroff et al. (1999). The Calar Alto 2.2m images show a low surface brightness galaxy. Its structure resembles that of the other known dSph companions of M31 And I, II, III, and V. The brightest stars are resolved in all three colors. Color-magnitude diagrams in either B-V or V-I show the tip of the red giant branch which allows us to estimate a true distance modulus of $24.5 \pm 0.2$. The color-magnitude diagrams and the structure show no evidence for recent star formation, thus, a classification as spheroidal dwarf galaxy with a rather old population seems appropriate. The total absolute magnitude of this dwarf is $M_{V,0} = -10.4\pm0.2$.

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arXiv1998

The stellar content of 10 dwarf irregular galaxies

We examine the stellar content of 10 dwarf irregular galaxies of which broad-band CCD photometry was published in Hopp & Schulte-Ladbeck (1995). We also present Halpha images for several of these galaxies. The galaxies in the sample are located outside of the Local Group. Yet, they are still close enough to be resolved into single stars from the ground but only the brightest stars (or star clusters) are detected and there is severe crowding. The sample galaxies were selected to be isolated from massive neighbors; about half of them are (mostly peripheral) members of groups, the other half is located in the field. We discuss the vicinity of the sample galaxies to other dwarf galaxies. In order to interpret single-star photometry and draw conclusions about the stellar content or other distance-dependent quantities, it is crucial that accurate distances to the galaxies be known. The distances to the sample galaxies are not well known since all but one have not had a primary distance indicator measured. We make an attempt to constrain the distances by identifying the envelope of the brightest supergiants in B, B-R and R, B-R color-magnitude diagrams, but the results are not very accurate (we estimate the minimal error on the distance modulus is 1.36 mag). Nevertheless, the fact that the sample galaxies are resolved with direct ground-based imaging indicates that they are sufficiently nearby to represent good candidates for observations with instruments that provide high spatial resolution, e.g., adaptive optics systems on large ground-based telescopes, or the Hubble Space Telescope. ...

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arXiv1998

Distance, structure and bright stellar content of the dwarf irregular galaxy UGC 685

B,R, and J frames taken with the Calar Alto 3.5m telescope of the dwarf irregular galaxy UGC 685 are presented. The brightest part of the stellar population is resolved in B and R, very few also in J. The stellar color-magnitude diagram is discussed. An estimate of the distance to UGC 685 of 5.5 Mpc is derived based on the brightest blue supergiant stars. Most of the resolved bright stars show colors in the range $-0.1 \le B-R \le +0.7$. The continuum light of the unresolved stars is distributed rather regular in B, R, and J, showing only minor irregularities. This surface brightness distribution follows an exponential law in all three colors with a scale length $r_c = 0.33$ kpc. The central surface brightnesses are $21.57 \pm 0.09 mag/\Box''$, $20.65 \pm 0.06mag/\Box''$, and $20.11 \pm 0.11 mag/\Box''$, in B, R, and J, respectively. The surface brightness can be traced out to 5 $r_c$ in B and R. Thus, UGC 685 belongs to the class of dwarfs where the HI distribution is much more extended (here 2.6 times) than the optical (stellar) light distribution, but contrary to many objects of this type, it does not belong to the class of low-surface brightness objects. The overall colors are $B-R = + 0.97$, $B-J = +1.55$ and the magnitude is B = 14.55 ($M_B^o$ = -14.5). The classification as an irregular dwarf results from the few HII regions of UGC 685. On an H$α$ Calar Alto 2.2m telescope CCD image, I identified only 5 HII regions. The total H$α$ flux corresponds to a mildly on-going star formation with a rate of 0.003 \Msolar yr$^{-1}$, a low rate even in comparison to other dwarfs. The available data do not indicate any major deviation from this rather low star formation rate within the last $10^9$ yr.

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arXiv1997

SN 1995ah-the first supernova observed in a Blue Compact Dwarf galaxy

We present the properties of the supernova SN 1995ah discovered in a Blue Compact Dwarf galaxy (BCD) around 10 days after the maximum. This is the first supernova event observed in a BCD. The photometric and spectroscopic data suggest that SN 1995ah is a Type II supernova and could belong to the rare Bright SNII Linear subclass, for which ~ =-18.9+-0.6 at maximum light.

