[Paper Review] The central region of the Fornax cluster -- I. A catalog and photometric properties of galaxies in selected CCD fields

M. Hilker, M. Kissler‐Patig+3 more|ArXiv.org|Jul 14, 1998

Impact of Light on Environment and Health1 references18 citations

TL;DR

This paper presents a deep $V$ and $I$-band photometric catalog of 1,144 galaxies in 11 CCD fields covering 0.17 deg² in the central Fornax cluster, reaching a limiting surface brightness of 24 mag arcsec⁻² and a total magnitude limit of $V \simeq 22$. It confirms that Fornax dwarf galaxies follow the same surface brightness–magnitude and color–magnitude relations as Local Group dwarfs, and rules out a true excess of dwarfs around NGC 1399, identifying a displaced overdensity as a background cluster at $z=0.11$. The study provides a comprehensive photometric catalog and surface brightness profile fits for a sub-sample, enabling detailed analysis of galaxy populations in the cluster core.

ABSTRACT

We present a photometric catalog (based on V and I photometry) of galaxies in the central regions of the Fornax galaxy cluster. Our 11 CCD fields cover 0.17 degrees in total. The limiting surface brightness is around 24 mag arsec^-2, similar to that of Ferguson's (1989, AJ 98, 367) catalog, whereas our limiting total magnitude is around V = 22 mag, about two magnitudes fainter. It is the surface brightness limit, however, that prevents us from detecting the counterparts of the faintest Local Group dwarf spheroidals. The photometric properties of all objects are presented as a catalog. The properties and fit parameters of the surface brightness profiles for a sub-sample are presented as a second catalog (both catalogs are available in electronic form at the CDS). We can only add 4 new dwarf galaxies to Ferguson's catalog. However, we confirm that the dwarf galaxies in Fornax follow a similar surface brightness-magnitude relation as the Local Group dwarfs. They also follow the color (metallicity) - relation seen in other galaxy clusters. A formerly suspected excess of dwarf galaxies surrounding the central cD galaxy NGC 1399 can finally be ruled out. An enhanced density of objects around NGC 1399 can indeed be seen, but it appears displaced with respect to the central galaxy and is identified as a background cluster at z = 0.11 in Paper II of these series, which will discuss spectroscopic results for our sample.

Motivation & Objective

• To create a deep, high-spatial-resolution photometric catalog of galaxies in the central Fornax cluster using $V$ and $I$-band CCD imaging.
• To measure the photometric properties of galaxies, including total magnitudes, surface brightness profiles, and colors, down to $V \simeq 22$ mag and surface brightness $\sim 24$ mag arcsec⁻².
• To investigate the spatial distribution and luminosity function of dwarf galaxies near the central cD galaxy NGC 1399, particularly testing for an excess population.
• To compare the photometric and structural properties of Fornax dwarf galaxies with those in the Local Group to assess similarities in scaling relations.
• To identify and characterize background structures, such as the $z=0.11$ cluster responsible for a displaced overdensity near NGC 1399.

Proposed method

• CCD imaging in $V$ and $I$ bands was conducted over 11 fields covering 0.17 deg² in the Fornax cluster, including fields centered on NGC 1399 and other bright galaxies.
• Source extraction and photometry were performed using SExtractor to derive total magnitudes, peak surface brightness, $V-I$ colors, ellipticity, position angle, and isophotal sizes.
• Surface brightness profiles were fitted using Sersic, de Vaucouleurs, and exponential laws to characterize galaxy structure, with Kormendy’s corrections applied for core radius and central surface brightness.
• Galaxy positions were cross-identified using the Guide Star Catalog with 3-plate constant transformations, achieving sub-0.3″ positional accuracy.
• A background estimation was performed using control fields (B1 and B2), though B2 is contaminated by Fornax members, limiting its use as a true background reference.
• Cross-identification with existing catalogs (FCC, C87, D88&I90) was performed to link new detections to prior work.

Experimental results

Research questions

• RQ1 Do dwarf galaxies in the Fornax cluster follow the same surface brightness–magnitude relation as those in the Local Group?
• RQ2 Is there a true excess of dwarf galaxies in the immediate vicinity of the central cD galaxy NGC 1399, as previously suspected?
• RQ3 What is the structural and photometric nature of the displaced overdensity observed near NGC 1399, and is it a background structure?
• RQ4 How do the $V$ and $I$ photometric properties of Fornax galaxies compare to those in other galaxy clusters?
• RQ5 What is the completeness and reliability of the photometric catalog in detecting faint, low-surface-brightness galaxies?

Key findings

• The photometric catalog contains 1,144 galaxies with $V < 22$ mag, covering 0.17 deg², and reaches a limiting surface brightness of 24 mag arcsec⁻².
• Only four new dwarf galaxies were added to Ferguson’s (1989) Fornax Cluster Catalog, indicating high completeness in previous surveys for this magnitude range.
• Fornax dwarf galaxies follow the same surface brightness–magnitude relation as Local Group dwarfs, supporting a common formation or evolutionary pathway.
• The color–magnitude relation (metallicity–magnitude relation) observed in other clusters is also present in the Fornax cluster, indicating similar metallicities for galaxies of similar luminosity.
• The previously suspected excess of dwarf galaxies around NGC 1399 is ruled out; the observed overdensity is instead identified as a background cluster at $z=0.11$ in Paper II.
• The surface brightness profile fits using Sersic, de Vaucouleurs, and exponential laws provide structural parameters such as effective radius, central surface brightness, and Sersic index $n$, enabling detailed morphological classification.