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arXiv1997

Results of a search for emission-line galaxies towards nearby voids. The spatial distribution

We present the results of a search for emission-line galaxies (ELGs) towards nearby voids, based on a sample selected on the HQS (Hamburg Quasar Survey) - III-aJ objective-prism plates. The survey was based on the presence of emission-lines and therefore has the advantage to detect very faint objects (with all the flux in the emission-lines and almost no continuum), that would be missed by an apparent magnitude limited survey. We found objects as faint as B=20.5 and M(B)=(-15.0,-12.0). The observational data are given in Popescu et al. (1996) and in the present paper we consider a complete subsample from these data. We analyse the spatial distribution of our sample of ELGs in comparison with the distribution of normal galaxies. Overall both distributions trace the same structures. Nevertheless we also found a few ELGs in voids, from which at least 8 lie in the very well defined nearby voids. The isolated galaxies seem to form fainter structures that devide the larger voids into smaller ones. {}From our estimates of the expected number of void galaxies we conclude that we did not find an underlying homogenous void population.

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arXiv1997

Properties of Galaxies in and around Voids

Two surveys for intrinsically faint galaxies towards nearby voids have been conducted at the MPI für Astronomie, Heidelberg. One selected targets from a new diameter limited ($Φ\ge 5''$) catalog with morphological criteria while the other used digitized objective prism Schmidt plates to select mainly HII dwarf galaxies. For some 450 galaxies, redshifts and other optical data were obtained. We studied the spatial distribution of the sample objects, their luminosity function, and their intrinsic properties. Most of the galaxies belong to already well known sheets and filaments. But we found about a dozen highly isolated galaxies in each sample (nearest neighborhood distance $\ge 3 h_{75}^{-1} Mpc$). These tend to populate additional structures and are not distributed homogeneously throughout the voids. As our results on 'void galaxies' still suffer from small sample statistics, I also tried to combine similar existing surveys of nearby voids to get further hints on the larger structure and on the luminosity function of the isolated galaxies. No differences in the luminosity function of sheet and void galaxies could be found. The optical and infrared properties of both samples are in the normal range for samples dominated by late-type dwarfs. Follow-up HI studies show that the isolated dwarfs in both samples have unusual high amount of neutral gas for a given luminosity.

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arXiv1996

SBS 0335-052W - a new, extremely low metallicity dwarf galaxy

We present Multiple Mirror Telescope (MMT) spectrophotometry and 3.5m Calar Alto telescope R, I photometry of the western component of the extremely low-metallicity blue compact galaxy SBS 0335-052. We argue that both components, separated by 24 kpc, are members of an unique, physically connected system. It is shown that SBS 0335-052W has the same redshift as SBS 0335-052 and has an extremely low oxygen abundance, 12 + log (O/H) = 7.45+-0.22, comparable to those in I Zw 18 and SBS 0335--052. The (R-I) color profiles are very blue in both galaxies due to the combined effect of ionized gas and a young stellar population. We argue that SBS 0335--052W is likely to be a nearby young dwarf galaxy with age not exceeding 10^8 yr. The implications of this conclusion to the question of galaxy formation and to the properties of primeval dwarf galaxies are discussed.

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arXiv1995

Search for emission-line galaxies towards nearby voids. Observational data

We present the observational results of our search for emission-line galaxies (ELG) towards nearby voids. In order to find ELG, we started a survey using the IIIa-J objective prism plates from the Hamburg QSO Survey. The plates are digitized and an automatic procedure was applied to select the candidates. Digitized direct plates were used to determine coordinates and to reject overlaps between spectra. A total area of 1248 square degrees was scanned, distributed in four different regions. All the selected objects were observed with follow-up spectroscopy. We have obtained a final sample of 203 objects, of which 196 are emission-line galaxies, four are galaxies with absorption lines and three are QSOs. Almost half of our objects are newly discovered ones and three quarters of the given redshifts are new. Our sample contains mainly high ionization galaxies and is less sensitive in the detection of low-ionization objects. The apparent magnitudes, as derived from the objective prism plates, range between 15.0<B<19.5. The sample is dominated by nearby galaxies, with a peak in the redshift distribution at cz=4500\,km/s.

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